IAEA-CN-194-012

Thermal ageing monitoring of primary circuit components at NPPs in the Slovak Republic

M. Brezina, L. Kupca

VUJE, Inc. Trnava Slovak Republic Abstract. The thermal ageing of nuclear power plant (NPP) primary circuit materials is one of the main issues especially in connection with NPP lifetime prolongation. The systematic evaluation of the primary circuit components resistance to thermal ageing at operational temperature of NPPs was not performed in past. The Nuclear Regulatory Authority of the Slovak Republic together with International Atomic Energy Agency requires to establish the ageing management program for operated NPP units. For that reason there were performed several detailed analysis of ageing mechanisms. Based on these requirements there were prepared programs for long-term monitoring of primary circuit structural material properties changes. Two approaches have been proposed for monitoring of the thermal ageing.

1. Introduction Ageing is a general process in which characteristics of an SSC gradually change over time or with use. This process may proceed by a single ageing mechanism or by a combination of several ageing mechanisms. Nuclear power plants experience two kinds of time dependent changes: (a) Physical ageing of SSCs is due to physical, mechanical, thermal, electrical, chemical,

irradiation and/or biological processes, which results in degradation, i.e. gradual deterioration in their physical characteristics;

(b) Non-physical ageing (obsolescence of SSCs), i.e. their becoming out of date in comparison with current knowledge, standards, codes and technology.

Evaluation of the cumulative effects of both physical ageing and obsolescence on the safety of nuclear power plants is a continuous process and is assessed in a periodic safety review or an equivalent systematic safety reassessment programme [1]. The operation requirements require that an NPP operating organization prepares and carries out a programme of maintenance, testing, surveillance and inspection of plant systems, structures and components important to safety to ensure that their level of reliability and effectiveness remains in accordance with the design assumptions and intent throughout the service life of the plant. The Nuclear Regulatory Authority of the Slovak Republic together with International Atomic Energy Agency requires establishing the ageing management program for operated NPP units [2]. There were developed numbers of detailed analysis describing of material properties changes and mechanisms of ageing [3 - 6]. From these papers follows that in addition to radiation embrittlement and corrosion, the most serious degradation mechanisms of primary component materials is thermal ageing, whose expression is embrittlement of steels due to long-term exposure at the operating temperature of 570 K. The thermal ageing of nuclear power plant (NPP) primary circuit materials is one of the important physical ageing processes. Systematic evaluation of the thermal ageing processes in

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the primary circuit component at the operating temperature was not performed at the Slovak nuclear power plants in past. Based on these knowledge and regulatory requirements there were prepared programs for long-term monitoring of primary circuit structural material properties changes. Two approaches have been proposed for monitoring of the thermal ageing: (a) Long-term exposure of blocks of original material that will be exposed to the same

thermal load as monitored components, (b) Sampling of material from operating components and subsequent detailed analysis.

2. Long-term exposure A simply system which allows long-term exposition of original materials at operational temperature was proposed for monitoring of thermal ageing processes. A special ring form rack with 15 metal boxes containing monitored materials (see Fig. 1a,b) will be installed on the hot leg of the primary piping under the thermal insulation (see Fig. 2). Every metal box of the ring contains a block of the original archive material. After a specified time of exposition (e.g. 5, 10, 15, 20, 25 years or according to the demands of operators) given boxes will be removed from the ring. Complete sets of testing specimens will be prepared from the blocks of materials according to knowledge at the time of testing. Following properties can be tested: tensile properties determined by a standard test or by a small punch test (SPT), impact strength, transition temperature and fracture toughness by standard tests or by

SPT, hardness and micro-hardness, detailed microstructure and sub-microstructure analyses, sensitization to intergranular corrosion, etc.

a) computer drawing b) testing installation of the ring FIG. 1. Ring of boxes containing monitored materials

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The main advantages of this conception are: zero induced activity of exposed materials, which enables preparing and evaluation of

the specimens without hot cells utilization, significantly lower price of designed system according to the other monitoring

possibilities, simplified access to placement, removing and/or changing of individual set of

materials, practically identical thermal exposition parameters and history during the unit operation, the possibility to evaluate all materials of safety related primary circuit components.

FIG. 2. Placement of the monitoring ring on the hot leg primary loop

As the experimental materials will be used new original materials and already operated materials – pieces of primary components after long-term operation, which had to be removed due to reconstruction, repairing or maintenance. Next type of experimental materials will be used for the thermal ageing monitoring: (a) reactor pressure vessel structural materials – steel 15Ch2MFA (base metal, weld metal,

heat affected zone), (b) main circulation pipeline of primary circuit – steel 08Ch18N12T (base metal, weld

metal, heat affected zone), (c) steam-generator shell – steel 22K (base metal). Sources of the experimental material are material archive of VUJE, rests of new parts, already operated primary circuit parts cut off due to maintenance etc. Chemical compositions of the experimental materials are summarized in the Table 1. An example of box with experimental material is shown in Figure 3. The first ring with 15 boxes filled by experimental materials has been already installed on the primary piping at Mochovce NPPs Unit #1 during the regular outage in 2012 (see Fig. 4). The second ring is planned to be installed at Bohunice NPPs Unit #3 during the regular outage in the same year.

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Table 1: Chemical composition of the experimental materials

Material Element content [mass %]

C Mn Si P S Cr Ni Mo Ti V As Co Cu

15Ch2MFA BM

(RPV)

0,13 ÷

0,18

0,30 ÷

0,60

0,17 ÷

0,37

max. 0,025

max. 0,025

2,50 ÷

3,00

max. 0,40

0,60

÷ 0,80

--- 0,25

÷ 0,35

max. 0,050

max. 0,020

max. 0,150

15Ch2MFA WM

(RPV)

0,03 ÷

0,12

0,55 ÷

1,30

0,20 ÷

0,65

max. 0,030

max. 0,030

1,10 ÷

1,60 ---

0,03 ÷

0,12 ---

0,20- ÷

0,65 ---

max. 0,025

max. 0,120

08Ch18N12T BM (PP)

max. 0,08

max. 2,0

max. 0,8

max. 0,035

max. 0,020

17,0 ÷

19,0

11,0 ÷

13,0 ---

5x%C ÷

0,60 --- ---

max. 0,05

max. 0,30

22K BM (SG)

0,19 ÷

0,26

0,75 ÷

1,05

0,20 ÷

0,45

max. 0,025

max. 0,025

max. 0,30

0,30 ÷

0,50

0,10 ÷

0,15

max. 0,05

0,02 ÷

0,05

max. 0,080 ---

max. 0,30

FIG. 3. Box with a block material FIG. 4. Installed monitoring ring

3. Direct material sampling from operating components A quick method to determine the current state of materials is a direct sampling of the primary circuit components and evaluation removed samples. Removing materials from operated devices using classical techniques represents an inappropriate interference with the integrity of assessed part. These “classical” destructive methods require subsequent repair of removed parts, including welding, heat treatment and a range of non-destructive testing. To avoid such complications it is proposed to use a special quasi “non-destructive” sampling system. For this purpose it is planned to apply a special Rolls Royce equipment SSamTM-2. This device is designed as a universal system for the sampling of different devices without

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significantly affecting of the component surface and without necessity to repair or modify the component. The SSamTM-2 sampling system provides a remotely operated material sampling capability. The machine’s unique hemispherical cutter is able to remove a sample without mechanical distortion or thermal degradation of the material. The sampling procedure produces a depression in the base material approximately 0,8 mm deeper than the sample thickness. A schematic depiction of the surface sample removal is shown in Fig. 5.

FIG. 5. Schematic depiction of the surface sample removal

SSamTM-2 machine by Rolls-Royce Company removes small slices of material using a cylindrical machine that has a diameter of approximately 70 mm and is 400 mm long (see Fig. 4). Samples are removed using a hemispherically-shaped cutter, rotating at 10,000 rpm, mounted at one end of the machine. The cutter “scoops” through the material to remove the samples, which are approximately 25 mm diameter and 2,5 mm thick. The resulting depression in the component is smooth and typically requires no post-sampling repair. The sampling system has been already several times successfully used e.g. in a chemical factory.

FIG. 6. Cutting head of the SSamTM-2 machine

It is planned to perform surface sampling of primary piping, steam generator shells and reactor pressure vessels at Bohunice and Mochovce NPPs. Removed samples will be analysed to determine the components actual state. Due to a small amount of removed materials it is planned to apply the SPT technique for determination of mechanical properties: tensile properties, transition temperature (FATT), fracture toughness.

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After cutting and testing the remains of the samples will be used for:

chemical analysis, micro hardness measurement, microstructure and sub-microstructure analysis. The first surface sampling of primary piping is planned in 2012. In the next two years will be performed sampling from steam generator shells and reactor pressure vessels at Bohunice and Mochovce NPPs. 4. Small punch test (SPT) technique The SPT technique is suitable method for testing of mechanical properties using a small amount of experimental materials. The principle of SPT testing procedure is a penetration of a disk shape specimen by a hemispheric rod and recording of loads and deflections during the test. Specimen dimensions: diameter: ø 8.00-0.05 mm thickness: 0.500± 0.005 mm Following characteristics of load - deflection curve of tested specimen are used for tensile properties estimation (see Fig. 7): Py [N] – load characterizing the transition from linearity to the stage associated with the

spread of the yield zone through the specimen thickness (plastic bending stage) Pm [N] – maximum load recorded during punch test dm [mm] – displacement corresponding to Pm Yield stress (Re) and Ultimate tensile strength (Rm) are well correlated with parameters Py and Pm obtained from the load – deflection curve (see Fig. 7).

FIG. 7. Load - Deflection curve

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Application of SPT testing has several advantages: (a) reduction of the high consumption of original materials, (b) better placing of SPT specimens in the limited space available for irradiation in nuclear

power reactors, (c) the determination of material properties, which are highly dependent on the local

microstructure like the heat affected zone (HAZ) material, (d) using the same geometry of samples for different types of testing or results. 5. Conclusions For the monitoring systems of the NPP`s material degradation causes by thermal ageing processes, it is possible to conclude, that systems mentioned above will meet the requirements of: Slovak Regulatory Authority (UJD SR), recommendations of IAEA, internationally approved standards used for evaluation of thermal ageing, the possibility to compare the results between the operated individual units not in the

Slovak Republic only, but with the other nuclear technologies in the world too.

The implementation of the described monitoring program of thermal ageing [7] will provide more reliable technical arguments for extending the planned lifetime of all operating units in the Slovak Republic.

REFERENCES

[1] IAEA: INTERNATIONAL ATOMIC ENERGY AGENCY, Ageing Management for Nuclear Power Plants, Safety Guide No. NS-G-2.12, IAEA, Vienna, 2009

[2] NUCLEAR REGULATORY AUTHORITY OF THE SLOVAK REPUBLIC, Safety Guide BNS I.9.2/2001 Ageing management of nuclear power plants - requirements

[3] KUPČA, Ľ. “Development of irradiation embrittlement monitoring in the Slovak Republic NPP's”, IAEA Technical Meeting on Irradiation Embrittlement and Life Management of Reactor Pressure Vessels (RPV) in NPPs, 18 - 22 October 2010, Znojmo, Czech Republic

[4] BREZINA, M. et al.: “Corrosion Monitoring Programs of NPP Safety Related Components”, IAEA Workshop on Erosion-Corrosion including Flow Accelerated Corrosion and Environmentally Assisted Cracking Issues in NPPs, 21-23, April 2009, Moscow, Russian Federation

[5] MAMAEVA, E. et al.: “Assessment of the microstructure and properties degradation for WWER-440 austenitic piping welded joints after modelling of long operating life”, Proceedings of the 26th ESReDA Seminar, Tampere, Finland, May 2004

[6] TIMOFEEV, B.T., NIKOLAEV, Y.K.: "About the prediction and assessment of thermal embrittlement of Cr–Ni austenitic–ferritic weld metal and castings at the ageing temperatures 260–425°C. International Journal of Pressure Vessels and Piping 1999, 76, 849–856

[7] PETZOVA J. et al.: Project of thermal ageing monitoring of primary circuit materials at EBO and EMO NPPs. (in Slovak), report VUJE No.: 0360/02/2010

[8] SSamTM-2 Surface Sampling System, Operation Manual, Rolls-Royce, Issue3, December 2005