ML19291G954
| ML19291G954 | |
| Person / Time | |
|---|---|
| Issue date: | 05/03/1983 |
| From: | Harold Denton Office of Nuclear Reactor Regulation |
| To: | Stello V NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| Shared Package | |
| ML19291G955 | List: |
| References | |
| REF-GTECI-A-49, REF-GTECI-RV, TASK-A-49, TASK-OR NUDOCS 8305110423 | |
| Download: ML19291G954 (36) | |
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,.+nd d w qim'9 May 3,1983 MEMORAfdDUlt TOR: Victor
- ello, Jr., Deputy Executive Director for wJional Operations and Generic Requirements Office of the Executive Director for Operations FROM:
Harold R. P ton, Director Office -)i )
w r Reactor Pegolet<cti
.s SbSJED.
CPGR REVIEJ OF PR0tOSE0 RUM UN WESTMZi.D Yp"Q ShfCK As you knew, last Decembe' the Conmiissi directed the staff to prepare a Noti;c of Propos ed Rolemaking or PTS, bcsed or our reconuitstion; in SECY-82-465, which the CRGR reviewed and en&>rsed. The Cunnission paper contairing the propesal notice and supporting naterial is due to the EDO on June 1.
The staff has rrepared a Federal Register flotit.e (Encir;;re A) cnd supporting Regulatory Assessms t (Er. closure B). TV propased rule would:
(1) establish e screening criterion related to the fracture resistarme of
- essu-ized watcr recator (PWR) vessels during pressurized thett.nl shock (PTS) cvents; (2) reqehe analyses and implementation of flux raductior.
prograns th6t are reasonably practicable to avoid exceeding the screening criterion; and (3) rr. quire detailed safety evaluaticns to be performed before "lant operstier, beyond the screening criterien wi'l be considered.
The Commission also asked that we give further consideration to.ne adequacy of the protesed PTS screeing criterion for B8W plants and provide our recomnrendations along with the flotice of Proposed Rulemakino We are working with the 88W Owners Grou to develop these recommend 3t. ions, but we do not expect to complete prervration of that portion of tne C~nmission prper until r.id-May.
On the basis of present inforration, we expect to be able to support use of the same screening criteriun for B&W Plants, and the 2nclosure; have been prepared assuming that outcome.
Since the f::.al stages of the review and concurrence on the Commission paper will be en an g
f extremely ticht time schedule, we request that CRGR review the proposed h t f, [
Sdeal 'legister Notice and the supporting Regulatory Analysir in advance of
/ [j b) the Commissicn po; er containing the results of the B&W study.
Inasmuch as
/1L/l CRGR has previously reviewed the technic :1 backgrourd of thi; issue and the value/ impact of the proposed actions as presented in SECY-82-465, ard since
% g)Ie,'3 the velue/ impact i,;aterial is now inu suded in the enclosed Regulatory
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Victor Stello, Jr. May 3,1983 Analysis, we have not included the usual background information package to CRGR review.
The actisns proposed are Category 2.
Therefore, it is requested th6t the CRGR complete review of the enclosures within two weeks.
For further infors.ation on these reports, contact Karl Kniel, Chief, Generic IsLaes Branch (Ext. 27359) or Roy Woods, Task Manager for A-49 (Ext. 24714).
orgind Sktof Bf H. R. Dett#D Harold F. Denton, Jirector Office af Nuclear Reactor Reo' Nt4on i
Ent esures:
A - Propcsed Federal Reaister Notice B - Regulatory Analysis cc:
E. Cese R. Minogue D. Eisenhut R. Ha'.tson T. Spais H. TFampson R. Vollmer M. Ernst G. Arlotto F. Schroeder K. Knici R. Woods F. Litton S. Stern D"tribution Copies:
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3 ENCLOSURE A Nuclear RegulLtory Commission 10 CrR Part 50 Analysis of Potential Pressurized Thermal Shock Events AGENCY: Nut.iear Poculatcry Commission.
ACTION:
Prcposed rule.
SUMMAkY: The Nucleai Regulatory Commission (NRC' is proposir.g to a.nend its regulations for light water nuclear pcwer plants to:
(1) establish a screening criterion related to the frt cture resistance of pressurized water reactor (PWR) ves'.elt during pressurized thermal shock (PTS) events; (2) requirc analyses and implementation of flux reduction programs that are reasor, ably practicable to avoid exceeding the screening. criterion; and (3) require detailed safety evaluations to
- e performed b'efore plant operatien beyond the screening criterion will be considered. The proposed changes acs intended, if adopted, to produce an improvement ir. the safety of PWR vessels by identifying those corrective actions that may be required to prevent or mitigate potential PTS evtnts.
Comments received after this DATE:
Comment period expires date will be considered if it is practical to do so, but assurance of consideration car. not be given except as to cc: aments received on or before thir. date.
. ENCLOSURE A ADDRESSES:
Send conmients to the Secretary of the Commission, U. S. Nuclear Regulatory Commission, Washington, DC 20555, Attention:
Docketing and Service Branch.
Copies of comments received may be examined at the Commission's Public Document Room at 1717 H Street, NW, Washington, DC.
FOR FURTHER INFORMATION CONTACT:
Roy Woods, Division of Safety Technology, Office of Nuclear Reactor Rsgulation, U.S. Nuclcar Regulatory Commission, Washington, DC.
20555, Telephone:
(301) 492-4714.
SUPPLEMENTAL INFORMATION: Transients and accidents can be postulated to occur in pressurizer water reactors (P'!Rs) that result in severe overcooling (thermal shock) of he reactor vesce'., cor. current with high pressure.
In these pressurized
,ctmal shock (PTS) events. renid cooling of the reactor vessel internal s.cface causes r temperature distribution across the reactor vessel wall. This temperature distribution produces a therma ~. stress on the reactor vessel with a maximum tensile stress at the inside surface of the vessel. The magnitude of the therr.;al stress var ist with the rate of changes of temperature, and with time during tha transient, and its effect is compounded by coincident pressure stresses.
Severe reactor system overcooling events with pressurization of the reactor vessel (PTS evenir) are postulated to result f roin a variety of causes. These include system transients, some of which are initiated by instrumentation and control system malfunctions (including stuck open valves in either the primary or secondary system), and postulated accidents such as small break loss-ef-coolant accidents, main steam line areaks, and feedwater line breaks.
As long as the fracture resistance of the reactor vessel material is relatively high, these events are not expected to cause vessel failure.
However, the fracture resistance of the reactor vessel material decreases with the integrated 'xposure to fast neutrons during the life of a nuclear power plant. The rate of decrease is dependent on the chemical composition of the
. ENCLOSUti A vessel wall and weld materials.
If the fracture resistance of the vessel has been reduced sufficiently by neutron irradiation, severe PTS events could cause small flaws that might exist near the inner surface to propagate into the vessel wall. The assumed initial flaw might be enlarged into a crack through the vessel wall of sufficient extent tu threaten vessel integrity and, therefore, ; ore cooling capability.
The toughness state of reactor vessei materials can be characterized by a
" reference temperature for nil ductility transition" (RTNDT). At normal operating temperatures, vessel materials ace quite tough and resistant to crack propagation. As the temperature decreases, the metal gradually loses toughness over a temperature range of about 100 F.
RT is a mea m e of NDT where this toughness transition occurr.
Its value dep2nds on the specific material in the vessel wall and the integrated neutron irradiation rcu ' m.
a the vessel. These effects are determined by destructive tests of materi<
specimens. Correlations, based on tests of irradiated specimens, have been developed to calculate the shift in RT as a function of neutron fluence NDT for various material compositions. The value of RT at a given time in a NDT vessel's life is used in fractuie mechanics calculations to determine whether assumed pre-existing flaws would propagate as cracks when the vessel is subjected to overcooling events The PTS issue is a concern only for PWRs. Boiling water reactors (BWRs) operate with a large portion of water inventory inside the pressure vessel at saturated conditions. Any sudden cooling will condense steam and result in a pressure decrease.
Simultaneous conditions of high pressure and low temp 2 ature are considered to be improbable.
Most BWRs also receive lower integrated fast neutron flux at the vessel inner wall, resulting in smaller RT shifts. BWRs are designed with a thinner-walled vessel, resulting in NDT lower thermal stress intensities for postulated cracks.
. ENCLOSURE A On the basis of generic studies of severe overcooling events that have occurred, calculations of postulated PTS events that could occur, and vessel integrity calculations, the NRC staff has concluded that a value of RTNDT can be selected so that the risk from PTS events for reacter vessels with smaller RT values is acceptable.
(The risk for vessels with higher values NDT of RT might also be shown to be acceptable, but the demonstration would NDT require detailed plant-specific evaluations and possibly modifications to existing equipment, systems, and procedures.) The staff recommended that a value for RT be established as a screening criterion that would determine NDT the need tur, and timing of, further plant-specific evaluations.
A detailed report presenting the bases for the staff's conclusions and recommendations was submitted to the Commission on November 23, 1982 (SECY-82-465)(2) After public meetings on December 1 and December 9,1982, the Commission directed the staff to develop a Notice of Proposed Rulemaking that would:
(1) establish an RT screening criterion; (2) require NDT licensees to submit present and projected values of RTNDT; (3) require.orly analysis and implementation of such flux reduction programs as are reasonably practicable to avoid reaching the screening criterion; and (4) require plant-specific PTS safety analyses before a plarit is 'within three calendar years of reaching the screening criterion, including analyses of alternatives to minimize the PTS problem.
The staff's approach to selection of the RT screening criterion is NDT described in detail in SECY-82-465.
In summary, the approach was to use a deterministic fracture mechanics algorithm to calculate the value of RT NDT (I)RT decreases with depth from the inside surface of an irradiated vebl'swall. This decrease is accounted for in vessel integrity analyses. As used in this document and in the screening criteria, RT NDT is the peak value at the inside surface of the vessel.
(p) Copies of this document are available for public inspection and copying at the Public Document Room at 1717 H Street, NW, Washington, DC 20555.
. ENCLOSURE A for which assumed pre-existing flaws in the reactor vessel would be predicted to initiate (grow deeper into the vessel wall) assuming occurrence of one of the severe ove-cooling events that have been experienced in U.S. PWRs. These
" critical" values of RT were related to the expected frequency of the NDT experienced severe overcooling events based on a limited data base, consisting of eight events in 350 reactor-years.
In addition, the staff considered a wide spectrum of postulated overcooling events that could occur. These events were grouped into categories, estimates were made of their expected frequency, and stylized characteriza-tions of the temperature and pressure time-histories were developed for each event category. The estimates presented in detail in SECY-82-465 are based on a generic study of Westinghouse-designed pressurized water reactor systems, and are considered also to be generally representative of PWR systems designed by Combustion Engineering.
Because there are some significant differences between those designs and PWRs designed by Babcock &
Wilcox that affect the characteristics and estimated frequencies of PTS events, information was also developed for the Babcock & Wilcox designs.
This information is described in detail in SECY A probabilistic treatment of the iracture mechanics calculations was ' developed to gain insight into the sensitivity of the fracture mechanics calculations to uncertainties in the various input parameters.
By combining the estimated frequencies of postulated events with the probabilistic fracture mechanics results, some estimates of the probability of vessel failure resulting from PTS events were developed. These estimates were used by the staff to better understand the residual risks inherent in the use of the screening criterion approach for further evaluations and resolution of the PTS issue.
On the basis of these studies, the NRC staff concluded that PWR reactor less than pressure vessels with conservatively calculated values of RTNDT 270*F for plate material and axial welds, and less than 300"F for circumferential welds, present an acceptably low risk of vessel failure from PTS events. T1ese values were chosen as the screening criterion in the
- ENCLOSURE A proposed new Section 50.61 of 10 CFR Part 50.
The conservative method chosen for the purpose of comparison by the staff for the calculation of the RTNDT with the screening criterion is presented in paragraph (b)(2) of the proposed Section 50.51. The bases for selection of this method are presented in Appendix E of Enclosure A to SECY 82-465.
Since the acceptability of the screening criterion was based on generic studies, and since the purpose of the criterion is to provide a defined and consistent threshold for initiating the submittal of plant-specific analyses, the proposed rule requires each licensee to assess the current and projected values of RT using the NDT methodology described in paragraph (b)(2) of the rule.
The Commission -
recognizes that alternative methods of determining RT may be justified for NDT use in plant-specific. evaluations af PTS events in the future.
Applicants for operating licenses would also be required to provide projected values of RT as part of the final safety analysis report as NDT provided in the proposed amendment to Section 50.34, " Contents of Applications: Technical Information."
On the basis of information currently available, it appears that the RT Uf NDT t eactor vessels for some plants will remain below the screening criterion throughout the service l' e.
For many other reactor vessels, fuel management programs could be instituted that would result in core configurations reducing neutron flux at critical locations, thereby slowing the increase of RT so that tne screening criterion would not be exceeded.
The Commission NDT recognizes that further refinements in materials information, analyses of PTS event frequencies and scenarios, and plant-specific analyses of alternative measures to reduce PTS risk may provide a basis for the acceptability of continued operation with RT values in excess of the screening criterion.
NDT The preparation and review of such analyses and determination of their acceptability will equire substantial time.
However, the effectiveness of
. ENCLOSURE A flux reduction programs depends on early implementation. The Commission has determined that re;sonably practicable flux reduction programs should be implemented promptly to maintain reactor vessel RT below the screening NDT criterion, without awaiting possible plant-specific determinations that higher values of RT are acceptaole.
Therefore, the proposed rule requires HDT licensees with pressure vessels for which the reference temperature is projected to exceed the screening criterion before the expiration date of the operating license to submit an analysis of such flux reduction programs as are reasonably practicable to avoid exceeding the screening criterion, and a suhedule for implementation of such programs. The NRC s;aff will review thrse submittals to confirm that all reasonably practicable Irograms have bttr, or will be, implemented. At the time of this submittal, or at a later timt, licensees may submit additional plant-specific analyses to justify (by information, improved analyses or evaluations of alternative measures) ra',
operation with less restrictive flux reduction programs in the future.
It is the Commission's intent that the effectiveness of flux reduction programs that are needed to avoid, or to delay, reaching the screening criterion, and that are reasonably practicable, should not be reduced by delay in implementation, pending review of these analyses. Accordingly, the Commission also directed the staff to meet with licensees of pla'nts for which a flux reduction factor of greater than two is required to avoid exceeding the screening criterion before end of licensed plant life. These meetings have already been held for the purpose of determining what plans these licensees have to implement flux reduction options before the PTS rule is in effect, and to encourage prompt implementation of reasonably practicable options.
When a licensee determines, and the Commission agrees, that reasonably practicable flux reduction measures Lave been or will be implemented, and that the vessel RT is still projected to exceed the screening cri,2rion NDT before expiration of the operating license, then the proposed rule requires the subnittal of a plant-specific safety analysis to determine what, if any, modifications to equipment, systems, and operation are necessary to prevent
. ENCLOSURE A potential failure of the reactor vessel as a result of postulated PTS events, if continued operation beyond the screening criterion is allowed.
Modifications to equipment or systems are preferable to reliance on procedure modifications. This analysis should be submitted at least three years before the screening criterion would be exceeded, to allow time for Commission review and licensee implementation of any proposed modifications.
The plant-specific PTS analysis would include a quantitative assessment of the PTS risk due to operation of the particular plant.
It would identify potential event sequences that contribute significantly to PTS risk.
It would consider both the expected frequency of these event sequences and the cenditional probability of vessel failure and subsequent core melt, given the occurrence of these event sequences. The analysis would include a refined assessment of the vessel fracture mechanics properties for the remainder of vessel life, including effects on PTS risk of possible changes in fuel loading patterns affecting the neutron flux at the vessel wall.
Finally, the analyses would estimate the effects of recommended corrective actions on PTS risk and would justify, partly on the basis of risk analysis, operation at RT values above the screening criterion after completion of corrective or NDT mitigative actions.
The Commission intends to develop additional guidance on the content of a plant-specific analysis and on the acceptance criteria the staff will use in reviewing the analysis. This guidance will be available before any licensee is required to submit such an analysis.
REGULATORY FLEXIBILITY ACT CERTIFICATION In accordance with the Regulatory Flexibility Act of 1980, 5 U.S.C. 605(b),
the Commission hereby certifies that this proposed rule, if promulgated, will not have a significant economic impact on a substantial number of saall entities. This proposed rule specifies minimum fracture toughness properties ENCLOSURE A of irradiated pressure vessel materials to amelioriate the effects of PTS events on nuclear facilities licensed under the provisions of 10 CFR 50.21(b) and 10 CFR 50.27. The companies that own these facilities do not fall within the scope of "small entities" as set forth in the Regulatory Flexibility Act or the small business size standards set forth in regulations issued by the Small Business Administration in 13 CFR Part 121.
PAPERWORK REDUCTION ACT STATEMENT This proposed rule amends information collection requirements that are-subject to the Paperwork Reduction Act of 1980 (44 U.S.C. 3501 et seq.).
This rule has been st_..iitted to the Office of Management and Budget for review and approval of the paperwork requirements.
ENVIRONMENTAL IMPACT STATEMENT The promulgation of this proposed rule would not result in any activity that significantly affects the environment. Accordingly, the Commission has det rmined that under the National Environmental Polic,v Act, and the criteria of 10 CFR Part 51, neither an environmental impact statement nor an environmental impact appraisal to support a negative declaration for the proposed rule is required.
REGULATORY ANALYSIS The Commission has prepared a regulatory analysis for this regulation. The analysis examines the costs and benefits of the rule as considered by the Commission. A copy of the regulatory analysis is available for inspection and copying for a fee at the NRC Public Document Room,1717 H Street, NW, Washington, DC 20555. Single copies of the analysis may be obtained from Mr. Karl Kniel, Office of Nuclear Reactor Regulation, U. S. Nuclear Regulatory Commission, Washington, DC 20555, Telephone, (301) 492-7359.
1 ENCLOSURE A LIST OF SUBJECTS IN 10 CFR PART 50 Antitrust, Classified information, Fire prevention, Intergovernmental relations, Nuclear power plarts and reactors, Peralty, Radiation protection, Reactor siting criteria, Reporting requirements.
For the reasons set out in the preamble and under the authority of the A+omic Energy Act of 1954, as amended, the Energy Rcorganization Act of 1974, as amended, and 5 U. S. C 553, notice is hereby given that adoption of the following amendment to 10 CFR Part 50 is contemplated.
PART 50-DOMESTIC LICENSING OF PROD;.CT.ON AND UTILIZATION FACILITIES 1.
The authority citation for Part 50 cortinues to read as follows:
AUTHORITY:
Secs. 103, 104, 161, 182, 183; 186, 189, 68 Stat. 936, 937, 948, 953, 954, 955, 956, as amended, sec. 234, 83 Stat. 1244, as amended (42 U.S.C. 2133, 2134, 2201, 2232, 2233, 2236, 2239, 2282); secs. 201, 202, 206, 88 Stat, 1242, 1244, 1246, as amended (42 U.S.C. 5841, 5842, 5846), unless otherwise noted.
(Section 50.7 also issued under Pub. L.95-601, sec. 10, 92 Stat. 2951 (42 U.S.C. 5851)). Section 50.58, 50-91, and 50.92 also issued under Pub. L.97-415, 96 Stat. 2073 (42 U.S.C. 2239). Sections 50.70 also issued under sec. 122, 68 Stat. 939 (42 U.S.C. 2152). Section 50.80-50.81 also issued under sec. 184, 68 Stat. 954, as amended (42 U.S.C.
2234). Sections 50.100-50.102 also issued under sec. 186, 68 Stat. 955 (42 U.S.C. 2236).
For the purposes of sec. 223, 68 Stat. 958, as amended (42 U.S.C. 2273),
5650.10(a),(b),and(c), 50.44, 50.46, 50.48, 50.54, and 50.8)(a) are issued under sec. 161b, 68 Stat. 948, as amended (42 U.S.C. 2201(b)); lb 50.10(b) and (c) and 50.54 are issued under sec.161i, 68 Stat. 949, as amended (42 U.S.C. 2201(i)); and 55 50.55(e), 50.59(b), 50.70, 50.71,
. ENCLOSURE A 50.72, and 50.78 are issued under sec.1610, 6C Stat. 950, as amended (42 U.S.C. 2201(c)).
2.
Subsection (b) of 950.34 is amended by adding a new paragraph (9) to read as follows:
(9) A description of protection provided against pressurized thermal shock events, including projected values of the reference temperature for reactor vessel beltline materials as defined in paragraphs (b)(1) and (b)(2) of 550.61 of this Part.
3.
A new $50.61 is added to read as follows:
650.61 Fracture Toughness Requirements for Protection Against Pressurized Thermal Shock Events.
(a) Definitions.
For the purposet of this section:
"ASME Code" means the American Society of Mechanical Engineers, boiler and Pressure Vessel Code,Section III, " Rules for the Construction of Nuclear Power Plant Components," edition and addenda as specified by 650.55a, Codes and Standards.
" Pressurized Thermal Shock Event" means an et.ent or transient in pressurized water reactors (PWRs) causing severe overcooling (thermal shock) concurrent with or followed by repressurization of the reactor vessel.
i
- 1? -
ENCLO50RE A
" Reactor Vessel Beltiine" means the region of the reactor vessel (shell material including welds, heat affected zones, and plates er forgings) that directly surrounds the effective height of the active core and adjacent regions of the reactor vessel that are predicted to experience sufficient neutron radiation damage to be considered in the selection of the most limiting material with regard to radiation damage.
"RT
" means the reference temperature for a reactor vessel, f4DT adjusted for the effects of neutron radiation for the period of service in question, calculated by the method given in paragraph (b)(2) of this section.
(b) Requirements.
(1) For each pressurized water nuclear power reactor for which an operating license has been issued, the licensee shall submit projected values of RTf4DT (at the inner vessel surface) of reactor vessel beltline materials giving values from the time of submittal to the expiration date of the operating license. The assessment must specify the bases for the projection, including the assumptions regarding core loading patterns. This assessment must be submitted by (three months after the effective date of this section), and must be updated whenever changes in core loadings, surveillance measurements, or other information indicate a sign"icant change in projected values.
(2) A PTS screening criterion is hereby established of 270 F for plates, forgings, and axial weld materials, or 300 F for circumferential weld materials.
For the purpose of comparison with this criterior, the value of RT f r the reactor vessel must be riDT calculated as follows: The calculation must be made for each weld and plate, or forging, in the reactor vessel beltline.
For each material, RT is the lower of the results given by Equations 1 and riDT 2.
, ENCLOSURE A
= I + M + [-10 + 470 Cu + 350 Cu Ni] f.270 0
Equation 1:
RTliDT Equation 2:
= I + M + 283 f NDT "I" means the initial reference temperature of the unirradiated material measured as definew in the ASME Code, Peragraph NB-2331.
If a measured value is not available, the following generic mean values must be used:
0 F for welds made with Linde 80 flux, and -56 F for welds made with Linde 0091, 1092 and 124 and ARCOS B-5 weld fluxes.
"M" means the margin to be added.
In Equation 1, M = 48*F if a measuied value of I was used, and M = 59 F if the generic mean value of I was used.
In Equation 2, M = 0*F if a measured value of I was used, and M =
34 F if the generic mean value of I was used.
"Cu" and "Ni" mean the weight percent of copper and nickel in the material. The source of these values must be reported. The relation-ship of the material on which any measurements were made to the actual material in the pressure vessel must be described.
19 "f" means the maximum neutron fluence, in units of 10 n/cm2 (E P.eV),
at the clad-base-metal interface on the inside surface of the vessel at the location where the material in question receives the highest fluence for the period of service in question.
(3) For each pressurized water nuclear power reactor for which the value of RT temperature is projected to exceed the PTS screening criterion NDT befoie the expiration date of the operating license, the licensee shall submit by (six months after the effective date of this section) an analysis and schedule for implementation of such flux reduction programs as are reasonably practicable to avoid exceeding the PTS screening criterion.
. ENCLOSURE A (4)
For each pressurized water nuclear power reactor for which the analysis required by paragraph (b)(3) of this section indicates that no reasonably practicable flux reduction program will prevent the value of RT from exceeding the PTS screening criterion before the expiration NDT date of the operating license, the licensee shall submit a safety analysis to determine what, if any, modifications to equipment, systems, and operation are necessary to prevent potential failure of the reactor vessel as a result of postulated pressurized thermal shock events if continued operation beyond the screening criterion is allowed.
This analysis must be submitted at least three years before the value of RT is projected to exceed the PTS screening criterion or by NDT (one year after the effective date of this section), whichever is later.
(5) After consideration of the licensee's analyses submitted in accordance with paragraphs (b)(3) and (b)(4) of this section, the Commission may, on a case-by-case basis, approve operation of the facility at values of RT in excess of the PTS screening criterion. The Commission will NDT consider the risk due to PTS events, and factors significantly affecting that risk, in reaching a decision.
(6)
If the Commission concludes, pursuant to paragraph (b)(5) of this section, that operation of the facility at values of RT in excess of NDT the PTS screenino criterion cannot be approved on the basis of the licensee's analyses submitted in accordance with paragraphs (b)(3) and (b)(4) of this section, the licensee must request and receive Commission approval prior to any operation beyond the criterion. The request may be based upon proposed modifications to equipment, systems, and operation of the facility that would reduce risk due to PTS events.
Dated at Washington, D.
C., this day of
,1983.
For The Nuclear Regulatory Commission Samuel J. Chilk, Secretary of the Commission.
. El$LOSURE B REGULATORY ANALYSIS OF PROPOSED PRESSURIZED THERMAL SHOCK (PTS) RULE The proposed rule woulo require collection and reporting of material properties data and analyses of flux reduction options for all PWR plants, and detailed PTS risk analyses for those relatively few plants that may reach a specified material condition during plant life. These required analyses would form part of the basis for any necessary future plant-specific regulatory actions. Those future actions could potentially require a choice among alternative corrective actions, which would then require a detailed value-impact study.
However, the present regulatory action requires only data reporting, the flux reduction analyses, and the later safety analyses, and does not require specific corrective actions at this time. This analysis therefore covers only the presently-proposed data reporting and analys'is requirements, which do not fall into the category requiring the most extensive regulatory analysis.
Statement of Problem The issue of pressurized thermal shock (PTS) arises because in pressurized water reactors (PWRs), transients and accidents can occur that result in severe overcooling (thermal shock) of the reactor pressure vessel, concurrent with high pressure.
In these PTS events, rapid cooling of the reactor vessel internal surface results in thermal stress with a maximum tensile stress at
. ENCLOSURE B the inside surface of the vessel. The magnitude of the thermal stress varies with time during the transient, and its effect is compounded by coincident pres sure stresses.
Severe reac';or system overcooling events simultaneous with pressurization of the reactor vessel (PT5 events) are postulated to result frcm a variety of causes. These include system transients, some of which are initiated by instrumentation and control system malfunctions (including stuck open valves in eit'ner the primary or secondary system), and postulated accidents such as small break loss-of-coolant accidents, main steam line breaks, and feedwater line breaks.
As long as the fracture resistance of the reactor ve sel material is relatively hi these events are not expected to cause vessel failure.
z However, the fracture resistance of reactor vessel materials decrease with integrated exposure to fast neutrons during the life of a nuclear power plant. The rate of decrease is dependent on the chemical composition of the vessel wall and weld material.
If the fracture resistance of the vessel has been reduced sufficiently by neutron irradiation, severe PTS events could cause small flaws that might exist near the inner surfuca to propagate into the vessel wall. The assumed initial flaw might be enlarged into a crack through the vessel wall of sufficient extent to threaten vessel integrity and, therefore, core cooling capability.
The PTS issue is a concern only for PWRs. Boiling water reactors (BWRs) are not a significant PTS concern. BWRs operate with a large portion of water inventory inside the pressure vessel at saturated conditions. Any sudden cooling will condense steam and result in a pressure decrease, so simultaneous conditions of high pressure and low temperature are considered to be improbable. Most BWRs also receive lower integrated fast neutron flux ENCLOSURE B at the vessel inner wall, and the use.
BWRs are designed with a thinner-walled vessel, resulting in lower thermal stress intensities for postulated cracks.
In the course of its review of the PTS concern for PWR plants, the NRC staff developed a two-step review process which involves a generic overview for PWR plants as a group using relatively conservative generic acceptance criteria, followed by individual plant-specific reviews for those plants wt ich do not meet the generic acceptance criteria.
Both steps involve a systematic examination of many potential events, with particular attention to the' probability and consequences of various possible operator actions and omissions, and equipment malfunctions.
The first step was completion of a generic analysis broadly applicable to all PWPs, and led to proposal of this rule. The rulemaking effort has examined categories of PTS events, including those that have occurred plus potential events that can occur, and has evaluated the expected frequency of these events and the probability of vessel failure if the events should occur.
The probability of vessel failure for any given event is strongly related to the relevant material properties of the vessel, which can be represented by a parameter called the Reference Temperature for the Nil-Ductility Transition (RTNDT). RT increases (failure from PTS becomes more likely) with fast NDT neutron irradiation which occurs during normal plant operation. These analyses have, therefore, been used to approximately determine public risk due to PTS-related events as a functior of RT This has allowed NDT.
identification of a value of RT below which the sta f has concluded that NDT PTS risk is acceptable.
Promulgation of the proposed rule is necessary at this time so that plants will collect the necessary data and perform the analyses necessary to identify needed corrective actions before operation at or above the identified RT value must be considered.
. ENCLOSURE B Objectives The objectives of the proposed new rule are:
(1) To define the material property (RTNDT) below which PTS risk is cont,idered acceptable; (2) To require each PWR licensee to compare its plant's RT to the above-NDT defined value; (3) To analyze and implement flux reduction options that are reasonably practicable and that will prevent or nlay the plant from operating above the defined RTNDT; and (4) To require plant-specific PTS risk analyses well before operation at the defined RT is considered, so that necessary corrective actions NDT are identified.
In the same order as presented above, these objectives are met by a proposed rule, containing the following requirements:
(1) A screening criterion is established in proposed $50.61(b)(2) for PWR pressure vessels based on the RT For plate and axially-oriented NDT.
weld materials, the screening criterion is an RT f 270 F.
For NDT circumferential1y-oriented weld materials, the screening criterion is an values RT f 300 F.
For purposes of this screening criterion, RTNDT NDT for PWR vessel materials are to be determined by the method described in proposed $50.61(b)(2).
(2) All licensees of PWR plants must submit an assessment of current and projected RT values for their vessels to the end of plant life, NDT utilizing the methods described in proposed $50.61(b)(2), and assuming future core loadings for which firm plans and commitments exist. The initial submittal shall be made within three months after the effective date of tha final rule, and shall be updated throughout plant life whenever core loadings, capsule analyses, or other items indicate a significant change has occurred in projected future values.
5-ENCLOSURE B (3) Each PWR licensee shall submit a detailed analysis of alternative flux reduction program", ar.d sha'$ propose a schedule for implementation of such programs as are reasenably practicable to avoid reaching the screening criterion before end of plant life, by six months af ter the effective date of the final rule.
(4) Before an individual plant is projected to be within three calendar years of reaching the RT screening criterion, the licensee shall NDT submit a plant-specific PTS risk analysis. The analysis must be based on state-of-the-art probability snd risk assessment techniques applied to the particular plant materials, systems, and operations, and as nearly as possible must idencify all categories of potential PTS events, their expected frequency, resulting probability of vessel failure given their occurrence, and resulting total PTS risk from all such categcries.
The study must also identify potential corrective actions that would reduce PTS risk to an acceptable level, and must justify any proposed operation with PT values above the screening values.
NDT Alternatives, Their Consecuences, and Decision Rationale The purpose of the proposed regulation is to assemble the data and analyses needed to identify the most effective corrective actions from among the many alternatives.
Future plant-specific regulatory action may be necessary for implementation of the necessary corrective actions, and the alternatives and their consequences will be properly considered and documented at that time.
There are multiple possible variations of the proposed rule, such as higher or lower values of the screening RT Discussion of such variations is NDT.
provided in SECY-82-465 where the particular choices made are justified.
. ENCLOSURE B They are not true "different approaches" of the type defined for required discussion in this section; rather they are variations of the same approach.
We do not believe that there are any acceptable alternatives to the timely and orderly assembly of the data and analyses needed to identify the most effective corrective actions, which is the objective of this proposed rule.
Unacceptable alternatives are to allow plants to continue to operate at an unevaluated risk once the screening RT is exceeded, to arbitrarily require t4DT specific corrective actions with no firm bases for those requirements, or to order plant shutdowns.
A semi-quantitative justification for this position is presented below.
Costs to Licensee (a) To assess RTf4DT (applies to all PWR licensees).
The basic materials property information is already available to each licensee. To meet this requirement would take about 25 licensee man-days and cost $10,000 based on a cost estimate.of $100,000 per man-year.
(b) To prepare and submit analysis of flux reduction programs (these costs apply to PWR licensees for which the prcl7cted RT is expected to riDT exceed.the proposed PTS screening criterion before expiration of the operating license).
Enclosure B to SECY-82-465 indicates that about rixteen plants are estimated to reach the screening criteria before end-of-life.
Estimated costs per plant are about $20,000 based on 50 licensee staff man-days (at $100,000 per man-year). There are a few plants discussed in item (c) below where costs may be much higher.
. ENCLOSURE B (c) To implement flux reduction operation (see item (b) above for applicability).
For several plants, action within the next few years to reduce the flux at critical welds by a factor of two or less will ensure that they do not exceed the screening criterion throughout their service life.
It appears that such a reduction can be achieved through installation of a low leakage core (installation of partially burned fuel assemblies in the periphery of the core in place of fresh fuel essemblies).
This fuel management option is already being implemented by some licensees at reportedly little or no additional cost.
For plants in this category, the staff believes that the rulemaking proposed above will provide a mechanism for ensuring prompt consioe,ation of approprT2 flux reduction measures, but no significant costs due to implement;.an will be incurred.
There is a group of about eight plants for which near-term action to reduce the flux at critical welds by factors of two to five would ensure that they do not exceed the screening criterion throughout their service life.
It appears to the staff that these flux reduction factors could be attained through the installation of a low leakage core and the replacement of a few (8-12) peripheral fuel assemblies by dummy assemblies or part length assemblies.
These measures seem practical and cost-effective to the staff, based on present knowledge, with some loss in margin to core overheating limits in certain postulated transients.
k'e estimate that there would be an engineering cost (redesign and safety analyses) of about $20 million per plant, and some small increase in fuel cycle or operating costs.
(d) To perform the plant specific PTS analyses due three years before exceeding the screening RTNDT (applies to only a few plants where flux reduction is not adequate).
. ENCLOSURE B An estimate of the cost for performing the plant-specific analyses was made for the staff. The items considered in the cost estimates are identified below. The items were reviewed and collectively discussed with reactor specialists in each of the engineering areas. A judgment of cost per plant was then made based upon an estimate of the time and facility requirement.
(1)
Identification and quantification of PTS events S 500K (2) Thermal hydraulic analysis S 400K (3) EPtter identification of vessel material properties S 70K (4) Deterministic and probabilistic fracture mechanics
$ 100K analyses (5) Flux reduction program analyses
$ 100K (6) Inservice inspections and nondestructive e'aluation 75K study (7) Plant modification study
$ 200K (8) Operating procedures and training study
$ 150K (9)
In-situ annealing study 5 50K
'0TAL S1645K Costs to Government The submittals by the licensee will be evaluated by the staff for each faci?ity.
(1) RT assessment NDT (2) Flux reduction The evaluation in these areas will be performed by the flaterials Engineering Branch and the Core Performance Branch, respectively.
Four man-weeks time is estimated at a cost of $10,000 for each evaluation.
. ENCLOSURE B (3) PTS Analysis The evalust on of the plant-specific analysis will be performed by the staff and a technical assistance contractor. The staff time is estimated at 180 man-days at a cost of $70,000. The contractor cost is estimated at 0.5 man-year for $50,000 plus $40,000 computer time.
Safety Benefits-Compared to Costs With the possible exception of a few plants where large flux reduction options may be initiated in the near future, the only significant costs for most plants are associated with performing the required plant specific analyses. There are no direct sefety benefits for performing the plant-specific analyses. The safety benefits will be derived from implementing any required corrective actions identified in these analyses.
The plent-specific analyses are a necessary step in this process. The operating experience record and probabilistic studies, as described in the Enclosure A of SECY-82-465, indicate that the potentiai safety benefits are large for initiating PTS corrective regulatory a'ction for plants approaching the r,creening criterion. A Value-Impact Assessment, will be prepared after receipt of the plant-specific analysis and the determination of the particuler corrective regulatory action necessary and expedient for a given operating facility.
In the anticipation that the safety benefits will prove to be large, and given the relatively low costs detailed above that are necessary preliminary steps in obtaining the safety benefits, the costs of the presently proposed actions are justified.
. ENCLOSURE B 1mplementation (a) Schedule The implementation schedule is explicitly stated in the proposed rule itself.
Referring to the items of the rule in the same order as given previously:
(1) The RT screening limit would be effective immediately upon-NDT final promulgation of the rule.
(2) All PWR licensees would submit the first assessment of their present and projected material condition within three months after the effecties date of the final rule and would submit re-evaluations whenever significant changes are indicated.
(3) All PWR licensees would submit analyses of alternative flux reduction programs and proposed implementation schedules for those programs that ere reasonably practicable within six months after eff ective date of the final rule.
(4) Licensees would submit detailed PTS safety analyses 3 calendar years before their plants are projected to exceed the RT screeni g NDT criterion.
Other schedules were considered; however, we believe the above-proposed schedule is achievable without undue burden on the licensees, and allows reasonable time for implementation of identified, necessary corrective actions to maintain PTS risk at
. ENCLOSURE B acceptable levels. Shorter schedules would tend to be unnecessarily burdensome; longer schedules would allow insufficient time for implementation of corrective actions.
(b) Relationship to Other Existino or Proposed Requirements Some of the Commission's regulations (Appendix G to 10 CFR Part 50,10 CFR 50.46, and possibly others) may not continue to reflect the staff's best understandino of the state of reactor vessel embrittlement and the potential for vessel failure as a result of PTS.
Should the research and analysis work that was started as a result of the PTS concerns so indicate, timely consideration will be given to the need for amendmir.ts to the regulations (as discussed in Section 8.6 of Enclosure A to SECY-82-465).
. ENCLOSURE B ANALYS]S REQUIRED TO SUPPORT THE IMPOSITION OF INFORMATION COLLECTION REQUIREMENTS 1.
Justification a.
Need for the Information Collection The issue of pressurized thermal shock (PTS) arises because in pressurized water reactors (PWRs) transients and accidents can occur that result in severe overcooling (thermal shock) of the reactor pressure vessel, concurrent with or followed by repressurization.
In these PTS events, rapid cooling of,the reactor vessel internal surface results in thermal stress with a maximum tensile stress at the inside surface of the vessel. The magnitude of the thermal stress depends on the temperature profile across the reactor vessel wall as a function of time. The effects of this thermal stress are compounded by pressure stresses if the vessel is pressurized.
Severe reactor system overcooling events which could be accompanied by pressurization or repressurizrtion of the reactor vessel (PTS events) can result from a variety of causes. T.hese include system transients, some of which are initiated by instrumentation and control system malfunctions including stuck open valves in either the primary or secondary system, and postulated accidents such as small break loss-of-coolant accidents, main steam line breaks, and feedwater pipe breaks.
. ENCLOSURE B As long as the fracture resistance of the reactor vessel material is relatively high, such events are not expected to cause vessel failure.
However, the fracture resistance of reactor vessel materials decreases with exposure to fast neutrons during the life of a nuclear power plant.
The rate of decrease is dependent on the metallurgical composition of the vessel wall and welds.
If the fractere resistance of the vessel has been reduced sufficiently by neutron irradiation, severe PTS events could cause propagation of fairly small flaws that might exist near the inner surface. The assumed initial flaw might initiate and propagate into a crack through the vessel wall of sufficient extent to threaten vessel integrity and, therefore, core cooling capability.
The data collection aspects of the proposed new rule are:
(1) to require each PWR licensee to determine the plant RTNDT according to a method uniformly defined for all plants; (2) to require analyses of flux reduction options that will prevent or delcy the plant from operating above the defined KTUDT; and (3) to require plant-specific PTS risk analyses be submitted before operation beyond the defined RT is considered, so that necessary NDT corrective actions are identified.
b.
Practical Utility of the Information Collection Collection of the information and analyses is nan ary to identify needed corrective actions before operation above the identified RT value must be considered.
-b-ENCLOSURE B The information ard anal; ses will be reported on the piant's docket through the NRC Licensing Project Manager (LPM). The LPM and the Ganeric Issues Branch Task Manager will coordi.. ate review of the information and analsces '; the appropriate branch (depending upon technical subjects covered) leading to a coordinated NRC staff recommendation to the Commission regarding necessary corrective values actions before plant operation can be cor.sidered at RTNDT above the screening value.
(cf Duplication with Other Collections of Information There are no other NRC requirements regarding analyses for flux reduction or plant PT5 safety analyses. However, materials value for the information leading to nlculatiot, of an RTNDT reactor vessel is submitted in response to the requirements of Appendices G and H, 10 CFR Part 50 For nes plants, it appears in the FSAR. During the operating iife, she info m tion is updated by the individual plant submittals that support requests for changes in the pressure-temperature limits given in Techrical Specifica-tions.
The new request for materials it. formation (RT values) conta'ned NDT in this proposed rule is required because:
(1) the calculatien of RT for PTS involves a new trend curve formula thai ontains HDT nickel as o.ne variable, and this represents a change from past practice which has yet to be adopted for normal operation; and (2) the calculation of RT f r PTS purposes requires precise, NDT updated data obtained in many cases by the licensee in response to NRC concerns regarding PTS.
In normal operation, there are cases where upper-bound estimates are used in the absence of complete data.
For PTS, this can, in snme cases, be unnecessarily
. ENCLOSURE B conservative, and an ertra effort to obtain the data is required.
For plants where complete data were available initially, this reouest will result in a verification (with quality assurance acceptable for PTS use) of earlier submittals.
(d) Consultations Outside NRC We have reviewed our overall PTS recommendations on several occasions with the Advisory Connittee on Reactor Safeguards (ACRS),
including the information gathering aspects.
The ACRS was in basic agreement with our recommendations (letter to Nunzio J. Palladino, Chairman, NR'., from P. Shewmon, Chairman, ACRS, October 14,1982).*
We have also reviewed our recommendations wit 1 consultants under contract with us at Pacific Northwest Laborattries. Their recommendations are similar to ours. (NUREG/CR 2837, July 1982.)*
(e) Other Suppcrting Information The staff recommen/'tions including the inf'ormation cathcring requirements are discussed and Justified in detail in SECY-82-465,*
with enclosures, November 23, 1982.
2.
Description of Informa tion Collection (a) Number and Typ_e of Respondents Licensees of all PWR plants would be subject to the requirements of the proposed rule if promulgated, including the information gathering portions.
. ENCLOSURE B (b) Reasonableness of Schehle for Collectino Information The schedule is stated in the proposed rule in three places.
(1) The initial RT determination "must be submitted by (three NDT months after the effective date o'< the final rule) and must be updated whenever changes in r. ore loadings, surveillance measurements, or other information indicate a significant change in projected values."
The staff believes that it is vital to quickly assess, with reliable information, which plants are nearest the screening criterion so that we know as early as possible which plants most quickly need to complete the flux reduction analyses (see 2 below) and the safety analyses (see 3 below) which results in identification of necessary corrective actions.
The information is already available to th! licensee.
It would require only verification and submitt:.1 to the NRC by letter to the docket. Therefore, the proposed schedule is reasonable.
(2) "For each pressurized water nuclear power reactor for which the value of RT is prcjected to exceed the PTS screening NDT criterion before the expiration date of the operating license, the licensee shall submit by (six mar.hs after the effective date of the final rule) an analysis and schedule for implementation of such flux reduction programs as are reasonably practicable to avoid exceeding the PTS screening criterion."
The flux reduction option must be implemented as soon as possible for maximum effectiveness.
Withomt this early reporting of flux reduction analyses, when the PTS safety analyses (see 3 below) are submitted, it may be too late to make use of this option.
. ENCLOSURE B Due to their own interest in safety and economy, licensees will have already analyzed flux reduction options before this rule is promulgated. Therefore, the schedule proposed to prepare and submit a report on the docket is reasonable.
(3) For each pressurized water nuclear power reactor for which the analysis required by proposed Q50.61(b)(3) " indicates that no reasonably practicable flux reduction program will prevent the values of RT fr m exceeding the PTS screening criterion f4DT before the expiration date of the operating license, the licensee shall submit a safety analysis to determine what, if any, modifications to equipment, systems, and procedures are necessary to provide acceptable protection against potential failure of the reactor vessel as a result of postulated pressurized thermal shock events. This analysis shall be submitted at least three years before the value of RT is f4DT projected to exceed the PTS screening criterion or by (one year after the effective date of the final rule),
whichever is later."
This is the final step to which all others lead, the identification of needed corrective actions. We believe the is three year " lead time" before the screening criterion RTf4DT exceeded represents the minimum time necessary to review the analyses, recommend actions, promulgate a requirement by Commission action (if necessary), and have the licensee implement the necessary corrective actions.
If less than three years are allowed and the required actions are not complete, plant shutdown could be necessary.
Since this would be a plant-specific analysis, the staff believes a report on the plant's docket to be the most efficient submittal.
. ENCLOSURE B (c) Method of Coliscting the Information The data and analyses are plant-specific and plant-unique and will be required from each plant. They are vitally necessary for the NRC staff's use in evaluating a potential safety concern and identifying corrective actions that may be required to alleviate that concern. The staff members that will perform the evaluation are in the Washington, D.C. (NRC Headquarters) area and are in several different NRC organizational units.
Reports filed on the plant docket and subsequently distributed to the reviewers appear to be the most efficient method.
The flux reduction analyses and the RT analyses would probably be NDT performed by different technical personnel within the licensee's (or vendor's) organization.
If the licensee wished to combine the two reports into a single report with two major sections, that would be acceptable.
We would distribute copies of the proper sections to the appropriate NRC organizations.
(d) Record Retention Period The RT history must be retained as a chronolo'gy for the life of the NDT plant.
The flux reduction and safety anal ses must also be retained until and 3
unless modified or revised.
(e) Reporting Period The RT and flux reduction information would be re-reported only when NDT significant changes are indicated, as already discussed.
. ENCLOSURE B (f) Copies Recuired to be Submitted Three copies submitted on the docket are acceptable.
If additional copies are required of portions of the report (s) due to the number of reviewers involved, then they would be made internally.
3.
Estimate of Burden (a) Estimated hours
- 2) Flux reduction analyses - 50 licensee staff man-days
- 3) PTS safety analysis - 10 licensee staff man-years (b) Estimated cost
Flux reduction analyses - $20,000 3)
PTS safety analysis - $1,645,000 (c) Source and method for estimating:
RT assessment and flux reduction analyses.
NDT The basic information is available to each licensee through ongoing reactor vessel integrity and surveillance programs. The method for i
estimating is based on engineering judgment by the NRC staff and our understanding by the assessment of the integrity of the vessel.
The cost estimate is based on $100,000 per man-year.
PTS safety analysis The estimate is based on the use of existing computer codes and modelling procedures. The estimate is based on the use of 10 man-years, plant-specific modelling time, and 12 transient
. ENCLOSURE B calculations.
Engineering judgment by the NRC staff and their consultants was the method used for the estimates.
(d) Reasonableness of estimate The estimates given above represent the best judgment of the NRC staff, and are based on actual experience with the costs of PTS analyses now being performed by NRC/ PES contractors at ORNL, INEL, and LANL.
4.
Estimate of Cost to Federal Government The submittals by the licensee will be evaluated by the staff.
- 2) Flux reduction The evaluation in these areas will be performed by the Materials Engineering Branch and the Core Performance Branch, respectively.
Four man-weeks time is estimated at a cost of $10,000 for each evaluation.
- 3) PTS Analysis The _ valuation of the PRA and plant-specific analysis will be performed by the staff and a technical assistsnce contractor. The staff time is estimated at 180 man-days at a cost of $70,000. The contractor cost is estimated at 0.5 man-year,for $50,000 plus
$40,000 computer time.