ML19221B113
| ML19221B113 | |
| Person / Time | |
|---|---|
| Issue date: | 03/31/1979 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| RTR-NUREG-75-087, RTR-NUREG-75-87 SRP-06.01.01, SRP-6.01.01, NUDOCS 7907120443 | |
| Download: ML19221B113 (11) | |
Text
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U.S. NUCLEAR REGULATORY COMMISSION
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STANDARD REVIEW PLAN
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OFFICE OF NUCLEAR REACTOR REGULATION SECTION 6.1.1 ENGIN2ERED SAFETY FEATURES MATERIALS REVIEW RESPONSIBILITIES Pr imary - Materials Engineering Branch (MTEB)
Secondary - Containment Systems Branch (CSB)
Accident Ar dlysis Branch ( AAB)
I.
AREAS CF REVIEW Engineered Safety Features (ESF) are proviced in nuclear plants to mitigate the conse-quences of design basis or loss-of-coolant accidents, even though the occurrence of these accidents is very unlikely. The General Design Criteria (GDC) 16, 34, 35, 38, 41 and 44 of Appendix A, 10 CFR Part 50, reaJire that certain systems be provided to serve as Engineered Safety Features (ESF). Lunt_inment systems, residual heat removal system, emergency core cooling systems, containment heat removal systems, containment atmosphere cleanup systems, and certain cooling water systems are typical of the systems that a"e required to be provided as ESF.
The steel containment is described in Section 3.8.2 of the Safety Analysis Report (SAR) and reviewed in accordance with Standard Review Plan (SRP) Section 3.8.2.
The metallic n.aterials are reviewed in this SRP section.
The emergency core cooling system, the containTent heat removal system, the containment cleanup systems and other ESF systems are described in Section 6 of the SAR and are reviewed in accordance with the SRPs for the individual systems. The materials for these sys'ims are reviewed in this SRP.
The cooling system for the reactor auxiliary systems is described in Section 9.2.2 of the SAR and reviewed in accordance with SRP Section 9.2.2.
The materials for these systems are reviewed in this SRP.
The areas relatirg to ge..eral materials considerations and ESF fluid chemistry in the design of the engineered safety features are reviewed:
1.
Materials and Fabrication The materials and f abrication procedures used in the engineet ed safety features are reviewed. The specific areas of review and review procedures are similar to those in Standard Review Plan Section 5.2.3, " Reactor Coolant Pressure Boundary Materials,"
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and to those in Standard Review Plan Section 10.3.6, " Steam and Feedwater System Materials." The purpose of the review is to assure compatibility of the materials with the specific fluids to which the materials are subjected. The review is per-formed to assure compliance with the General D2 sign Criteria and with the ASME Boiler and Pressure Vessel Code (hereafter "the Code"),Section II, parts A, B, and C,Section III, Division 1 and 2, and Section IX.
Areas that are reviewed include:
mechanical properties of materials (including fracture toughness), control of welding procedures, control of low hydrogen welding materials, control of the welding of low alloy steel (including minimum specified preheat), and control of the use of sensitized austenitic stainless steels.
2.
Composition and Compatibility of Engineered Safety Features Fluids l
The composition of the containment and core spray coolants must be controlled to ensure their compatibility with materials in the containment building, including the reactor vessel, reactor internals, piping, and structural and insulatir.g materials. The methods and procedures to control the chemical composition of solutions recirculated witkin the containment after design basis accidents (DBA) must be selected (a) to maintain the integrity of the reactor coolant pressure boundary, by preventing stress corrosic cracking of safety-related components, (b) to insure that adequate solution mixing of ESF fluids will occur, and (c) to prevent evolution of excessive amounts of hydrogen within the containment in the unlikely event of a design basis accident.
The time history of the pH of the fluids, including the source and quantity of all soluble acids and bases in the containment after a design basis accident, is reviewed.
Cantainment and core spray solutions must be stable under long-term storage condi-tions and during prolonged operation of the sprays. Some of these solutions contain boron for reactivity control and other additives (such as thiosulfates) for reacting with gaseous fission products.
In many instances the ESF coelant solutions are stored in more than one form (such as boric acid solution and a sodium hydroxide solution) and mixed only when the ESF are called upon to operate during an emergency. In some plants, the coolant is stored as a boric acid solution that is neutralized by (dry) sodium phosphates mounted in baskets inside the containment after the ESF sprays are activated.
The controls on contaminants, such as chlorides, lead, zinc, sulfur, or mercury, in the ESF fluids are reviewed. Potential sources of these cortamirants, such as coatings and nonmetallic thermal insulation, that will be exposed to ESF fluids in DBA environments are evaluated.
Peeling, flaking or delamination of coatings can result in clogging of ESF system strainers and spray nozzles and thereby stop or slow down the flow rates of the ESF fluids. The qualification program for the protective coatings is reviewed.
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Rev. 1 6.1.1-2
The Containtnent Systems Branch (CSE) is responsible for reviewing hydrogen buildup in the containment as described in SRP Section 6.2.5, " Combustible Gas Control in Containment." The Materials Engineering Branch (MTEB) reviews only c;rrcsion and hydrogen evolution rates as submitted in the SAR.
The procedures, methods, and liqte.d pathways usec in the control of pH, conductivity, and chlorides are reviewed by the Accident Analysis Branch (AAB).
MTEB has secondary review responsibility for corrosion and integrity of Containment Materials as described in SRP Section 3.8.2.
II.
ACCEPTANCE CRITERIA The acceptance criteria for the areas of review described in Section I of this plan are as follows:
1.
Materials and Fabrication Materials for use in ESF must be selected for their compatibility with ECCS ana cc itainment spray solutions and other ESF fluids. The materials specified for use in these systems must be as given in Appendix 1 to Section III of the Code, and parts A, B and C of Section II of the Code. Fracture toughness of the materials shall be as stated in SRP Section 10.3.6, Subsection II.l.
Cold worked austenitic stainless steels must have a maximum 0.2% offset yield strength of 90,000 psi to reduce the probability of stress corrosion cracking in these systems.
Regulatory Guide 1.85, " Code Case Acceptability ASME Section III Materials," describes acceptable Code cases that may bi ased in conjunction with the above specifications.
Regulatory Guide 1.44, " Control of the Use of Sew isized Stainless Steel," describes acceptable criteria for preventing intergranular corrosion of stainless steel components of the ESF. Furnace-sensitized material should not be allowed in the ESF, and methods described in this guide should be followed for cleaning and protecting austenitic stainless steels frcm contamination during handling and storage, for testing the materials prior to fabrication, and for ensuring that no deletericus sensitization occurs curing welding.
Components and systems are to be cleaned in conformance with the requirements of Regulatory Guide 1.37, " Quality Assurance Requirements for Cleanirg of Fluid Systems and Associated Components of Water-Cooled Nuclear Power Plants."
Branch Technical Position MTFB d-7, " Material Selection and Processinc, Guidelines for BWR Caolant 'ressure Boundary Piping," describes acceptable criteria for the use of austenitic stainless steel piping in boiling water reactors. (See SRP Section 5.2.3.)
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ECV-I 6.1.1-3
Regulatory Guide 1.31, " Control of Ferrite Content in Stainless Steel Weld Metal,"
l describes acceptable criteria for assuring the integrity of welds in austenitic stainless steel ESF components. The control of delta ferrite content of weld filler metal is specified in this guice, which sets forth an acceptable basis for delta ferrite content of weld filler metal.
The acceptance criteria for ferritic steel welding are as follows:
a.
The amount of minimum spec;fied preheat must be in accordance with the recom-mendations of the Code,Section III, Appendix D, Article 0-1000, and Regulatcry Guide 1.50, " Control of Preneat Temperature for Welding Low-Alloy Steel,"
unless an alternate procedure is justified.
b.
Moisture control on low hydrogen welding materials shall conform tu the require-ments of the Code,Section III, Articles NB, NC, ND-2000 and 4000, and AWS DI.1,
" Structural Welding Code," unless alternate pr)cedures are justified.
c.
For areas of limited accessibility, the criteria of SRP Section 10.3.6. II.2.c shall apply.
The composition of nonmetallic thermal insulation for components of ESF should be controlled as described in Regulatory Guide 1.36, " Nonmetallic Thermal Insulation for Austenitic Stainless Steel." Concentrations of leachable cchtuminants and added inhibitors should be controlled as specified in position C.2.b and Figure 1 of this guide to reduce the probability of stress corrosinn cracking of austenitic l
stainless steel components.
The use of nonmetallic insulation on nonaustenitic stainless steel components should be controlled as above. The moisture dripping from wet insulation on any component can affect austenitic stainless steel that is at a physically lower elevation.
The criteria for coatings to be used in containments are described in Regulatory Guide 1.54, " Quality Assurance Requirements for Protective Coatings Applied to Water-Cooled Nuclear Power Plants." Identified quantities of soluble acids and bases within the containment must not be great enough to cause excessive hydrogen generation or deleterious corrosion.
2.
Composition and Compatibility of Engineered Safety Features Fluids The qualification program for coating systems should confirm that the systems used on ESF will not possibly stop or slow down the flow rates of the ESF fluids during a design basis accident. The compositions of containment spray and core cooling water for PWRs should be controlled to ensure a minimum pH of 7.0, as given in Branch Technical Position MTEH 6-1 attached to this SRP section. Experience has shown that maintaining the pH of borated solutions at this level will help to Rev. I 6.1.1-4
inhibit initiation of stress corrosion cracking of austenitic stainless steel componen's.
Hydrogen generation results from the corrosion of materials by the containment sprays during design basis accident. Hydrogen release within the containment should be controlled as described in Regulatory Guide 1.7. " Control of Combustible Gas Concentrations in Containment Following a loss-of-Coolant Accident."
a.
Pressurized Water Reactors (PWRs)
The hydrogen generation from the corrosion of materials within containment, such as al uinum and zinc, depends upon the corrosion rate which in turn depends upon such factors as the coolant chemistry, the coolant pH, the metal and coolant temperature, and the surface ar a exposed to attack by the coolant.
The reviewer compares the assumed corrosion rates of materials in containment with standard corrosion rate data.
b.
Boiling Water Reactors (BWRs)
Water used in the engineered safety feature systems should be controlled to provide assurance against stress corrosion cracking of unstabilized austenitic stainless steel components. Water used for emergency core cooling systems and spray systems should be controlled to ensure the following limits:
Conductivity = 3 to 10 pmhos/cm @ 25 C Chloride (Cl-) < 0.50 ppm pH = 5.3 to 8.6 @ 25 C Hydrogen generation in BWR containments is assumed to follow the same charac-teristics as in PWRs in that the "ates of hydrogen generation will rise with increasing zinc corrosion as the tempe^ature rises, and will change with any change in pH.
III. REVIEW PROCEDURES The reviewer will select and emphasize material from the procedures described below, as may be appropriate for a particular case.
To ascertain that the acceptance criteiia given in Section II are met, the reviewer examines each of the review areas given in Secticn I for the required information, using the following procedure:
1.
Materials and Fabrication The reviewer examines the information on the compatibility of the ESF materials of construction with the ESF fluids to verify that all materials used are compatible.
147 186 6.1.1-5 Rev. 1
The reviewer considers the composition of the sprays and any mixing processes that might occur during operation of the sprays.
fhe reviewer verifies that the materials proposed for the ESF are in conformance with f,ppendix i of Section III of the Code, ano with parts A, B and C of Section II of the Code.
He verifies that cold worked austenitic stainless steels used in fabricaticn of the ESF are in conformance with Section 11.1.
The reviewer verifies that the controls of ferritic steel weldirg are in conformance f the with Subsection II.l.
The reviewer verifies that the fracture toughness o mattrials is in accordance with the requirements of the Code.
The inethods of controlling sensitized stainless steel in the ESF systems are examined by the reviewer who verifies that :he methods are in conformance with Regulatory Guide 1.44.
This applies espec'aliy to the cleaning and protection of stainless steel from contamination during handling and storage, to the verification of nonsensitization of the materials, and to the qualification of welding procedures using ASTM A262.
If alternative mecTods of testing the qualificatior, welds for degree of sensitization are pro: > sed by the applicant, the reviewer determines if these are satisfactory, based on the degree to which the alternate methods previde the needed results.
The methods for controlling the amount of delta ferrite in stainless steel weld deposits are examined by the reviewer in accordance with Regulatory Guide 1.31,
" Control of Ferrite Content in Stainless Steel Weld Metal."
The reviewer determines whether nonmetallic thermal insulation will be used on components of the ESF, and, if it is, he verifies that the amount of leachable impurities in the specified insulation lie within the "acceptule analyfis" area of figure 1 of Regulatory Guide 1.36, as discussed in the acceptance criteria,Section II.l.
The reviewer verifies that tne coatings used in the containment conform with Regu atory Guide 1.54.
i 2.
Composition and Compatibility of Engineered Safety Features Fluids f
a.
PressurizedWaterReactors(PWRs_]
l The reviewer determines that the coolant spray will have a minimum pH of 7.0 and reviews the methods of ascertaining that the pH will remain above this minimum during the operation of the sprays. The reviewer examines the control of pH of such coolants to evaluate the short-term (during the mixing process) compatibility and long-term compatibility of these sprays with all safety-related components within the containment.
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The AAB reviews the chemical and radiation environment discussed in SRP Section 3.11 Appendix, " Chemical and Radiological Environment in Containment During Postulated Accidents" and in SRP Section 6.5.2, " Containment Spray as a fission Product Cleanup System." The AAB examines the paths that the solutions would follow in the containment from sprays and emergency core cooling systems to the sump, for both injection and recirculation phases to verify that no areas accumulate very high or low pH solutions and that any assumptions regarding pH in the modeling of containment spray fission product removal are valid.
reviewer examines the methods of storing the ESF fluids to determine "a
whether deterioration will occur either by chemical instability or by corrosive attack on the storage vessel. The reviewer determines what effects such deterioration could have on the compatibility of these ESF coolants with both the ESF materials of construction and the other materials within the containment.
b.
Boiling Water Reactors (BWRs)
The reviewer verifies that the chemistry of the water used for the emergency core cooling systems and the containment spray systems is controlled to the limits given in Subsection II.2.b.
IV.
EVALUATION FINDINGS The reviewer verifies that the information provided supports conclusions of the following type, which are to be included in the staff's safety evaluation report:
"The materials selected for the Engineered Safety Features satisfy Appendix I of Section III of the ASME Code, and Parts A, B, and C of Section II of the Code, and the staff position that the yield strength of cold-worked stainless steels shall be less than 90,000 psi. Fracture toughness of the ferritic materials meets the requirements of the Code.
"The controls on the pH and chemistry of the reactor containment sprays and the l
Emergency Core Cooling water following a postulated loss-of-coolant or design basis accident, are adequate to reduce the probability of stress corrosion cracking of the austenitic stainless steel components and welds of the Engineered Safety Features systems in containment throughout the duration of the postulated accident to comple-tion of cleanup. The controls on the use and fabrication of the austenitic stainless steel of the systems satisfy the requirements of Regulatory Guide 1.31, Control of Ferrite Content of Stainless Steel Weld Metal', and Regulatory Guide 1.44,' Control of the Use of Sensitized Stainless Steel'.
Fabrication and heat treatment practices performed in accordance with these requirements provide added assurance that the probability of stress corrosion cracking will be reduced during the postulated accident time interval. The controls placed on concentrations of leachable impurities I in nonmetallic thermal insulation used on components of the Engineered Safety eatures are in accordance with Regulatory Guide 1.36,' Nonmetallic Thermal Insulation r
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for Austenitic Stainless Steel'.
The control of the pH of the sprays and cooling water, in conjunction with controls on selection of containment materials, is in accordance with Regulatory Guide 1.7,'Contral of Combustible Gas Concentrations in Containment Following a !nss of-Coolant Accident', and provides assurance that the sprays and cooling water will not give rise to excessive hydrogen gas evolution resulting f rom corrosion of containment metal or cause serious deterioration of the materials in containment. The protective coating systems have been qualified by tests acceptable to the staff. This qualification provides reasonable assurance that the coating systems will not degrade the operation of the ESF by delaminating, flaking or peeling. Conformance with the Codes and Regulatory Guides ano with the staff positions mentioned above, constitute an acceptable basis for meeting in part the requirements of General Design Criteria 16, 34, 35, 38, 41 and 44."
V.
REFERENCES 1.
10 CFR Part 50, Appendix A, General Design Criteria.
2.
ASME Boiler and Pressure Vessel Code,Section II, Parts A, B, and C,Section III, Division 1, including Appendix I,Section III, Division 2, and Section IX, American Society of Mechanical Engineers.
3.
ASTM A-262, " Detecting Susceptibility to Intergranular Attack in Stainless Steel,"
Annual Book of ASTM Standards, Part 3, American Society for Testing and Materials.
4.
AWS 01.1, " Structural Welding Code," American Welding Society.
5.
Regulatory Guide 1.7, " Control of Combustible Gas Concentrations in Containment following a Loss-of-Coolant Accident."
6.
Regulatory Guide 1.31, " Control of Ferrite Content in Stainless Steel Weld Metal."
7.
Regulatory Guide 1.36, " Nonmetallic Thermal Insulation for Austenitic Stainless Steel."
8.
Regulatory Guide 1.37, " Quality Assurance Requirements for : leaning of Fluid Systems and Associated Components of Water-Cooled Nuclear Power Plants."
9.
Regulatory Guide 1.44, " Control of the Use of Sensitized Steel."
10.
Regulatory Guide 1.50, " Control of Preheat Temperature for Welding Low-Alloy Steel."
11.
Regulatory Guide 1.54, " Quality Assurance Requirements for Protective Coatings Applied to Water-Cooled Nuclear Power Plants."
12.
Standard Review Plan 3.11, Appendix, " Chemical and Radiological Environment in Containment During Postulated Accidents."
Rev. 1 6.1.1 -8 147 189
13.
Standard Review Plan 5.2.3, " Reactor Coolant Pressure Boundary Materials."
14.
Standard Review Plan 6.2.5, " Combustible Gas Control in Containment."
15.
Standard Resi., ol:n 6.5.2, " Containment Spray as a Fission Product Cleanup System."
f 16.
Standard Revic 9'an 10. 3.6, " Steam and Feedwater Systems Materials."
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17.
Branch Technical Position MTEB 5-7, " Material Selection and Processing Guidelines f
for BWR Coolant Pressure Boundary Piping."
18.
Branch Technical Position MTEB 6-1, "pH for Emergency Coolant Water," appended.
147 190 pe v, 1 6.1.1-9
BRANCH TECHNICAL POSITION MTE8 6-1 pH FOR EMERGENCY COOLANT WATER FOR PWRs A.
Background
To establish the minimum value of pH in oost accident containment sprays in Pressurized Water Reactors, the Materials Engineering Branch has reviewed the available information and recommended the criteria listed in the Branch Technical Position below.
The minimum pH value of 7.0 follows from the Westinghouse report (Ref. 1) conclusion that, in ECCS solutions 'djusted with NaOH to pH 7.0 or greater, no cracking should be observed at chloride concentrations up to 1000 ppm during the time of interest. Figure 7 of the Westinghouse report shows that time for initiation of cracking of sensitized and nonsensitized U-bend specimens of Type 304 austenitic stainless steel in solutions of 7.0 pH having 100 ppi chloride was seven and one half months and ten months, respectively.
These time periods are more than ample time to allow cleanup after the hypothetical design basis accident.
The great majority of tests reported in the Oak Ridge report, Reference 2, were per-l formed with pH of 4.5, and only two tests were conducted with pH values other than 4.5.
Some cracking was observed at pH 7.5 in the sensitized 304 stainless steel U-beni specimens after two months exposure to pH 7.5 and chloride concentration of 200 ppm.
All of the 316 stainless steel specimens shcwed no evidence of cracking. Considerinj the fact that in U-bend specimens the material was sensitized, stressed beyond yiels, and plastically deformed, we conclude that the reported test conditions were much more severe than the stress conditions likely to exist in the post-accident emergency coolant systems.
We agree with the Oak Ridge conclusion that absolute freedom from failure of any complex system such as a spray system can never be guaranteed, but, by proper design, fabrica-tion, and control of the corrosive environment, the probability of failure can be significantly reduced. Our recommended minimum pH of seven is somewhat higher than the Oak Ridge recommendation of a minimum of 6.5.
B.
Branch Technical Position MTEB criteria for pH level of post-accident emergency coolant water to reduce the prob-ability of stress-corrosion crackinq of austenitic stainless steel components, non-sensitized or sensitized, non-stressed or stressed, are as follows:
1.
Minimum pH should be 7.0.
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2.
The higher the pH (in the 7.0 to 9.5 range), the greater the assurance that no stress corrosien cracking will occur.
3.
If a pH greater than 7.5 is used, consideration should be given to the hydrogen generation problem from corrosion of aluminum in the containment.
C.
References 1.
D. D. Whyte and L. F. Picone, " Behavior of Austenitic Stainless Steel in Post Hypothetical loss of Coolant Environment," WCAP-7798-L, Westinghouse Nuclear Ener gy Systems, November 1971 (NES Proprietary Class 2).
2.
J. C. Criess and E. E. Creek, " Design Considerations of Reactor Containment Spra, Systems - Part X, The Stress Corrosion Cracking of Types 304 and 316 Stainless Steel in Boric Acid Solutions," ORNL-TM-2412, Part X, Oak Ridge National Laboratory, May 1971.
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t-S 6.1.1-11 Rev. 1