Stress Corrosion Cracking of Type 304 Stainless Steel Exposed to

Atmospheric Ammonium Nitrate and Sodium Chloride Mixtures

Xihua (Shē-wă) He,1 Roberto Pabalan,1 Todd Mintz,1 Greg Oberson,2 Darrell Dunn,2 Tae Ahn2

1Center for Nuclear Waste Regulatory Analyses

Southwest Research Institute®

San Antonio, Texas

2U.S. Nuclear Regulatory Commission Washington, DC

NACE 2014, San Antonio, Texas

March 9–13, 2014

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Outline

Background Objective

Experimental approaches Results and discussion Summary

Dry Cask Storage Systems

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Spent nuclear fuel may be placed in dry storage after several years of cooling in spent fuel pool The dry storage systems may consist of welded austenitic stainless steel canister inside concrete vault or overpack at independent spent fuel storage installation (ISFSI) sites

Potential for Stress Corrosion Cracking of Dry Storage Canisters

Canisters may be exposed to atmospheric particulates through external vents in the vault or overpack Austenitic stainless steel may be susceptible to stress corrosion crackling (SCC) caused by deliquescence of chloride-rich atmospheric salts Austenitic stainless steel does not appear susceptible to SCC from deliquescence of non-chloride species such as NH4NO3, NH4HSO4, (NH4)2SO4, and fly ash Little is known about SCC susceptibility for mixtures of chloride and non-chloride bearing species

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Objectives Identify typical atmospheric concentrations of chloride and non-chloride species at a range of locations throughout the U.S. Focus primarily on inland areas where chloride should be less abundant than near ocean, such as regions of industrial, commercial, and agricultural activities Evaluate SCC susceptibility of austenitic stainless steel exposed to representative salt mixtures

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Literature Survey of Non-Chloride Salts to Chloride Ratio

Mole Ratio of Nitrate to Chloride and Sulfate to Chloride in Fine Particulate Matter Collected at Five IMPROVE Monitoring Sites

Site Location NO3

−/Cl− Mole Ratio

SO42−/Cl−

Mole Ratio SO4

2−/NO3−

Mole Ratio Arendtsville, Pennsylvania 12.1 33.1 2.7 Bondville, Illinois 21.1 24.0 1.1 Great River Bluffs, Minnesota 12.2 18.3 1.5 Great Smoky Mountains National Park, Tennessee

5.8 52.2 8.9

Phoenix, Arizona 2.6 3.4 1.3

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SCC tests were conducted only for a nitrate and chloride salt mixture because of interest in the potential inhibiting effects of nitrate NO3

−/Cl− mole ratios of 3.0 and 6.0 were selected for the SCC tests

Technical Approaches Deliquescence and efflorescence tests – Contained mixtures of nitrate and chloride in beakers compared to

pure salts – Exposed in environment chamber at 45 oC [113 °F] and relative

humidity (RH) increased from 10% to 59% at 3% intervals, then subsequently decreased to 10% RH also at 3% intervals

SCC tests – Triplicate as-received and sensitized Type 304 stainless steel single

U-bend specimens – Deposited pre-mixed salt mixtures on heated specimens – Exposed in test chamber at 45 oC [113 °F] and 44% RH

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Surface Salt Amount

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

Deliquescence Test Results Salts tested: – NH4NO3 and NaCl pure salts – Mixtures of NH4NO3 and NaCl with mole ratios of 3 and 6

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Before Test

28% RH

31% RH

(a) NH4NO3/NaCl = 6

Before Test

31% RH

34% RH (b) NH4NO3/NaCl = 3

Before Test

46% RH

49% RH

(c) NH4NO3 Observation From Beakers at 45 °C for Deliquescence Tests at Increasing

Relative Humidity: (a) NH4NO3/NaCl = 6, (b) NH4NO3/NaCl = 3, and (c) NH4NO3

• NaCl was not deliquesced up to 59% RH

• Deliquescence

RH of mixtures of NH4NO3 and NaCl is lower than that of pure salts

Efflorescence Test Results Salts used: – NH4NO3 pure salt – NaCl pure salt – Mixtures of NH4NO3 and NaCl with mole ratios of 3 and 6

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40% RH

37% RH

31% RH

28% RH

(a) (b)

Observation From Beakers at 45 °C for Efflorescence Tests at Decreasing Relative Humidity: (a) NH4NO3 and (b) Mixtures NH4NO3/NaCl = 3 and 6

• Efflorescence RH of mixtures of NH4NO3 and NaCl is lower than that of pure salts

Results of SCC Tests

Pitting corrosion was observed on all specimens within 47 days, with large pits at the apex and more pits on the sensitized specimens compared to the as-received specimens

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NH4NO3 + NaCl (NO3-/Cl- = 3.0) NH4NO3 + NaCl (NO3

-/Cl- = 6.0) As-Received Sensitized As-Received Sensitized

(a)

NH4NO3 + NaCl (NO3

-/Cl- = 3.0) NH4NO3 + NaCl (NO3-/Cl- = 6.0)

As-Received Sensitized As-Received Sensitized

(b)

(a) All Test Specimens after Exposure for 47 Days at 45 °C and 44% Relative Humidity

and (b) Four Specimens After Exposure Removed from Chamber after 47 Days

Observations From Sensitized Specimens

11 NH4NO3 and NaCl Mixture (NO3-/Cl- = 6.0)

NH4NO3 and NaCl Mixture (NO3-/Cl- = 3.0)

Surface Cross section After testing for 47 days After testing

for 8 weeks

Observations From As-Received Specimens After 4 months, more pitting and clearly evident cracks on specimens deposited with NH4NO3 and NaCl salt mixture with NO3

−/Cl− mole ratio of 3.0

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NH4NO3 + NaCl (NO3-/Cl- = 3.0)

NH4NO3 + NaCl (NO3-/Cl- = 6.0)

Discussion―SCC from Low pH and Deliquescence Relative Humidity of Salt Mixtures

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Summary SCC tests were conducted to evaluate the susceptibility of Type 304 stainless steel to SCC initiation caused by deliquescence of mixtures of NH4NO3 and NaCl In previous tests, SCC initiation was not observed on specimens exposed only to NH4NO3, but in this study, cracking was observed for specimens exposed to the mixtures with NO3

−/Cl− mole ratios of 3.0 and 6.0 The extent of SCC appeared to be higher at the higher chloride concentration The results indicate that chloride has a large effect on SCC susceptibility and that the presence of nitrate did not inhibit SCC at the tested NO3

−/Cl− mole ratios SCC of Type 304 stainless steel in nitrate-chloride mixtures could be attributed to the low pH and dissolution of chloride in the deliquescence solution 14

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Disclaimer This presentation describes work performed by the Center for Nuclear Waste Regulatory Analyses (CNWRA) and its contractors for the U.S. Nuclear Regulatory Commission (USNRC) under Contract No. NRC–02–07–006. The activities reported here were performed on behalf of the USNRC Office of Nuclear Regulatory Research. This presentation is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of the USNRC. The USNRC staff views expressed herein are preliminary and do not constitute a final judgment or determination of the matters addressed or of the acceptability of any licensing action that may be under consideration at USNRC.