© 2015 Electric Power Research Institute, Inc. All rights reserved.

John Broussard

Dominion Engineering, Inc.

Shannon Chu

EPRI

Susceptibility Assessment 

Criteria for Chloride-Induced 

Stress Corrosion Cracking 

(CISCC) of Welded Stainless 

Steel Canisters for Dry Cask 

Storage Systems
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

Overview

 Background

 Project Timeline

 Progression of Modeling

 Key Results

 Susceptibility Criteria Format

 Steps Beyond the RIRP
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

 Chloride Induced Stress Corrosion Cracking (CISCC) of stainless steel reactor 

components has occurred when three elements are all present:

– Elevated stress

– Susceptible Material 

– Corrosive Environment

 Surface contamination by atmospheric chlorides

 Sufficient humidity

 EPRI has a multi-year project to 

– Evaluate susceptibility to CISCC for welded stainless steel used fuel canisters

– Develop related aging management guidelines 

Background

Tensile 

Stress

Susceptible 

Material

Corrosive 

Environment

SCC
2013 2014 2015 2016

Key Reports

Failure Modes and 

Effects Analysis

Industry Susceptibility 

Assessment Criteria

Aging Management 

Guidelines

Assess Mitigation Techniques

Assess Field Data

EPRI Efforts and Literature Search 

Documentation of

Technical Basis Flaw Growth & 

Flaw Tolerance 

Assessment

Canister Confinement 

Integrity Assessment

Continue Test Programs and Model Development

Collaborative 

Efforts (Multiple 

Contributors)

Voluntary Inspections and Environmental Monitoring

Evaluate NDE Options

Develop Thermal Models

Develop NDE Techniques & Delivery Systems

EPRI Project Objectives and

Project Timeline
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

Progression of Modeling

Failure Modes and Effects Analysis

– Qualitative judgments of frequency/probability, detectability, & severity over time based on 

literature and limited calculations

– Define boundaries of the degradation problem & guide approach to subsequent assessments

Canister Flaw Growth and Flaw Tolerance

– Model flaw growth to through-wall 

– Determine consequential flaw sizes 

Susceptibility Assessment Criteria

– Model the process of canister degradation based on the pathways identified in the 

FMEA

Canister Confinement Integrity Assessment

– Probabilistic analysis to determine the benefit of various monitoring, mitigation, & inspection 

regimes
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

Susceptibility Criteria Format

 ISFSI Susceptibility Ranking (ZISFSI) 

– Direct resources to locations where CISCC is more likely to occur

– Results from sites identified as more susceptible may help to refine aging 

management recommendations (improve technical basis, identify bounding 

locations)

– Proximity to chloride source and local absolute humidity are key variables

 Canister susceptibility ranking (HCAN and VCAN) intended to identify 

canister(s) to be inspected at a given site and to guide scope 

expansion if needed

– Geometry (horizontal or vertical) affects locations of maximum chloride 

deposition and locations of minimum temperature

– Canister material, storage duration, and fuel load power are key variables
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

ISFSI Susceptibility Ranking

ZISFSI = Cl

starting + Cladj + AHadj

Chloride starting value is based on proximity to a marine 

shore

Chloride adjustments are made for elevation, for proximity to 

a cooling tower (saline/non-saline), and for proximity to 

salted roads

Absolute humidity adjustment is based on local atmosphere 

annual average data, this affects the amount of time a 

surface is likely to support deliquescence 
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

ISFSI Susceptibility Ranking

Ranking value will range from 1 to 10

Ranking value will be constant with time

Ranking value will be an input to Horizontal and Vertical 

Canister Rankings
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Canister Susceptibility Ranking

Ranking value will range from 1 to 10

Ranking value will increase with time

Ranking will be different for vertical and horizontal canisters

Within a given geometry, ranking values may be used to 

identify bounding canisters at different sites

– Canister rank must be greater by at least 2

– ZISFSI at bounding canister site must be equal or greater

Additional considerations are provided for identification of 

canister(s) to be inspected
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

HCAN , VCAN = Deposition + Material + Power

Separate paths are needed because the different geometries 

have different specific locations of concern and different heat 

profiles

Rankings cannot be used to make any comparison of the 

susceptibility of horizontal canisters relative to the 

susceptibility of vertical canisters

Although the canister geometries have different deposition 

profiles, they are weighted in the same manner due to high 

variability and uncertainty in deposition rate 

Horizontal and Vertical Canister Ranking
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

Canister Locations of Higher Susceptibility

Factor for CISCC 

Susceptibility Locations on Horizontal Canister Locations on Vertical Canister

Tensile Stresses on OD Regions in the vicinity of welds 

(e.g. within about 2 thicknesses)

Regions in the vicinity of welds 

(e.g. within about 2 thicknesses)

Low Surface 

Temperature

Lids; shell along canister underside 

and along ends Lower region of canister OD

Elevated Chloride 

Deposition

Upward-facing surfaces of canister 

shell

Top lid; possibly the areas in the 

vicinity of the overpack inlets

Crevice-like Geometry Support rail contact region Areas where canister contacts the 

overpack channels/standoffs*

Material Condition Areas of heavy grinding or 

mechanical damage (e.g. gouges)

Areas of heavy grinding or 

mechanical damage (e.g. gouges)

More Susceptible 

Location(s)

Shell welds at canister ends (top 

surface); support rail interface near 

welds

Canister sides near welds at the 

bottom of the canister

* These features are not present in all overpack designs for vertical canisters.
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

Canister Ranking Parameters - Deposition

Deposition parameter reflects both canister age and 

environmental characteristics

The parameter is established based on the value of XCl, 

which is calculated using:

– The storage duration of the canister

– The ZISFSI value for the canister site

At higher ZISFSI values, XCl increases faster (i.e., higher XCl for 

a given storage duration)
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

Canister Ranking Parameters - Material

The material alloy factor provides credit to canisters 

constructed from alloys that are more resistant to CISCC 

initiation.

Different values are provided for 304, 304L(N), 316, and 

316L(N).

The primary differences among these alloys are the limitation 

of the carbon content to reduce the potential for sensitization 

during welding (–L/–LN) and the addition of molybdenum 

(type 316) to increase the passivity of the surface oxide 

layer. 
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

Canister Ranking Parameters - Power

Decreasing thermal power causes decreasing canister 

surface temperature.

Deliquescence for a significant portion of time each year at 

atmospheric levels of absolute humidity tends to begin at 

25oC above atmospheric temperature.

The relationship between thermal power levels and 

temperature at various points on the canister is different for 

horizontal and vertical canisters.
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

Qualitative Considerations for Identifying Candidate(s) for 

Canister Inspection Among Equally Ranked Canisters

Canister in storage the longest

Specific canister placement 

Pre-load storage and installation experience

Fabrication record information
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

Susceptibility Assessment Criteria Schedule

Susceptibility Assessment Criteria draft report currently in 

review cycle

– EPRI staff

– Advisory panel (comprised of vendors, DOE, NEI, utilities)

– EPRI High Level Waste Technical Advisory Committee

Publication anticipated in September 2015
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

Steps Beyond the RIRP Resolution

Canister Confinement Integrity Assessment

– Probabilistic assessment of flaw growth 

– Compare various inspection regimes (optimize resources, aiming 

for detection prior to 75% through-wall)

Aging Management Guidelines

Continue to follow CISCC testing, modeling and inspection 

results
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© 2015 Electric Power Research Institute, Inc. All rights reserved.

Together...Shaping the Future of Electricity
Example AMP for Localized Corrosion and 

Stress Corrosion Cracking of Welded 

Stainless Steel Dry Storage Canisters

Darrell S. Dunn, 

NRC/NMSS/DSFM/RMB

Chloride-Induced Stress Corrosion Cracking 

Regulatory Issue Resolution Protocol Meeting

April 21, 2015
Outline

• Atmospheric chloride-induced stress corrosion 

cracking (CISCC)

• CISCC calculations:

– Effect of temperature 

– Deliquescence of chloride salts

– Conservative estimation of CISCC growth

– Key points 

• Example aging management program (AMP)

• Summary 

04/21/2015 CISCC RIRP Meeting 2
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Atmospheric CISCC

• Cl salts transported and 

deposited by flow of ambient air

• Salt deliquescence dependent on 

composition and relative humidity 

• Pitting and crevice corrosion 

initiation sites for CISCC

04/21/2015 CISCC RIRP Meeting 3

• Atmospheric CISCC of welded 

stainless steel components 

observed in operating power 

plants 

– Piping systems

– Storage tanks 

• CISCC complex process with 

multiple dependencies

– Surface temperatures 

– Composition of deposited salts

– Surface concentration of salts

– Site specific environmental 

parameters

– Residual stress profiles 

References:

NRC IN 2012-20 (ML12319A440)

NUREG/CR-7170 (ML14051A417)

NUREG/CR-7030 (ML103120081)
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CISCC Growth Rate vs Temperature

04/21/2015 CISCC RIRP Meeting

• Baseline rate of 0.29 mm/yr at 

23oC from Kosaki (2008) 

• Activation energy of 31 kJ/mol 

from Hayashibara et al. (2008)

• DOES NOT show crack growth 

rates of actual components

– Composition and deliquescence 

behavior of atmospheric deposits 

– Site specific environmental data 

– Residual stress profile

• Plant operating experience*

– Turkey Point: 0.11 mm/yr

– San Onofre: 0.25 mm/yr

– St. Lucie: 0.39 mm/yr

*Assuming crack initiation at the start of 

plant operation and continuous growth 
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Deliquescence of Deposited Salts

04/21/2015 CISCC RIRP Meeting

• Deliquescence of 

chloride salts dependent 

on composition of salts, 

relative humidity, and 

temperature

• NUREG/CR-7170
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Deliquescence of Chloride Salts

04/21/2015 CISCC RIRP Meeting

• Minimum absolute humidity 

(AH) for deliquescence as a 

function of temperature

• AH value of 30 g/m3 used as a 

maximum for natural conditions

• Maximum AH values based on 

2014 National Oceanic and 

Atmospheric Administration 

(NOAA) data

– Vandenberg AFB*, CA: 16.3 g/m3

– Witham Field*, FL: 24.8 g/m3

– Groton*, CT: 21.5 g/m3

*None of these sites have been 

determined to be representative of 

any NRC licensed facilities
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Deliquescence of Chloride Salts

04/21/2015 CISCC RIRP Meeting

• Calculations for ambient air 

contacting a heated surface 

with salt deposits (CaCl2, 

MgCl2, NaCl)

• Surface temperatures range 

from 20 to 65oC

• Determine the time that an 

aqueous solution with Clions 

may be in contact with a 

surface at temperature using 

NOAA weather station data 

and salt deliquescence curves

• Fraction of the month where 

RHSurface (T) > RHD, Salt for 2014

NOAA data

Ambient Air

• Temperature 

• Relative Humidity 

• Dew Point

Deliquescence of deposited 

salts occurs if the relative 

humidity (RH) at a surface 

at temperature (T) is 

equal to or greater than the 

relative humidity necessary 

for salt deliquescence

RHSurface (T) > RHD, Salt
04/21/2015 CISCC RIRP Meeting 8

Deliquescence of Chloride Salts 

Solubility at 50oC

CaCl2: 132g/100g water

CaCO3: 0.00077g/100g water

Ca(OH)2: 0.131g/100g water

CaSO4 2H2O: 0.255g/100g water
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Deliquescence of Chloride Salts 

04/21/2015 CISCC RIRP Meeting

Solubility at 35oC

MgCl2: 57g/100g water

MgCO3: 0.04g/100g water

Mg(OH)2: 0.001g/100g water

MgSO4: 42g/100g water
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Deliquescence of Chloride Salts 

04/21/2015 CISCC RIRP Meeting
CISCC Calculation Assumptions

• Conservative Assumptions (not necessarily representative):

– Sufficient through wall tensile residual stresses for CISCC initiation and growth

– Sufficient surface salt concentrations for CISCC initiation and growth

– Composition of surface deposits do not change with time 

• No changes as a result of precipitation or decomposition reactions

• No effects of corrosion product accumulation

– Deliquescence is instantaneous when RHSurface (T) > RHD, Salt

– CISCC initiation instantaneous when RHSurface (T) > RHD, Salt

– Crack growth at all times when RHSurface (T) > RHD, Salt

– CISCC re-initiation instantaneous when RHSurface (T) > RHD, Salt

– Maximum CISCC rate at temperature when RHSurface (T) > RHD, Salt

• Assumptions under further evaluation:

– CISCC stops when RHSurface (T) < RHD, Salt

– CISCC growth rate is independent of surface salt concentration

– CISCC growth rate independent of crack depth 

04/21/2015 CISCC RIRP Meeting 11
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Conservative Estimations of 

CISCC Crack Growth 

04/21/2015 CISCC RIRP Meeting 12

• Evaluating seasonal/annual environmental variations with composition of 

actual atmospheric deposits to eliminate overly conservative assumptions for 

more representative CISCC growth rates 
Key Points

• CISCC initiation and growth a complex aging mechanism with 

multiple coupled parameters including deposit compositions, 

site specific environmental conditions and surface temperatures

– When and where deliquescence could occur

– Fraction of time when cracking may occur 

– Crack growth rates

• Conservative approach for CISCC calculations

– Conservative assumptions used for CISCC initiation and growth

– Multiple assumptions likely overestimate CISCC growth rates 

• CISCC rates limited by site specific environmental conditions

– CISCC not expected above ~55oC

• Aging Management Program appropriate for managing possible 

aging effects as a result of CISCC initiation and growth

04/21/2015 CISCC RIRP Meeting 13
Example AMP for Welded Stainless 

Steel Canisters 

• Example AMP included in NUREG-1927 Revision 1

– American Society of Mechanical Engineers Boiler and Pressure 

Vessel (ASME B&PV) Code Section XI - Rules For Inservice 

Inspection Of Nuclear Power Plant Components

• AMP Elements:

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1. Scope of the Program

2. Preventive Actions

3. Parameters Monitored/Inspected

4. Detection of Aging Effects

5. Monitoring and Trending

6. Acceptance Criteria

7. Corrective Actions

8. Confirmation Process 

9. Administrative Controls

10. Operating Experience

04/21/2015 CISCC RIRP Meeting
AMP Element 1 

Scope of the Program 

• Inservice inspection of external surfaces of welded 

austenitic stainless steel canisters for localized corrosion 

and SCC 

– Fabrication and closure welds 

– Weld heat affected zones

– Locations where temporary supports or fixtures were attached by 

welding

– Crevice locations 

– Surfaces where atmospheric deposits tend to accumulate 

– Surface areas with a lower than average temperature

04/21/2015 CISCC RIRP Meeting 15
AMP Element 2

Preventative Actions

• Example AMP is for condition monitoring. 

– Preventative actions are not incorporated into the example AMP 

contained in NUREG-1927 Revision 1

• Preventative actions for welded stainless steel canisters 

may include: 

– Surface modification to impart compressive residual stresses on 

welds and weld heat affected zones

– Materials with improved localized corrosion and SCC resistance

04/21/2015 CISCC RIRP Meeting 16
AMP Element 3

Parameters Monitored/Inspected

• Canister surfaces, welds, and weld heat affected zones 

for discontinuities and imperfections 

• Appearance and location of atmospheric deposits on the 

canister surfaces

• Size and location of localized corrosion (e.g., pitting and 

crevice corrosion) and stress corrosion cracks

04/21/2015 CISCC RIRP Meeting 17
AMP Element 4

Detection of Aging Effects 

• Qualified and demonstrated technique to detect evidence 

of localized corrosion and SCC:

– Remote visual inspection, e.g. EVT-1, VT-1, VT-3

• Suspected areas of localized corrosion and/or SCC 

require additional evaluation 

• Sample size

– Minimum of one canister at each site (greatest susceptibility) 

• Data Collection

– Documentation of the canister inspection 

– Location and appearance of deposits, localized corrosion, SCC

• Frequency

– Every 5 years

• Alternative methods or techniques may be provided 

04/21/2015 CISCC RIRP Meeting 18
AMP Element 5

Monitoring and Trending

• Reference plans or procedures to establish a baseline 

• Document canister condition particularly at welds and 

crevice locations using images and video that will allow 

comparison in subsequent examinations 

• Changes to the size and number of corrosion product 

accumulations 

• Track parameters and aging effects such as location and 

sizing of localized corrosion and SCC 

04/21/2015 CISCC RIRP Meeting 19
AMP Element 6 

Acceptance Criteria 

• No indications of: 

– Pitting corrosion, crevice corrosion, or SCC 

– Corrosion products on or adjacent to fabrication welds, closure 

welds, and welds for temporary supports or attachments

• Locations with corrosion products require additional 

examination for localized corrosion and/or SCC

• Canisters with localized corrosion and/or SCC must be 

evaluated for continued service. 

• Example AMP uses ASME B&PV Section XI Criteria 

– IWB-3514 

– IWB-3640

• Alternative acceptance criteria may be provided 

04/21/2015 CISCC RIRP Meeting 20
AMP Element 7

Corrective Actions

• Applicants may reference the use of a Corrective Action Program 

(CAP), which is consistent with the quality assurance (QA) 

requirements in either 10 CFR Part 50, Appendix B, or 10 CFR Part 

72, Subpart G

• Perform functionality assessments

• Perform apparent/root cause evaluations

• Address the extent of condition

• Determine actions to prevent recurrence 

• Justifications for non-repairs

• Trend conditions

• Identify operating experience actions, (e.g., AMP changes) 

• Determine if the condition is reportable to the NRC

04/21/2015 CISCC RIRP Meeting 21
AMP Element 8

Confirmation Process

• Confirmation process should be commensurate with the 

specific or general licensee Quality Assurance (QA) 

Program and consistent with 10 CFR Part 72, Subpart G 

or 10 CFR Part 50, Appendix B. 

• QA Program ensures that the confirmation process 

includes provisions to preclude repetition of significant 

conditions adverse to quality.

• The confirmation process describes or references 

procedures to:

– Determine follow-up actions to verify effective implementation of 

corrective actions 

– Monitor for adverse trends due to recurring or repetitive findings

04/21/2015 CISCC RIRP Meeting 22
AMP Element 9

Administrative Controls

• The specific or general licensee QA Program must be 

commensurate with 10 CFR Part 72, Subpart G or 10 

CFR Part 50, Appendix B and specifically addresses: 

– Instrument calibration and maintenance

– Inspector requirements

– Record retention requirements

– Document control

• The administrative controls describes or references: 

– Frequency/methods for reporting inspection results to the NRC

– Frequency for updating AMP based on industry-wide operational 

experience

04/21/2015 CISCC RIRP Meeting 23
AMP Element 10

Operational Experience

• References and evaluates applicable operating 

experience, including:

– Internal and industry-wide condition reports,

– Internal and industry-wide corrective action reports,

– Vendor-issued safety bulletins,

– NRC Information Notices, and

– Applicable DOE or industry initiatives (e.g., EPRI or DOE 

sponsored inspections)

• References the methods for capturing operating 

experience from other ISFSIs with similar in-scope SSCs

04/21/2015 CISCC RIRP Meeting 24
AMP Element 10

Operational Experience

• Identifies any degradation in the referenced operating 

experience as either age-related or event-driven, with 

proper justification for that assessment

• Past operating experience supports the adequacy of the 

proposed AMP, including the method/technique, 

acceptance criteria, and frequency of inspection

• Example AMP also references past operating experience 

04/21/2015 CISCC RIRP Meeting 25
Summary

• Atmospheric CISCC has been observed in welded 

austenitic stainless steel components 

• Limited data on atmospheric CISCC growth rates and 

composition of atmospheric deposits 

• Composition of deposits, operating environment and 

surface temperature of the canister are significant 

• Example AMP for welded austenitic stainless steel 

canisters included in NUREG-1927 Revision 1 

• Adaptation of inspection methods used for operating 

reactor pressure boundary components and development 

of inspection delivery systems for canister inspections 

are necessary to improve canister inspection capabilities 

04/21/2015 CISCC RIRP Meeting 26
Acronyms

AH: Absolute Humidity (grams of water per 

cubic meter of air)

AMP: Aging Management Program 

ASME B&PV Code: American Society of 

Mechanical Engineers Boiler and Pressure 

Vessel Code 

CAP: Corrective Action Program

CISCC: Chloride-Induced Stress 

Corrosion Cracking

CFR: Code of Federal Regulations

DOE: Department of Energy

EPRI: Electrical Power Research Institute

EVT-1: Enhanced Visual Testing-1 (Boiling 

water reactor vessels and internals 

project, BWRVIP-03)

ISFSI: Independent Spent fuel Storage 

Installation

NOAA: National Oceanic and Atmospheric 

Administration 

QA: Quality Assurance

T: Temperature

TLAA: Time-Limited Aging Analysis

RH: Relative Humidity

SCC: Stress Corrosion Cracking

SSC: Structure, system or component

VT-1: Visual Testing-1 (ASME B&PV code 

Section XI, Article IWA-2200) 

VT-3: Visual Testing-3 (ASME B&PV code 

Section XI, Article IWA-2200)

04/21/2015 CISCC RIRP Meeting 27