Stress Corrosion Cracking of Spent Nuclear Fuel Dry Storage Canisters

Greg Oberson, Materials Engineer U.S. Nuclear Regulatory Commission Office of Nuclear Regulatory Research

Meeting with Fuel Cycle and Materials Administration September 16-19, 2013

Outline

• Background

• NRC activities − Regulatory approach − Research programs

• Industry activities

• Summary

2

BACKGROUND

3

Summary of Issue

• Most spent nuclear fuel dry storage canisters in the U.S are fabricated from austenitic stainless steel

• Canisters in vented overpacks may be exposed to airborne chloride salts or other atmospheric species

• Deliquescence of salts at high relative humidity (RH) could create

corrosive brine on canister surface

• Stress corrosion cracking (SCC) could initiate at stressed locations

such as welds

4

Conceptual Scenario

Overpack

Canister

Salt or other airborne species

Airflow through vents

Weld

Species on surface

Deliquescent brine

Canister wall

5

Atmospheric Chlorides

Map of atmospheric chloride concentration Map of ISFSI locations

National Atmospheric Deposition Program/National Trends Network

NUREG-1350, Volume 24, “2012-2013 Information Digest

6

Technical History • Susceptibility of austenitic stainless steel to chloride–

induced SCC has been recognized for many years

− Outdoor atmospheric testing o Kure Beach, USA1,2

o Okinawa and Okitsu, Japan3

− Laboratory testing o Shoji and Ohnaka4

o Hirshfeld, et al.5,6

− Case studies o Stainless steel rock climbing equipment7

o Outdoor vessels8

• Useful review articles are already available9,10

7

Operational Experience SCC of stainless steel components attributed to atmospheric chloride exposure has been reported at nuclear power plants

• St. Lucie, Unit 2 – Florida, U.S.11

− Through-wall cracking of Type 304 piping for refueling water storage tank (RWST) exposed to outdoor air

− Pipe wall thickness 0.25”, operating at 30 psig at about 50oC

• Turkey Point, Unit 3 – Florida, U.S.12

− Through-wall cracking of Type 304 spent fuel pool cooling line exposed to outdoor air

− Located in heat-affected zone (HAZ) near flange butt weld

• San Onofre Nuclear Generating Station – California, U.S.13

− Through-wall cracking of Type 304 emergency core cooling system piping exposed to outdoor air

− Located in weld HAZ

8

Operational Experience (cont.)

• Koeberg Nuclear Power Station – near Cape Town, South Africa14

− Extensive cracking near welds in reactor cavity and spent fuel cooling system tanks exposed to outdoor air

− Tank water temperature maintained between 7 and 40 oC

• Ohi, Unit 1 – Fukui Prefecture, Japan15

− Cracking near backplate weld on RWST exposed to outdoor air − Tank installed in 1974 without coating, then painted in 1981 − Cracks were identified after approximately 30 years in service.

9

CRIEPI Studies Significant recent work on chloride-induced SCC of stainless steel has been performed by Japan Central Research Institute of the Electric Power Industry (CRIEPI)16-19

• Effects of applied stress level

• Effects of temperature and humidity

• Effects of chloride concentration

• Salt deposition rate

• Stress mitigation

10

Notable CRIEPI Results • Crack initiation observed at less than material yield stress

• Crack initiation observed at surface chloride concentration

of 0.8 g/m2

• Crack initiation observed up to 80oC for RH in range of 16

to 20%

• Decreasing salt deposition rate with increasing temperature

• Peening may be considered for relaxing weld residual stress

11

NRC REGULATORY APPROACH

12

References

• Title 10, Code of Federal Regulations (CFR), Part 72, “Licensing Requirements for the Independent Storage of Spent Nuclear Fuel, High-Level Radioactive Waste, and Reactor-Related Greater than Class C Waste”

• NUREG-1536, “Standard Review Plan for Spent Fuel Dry Storage Systems at a General License Facility”

• NUREG-1927, “Standard Review Plan for Renewal of Spent Fuel Dry Cask Storage System Licenses and Certificates of Compliance”

• Information Notice (IN) 2012-20, “Potential Chloride-Induced Stress Corrosion Cracking of Austenitic Stainless Steel and Maintenance of Dry Cask Storage System Canisters”

13

10 CFR, Part 72 • 10 CFR 72.122(b)(1) – structures, systems, and components must be

designed to accommodate the effects of site characteristics and environmental conditions

• 10 CFR 72.122(h)(1) – cladding must be protected during storage against degradation that leads to gross ruptures

• 10 CFR 72.122(h)(4) – confinement systems must have the capability for periodic monitoring

• 10 CFR 72.236(l) – cask and safety systems must maintain confinement of radioactive material under normal, off-normal, and credible accident conditions.

• 10 CFR 72.240(c)(3) – licensee must provide description of aging management programs for license renewal

14

NUREG-1536 • Section 5.4.2 – Confinement Monitoring Capability

− Welded canisters not typically monitored − Initial staff review ensures the integrity of the confinement

boundary for the licensing period

• Section 8 – Materials Review − Section 8.4.3 – Environment − Section 8.4.5 – Material Properties − Section 8.4.6 – Coastal Marine ISFSI Sites − Section 8.4.7 – Weld Design/Inspection − Section 8.4.21 – Periodic Inspection

o For license renewal, one-time inspection of normally inaccessible portions of the canister exterior

o Canister selected based on criteria such as time in service and temperature, and used to “bound” other canisters

15

NUREG-1927 • Section 3.4 – Materials, Service Environment, Aging Effects, and

Aging Management Activities − Section 3.4.1 – Identification of Materials and Environments − Section 3.4.2 – Identification of Aging Effects − Section 3.4.3 – Aging Management Activity

• Section 3.6 – Aging Management Program (AMP) − Intended to ensure that aging effects do not affect the functionality

of structures, systems, and components important to safety − Conceptually similar to AMPs for reactor license renewal

described in NUREG-1801, “Generic Aging Lessons Learned (GALL) Report”

− Types of AMPs: o Preventative maintenance (e.g., coating of exposed surfaces) o Mitigation (e.g., cathodic protection) o Condition monitoring (e.g., periodic visual examination) o Performance monitoring (e.g., radiation monitoring)

16

NUREG 1927 – AMP Elements

• Scope –structures and components are subject to aging • Preventative actions – actions taken to prevent/mitigate aging • Parameters monitored or inspected – what is being looked for • Detection of aging effects – how aging effects will be detected • Monitoring and trending – how monitoring/trending will be

used to inform corrective actions • Acceptance criteria – criteria used to evaluate need for

corrective actions • Corrective actions – what will be done if aging is detected • Confirmation process – how it will be determined that

preventative and corrective actions are appropriate • Administrative controls – integration into quality assurance

program • Operating experience – consideration of industry-wide

operating experience

17

NUREG-1927 – Lead Canister Inspection • Remote visual inspection is staff-accepted way to

demonstrate canister condition

• Canister selected on basis of time in service, thermal load, or other factor that affects degradation

• Expected to be performed before submittal of the license renewal application

• Repeat inspection may be conducted at 20-year intervals as a license condition for renewal

18

IN 2012–20 • Informs licensees of a potential issue but does not convey

regulatory requirement

• Technical bases − Operational experience − Laboratory studies

• Recommendations − Evaluate applicability to particular sites based on location,

design, and other characteristics − Develop monitoring and inspection capabilities

• Acknowledges need for further research and development − Limited understanding of canister conditions in service − Need for inspection and monitoring techniques

19

Summary of Current Regulatory Position • Potential for SCC to affect canisters should be addressed in initial

license applications and as part of AMPs for license renewal.

• Licensees must demonstrate that structures, systems, and components important to safety can perform their intended function during the licensing period.

• Periodic in-service inspection or active monitoring of welded canisters is not currently required.

• Visual canister inspection may be conducted for license renewal but no requirement that the inspection be qualified to detect indications of SCC.

20

NRC RESEARCH

21

Motivation for Research Activities

• Understand scope of potential safety issue

• Evaluate technical bases for current NRC regulatory guidance

• Determine whether further actions by industry are needed to manage potential degradation

22

Research Areas

• Evaluation of SCC susceptibility conditions20-24

• Weld residual stress25

• Non-destructive examination

• Functional monitoring

23

Evaluation of SCC Susceptibility Conditions • Objective: Identify conditions where austenitic stainless

steel is susceptible to SCC initiation when exposed to chloride-rich and non-chloride-rich salts

• Systematic evaluation of various parameters:

− Temperature and humidity − Salt quantity and chemistry − Stress level − Material condition

• Primary focus on crack initiation, not propagation or

growth rates

24

Previous NRC-Sponsored Work – NUREG/CR-7030

Exposed stainless steel U-bend specimens to salt fog conditions at 43, 85, and 120oC to evaluate susceptibility for cracking. Specimens had high surface salt concentration.

25

Specimens examined after 4, 16, 32, and 52 weeks exposure. Only specimens tested at 43oC showed cracking.

Exposed 52 weeks, 43oC Exposed 52 weeks, 120oC

Previous NRC-Sponsored Work – NUREG/CR-7030

26

• At 85 and 120oC, low RH and no deliquescence.

• At 43oC, high RH and deliquescence of salt.

SCC

No SCC

Previous NRC-Sponsored Work – NUREG/CR-7030

27

Need for Further Research • Very high absolute humidity (AH) used for NUREG/CR-

7030 tests. May not be representative of natural conditions.

• Limited control on quantities of salt deposition

• Lack of testing between 43 and 85oC, which may be important temperature range.

• Only use of highly-strained U-bend test specimens

• Only tested using chloride-rich salt, no evaluation of other potential atmospheric species

28

Scope of Tests for Current Research Program • Tests with chloride salts

− Deliquescence testing − Cyclic humidity SCC testing − Elevated temperature SCC testing − High humidity SCC testing − Variable strain testing

• Tests with non-chloride rich species − Deliquescence testing − Static humidity SCC testing

• Final NUREG/CR report expected October 2013

29

Chloride Salts Deliquescence Testing • Test objective: Confirm humidity above which sea salt and constituents

will deliquesce (DRH) and compare to calculated values.

• Test methodologies:

Low RH: Salt is dry High RH: Salt has deliquesced

Salts in beakers Conductivity cell impedance measurement

– Place dry salts in beakers – Gradually increase, then decrease humidity – Observe beakers for moisture absorption and dry out

– Place on filter paper attached to electrodes – Gradually increase, then decrease humidity – Measured impedance drop when salt deliquesces

30

Example Test Results at 45oC RH = 16% RH = 19%

RH = 31% RH = 34%

RH = 25% RH = 31%

Sea Salt Composition NaCl MgCl2⋅6H2O Na2SO4 CaCl2 KCl NaHCO3 KBr H3BO3 SrCl2⋅6H2O NaF

58.490 26.460 9.750 2.765 1.645 0.477 0.238 0.071 0.095 0.007

31

Test Results • For CaCl2, MgCl2, NaCl, and

Na2SO4, the measured DRH was close to values calculated by thermodynamic software (OLI Analyzer), indicated by solid lines on figure.

• Measured DRH and efflorescence RH (ERH) for simulated sea salt were between DRH for CaCl2 and MgCl2

32

Cyclic Humidity SCC Testing

• Test objectives: – Identify whether SCC can initiate at AH limited to about 30 g/m3, which is

reference point for limit in natural conditions – Investigate effects of surface salt concentration and material condition on

SCC susceptibility

• Test methodology: – Deposited 0.1, 1, or 10 g/m2 of sea salt on ASTM G30 U-bend specimens

o Expose to salt fog for various times o Quantity determined by control specimen weight gain

– Specimens were Type 304 in as-received or furnace sensitized (2 hours at 650oC) conditions

– Specimens were exposed in air to cyclic AH between about 15 and 30 g/m3 at various temperatures

– Specimens were not contacted by liquid water during exposure so SCC would only occur by salt deliquescence.

33

Test Specimens

Chemical Composition of Type 304 Stainless Steels Material C S Mn P Si Cr Mo Ni Cu N Fe

Type 304 Heat 2N739

0.039 0.002 1.21 0.026 0.55 18.19 N/A 8.07 N/A 0.042 Bal

Type 304 Heat 257524

0.046 0.003 1.06 0.021 0.50 18.41 0.01 8.23 0.04 0.050 Bal

As-received

Sensitized

U-bend specimen

Deposited specimens

34

Test Conditions

AH = 30 g/m3

AH = 15 g/m3

27oC

52oC 45oC

35oC

60oC

DRH Na2SO4

DRH NaCl

DRH MgCl2

DRH CaCl2

Chamber humidity cycle Test temperatures

• Four humidity cycles per day • Temperature above refers to ambient

test chamber temperature • Individual specimen temperatures

controlled by cartridge heaters

35

Test Results Temperature

(oC) Exposure Time SCC Observed? Lowest salt concentration at which SCC was observed

27 8 months No N/A 35 4 – 12 months Yes 0.1 45 4 – 12 months Yes 0.1 52 2.5 – 8 months Yes 1 60 6.5 months Yes 10

Pitting on specimens at 10 g/m2 (top), 1 g/m2 (middle), and 0.1 g/m2 (bottom)

Cross section of sensitized, 0.1 g/m2 specimen at 45oC after 4 months

200 µm Top view of sensitized, 10 g/m2 specimen at 60oC for 6.5 months

Salt deliquesced and drained off of specimens

36

Elevated Temperature SCC Testing

• Test objective: Evaluate SCC susceptibility in the temperature range of 60 to 80oC

• Test methodology: – Deposited 10 g/m2 of sea salt on ASTM G30 U-bend specimens – Exposed specimens in air at different humidity levels

Test Conditions Temperature

(°C) Relative Humidity

(%) Absolute Humidity

(g/m3) Maximum Test Duration

(Months)

60

22 29 1 25 33 2.75 30 39 5.75 35 46 1 40 52 1.5

80 28 82 2.5 35 102 2.25 40 117 1

37

Test Results

• SCC initiation observed at RH as low as 25% at 60oC and 28% at 80oC

• Sensitized specimens showed greater extent of cracking

Sensitized, 60oC, 30% RH As-received, 80oC, 35% RH Sensitized, 80oC, 28% RH

38

High Humidity SCC Testing • Test objective: Equilibrium chloride concentration in saturated solution decreases

with increasing RH.26,27 Dilution of chlorides at high RH could reduce SCC susceptibility. Tests were performed at high RH to determine whether SCC could initiate.

• Test methodologies: – Immersed U-bend specimens in prepared saturated solutions at 30oC and 90% RH – Deposited 10 g/m2 of sea salt on U-bend specimens for exposure at 30oC and 90% RH

Calculated chloride concentration in saturated sea salt solution as function of RH at 30oC

U-bend specimens immersed in solution

39

Test Results • For specimens with deposited salt, salt quickly deliquesced and ran off

sides of specimens with no SCC observed. • For immersed specimens in sea salt, NaCl, MgCl2 and CaCl2, pitting and

SCC were observed within 5 weeks. Chloride and Salt Concentrations in Saturated Solutions at 30oC and 90% RH

Chloride Concentration (mol/kg H2O) Salt Concentration (g/kg H2O)

Solution

Sea salt 2.71 203 NaCl 2.79 163 MgCl2 3.01 306 CaCl2 3.16 232

Specimens immersed in sea salt after 5 weeks, as received (L); sensitized (R) Cracking on surface of

specimen immersed in MgCl2

100 µm 50 µm Cracking on surface of specimen immersed in sea salt

40

C-Ring SCC Testing • Test objective: U-bend specimens represent a highly strained state, 13-14% at the

apex. These may not be representative of canister conditions. SCC initiation at lower strain levels was evaluated using C-ring specimens.

• Test methodologies: − Specimens fabricated following ASTM G38-01 and deposited with 1 or 10 g/m2 of

salt − Specimens were strained to slightly above yield stress (~0.4% strain) or 1.5% strain,

as measured by strain gage. − Specimens were exposed at conditions of 35oC and 72% RH, 45oC and 44% RH, and

52oC and 32% RH (AH ~ 30 g/m3 at each temperature)

41

Test Results Temperature

(°C) RH (%) AH (g/m3) Salt Concentration (g/m2)

Strain (%)

Exposure Time (months) Crack Initiation

35 72 29 1 0.4 2 No 10 0.4 3 Sensitized only

45 44 29 1 0.4 3 No

10 0.4 3 No 1.5 2 Sensitized and as-received

52 32 29 1 0.4 2 Sensitized and as-received

10 0.4 3 Sensitized only 1.5 2 Sensitized and as-received

Sensitized, 35oC, 0.4% strain, 10 g/m2 salt

As-received, 45oC, 1.5% strain, 10 g/m2 salt

Sensitized, 52oC, 0.4% strain, 1 g/m2 salt

42

Visualization of Test Results Immersion test

C-ring specimens

43

Conclusions from Chloride Salt Tests • Between 35 and 80oC, SCC initiation with simulated sea salt

is observed when RH is higher than about 25 to 30%, which is close to DRH for CaCl2 and MgCl2.

• Susceptibility appears to increase with decreasing temperature because RH will be higher for the same AH.

• SCC initiation is observed at salt quantity as low as 0.1 g/m2, but seems more extensive at higher amounts.

• SCC initiation is observed at strain as low as 0.4, but seems more extensive at higher levels.

• Sensitized material seems more susceptible than material in as-received condition.

44

Tests with Non-Chloride-Rich Species • Besides chloride salts, other atmospheric species could arise from

industrial, agricultural, and commercial activities near ISFSI sites

• Survey of atmospheric monitoring data in U.S. identified common species containing ammonium, sulfate, and nitrate ions

• Representative set of species were selected for testing: − Ammonium sulfate – (NH4)2SO4 − Ammonium bisulfate – NH4HSO4 − Ammonium nitrate – NH4NO3 − Fly ash – class F, mostly alumina, silica, and iron, less than 20%

lime

• Tests were also performed with chloride and non-chloride-rich salt mixtures: NH4NO + NaCl

45

Deliquescence Testing

• Beaker and impedance cell methodologies, similar to chloride salt testing

• Tests performed between 35 and 60oC

• Species tested − (NH4)2SO4 − NH4HSO4 − NH4NO3 − Mixtures of (NH4)2SO4 + NH4NO3 with SO4

2–:NO3– mole ratios of 0.5, 1.0, and 3.0

− Fly ash − Mixtures of NH4NO3 + NaCl with NO3–:Cl– mole ratios of 3.0 and

6.0

46

Example Test Results Impedance tests at 45oC

NH4NO3 + NaCl at 45oC, 46% RH

NH4NO3 + NaCl at 45oC, 49% RH

47

Deliquescence Test Results

DRH in Temperature Range of 35 – 60oC (%)

Pure Salts and Salt Mixture NH4HSO4 NH4NO3 (NH4)2SO4

Fly Ash

(NH4)2SO4 + NH4NO3 Mole Ratio

NH4NO3 + NaCl Mole Ratio

0.5 1.0 3.0 3.0 6.0 Calculated DRH, percent

35-40 45-55 75-80 N/A 50-65 50-65 50-65 N/A N/A

DRH by Conductivity Cell, percent

35-45 40-55 ~80 No deliq.

50-60 50-60 50-60 N/A N/A

DRH by Beaker, percent

30-45 40-60 ~80 No deliq.

50-70 50-70 50-70 30-35 30-35

• Observations are consistent with calculations made by thermodynamic software

• (NH4)2SO4 and fly ash have very high DRH

• Other species could deliquesce between 35 and 60oC at AH less than 30 g/m3

• DRH of mixture may be lower than that of pure constituents

48

SCC Testing – Non-Chloride Salts Only • Test objective: Determine if SCC could initiate for austenitic stainless

steel exposed to the non-chloride-rich species

• Test methodology: – Deposited large quantities (>100 g/m2) of species on ASTM G30 U-bend

specimens by spray bottle. Other specimens were buried in bins of solid salt. – Exposed specimens in air at 45oC and 44% RH for 6 weeks followed by

35oC and 72% RH for 1 month (AH ~ 30 g/m3 at both temperatures)

49

Test Results • No cracking observed on specimens exposed to any species

• Most specimens appeared near pristine after removing salt, except for extensive general corrosion on specimens exposed to NH4HSO4 − Deliquescent solution pH for most species is in range of about 4 to 5 − Deliquescent solution pH for NH4HSO4 is about -1 to -2

Specimens exposed

to NH4NO3 Specimens exposed to

NH4NO3 + (NH4)2SO4 mixture

Specimens exposed to NH4HSO4

50

SCC Testing – Chloride and Non-Chloride Salt Mixtures • Test objective: Determine if SCC could initiate for austenitic stainless

steel exposed to chloride-rich and non-chloride-rich mixed salts

• Test methodology: – Deposited NH4NO3 + NaCl mixtures on ASTM G30 U-bend specimens by

spray bottle. Solution pH was about 3.5 to 4. – Amount of chlorides on specimens were less than 10 g/m2. – Exposed specimens in air at 45oC and 44% RH for up to 4 months

Specimen Type Molar Ratio of

NH4NO3 to NaCl Amount of NH4NO3 and NaCl

Deposited (g/m2)

Calculated Amount of NaCl Deposited

(g/m2) As-Received 3 54 6.4

6 74 4.9 Sensitized 3 62 7.4

6 83 5.5

51

Test Results • Extensive cracking on specimens, larger cracks than any other

specimens in test program

• Presence of nitrate does not appear to inhibit cracking in these test conditions.

Specimens exposed to NH4NO3 + NaCl with

NO3–:Cl– mole ratio of 3.0

Specimens exposed to NH4NO3 + NaCl with

NO3–:Cl– mole ratio of 6.0

52

Conclusions from Non-Chloride Salt Tests

• Austenitic stainless steel did not appear susceptible to SCC when exposed to the ammonium, sulfate, and nitrate bearing species or fly ash evaluated in this test program, even above the species’ DRH.

• Crack initiation was observed for mixtures of chloride-rich and non-chloride-rich salts, even with the presence of a large quantity of nitrate.

53

OTHER NRC RESEARCH PROGRAMS

54

Welding Residual Stress (WRS)

• Objective: Calculation of potential WRS using finite element methodology

• Longitudinal and circumferential welds modeled − Double-V joint − Two weld passes on inner diameter − Back-gouge then three weld passes on outer diameter

• Welding process is not well characterized so assumptions made include: − Low constraint and high constraint boundary conditions − Weld parameters for submerged arc welding

• Calculations made using ABAQUS

• Benchmarked against stainless steel pipe girth weld

55

• Circumferential weld: high through-wall residual hoop stresses in weld and HAZ

• Longitudinal weld: high through-wall residual axial stresses in weld and HAZ

• Calculations may be refined using actual welding details

WRS Modeling Results

56

Evaluation of NDE Techniques • Objective: Evaluation of ultrasonic (UT) and eddy current testing

(ECT) as methods for detecting SCC of canisters

• Canister could be accessed by vents in overpack, but may be limited by tortuous path and narrow annulus − Probes on end of flexible wand − Robotic crawlers

• Probes and cables should be able to withstand temperature and radiation fields for limited time of inspection

• UT and ECT should be capable of detecting cracks before wall is penetrated − ECT may be better for detecting shallow (< 1mm) flaws − UT may be better for depth sizing

• Final report expected September/October 2013

57

Functional Monitoring • Objective: Evaluation of technologies that could be used for

continuous or periodic monitoring of conditions of canister

• Temperature − Thermocouples − Resistance temperature detectors − Ultrasonic

• Humidity − Ceramic oxides − Leakage monitoring systems

• Chlorides − SaltSmart − Outdoor air sampling

• Final report expected late 2013

58

Topics Not Yet Addressed by NRC Research

• Crack growth and propagation

• Mitigation methods • Risk assessment

59

NRC Engagements with Industry and Stakeholders • Nuclear Energy Institute (NEI) Regulatory Issue Resolution

Protocol − Regular, public exchange of information with industry on

ongoing research and regulatory issues − Framework for development of Electric Power Research

Institute (EPRI) Research and Development Roadmap

• EPRI Extended Storage Collaboration Program (ESCP) − Participation by EPRI, NRC, Department of Energy (DOE),

NEI, utilities, vendors, university, and international organizations

− Working group on marine-atmosphere SCC

60

INDUSTRY ACTIVITIES

61

EPRI Roadmap • Industry actions to address SCC are guided by the EPRI

Roadmap28

• Objective of roadmap is to develop a susceptibility

assessment to: − Identify conditions required for SCC to occur, and − Recommend models and inspection/monitoring approaches

to utilities for managing the issue.

• Elements of the Roadmap include: − Failure modes and effects analysis (FMEA) − Literature review − Degradation model development − Voluntary inspections

62

FMEA • Focus on aging-related degradation of welded canisters

• Identify range of effects for failure modes in normal and

accident conditions − Fault tree analysis to evaluate initiating events − Assess consequences for each failure mode − Consider probability of failure modes over time − Utilize existing information to extent possible

• Schedule

− Draft: September 2013 − Final: December 2013

63

Modeling • Multiple models proposed to assess degradation

susceptibility: − Thermal − Crack initiation and propagation − Weld residual stress − Salt deposition rate and chemistry

• Support envisioned from EPRI, DOE, vendors, international

community

• Potential need for additional laboratory studies

• EPRI anticipates results by late 2014 – mid–2015

64

Canister Inspections

• Voluntary inspections of in-service canisters

• Focus on identifying condition of canisters − Visual indications of corrosion − Temperature − Quantity/chemistry of species deposited on canister − Ambient atmospheric conditions

• Coordinated effort by EPRI, DOE, vendor, and

utilities

65

Proposed Inspection Locations

Calvert Cliffs TN NUHOMS Summer, 2012

Hope Creek Holtec Hi-Storm Summer/Fall, 2013

Diablo Canyon Holtec Hi-Storm TBD

66

Inspection Approach for Calvert Cliffs • Selected cold canister for inspection

− Loaded November 1993 − Estimated current 4.2 kW heat load

• Visual examination29

− Insert camera through vent − Open shield door and insert camera on pole

• Temperature measurements at several locations30

• Analysis of particulates31

− Dry: scrape and vacuum deposits for laboratory analysis − Wet: SaltSmart analysis

o Flow extraction o Conductivity measurement

67

Probe for particulate measurements was inserted through open door with temporary shielding

Camera lowered through vents for visual examination

Inspection Approach for Calvert Cliffs

SaltSmart probe

http://www.saltsmartonline.com/device_description.php

68

Visual Findings

“Dusty” particulate matter on surfaces Apparent water stains on canister

Scattered rust spots Some stalactites on roof

69

Temperature Measurements

• After opening shield door, inserted thermocouple on end of extension tool

• Temperature measurements at several axial and radial locations

• Measured temperatures in range of about 40 – 49oC

• Findings will be benchmarked against existing thermal models

• Uncertain effects of ambient air temperature, time door is open prior to measurement, deposits on canister surface, and others

70

Chloride Measurements

• One SaltSmart measurement was made. Licensee indicated that SaltSmart did not get a valid chloride concentration measurement because it cannot distinguish chlorides from other ions.

• Laboratory analysis of dry deposits indicated other species in greater abundance than chloride, including calcium, silicon, aluminum, and magnesium. Difficulties in analysis included interference by specimen backing material and presence of amorphous phases.

71

Inspection Challenges

• Physical accessibility

• Allowable dose for “non-mandatory” inspections • Lack of qualified or benchmarked techniques

• Difficulty in interpreting results:

− No baseline for comparison

− Uncertain how representative are measurements with limited data set

72

SUMMARY

73

Summary • Austenitic stainless steel is susceptible to SCC when

exposed to chloride-rich salts in certain conditions.

• Operating plants have experienced chloride-induced SCC for outdoor stainless steel components.

• Susceptibility to SCC appears to be greater at lower temperatures (<80oC) because RH may be high enough to cause deliquescence of salts.

• In laboratory studies, crack initiation was observed with salt quantity as little as 0.1 g/m2 and stress near the material yield stress.

74

Summary (cont). • Less information is available about SCC susceptibility for

exposure to non-chloride-rich species. Tests with ammonium-, nitrate-, and sulfate-rich salts did not indicate cracking unless chloride was also present.

• Currently available in-service inspection and monitoring techniques may be useful detecting SCC but further research and development is needed to determine their applicability to canister systems.

• NRC remains engaged with industry and other stakeholders through regular information exchanges

75

Summary (cont). • Industry has proposed a Roadmap for an SCC susceptibility

assessment considering the need for model development and field data.

• Canister inspections present a number of challenges including lack of physical accessibility, dose considerations, lack of qualified and benchmarked techniques, and interpreting the significance of finding.

76

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H. Ailor, Ed., John Wiley and Sons: New York (1982) 461.

2. R.M. Kain, “Marine Atmosphere Stress Corrosion Cracking of Austenitic Stainless Steel,” Material Performance, 29 (1990) 60.

3. Y. Toshima, Y. Ikeno, “Long-Term Exposure Test for External Stress Corrosion Cracking on Austenitic Stainless Steels in Coastal Areas,” NACE Corrosion 2000, Paper Number 597.

4. S. Shoji, N. Ohnaka, “Effects of Relative Humidity and Chloride Type on Stainless Steel Room Temperature Atmospheric Stress Corrosion Cracking,” Corrosion Engineering, 38 (1989) 111.

5. D. Hirschfeld, H. Busch, I. Stellfeld, N. Arlt, E. Michel, D. Grimme , G. Steinbeck, “Stress Corrosion Cracking Behaviour of Stainless Steels with Respect to their Use in Architecture, Part I: Corrosion in the Active State,” Steel Research, 64 (1993), 461.

6. D. Hirschfeld, H. Busch, I. Stellfeld, N. Arlt, E. Michel, D. Grimme , G. Steinbeck, “Stress Corrosion Cracking Behaviour of Stainless Steels with Respect to their Use in Architecture, Part I: Corrosion in the Passive State,” Steel Research, 64 (1993), 526.

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