Durability of High Level Waste and Spent Fuel Disposal ...

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Appendix C: Container Fabrication

Durability of High Level Waste and Spent Fuel Disposal Containers – an overview of the combined effect of chemical and mechanical degradation mechanisms Appendix C: Container Fabrication

Fraser King

AMEC Report Reference 17697/TR/03 Appendix C

Partner Reference QRS-1589A-R1_AppC

Client Name RWM

Issue Number Issue 2

Report Date December 2016

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DOCUMENT ISSUE RECORD

Document title Container Durability Report: Appendix C: Container Fabrication

Project Reference 17697/TR/03

Purpose of Issue External Publication

Security Class Official

Issue Description Author Checker Approver Date

Draft 1 Draft for comment

Fraser King David

Sanderson, Sarah Watson

October

2015

Issue 2

Updated in response to RWM review comments

Fraser King Sarah Watson December

2016

Previous issues of this document shall be destroyed or marked SUPERSEDED

©Amec Foster Wheeler Nuclear UK Limited 2016

This report was prepared exclusively for RWM by Amec Foster Wheeler Nuclear UK Limited. The quality of information, conclusions and estimates contained herein is consistent with the level of effort involved in Amec Foster Wheeler’s services and based on: i) information available at the time of preparation, ii) data supplied by outside sources and iii) the assumptions, conditions and qualifications set forth in this report. This report is intended to be used by RWM only, subject to the terms and conditions of its contract with Amec Foster Wheeler. Any other use of, or reliance on, this report by any third party is at that party’s sole risk.

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CONTENTS C.1 Introduction ................................................................................................................ 7

C.2 Container design ....................................................................................................... 7

C.2.1 Single wall .............................................................................................................. 8

C.2.2 Dual wall ............................................................................................................... 11

C.2.3 Supercontainers .................................................................................................... 14

C.3 Fabrication of container components ....................................................................... 17

C.3.1 Container body ..................................................................................................... 17

C.3.2 Container lids ........................................................................................................ 21

C.3.3 Internal furniture .................................................................................................... 23

C.4 Container sealing .................................................................................................... 23

C.4.1 Welding ................................................................................................................. 23

C.4.2 Inspection ............................................................................................................. 24

C.4.3 Inerting and Leak Testing...................................................................................... 25

C.4.4 Residual stress and post-weld stress relief ........................................................... 26

C.4.5 Defects, flaws, and discontinuities ........................................................................ 28

C.4.6 Juvenile Failures ................................................................................................... 29

C.5 International programmes ........................................................................................ 30

C.5.1 Sweden ................................................................................................................. 30

C.5.2 Finland .................................................................................................................. 31

C.5.3 France .................................................................................................................. 32

C.5.4 Belgium ................................................................................................................. 34

C.5.5 Canada ................................................................................................................. 35

C.5.6 Switzerland ........................................................................................................... 36

C.5.7 Japan .................................................................................................................... 37

C.5.8 Korea .................................................................................................................... 39

C.5.9 USA ...................................................................................................................... 40

References ......................................................................................................................... 41

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TABLES AND FIGURES Figure C.1: Proposed Design for a Carbon Steel Variant 2 Disposal Container for AGR Spent Fuel (Arup 2014). ........................................................................................................ 9

Figure C.2: Photograph of a Packed-particulate Titanium Shell Container (Forsberg 1997). The container is positioned on a vibratory bed to ensure proper packing of the particulate material. .............................................................................................................................. 10

Figure C.3: Proposed Designs for a Ceramic HLW Container (Baroux and Martin 2013). ... 11

Figure C.4: Individual Components of the KBS-3 Copper Cast Iron Dual-wall Container Design. ............................................................................................................................... 12

Figure C.5: Transport, Aging, and Disposal (TAD) Waste Package Design for Commercial Spent Fuel for the Yucca Mountain Project (DOE 2008). .................................................... 13

Figure C.6: Canadian Copper-coated Steel Container Design (Hatton 2015). ..................... 14

Figure C.7: KBS-3H Supercontainer Components (Anttila et al. 2008). ............................... 15

Figure C.8: Assembly of a Prototype KBS-3H Supercontainer (Anttila et al. 2008). Anticlockwise from top left – assembly of the bentonite rings over the vertical copper container, the completed supercontainer assembly with perforated steel shell, and the supercontainer inside the transport container. ..................................................................... 16

Figure C.9: Conceptual Design of the Belgian Supercontainer (Bel et al. 2006). ................. 17

Figure C.10: Extrusion of a Copper Tube (SKB 2010). ........................................................ 18

Figure C.11: Sequence of Fabrication Steps in the Manufacture of an Alloy 22 Waste Package from Rolled and Welded Plate Material (DOE 2008). ............................................ 19

Figure C.12: Quenching of a Prototype Alloy 22 Waste Package Following Solution Annealing at 1120°C (2050°F) (Sandia 2007a). .................................................................. 20

Figure C.13: A Full-scale Copper-coated Steel SF Container for CANDU® Spent Fuel (P. Keech, NWMO, private communication). The top head and body of the carbon steel inner vessel were coated separately using a cold spray technique, welded together, cold spray was applied to the final closure weld and then the entire container was machined. ............. 21

Figure C.14: Fabrication of Copper Lids by Forging and Machining (SKB 2010). ................ 22

Figure C.15: Cast Iron Insert and Carbon Steel Lid for the KBS-3 Container (SKB 2010). .. 23

Figure C.16: Cross Section Through a Friction Stir Weld Between a KBS-3 Container Lid and Wall Mock-up (Andrews 2004). .................................................................................... 24

Figure C.17: Radiographic Inspection of a KBS-3 Copper Container Closure Weld Mock-up (Pitkänen 2010). .................................................................................................................. 25

Figure C.18: Ultrasonic Inspection of a KBS-3 Copper Container Closure Weld Mock-up (Pitkänen 2010). .................................................................................................................. 25

Figure C.19: A Partly Installed Purge Port Plug in the Inner Stainless Steel Lid of YMP Waste Package Highlighting the Holes for Inerting the Container (Skinner et al. 2005). ...... 26

Figure C.20: Test Rig for Leak Testing the Inner Lid Welds Following Inerting and Prior to Welding the Outer Lid (Skinner et al. 2005). ........................................................................ 26

Figure C.21: Schematic Illustration of the Laser Peening Process for the Reduction of Weld Residual Stress (Hill et al. 2003). ........................................................................................ 27

Figure C.22: Schematic Illustration of the Low Plasticity Burnishing Process for the Reduction of Weld Residual Stress (BSC 2005). ................................................................. 28

Figure C.23: Possible Flaw Types in KBS-3 Copper Container Friction Stir Welds (SKB 2010). ................................................................................................................................. 29

Figure C.24: Interior View of SKB’s Canister Laboratory at Oskarshamn. ........................... 31

Figure C.25: Pierce and Drawing of the Carbon Steel Shell of a Four Fuel Assembly Spent Fuel Container (Andra 2005). .............................................................................................. 32

Figure C.26: Grey Cast Iron Inserts for Four Fuel Assemblies With (Left) or Without (Right) Cladding (Andra 2005). ....................................................................................................... 32

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Figure C.27: Polished Cross Section Through an Electron Beam Weld Between the Container Lid and Shell (Andra 2005). Although no scale is provided in Andra (2005), the “usefull thickness” (sic) indicated in the figure would correspond to the container shell thickness of 110 or 120 mm. ............................................................................................... 33

Figure C.28: Sintered Half-scale Alumino-silicate Ceramic HLW Container (Baroux and Martin 2013). The tape measure shown as a scale is extended to 1000 mm. .................... 34

Figure C.29: The Four Stages in the Manufacture of the Belgian Supercontainer (Bel et al. 2006). ................................................................................................................................. 35

Figure C.30: Calculated Distribution of Residual Stress (in MPa) in the Closure Weld of a Carbon Steel Container Following Electron Beam Welding (Patel et al. 2012). a) axial stress, b) hoop stress, c) through-thickness stress. ............................................................. 36

Figure C.31: Calculated Distribution of Residual Stress (in MPa) in the Closure Weld of a Carbon Steel Container Following NG-GTA Welding (Patel et al. 2012). a) axial stress, b) hoop stress, c) through-thickness stress. ............................................................................ 37

Figure C.32: Prototype Carbon Steel Container (JNC 2000). .............................................. 38

Figure C.33: Results of Welding Trials on Full-size Carbon Steel Weld Mock-ups (Asano et al. 2006b). Surface appearance (upper left) and cross-section (lower left) of TIG weld and surface appearance (upper right) and cross-section (lower centre and right) of EB weld. ... 39

Figure C.34: Comparison of the stress-strain behaviour (left) and elastic modulus (right) of three copper cold spray samples (#1 low-purity Cu on stainless steel substrate, #2 high-purity Cu on stainless steel substrate, #3 low-purity Cu on DCI cast iron substrate), and extruded copper (sample #4) (Lee et al. 2011). ................................................................... 39

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GLOSSARY OF TERMS

Acronym Definition

AECL Atomic Energy of Canada Limited

BWR Boiling Water Reactor

C-steel Carbon steel

EB Electron Beam (Welding)

EBS Engineered Barrier System

FSW Friction Stir Welding

GDF Geological Disposal Facility

GMAW Gas Metal Arc Welding

GTAW Gas Tungsten Arc Welding

HAZ Heat Affected Zone

HLW High Level Waste

MAG Metal Arc Welding

MIG Metal Inert Gas

NG-GTA Narrow-gap Tungsten Inert Gas (Welding)

NWMO Nuclear Waste Management Organisation (Canada)

PWHT Post Weld Heat Treatment

PWR Pressurised Water Reactor

SCC Stress Corrosion Cracking

SF Spent Fuel

TIG Tungsten Inert Gas (Welding)

UT Ultrasonic Testing

WMO Waste Management Organisation

YAG Yttrium Aluminium Garnet (Welding)

YMP Yucca Mountain Project

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C.1 Introduction

In a geological disposal facility (GDF), the container is one of the key engineered barriers in the overall multi-barrier system for the disposal of high-level waste (HLW) and spent fuel (SF). As such, the fabrication of the containers will be an important component of the eventual operation of a (GDF), regardless of whether fabrication occurs at a location away from the disposal site or fabrication and packaging occurs at the disposal site.

There is a strong link between the container design and container fabrication. In addition to fulfilling the safety functions associated with it, the container design must also clearly be amenable to fabrication. Container fabrication must be both feasible and economic. In terms of the feasibility of container fabrication, the design must be such that:

• the container can be fabricated and sealed using existing, proven methods; • sealing and inspection of the containers can be performed remotely in a shielded

facility; • the containers, and in some cases the associated buffer material, can be produced

with sufficient quality; • any adverse effects of fabrication on the subsequent performance of the container

are minimised; and • loaded containers ready for emplacement in a GDF can be produced at a sufficient

rate (typically of the order of one container per day).

From an economic standpoint, in addition to the cost of the raw materials, the container design and selected fabrication method should:

• minimise the number and duration of procedures required to be performed remotely; and

• not be so specialised that there are a limited number of suppliers.

Here, the available information on container fabrication from the nuclear waste management literature is reviewed. It is difficult to discuss container fabrication without first considering the design of the container. Different types of container design are described in Section C.2, although the intent of this section is not to describe in detail all of the different container designs that have been proposed. The description of the container fabrication procedures is divided into those that can be done outside of the hot cells prior to loading the HLW/SF (Section C.3) and those that must be performed remotely (Section C.4). Thus, Section C.3 covers aspects such as fabrication of the container body and closure lids, as well as fabrication of the internal furniture that holds the SF (or HLW containers). Section C.4 includes discussion of welding and inspection procedures, the presence and possible post-weld relief of residual stress, and the nature of flaws that may be present in the containers at the time of disposal. Finally, the status of HLW/SF container fabrication in various international nuclear waste management programmes is described in Section C.5.

C.2 Container Design

The design of the container can have a significant impact on how it is fabricated. However, in many international nuclear waste management programmes, the design has been driven by the safety function requirements and the expected mechanical and corrosion performance of the container, rather than by any consideration of its fabricability. Generally, considerations of how to manufacture, seal, and inspect the container have come later in the overall programme development.

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When developing a design for a HLW/SF container, it is common to define a number of design requirements. Many of these requirements relate to the container safety functions, but some relate specifically to manufacture of the container, for example (JNC, 2000; Patel et al., 2012):

• in general, the design should be such that the container can be manufactured using existing technology or technology that may reasonably be expected to be developed in the future;

• sealing should be possible remotely and not result in damage to the SF or HLW; • the level of residual stress from the sealing process should not increase the

probability of environmentally assisted cracking or should be reduced following sealing using a suitable technique which does not result in damage of the SF or HLW;

• the sealing procedure should not result in remaining near-critical defects; • a radiation-hardened, remote inspection procedure capable of detecting sub-critical

defects should be used; • the design should permit production of the containers in the encapsulation facility at a

sufficient rate, generally in the range of 1-2 containers/day; • an internal structure should be provided to support the SF or HLW.

C.2.1 Single Wall

In single-wall designs, the container shell generally acts as both the corrosion barrier and as the structural member. The minimum wall thickness is thus defined by the sum of the corrosion allowance necessary to provide containment of the radionuclide inventory for the required period and a “mechanical allowance” necessary to provide structural stability under the expected external loads. An additional allowance may also be added to the wall thickness to provide radiation shielding (JNC, 2000).

The most common single-wall design proposed to date is a thick-walled carbon steel (C-steel) container, as typified by designs developed in Japan (JNC, 2000), Switzerland (Patel et al., 2012), and the UK (Arup, 2014) (Figure C.1). The wall thickness is typically in the range of 120-200 mm and is sufficient to provide both mechanical and corrosion allowances and radiation shielding.

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Figure C.1: Proposed design for a carbon steel Variant 2 disposal container for AGR spent fuel (Arup, 2014).

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In some early designs, the use of some type of filler material was proposed to either create a monolithic structure or to support a relatively thin-walled shell. Figure C.2 shows a so-called packed-particulate container design developed in Canada (Johnson et al., 1994a,b) in which a particulate material (e.g., glass beads) was used to support the 4.76-mm-thick Ti Grade 2 shell. In a variant of this design (the so-called structurally supported container), the tubes holding the CANDU® spent fuel bundles were closed with welded caps and were designed to support the external load (Johnson et al., 1994b). In this case, the packed particulate serves to transfer the external load to the internal load-bearing tube structure. Various metallic materials have also been investigated as filler materials with the aim of producing a monolithic structure. The hot isostatic pressing of copper powder inside a copper shell was investigated by SKB as a means of fabricating a monolithic copper container (Lönnerberg et al., 1983; SKB, 1992). Lead and other low-melting-point alloys were also investigated as possible filler materials (Johnson et al., 1994b; Lönnerberg et al., 1983; SKB, 1992), although there would inevitably be shrinkage of the metal matrix upon cooling which left the top of the container unsupported.

Figure C.2: Photograph of a packed-particulate titanium shell container (Forsberg, 1997). The container is positioned on a vibratory bed to ensure proper packing of the particulate material.

A single-wall design has also been proposed for a ceramic HLW container (Baroux and Martin, 2013). Figure C.3 illustrates a number of preliminary designs based on a wall thickness of 40-50 mm and an overall length of approximately 1.75 m, suitable for accommodating a single stainless steel HLW canister. The container is designed with a hemi-spherical head to take advantage of the excellent compressive strength of ceramics.

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Figure C.3: Proposed designs for a ceramic HLW container (Baroux and Martin, 2013).

C.2.2 Dual Wall

In a dual-wall design, an outer corrosion barrier is supported by an internal vessel which itself may provide additional containment. Unlike a single-wall design where the container wall acts as both the corrosion barrier and load-bearing structure, these two functions are separated in the dual-wall design.

The most well-known example of a dual-wall container design is the KBS-3 copper-cast iron container (Figure C.4) (SKB, 2010). In this case, the outer corrosion barrier is a 50-mm-thick oxygen-free with added phosphorus copper shell with an inner ductile cast iron load-bearing insert. In this design, the 1-2 mm gap between the insert and the copper shell will close by creep deformation once an external load is imposed in the GDF, effectively transferring the load onto the insert. Because the lid of the cast iron insert is bolted with the aid of a gasket, the containment function for the KBS-3 design is only provided by the welded copper shell.

Another example of a dual-wall design with separate inner and outer vessels is illustrated in Figure C.5. In this case, the transportation, aging, and disposal waste package for commercial SF in the Yucca Mountain Project (YMP) comprises a 25.4 mm (1 inch) thick outer Hastelloy C22 (Alloy 22) corrosion barrier and a 50.8 mm (2 inch) thick Type 316 stainless steel inner vessel. Both the outer corrosion barrier and the inner vessel are sealed by welding, thus providing two containment barriers. When the container is laid horizontally, the radial gap between the inner and outer vessels varies from zero at the bottom to approximately 10 mm at the top. When the container is stood vertically, the inner vessel is held above the bottom of the outer barrier by a combined interface/support ring so that there is no direct contact between the two vessels.

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Figure C.4: Individual components of the KBS-3 copper cast iron dual-wall container design.

An alternative to the use of two separate vessels for the dual-wall design is to apply a thin coating directly to the inner load-bearing structure. In the early years of the development of HLW/SF disposal programmes, some thought was put into the development of thin organic- or ceramic-based coatings, but it was soon realised that it would be difficult to avoid damage to the coating during handling and that a defected coating could be worse than no coating at all (Landolt et al., 2009). More recently, however, some effort has gone into the development of a relatively thick (3-5 mm) copper coating applied to a C-steel substrate (Choi et al., 2010; Hatton, 2015, Jakupi et al, 2015; Keech et al., 2014; Lee et al., 2011). The coating can be applied using a cold-spray technique or by electrodeposition. Figure C.6 shows a full-scale copper-coated container developed in Canada, along with the internal furniture used to hold 48 CANDU® SF bundles. The coated dual-wall design has a number of advantages over the double-vessel design, including the absence of a final closure weld in the corrosion barrier (the joint between the head and the shell can be coated in a shielded facility after the container has been loaded with SF) and the absence of a gap between the inner and outer barriers. Thus, there is no deformation of the copper corrosion barrier and no concern over creep. The particular design illustrated in Figure C.6 also has the advantage that the C-steel body is standard pipe material and the head can be welded to the body before final coating, providing an extra level of containment. In addition, the hemispherical head ensures compressive loading of the container during the post-closure period in the GDF. In principle, other corrosion-resistant materials could be coated onto the C-steel substrate. Furthermore, any material could be applied to a steel vessel using one of a number of widely used cladding methods (Holdsworth et al., 2014), although a cladded dual-wall container design has not yet been proposed by any waste management organisation (WMO).

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Figure C.5: Transport, aging, and disposal waste package design for commercial spent fuel for the Yucca Mountain Project (DOE, 2008).

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Figure C.6: Canadian copper-coated steel container design (Hatton, 2015).

C.2.3 Supercontainers

A supercontainer1 is the combination of a metallic container and pre-applied buffer material, fabricated and then emplaced as a single component. Such a design has the advantage of better quality control over the placement of the buffer, but has the disadvantage that the combination of container and buffer results in a large and heavy object to handle and emplace. Two supercontainer designs have been proposed to date; a copper-compacted bentonite buffer design developed in Finland and Sweden (Anttila et al., 2008) and a C-steel-cementitious backfill design developed in Belgium (Bel et al., 2006).

In the KBS-3H supercontainer design, rings of compacted bentonite buffer are placed around a copper container and the entire assembly supported by a perforated steel shell (or other suitable arrangement, Figure C.7). One of the advantages of this approach is that the bentonite rings can be pre-fabricated in an un-shielded facility, providing better quality assurance over the placement of the buffer than if the operation has to be done in situ as is envisaged for the KBS-3V vertical emplacement of containers in boreholes. A prototype supercontainer of this type has been fabricated (Figure C.8) (Anttila et al., 2008).

The Belgian supercontainer design arose from concerns regarding the long-term corrosion behaviour of stainless steel HLW containers in a GDF in Boom Clay (Ondraf/Niras, 2004). To prevent corrosion due to thiosulphate and other aggressive species produced by oxidation of pyrite in the Boom Clay, it was argued that the near field should be conditioned through the use of a cementitious backfill to ensure passivation of the container material. The supercontainer comprises a C-steel container (overpack), a Portland cement based buffer, and a Type 316L stainless steel liner (Figure C.9).

1 RWM uses the term prefabricated emplacement module (PEM) for such disposal assemblies.

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Figure C.7: KBS-3H supercontainer components (Anttila et al., 2008).

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Figure C.8: Assembly of a prototype KBS-3H supercontainer (Anttila et al., 2008). Anticlockwise from top left – assembly of the bentonite rings over the vertical copper container, the completed supercontainer assembly with perforated steel shell, and the supercontainer inside the transport container.

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Figure C.9: Conceptual design of the Belgian supercontainer (Bel et al., 2006).

C.3 Fabrication of Container Components

C.3.1 Container Body

Large cylindrical vessels, sometimes with a substantial wall thickness, are routinely manufactured for a number of industries. In this sense, the manufacture of HLW/SF containers does not represent a significant technological challenge, although some specific developments would undoubtedly be required. For example, various technologies exist for the manufacture of the container body, including (Patel et al. 2012)2:

• Casting; • Forging (of a single piece or of multiple pieces, with or without an integral base); • Pierce and draw (with or without an integral base); • Rolled plate.

Because of the possibility of preferential corrosion of welds, methods capable of producing a single long tube have an advantage over methods where welding would be required to join several shorter tubes (short forging) or to weld two or more rolled plates. Fabrication of a container body with an integral base would also reduce the number of welds required, although control of the metallurgical properties of the material may be challenging because of different rates of cooling of the body and integral base. Patel et al. (2012) concluded that all of the above methods were suitable for the fabrication of a thick-walled (180 mm thick)

2 Blank backward extrusion was also initially considered by Patel et al. (2012) but later excluded from the list of feasible methods for a carbon steel container because of size restrictions and the limited number of available suppliers.

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carbon steel HLW/SF container of the design specified by Nagra, although a long-forged (i.e., a single piece) body with a welded base was identified as the most favoured method at this time because of the high quality possible with hollow forgings.

The reference method selected by SKB for manufacturing the outer copper shell of the KBS-3 container design is to extrude a tube (Figure C.10) from a copper ingot and to weld on forged bottom and top lids.

Figure C.10: Extrusion of a copper tube (SKB 2010).

In the U.S. YMP, the Alloy 22 corrosion barrier was to be fabricated from rolled plate material, with longitudinal and circumferential welds to join the components for the body and additional circumferential welds to join the lids (Figure C.11). This, of course, has the disadvantage of introducing a number of longitudinal and circumferential welds into the corrosion barrier. A single prototype waste package was fabricated prior to the winding down of the programme (Sandia, 2007a).

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Figure C.11: Sequence of fabrication steps in the manufacture of an Alloy 22 waste package from rolled and welded plate material (DOE, 2008).

Prior to loading the HLW/SF, all of the pre-fabricated container components can be inspected, repaired (if necessary), machined, and stress relieved to ensure the desired quality and initial condition. For example, thermal treatments can be applied to either relieve residual stress or to produce the desired microstructural features without concern for violating the thermal constraints for the HLW/SF wasteform. Figure C.12 shows the transfer of an annealed Alloy 22 prototype waste package to a quench tank in order to produce the desired solution annealed treatment to optimise the localised corrosion properties of the outer corrosion barrier (Sandia, 2007a). Quenching from both sides of the container can bring the temperature down to 370°C (700°F) in 9 minutes to prevent distortion.

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Figure C.12: Quenching of a prototype Alloy 22 waste package following solution annealing at

1120°°°°C (2050°°°°F) (Sandia, 2007a).

As noted above, a different approach is being taken to the fabrication of a copper corrosion barrier supported by an inner iron-based structural component in the Canadian programme. Sufficient copper (of the order of 3-5 mm) would be coated on to a carbon steel inner vessel that would provide the structural support against the external loads. Both electrodeposition and a cold spray technique are being investigated for applying the coating and a combination of the two methods (electrodeposition of the head and main vessel with cold spray used to coat the final closure weld) could be used. Although a wide range of carbon steel structures could be coated, the Nuclear Waste Management Organisation of Canada (NWMO) are investigating the use of commercially available thick-walled carbon steel pipe as a cost-effective option for the inner vessel.3 Figure C.13 shows a full-scale copper-coated container recently fabricated by NWMO. In this case, both the hemispherical head and the body assembly (comprising a hemispherical bottom head welded to a 1.95 m length of NPS 22 Schedule 120 carbon steel pipe) were pre-coated using a high-pressure cold spray technique. The hemispherical head was then welded to the body assembly and the closure weld coated using cold spray. The entire container was then machined to produce the required surface finish. The manufacturing technology for a copper-coated container is still under development but the prospects for being able to manufacture cost-effective containers at the required rate and to the required quality look promising.

3 NWMO have considerable flexibility in the size of the container because the small size of a CANDU® spent fuel bundle (approximately 50 cm long by 10 cm diameter) lends itself to a number of packaging configurations.

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Figure C.13: A full-scale copper-coated steel SF Container for CANDU® spent fuel (courtesy of NWMO). The top head and body of the carbon steel inner vessel were coated separately using a cold spray technique, welded together, cold spray was applied to the final closure weld and then the entire container was machined.

C.3.2 Container Lids

Manufacture of the top (and bottom) lids for the container can be achieved through a number of conventional fabrication techniques. The technique commonly selected for thick heads is hot forging, where a billet of material is heated and then forged to the approximate shape (Figure C.14(a)). The rough blank would then be machined and, if necessary, heat treated to produce the desired material properties (Figure C.14(b)). Thinner heads could be cut from flat plate material and, of course, the hemispherical heads for the copper-coated container in Figure C.13 would require special machining.

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(a) Forging of a copper lid blank

(b) Machined copper lid

Figure C.14: Fabrication of copper lids by forging and machining (SKB 2010).

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C.3.3 Internal Furniture

Virtually all HLW/SF container designs include some sort of internal structure to ensure a stable configuration for the wasteform and, in some cases, to provide structural support to the outer corrosion barrier. As for the outer shell, existing manufacturing technologies can be either used or adapted for the fabrication of the internal furniture. For example, a system of welded box sections with welded top and bottom plates was proposed by Patel et al. (2012) as a simple and cost effective method to support pressurised water reactor (PWR) or boiling water reactor (BWR) fuel assemblies inside a thick-walled carbon steel container. A more substantial structure is required when the internal furniture is required to support the external load that develops in a GDF. A cast iron insert is used in the KBS-3 container design (Figure C.15) and more than 50 such inserts have been fabricated by SKB (SKB, 2010).

Figure C.15: Cast iron insert and carbon steel lid for the KBS-3 container (SKB 2010).

C.4 Container Sealing

All of the fabrication processes to the time of sealing can be done outside of shielded facilities using (generally) conventional manufacturing processes. The loading of the HLW/SF and all subsequent fabrication processes, however, must be performed in a shielded facility. The requirement to work remotely introduces an extra level of complexity to the manufacturing process and may limit the use of some technologies that are either not suited to remote operation or which cannot be radiation hardened.

C.4.1 Welding

Following loading of the container with the HLW/SF wasteform, the container lid is welded on to the container body in the hot cell. A range of welding methods have been investigated for the remote sealing of HLW/SF containers (Asano and Ito, 2008; Asano et al., 2005, 2006a,b; Pike et al., 2010; Johnson et al., 1994b; SKB, 2010; Meuronen and Salonen, 2010). Both SKB and Posiva have selected friction stir welding (FSW) for the final closure weld of the copper corrosion barrier of the KBS-3 container (SKB, 2010), although Posiva also

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considered the use of electron beam (EB) welding (Meuronen and Salonen, 2010). Trials have shown that both techniques can produce high quality welds in 5-cm-thick copper (Figure C.16), even though copper is challenging to weld because of its high thermal conductivity. Because it does not involve melting, FSW has the advantage over EB welding that there is limited grain growth of the copper (note the similar grain sizes of the weld and parent material in Figure C.16) and lower levels of residual stress.

Figure C.16: Cross section through a friction stir weld between a KBS-3 container lid and wall mock-up (Andrews, 2004).

Asano and co-workers investigated the use of EB, gas-tungsten arc welding (GTAW, also referred to as tungsten inert gas TIG), and gas metal arc welding (GMAW, also referred to as metal active gas MAG) for the welding of carbon steel up to 190 mm thick (Asano et al., 2005, 2006a,b). All three methods were found to be feasible in mock-up tests. For a similar application, Pike et al. (2010) concluded that narrow-gap GTAW and EB welding were the most suitable methods for welding thick-section carbon steel in the Nagra programme.

C.4.2 Inspection

After sealing, the final closure weld must be inspected, again remotely in a hot cell environment. The aim of the inspection step is to determine whether there are flaws present that exceed a specified size or population, which might lead to premature failure of the container. Weld flaws could be in the form of various types of inclusion, lack of fusion, solidification cracking, or excess porosity (Patel et al., 2012). Suitable techniques for the detection of sub-surface flaws include ultrasonic testing (UT) and radiographic techniques. Eddy current testing can be used for surface-breaking flaws. Figure C.17 and Figure C.18 show equipment for radiographic and ultrasonic inspection of welds in KBS-3 copper container mock-ups, respectively (Pitkänen, 2010).

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Figure C.17: Radiographic inspection of a KBS-3 copper container closure weld mock-up (Pitkänen 2010).

Figure C.18: Ultrasonic inspection of a KBS-3 copper container closure weld mock-up (Pitkänen 2010).

C.4.3 Inerting and Leak Testing

Some container fabrication processes require inerting (the addition of an inert gas inside the container) and leak testing. A number of WMOs specify the drying of the internal

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atmosphere of SF containers to remove any water carried over from wet storage in fuel assemblies previously stored under water. All of these processes require the provision of some sort of purging or filling port, generally on an inner vessel of a dual-wall container design.

Figure C.19 and Figure C.20 shows examples of a purge port and a leak test assembly developed in the YMP (Skinner et al., 2005).

Figure C.19: A partly installed purge port plug in the inner stainless steel lid of a YMP waste package highlighting the holes for inerting the container (Skinner et al., 2005).

Figure C.20: Test rig for leak testing the inner lid welds following inerting and prior to welding the outer lid (Skinner et al., 2005).

C.4.4 Residual Stress and Post-weld Stress Relief

Apart from the possible introduction of flaws, sealing the container may introduce a high level of residual stress into the closure weld. Post-weld stress relief may then be necessary to reduce the probability of environmentally assisted cracking. The usual practice of post-

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weld heat treatment (PWHT) may not be possible because there are thermal constraints for the HLW/SF, which may limit the PWHT soaking temperature (the temperature at which the container is maintained to relieve the stresses) and exposure time.4 Preliminary simulations for a Nagra carbon steel container with a PWHT soak temperature of 550-600°C indicated that the thermal limit of 450°C for HLW would be exceeded (Patel et al. 2012). However, the results of the analysis suggested that optimisation of the heat treatment process or changes to the container design (e.g., by increasing the distance between the heated band and the HLW/SF) should result in suitable stress relief without exceeding the thermal limits.

In the YMP, two non-thermal post-weld stress relief methods were investigated, laser peening and plasticity burnishing (BSC, 2005). In laser peening, the application of a laser to the surface of the test piece creates a high pressure plasma at the surface that imparts a shock wave into the material and creates a compressive layer (Figure C.21) (BSC, 2005; Hill et al., 2003). Plasticity burnishing involves plastically deforming the surface of the test piece using a hydraulically loaded ball. As the load is removed after the ball has passed over the surface, the surface relaxes into a compressive state (Figure C.22) (BSC, 2005).

Both methods are capable of imparting a surface layer of compressive residual stress with a minimum depth of 1.9 mm in Alloy 22 (BSC, 2005). For a passive material such as Alloy 22 with a correspondingly low rate of general corrosion, such a relatively thin compressive zone may be sufficient to suppress the initiation of environmentally assisted cracking for a considerable length of time. However, for a material that corrodes at a higher rate, such as carbon steel, it is likely that the layer of surface compressive residual stress would be lost by corrosion in a relatively short period.

Figure C.21: Schematic illustration of the laser peening process for the reduction of weld residual stress (Hill et al. 2003).

4 Nagra have defined thermal limits of 400°C for SF and 450°C for HLW (Patel et al. 2012).

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Figure C.22: Schematic illustration of the low plasticity burnishing process for the reduction of weld residual stress (BSC, 2005).

C.4.5 Defects, Flaws, and Discontinuities

It is inevitable that the sealed container will contain flaws that may or may not be detected by the inspection methods employed. Whether these flaws are large enough to cause an unacceptable loss of durability of the container (i.e., whether they should be classified as defects5) depends on the properties of the container material and the loads to which the container will be exposed during storage, handling, and disposal.

The nature of the flaws in the closure weld will depend on the welding procedure, weld design, and the material properties. For example, FSW of copper leads to two types of defect: joint line hooking and wormholes (Figure C.23) (SKB, 2010). Under non-optimised welding conditions, joint line cracks with a radial extension of 1.4-5.4 mm were found by destructive examination. The circumferential extent of these flaws (i.e., in the direction parallel to the side of the container) could be several tens of cm in length. Again under non-optimised welding conditions, wormholes with a radial extent of up to 10 mm have been observed. However, under optimised welding conditions, only joint line hooking with a maximum extent of 0.4-1.5 mm has been found. Electron-beam welds in copper lids were reported to be “almost defect-free” (Meuronen and Salonen, 2010). Further testing of lid welds indicated that all the flaws met the acceptance criteria (Raiko et al., 2012), which 5 The conventional NDT terminology is used here, in which the term “flaw” or “discontinuity” is used to refer to a feature that may or may not be detectable but which may be acceptable and the term “defect” is used to refer to one or more flaws whose aggregate size, shape, location, or orientation do not meet specified acceptance criteria.

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range from 3 mm for cracks to 8-10 mm for porosity, joint line hooking, lack of fusion, and inclusions (all dimensions in the radial direction) (Pitkänen, 2010).

Figure C.23: Possible flaw types in KBS-3 copper container friction stir welds (SKB 2010).

In the YMP, studies have been performed involving the UT and destructive examination of sixteen test welds on Alloy 22 mock-ups (Sandia, 2007a). The dimensions of the weld flaws found by UT inspection varied from 1.6-14 mm in depth, 1.6-3.2 mm in width, and 3-35 mm in (circumferential) length. Destructive examination did not locate any larger flaws and the only flaws found that were not detected by UT were minor gas pores less than 0.08 mm in diameter (Sandia, 2007a). Although the weld procedure was not fully developed, an acceptable flaw size of 1.6 mm (1/16 inch) was proposed, above which weld repair would be necessary.

C.4.6 Juvenile Failures

One consequence of the fabrication process is that there is a small but finite probability that a container could be emplaced in the GDF with an undetected, sufficiently large defect that a through-wall penetration would occur soon after closure. For performance assessment purposes, such failures are referred to as “juvenile failures” or the “initial failure fraction.” The fraction that may fail at short times is typically estimated as 1 in 1000 to 1 in 10,000 containers, based on the probability of defects in similar components not being detected during inspection.

There are various methods for estimating the potential fraction of juvenile failures. Doubt (1984) surveyed the early failure rate of similarly mass-produced objects from the nuclear and similar industries, such as CANDU® pressure tubes. An initial failure fraction of 1 in 5000 containers was estimated. Sandia (2007a) carried out a detailed study of the types of weld flaw that might exist and the reliability of non-destructive examination techniques for detecting such defects. The analysis lead to not only probability distributions for juvenile

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failures (median of 1 in 24,000 for waste packages and 1 in 2.3 million for drip shields), but also distributions of flaw size.

C.5 International Programmes

C.5.1 Sweden

Sweden (SKB) has by far the most developed container fabrication programme of any international WMO. The SKB container development programme covers all aspects of the fabrication process, including:

• Design and specification of material properties, including the assessment of fabrication feasibility for various container design options (SKB, 1992, 2010);

• Fabrication of prototype copper shells and lids and cast iron inserts (Andersson, 2002; Andersson et al., 2004; SKB, 2010);

• Encapsulation, transportation, and handling of the container (SKB, 2010); • Container joining studies using electron-beam and friction-stir welding (Andrews,

2004; Cederqvist, 2004; Claesson and Ronnetag, 2003; SKB, 2010); • Inspection of closure welds and inserts by ultrasound and eddy current techniques

(Lingvall et al., 2003; Stepinski, 2004; Stepinski et al., 2001, 2004a,b; Wu et al., 2000);

• Assessment of the reliability of the detection and sizing of defects (Müller and Öberg, 2004; Müller et al., 2006; Ronnetag et al, 2006);

• Assessment of weld defects (King 2004, Raikko et al. 2010); • Mechanical and corrosion testing of weld material (Andersson et al., 2005; Gubner

and Andersson, 2007; Gubner et al., 2006); • Pressure testing of container and insert (Nilsson et al., 2005).

These studies have been carried out over a period of over 20 years and have resulted in the fabrication of 55 cast iron inserts as of 2008 (47 for BWR SF and 8 for PWR SF) (SKB, 2010). In addition, a total of 40 large copper ingots and 27 extruded copper tubes had also been fabricated. A specific Canister Laboratory (CLAB) has been established at Oskarshamn (Figure C.24) at which the container fabrication technology is developed. SKB also have plans for an encapsulation plant that would be built close to the interim storage site CLAB. The plant would be capable of delivering approximately 200 loaded containers per year, which would then be shipped by sea to the proposed GDF site at Forsmark.

Further details of the SKB container fabrication programme can be found in Sections C.3 and C.4 or in the references cited above.

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Figure C.24: Interior view of SKB’s canister laboratory at Oskarshamn.

C.5.2 Finland

Although the Finnish programme is closely linked to SKB’s programme, Posiva has also conducted separate fabrication studies. In addition to extensive work on the development of the KBS-3H supercontainer concept (Figure C.7), Posiva has also:

• Developed specific container insert designs for VVER SF (Raikko, 2013); • Investigated various welding techniques for thick copper sections, including narrow

gap arc welding, EB welding, and FSW (Meuronen and Salonen, 2010; Pohja et al., 2003);

• Fabricated a number of copper shells and cast iron inserts (Koivula, 2005; Nolvi ,2009; Raikko, 2003, 2005);

• Investigated visual, ultrasonic, radiographic, and eddy current methods for inspecting bottom and lid welds for copper containers, as well as reliability and quality control protocols (Pikänen, 2010, 2012; Pitkänen et al., 2007);

• Developed specifications and protocols for the fabrication of containers (Raikko et al. 2012);

• Measured residual stress levels in copper plate and EB welds (Gripenburg, 2009).

Posiva’s reference procedure for the fabrication of the copper container is the pierce and draw method, in which the base is formed as an integral part of the body and only the lid needs to be welded to the copper shell (Nolvi, 2009). At one time, Posiva favoured EB welding over FSW (the reference technique for SKB), but the finding of relatively high residual stresses (Gripenberg, 2009) caused Posiva to switch to FSW for the closure weld.

As a result of their complementary container development programmes, Posiva and SKB have been able to demonstrate that it is feasible to fabricate containers of the KBS-3 design with the required quality.

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C.5.3 France

The development of fabrication procedures for C-steel HLW and SF containers in the French programme is summarised in Andra (2005). A non-alloy steel (Grade P235) with minimum room temperature yield and ultimate tensile strengths of 200 MPa and 360 MPa, respectively, was selected for all container types. Two types of SF container design have been developed, a small-diameter version capable of holding a single fuel assembly and a wide-diameter version to hold four fuel assemblies. The container wall thickness varies from a minimum of 55 mm for the Type C HLW container to 110-120 mm for the SF containers. The fabrication methods would be similar for both types of container, although the focus here is on the wide-body SF design, of which a prototype container has been manufactured.

Two suitable methods have been identified for manufacturing the container body, piercing and drawing the shell and attaching a bottom lid and drilling and drawing the shell with an integral base, with the former method selected for the prototype demonstration (Figure C.25). A grey cast iron insert is used to hold the fuel assemblies (both circular and square cross section) (Figure C.26), but is not required to provide structural strength which is provided by the shell itself.

Figure C.25: Pierce and drawing of the carbon steel shell of a four fuel assembly spent fuel container (Andra, 2005).

Figure C.26: Grey cast iron inserts for four fuel assemblies with (left) or without (right) cladding (Andra 2005).

Once the fuel has been loaded into the container, a closure plug consisting of a 10-mm-thick C-steel plate would be attached to the container by metal inert gas (MIG) welding. Welding of the closure plug would be performed in a shielded facility, but not under vacuum. The container would then be rotated and partially inserted into a vacuum chamber and the lid

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attached to the base using EB welding. Andra (2005) considered several welding techniques for the final closure weld, including MIG, TIG, MAG, yttrium aluminium garnet (YAG) laser, FSW, and EB welding. Of these techniques, FSW has not yet been demonstrated for C-steel, YAG laser welding is still under development and currently limited to a maximum component thickness of 10 mm, and MIG, TIG, and MAG are multipass welding techniques requiring the use of a filler material and creating extensive heat affected zones (HAZ). The need for multiple weld passes would slow the rate at which containers could be sealed in the shielded facility and the use of a filler material introduces a potential source of hydrogen (from the welding rod and the flux used to prevent oxidation of the weld surfaces), which could lead to cold cracking. Therefore, the preferred technique is EB welding, which is faster (single pass), has been demonstrated for thicknesses up to 200 mm, has a smaller HAZ, and has a lower H input (as it is performed in vacuum and because no filler material is required). Figure C.27 shows a cross section through a completed EB weld between the head and shell of the prototype container. Other than stating that no macroscopic or microscopic defects were observed, Andra (2005) provides no information about inspection methods or the possibility of weld defects.

Figure C.27: Polished cross section through an electron beam weld between the container lid and shell (Andra, 2005). Although no scale is provided in Andra (2005), the “usefull thickness” (sic) indicated in the figure would correspond to the container shell thickness of 110 or 120 mm.

As part of their programme on ceramic container materials, Andra have produced half-scale ceramic container components (Baroux and Martin, 2013). The body and head shown in Figure C.28 were first cast in a plaster mould using a ceramic slurry under pressure to ensure that the mould was completely filled and to prevent voids. The mould is oversized to account for the 11% shrinkage that occurs when the ceramic is fired. The slurry “sets up” as water is absorbed into the mould. A further drying step eliminates additional water, after which the ceramic is fired in a two-stage process. Initial low-temperature (bisque) firing at 900°C starts to develop the glass structure and also reveals casting defects as cracks or scales in the fired component. The second, high-temperature (1280°C) sintering stage fully develops the ceramic grain structure.

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Figure C.28: Sintered half-scale alumino-silicate ceramic HLW container (Baroux and Martin 2013). The tape measure shown as a scale is extended to 1000 mm.

Baroux and Martin (2013) also carried our preliminary tests on sealing the ceramic lid to the base. Three bonding techniques were investigated, namely: laser sintering, spark-plasma sintering, and cold bonding. However, all techniques require temperatures in excess of the thermal limit for the HLW (450°C), and there is currently no demonstrated method for joining the ceramic parts of the container without violating this thermal limit.

Preliminary inspection trials were also performed using ultrasonic methods and manufactured defects (Baroux and Martin, 2013).

Despite their progress to date, Baroux and Martin (2013) concluded that the production of a full-scale ceramic container is still some time away, not least because of the joining issues.

C.5.4 Belgium

As discussed in Section C.2.3, the Belgium programme is based on a supercontainer concept comprising a C-steel container (overpack) surrounded by a cementitious buffer. In one design variant, a modified Type 316L stainless steel (with an enhanced Mo content) liner would be used to surround the cement buffer. Portland cement buffer is poured into a mould created by the stainless steel liner to create a pre-fabricated cementitious vessel (Stage 1, Figure C.29) (Bel et al. 2006). This vessel would then be transferred to the shielded facility and the loaded and sealed overpack would be placed inside (Stage 2) and the gap between the overpack and cement buffer filled with a filler material (Stage 3). Finally, additional concrete would be poured over the top of the overpack to create the cementitious buffer (Stage 4). Trials have been conducted to demonstrate the feasibility of forming the concrete buffer (Van Humbeeck et al. 2007) and a half-scale prototype supercontainer, with a heater to simulate the overpack, has been constructed (Kursten et al. 2014).

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Figure C.29: The four stages in the manufacture of the Belgian supercontainer (Bel et al. 2006).

C.5.5 Canada

The early container fabrication studies performed in Canada by Atomic Energy of Canada Limited (AECL) are summarised by Johnson et al. (1994b). These studies were focussed on various Ti Grade-2 container designs for spent fuel and included:

• Investigation of various welding techniques, including GTAW, EB welding, GMAW, and resistance-heated diffusion bonding;

• Application of closure-weld inspection methodologies, primarily various UT technologies. In these preliminary studies, it was shown that UT methods could be used remotely (as would be required in a hot cell), but no measurements of the probability of detection and sizing of flaws were reported

• Fabrication of full- and half-scale containers from Ti Grade-2 plate material with longitudinal and circumferential welds, although alternative methods might be used for large-scale manufacturing;

• Pressure testing of full- and half-scale containers in a hydrostatic test facility at temperatures up to 150°C and pressures of 10 MPa;

• A desk-based study of leak-detection methods; • Study of different particulate materials and the development of packing techniques

(e.g., vibratory beds) for the packed-particulate container design; • The study of metal matrix materials and the development of casting technology. • Reliability analysis to estimate the fraction of containers that would contain

undetected defects sufficiently large to cause failure soon after emplacement (Doubt 1984).

A limited number of EB welds were produced in 25-mm thick copper test pieces, but no other fabrication studies were done by AECL on their alternative copper container concept (copper was considered as an alternative to titanium) (Johnson et al. 1994b).

More recently, the focus of the Canadian programme has shifted to the use of copper containers, with particular attention on the development of a copper-coated C-steel design

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(Figure C.6) (Hatton 2015). There have been few published details of the development of the cold-spray and electrodeposition techniques, which are still under development. Some work on the characterisation of the coatings has been published, for example, on the corrosion behaviour (Keech et al. 2014) and on the microstructural properties and steel-coating bond strength (Jakupi et al. 2015).

C.5.6 Switzerland

Container fabrication studies in Switzerland have been limited to desk-based studies of suitable welding techniques (Pike et al. 2010) and of general manufacturing feasibility as part of a container design exercise (Patel et al. 2012). Of the various welding techniques reviewed, narrow-gap tungsten inert gas (NG-GTA) and EB welding techniques were preferred (Pike et al. 2010).

Although only a paper exercise, the design study of Patel et al. (2012) provides some useful information about container fabrication technology. Patel et al. (2012) predicted the distribution of residual stress around the closure weld of a C-steel container following either EB (Figure C.30) or NG-GTA welding (Figure C.31). Both techniques were predicted to result in yield-level residual stresses with high tensile stresses in the hoop direction, but the distribution of stresses was quite different. The NG-GTA weld resulted in high tensile stresses on the container surface, but low tensile or compressive stresses at the weld root on the internal surface of the container (Figure C.31). Conversely, the highest tensile stresses for the EB weld were mid-wall, with tensile stresses extending to the weld root (Figure C.30). Notwithstanding the assumptions and caveats underlying the calculations, this difference in stress distribution could have different consequences for the performance of external and internal defects (see Section 5 of the main text).

With such potentially high levels of residual stress, some form of post-weld stress relief would be required (particularly for NG-GTA welding). Patel et al. (2012) simulated the effect of a local PWHT to a temperature of 600°C on the temperature distribution within a HLW container. Preliminary analyses suggested internal temperatures exceeding the 450°C thermal limit for HLW, although it was considered likely that lower temperatures could be obtained by optimising the heat treatment process and/or by modifications to the container design, whilst still achieving the desired stress reduction.

Figure C.30: Calculated distribution of residual stress (in MPa) in the closure weld of a carbon steel container following electron beam welding (Patel et al. 2012). a) axial stress, b) hoop stress, c) through-thickness stress.

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Figure C.31: Calculated distribution of residual Stress (in MPa) in the closure weld of a carbon steel container following NG-GTA welding (Patel et al. 2012). a) axial stress, b) hoop stress, c) through-thickness stress.

A detailed review of inspection technologies and their suitability for remote operation in a hot cell was also carried out (Patel et al. 2012), and concluded that:

• Conventional UT techniques should be able to reliably detect planar flaws with a size of 4 mm (through wall extent) by 15 mm in length, with advanced UT techniques capable of detecting flaws of 2 mm through wall extent;

• The use of radiography would require a more complex container design and was not considered a worthwhile technique;

• Eddy current testing would be a suitable complementary technique to UT and would improve the reliability of detecting near-surface flaws;

• Eddy current testing would require dressing of the weld cap to improve performance, but should then be able to detect flaws 1 mm through wall extent by 5 mm long.

In terms of the fabrication method for the body and lids of the container, it was concluded that any of the following techniques would be suitable (Patel et al. 2012):

• Casting; • Long forging with welded base; • Pierce and draw (with or without an integrated base); • Pressed, rolled, and welded plates.

Of these methods, long forging was preferred primarily because of the known high quality forgings that can be produced.

Several options for fabricating the internal furniture required to hold the SF assemblies were considered, but a relatively simple method based on welding steel box sections was considered to be a suitable and cost effective solution (Patel et al. 2012).

Some thought was also given to the need for an identification system that would not impact the performance of the container (for instance, due to galvanic corrosion or by acting as a stress raiser) but which would still be readable after a period of corrosion. A binary dot matrix system involving a sequence of drilled holes was suggested (Patel et al. 2012).

C.5.7 Japan

In the H12 report describing progress in the development of a HLW disposal system in Japan, JNC (2000) briefly described the status of the container fabrication activities in the Japanese programme. Figure C.32 shows a prototype C-steel container in which the body and integral base were formed as a single piece, although the alternative of forming a cylindrical shell with a welded base was also considered technologically feasible. Because of the required wall thickness of 190 mm, JNC (2000) considered EB welding, with its

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concentrated heat input and narrow HAZ, to be the most appropriate technique for the closure weld. JNC (2000) discussed possible inspection methods but did not report any specific studies that had been performed in the Japanese programme.

Since that time, Asano and co-workers have studied various welding techniques as part of a broader programme aimed at assessing the long-term integrity of the closure weld (Asano et al. 2005; 2006a,b). Various test welds in closure weld mock-ups have been produced using EB welding, GTAW, also referred to as TIG, and GMAW, also referred to as MAG. Figure C.33 shows the results of TIG and EB welding trials on 190-mm-thick test pieces. Compared to EB welding, TIG welding produced excellent quality welds with no visible flaws but it took over 24 hours for the 54 passes required to complete the weld. In contrast, the single-pass, full-penetration EB weld was completed in 26 minutes, but there was a lot of roughness and surface porosity as well as large voids (up to 40 mm in dimension) at the start and finish of the weld. MAG welding seems to offer a compromise, with weld quality similar to the TIG welds and a welding time of 2.4 hours. In all cases, welding produced residual stresses of yield strength magnitude either on the surface or at some location within the weld, suggesting that some form of post-weld stress relief would be beneficial.

Asano et al. (2006b) also investigated different inspection methods using machined defects in simulated welds. Ultrasonic inspection in the creeping wave and time-of-flight-diffraction modes was investigated as well as the alternating current field magnetic method. The alternating current field magnetic method is a surface-sensitive method and was the most reliable method for detecting and sizing surface flaws. Creeping wave diffraction was capable of sizing flaws at depths in the range 5-10 mm and time-of-flight-diffraction was the most reliable technique for sizing flaws at depths of 10-190 mm below the surface. Therefore, a combination of the three methods should provide reliable flaw detection for the complete depth of the weld.

Figure C.32: Prototype carbon steel container (JNC 2000).

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Figure C.33: Results of welding trials on full-size carbon steel weld mock-ups (Asano et al. 2006b). Surface appearance (upper left) and cross-section (lower left) of TIG weld and surface appearance (upper right) and cross-section (lower centre and right) of EB weld.

C.5.8 Korea

Like Canada, Korea has been investigating the use of cold-spray techniques for producing copper-coated containers (Choi et al., 2010; Lee et al., 2011). To date, studies have been published on coated samples and on a one tenth scale container. As is common with the cold-spray technique, the as-deposited coating is relatively hard and brittle. Figure C.34 compares the stress-strain behaviour of various as-deposited copper cold spray samples with that of an extruded copper (Lee et al., 2011). These results suggest that some form of heat treatment will be required to improve the ductility of the as-deposited coatings.

Figure C.34: Comparison of the stress-strain behaviour (left) and elastic modulus (right) of three copper cold spray samples (#1 low-purity Cu on stainless steel substrate, #2 high-purity Cu on stainless steel substrate, #3 low-purity Cu on ductile cast iron substrate), and extruded copper (sample #4) (Lee et al. 2011).

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C.5.9 USA

Development of fabrication technologies for the Alloy 22 containers (waste packages) and Ti Grade-7 drip shields was at an early stage when the YMP was scaled back following the submission of the license application in 2008. Plans were in place for the fabrication of six prototype waste packages (BSC, 2006), but the available evidence suggests that only one of these was ever fabricated. No drip shields were fabricated prior to the effective cessation of the programme. However, there was significant development of sealing and inspection techniques and of methods for treating residual stresses.

Although little actual fabrication of prototype containers was performed prior to license application, the accompanying documentation did define a detailed fabrication process (BSC, 2007; DOE, 2008; Sandia, 2007b). Procedures were defined for the following fabrication processes:

• Cutting and machining of plate material and weld surfaces (Figure C.11); • Fitting and alignment requirements and dimensional verification; • Waste package marking and identification; • Acceptable welding procedures for the Alloy 22 outer barrier and stainless steel inner

vessel, including the required preheat and interpass temperatures, workmanship and visual weld quality requirements, and acceptable weld repair methods;

• Heat treatment (solution annealing) of the Alloy 22 outer barrier (soak temperature of 2050°F (1120°C) for a minimum of 20 minutes, with a specified minimum water cooling rate (≥275°F/min (≥135°C/min)) down to a temperature of below 700°F (370°C) (Figure C.12). No heat treatment was specified for the stainless steel inner vessel;

• Cleaning and surface preparation, including degreasing, removal of weld roughness, and oxide from the solution annealing process. The target outer barrier surface roughness was specified as at least 125 µinch with surface damage (scratches) no greater than 1/16th inch (1.6 mm) in depth and with dents having a width:depth ratio of at least 5;

• Non-destructive examination by a combination of visual inspection, eddy current testing and ultrasonic inspection to detect weld flaws with a detection criterion of ≥1/16th inch (≥1.6 mm);

• Residual stress mitigation of the final closure weld on the Alloy 22 outer barrier using low-plasticity burnishing (Figure C.22);

• Hydrostatic and pneumatic pressure testing of the inner vessel (Figure C.19); • Helium leakage testing of the inner vessel (Figure C.20).

The one waste package that was fabricated was designed to contain 21 PWR fuel assemblies (with neutron absorber plates). A detailed testing programme was planned for the prototype container (including residual stress measurements, metallographic examination, and measurements of mechanical and corrosion properties), but there is no documentary evidence that such tests were performed. From the one prototype waste package that was fabricated, it was learnt that the automated GTAW process for the closure weld was successful, that there were challenges with final machining of the Alloy 22, and that additional wall thickness should be allowed for post-heat-treatment machining.

Development work was also undertaken on remote welding and handling procedures that would be required for eventual hot cell fabrication (Barker et al., 2008; Skinner et al., 2005).

Work on the detection of weld flaws is described in Section C.4.5 (BSC, 2003; Sandia, 2007a).

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Andersson, H.C.M., F. Seitisleam, and R. Sandström. 2005. Creep testing of thick-wall copper electron beam and friction stir welds at 75, 125 and 175°C. Swedish Nuclear Fuel and Waste Management Company Technical Report, SKB TR-05-08.

Andra. 2005. Dossier 2005 Argile. Architecture and management of a geological repository. Agence Nationale pour la Gestion des Déchets Radioactifs.

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