ML20141K411
| ML20141K411 | |
| Person / Time | |
|---|---|
| Issue date: | 05/23/1997 |
| From: | Callan L NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| To: | Diaz N, Dicus G, Shirley Ann Jackson, Mcgaffigan E, Rogers K, The Chairman NRC COMMISSION (OCM) |
| References | |
| COMSECY-97-013, COMSECY-97-13, NUDOCS 9705290083 | |
| Download: ML20141K411 (7) | |
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S RELEASED TO THE PDA g- i NUCLEAR REG TORY COMMISSIOh WASHINGTON, D.C. 2066M*H . e IJ
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MEMORANDUM 10: Chairman Jackson Commissioner Rogers Commissioner Dicus Commissioner Diaz Commissioner McGaffi n FROM:
L. Joseph Callan o
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Executive Directo
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SUBJECT:
STEAM GENERATOR RULE KING In SECY-95-131, the staff proposed to develop a steam generator (SG) rule to update the current regulatory framework for ensuring SG tube integrity.
Subsequently, the staff developed a draf t rule and draft reg for the development and implementation of appropriate measures to ensure the consistency and quality of inspection methods, repair criteria, and tubeAs part condition assessment, while giving appropriate consideration to risk.
of the rulemaking process, the staff estimated the risk associated with SG tube degradation and utilized the results to provide the insights required for performing a regulatory analysis of the proposed rulemaking approach.
Risk Assessment Summary Risk associated with SG tube integrity can be divided into three principal contributors: 1) risk from core damage caused by spontaneous steam generator tube ruptures (SGTRs), 2) risk from core damage caused by SGTRs induced by pressure transients, and 3) risk from core damage sequences that induce SG The contributions to risk from core damage caused by spontaneous and pressure transient-induced SGTRs were previously assessed i More recent assessments have yielded similar integrity in September 1988. /
results for the risk from spontaneous and pressure transie /
ruptures. /
and by licensees on a case-by-case basis to ensure tube integrity consis with the regulations and plant licensing bases.
approach would maintain required margins against tube 1, 'V75 D30009 , q Apg uyng CONTACT: Timothy A. Reed, NRR (301) 415-1462 lllllI.llOl.lOl[Il!00jlll%ll
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prior requirements, the staff concluded that no significant increase in risk ,
from spontaneous and pressure transient-induced tube ruptures would result. '
The majority of the staff's effort was aimed at gaining a better understanding This is because the SG tube of the likelihood of core damage-induced SGTRs. :
functions as both the reactor coolant pressure boundar*y and as part of the containment boundary, and a breach of a SG tube during a severe accident This can ,
result in a large early release of radioactivity to the environment. l assessment focused on the performance of degraded SG tubes during postulated !
severe accidents and estimated the large early release frequency (LERF) i associated with containment by-pass. The 4LERF from SGTRs induced by core l per reactor-year range for the '
damage sequences is in the low-to-mid 10From our assessment, we gained :
example plant analyzed by the staff.
significant insights into the behavior of the primary system and SG tubes i under severe accident conditions. the frequency of certain types the results are plant-specific, depending on:
of core damage sequences, the thermal-hydraulic response to core damage l progression, and the type and degree of steam generator vary over a wide range throughout the population of pressurized water reactors l (PWRs) and may also change as a function of time.
Finally, the staff concludes thatHowever, relaxation of existing tube repair criteria the increase in risk cannot be i
could result in increased risk.
estimated without first knowing the nature of the proposed relaxation and its j relationship to the type and level of degradation that might be left in service.
Major Conclusions From Reaulatory Analysis t l
The regulatory analysis was conducted to determine if the beneTits to public -
' healto and safety associated with implementing the requirements and guidance ;
in the draft rule and draft regulatory guide satisfy the 10 CFR 50.109 :
l criteria for a generic backfit. From this ar-lysis, the staff reached the f following two major conclusions:
l 1.
Necessary revisions to requirements for inspecting, monitoring, and f l
assessing the condition of the SG tebes, and for maintaining SG The tube integrity are believed to be implemantable as compliance backfits.
staff noted that the current regulations are adequate to ensure continued safe steam generator operation, but to properly address newly !
evolving 'and/or changing tube degradation mechan ;
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~ tube surveillance and maintenance are accomplished consistent with 10 CFR Part 50, Appendix B; 10 CFR 50.55a; guidance in 10 CFR Pa)
Appendix A; and plant licensing bases. ;
addressed on a case-by-case basis. ,
2.
Based on the example plant risk assessment, requirements for licensees j to take generic actions to reduce risk do not satisfy the backfit !
- However, additional investigation is !
criteria of 10 CFR 50.109.
required to determine if plant-specific backfits are justified for some pl ants. As part of the individual plant examination (IPE) follow-up
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. 3 l process, the staff will use IPE accident sequence frequencies, steam !
generator inspection results, information on remaining plant life, and 3 site characteristics to identify plants which might warrant backfits.
The final plant-specific determinations would require understanding the In ;
plant-specific thermal-hydraulic response during severe accidents. j addition, specific new mitigation capabilities would have to be i identified that could mitigate enough of this risk to be cost-Justified. l This approach is consistent with the Commission guidance for GSI-23 (reactor coolant pump seals) in which the staff was In directed to address addition, the the pump seal issue on an individual plant basis.
appro v h of addressing plant-specific design and performance information to resolve potential safety concerns is consistent with the staff's IPE follow-up process recently discussed with the Commission and documented in the PRA Implementation Plan. )
l implications for SG Rulemaking l Given the conclusions discussed above, the staff reassessed whether a rule is the appropriate regulatory vehicle for addressing the problems associated with !
SG tube integrity. It should be recognized that the staff found the current regulations governing SG tube integrity provide an adequate basis to ensure public health and safety due to SG operation; however, the i I
meet these regulations. Addressing this issue, which involves the plant technical specifications, is amenable to a generic letter approach.
Additionally, the staff noted that although backfitting of plants _to reduce !
i the risk associated with SG tube failure under severe accident conditions could not be generically imposed on PWR licensees, licensees who proposed alternate repair criteria would need tc address risk considerations, since the staff's technical Thiswork didcan situation indicate also bethat risk could addressed increase by providing under guidance to such i
circumstances.
licensees on how risk should be considered when pursuing alternate SG tube repair criteria. Given these considerations, the staff concluded that promulgation of a new rule was not necessary, and instead is proposing a revised approach as described below.
j Prooosed Reoulatory Acoroach !
l Based on the above, the staff plans to pursue the following approach in lieu l of steam generator rulemaking: t
- 1. Complete development of a SG tube integrity regulatory guide which ,
describes an acceptable performance-based program for ensuring l adequate tube inspection, monitoring, and assessment.
- 2. Request licensees, through a generic letter, to propose performance-based technical specification changes to address the issues regarding inspection, monitoring, and assessment of j ;
with the plant licensing basis. The SG tube integrity RG would provide guidance to licensees on an acceptable approach and program l for addressing these issues.
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- 3. Provide licensees with an option to change current SG tube repair criteria and implement a degradation-specific management auproach, if it can-he demonstrated that risk will be maintained at an t acceptable level. Draft regulatory guide DG-1061 "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Current Licensing' Basis," in conjur.ction with an application-specific regulatory guide would provide guidance on acceptable approaches for proposing changes to SG tube integrity criteria and assessing changes in risk associated
- with relaxation of tube integrity criteria. Licensees would not be able to implement alternate repair criteria until an appropriate risk assessment is submitted and found acceptable by the staff.
- 4. As part of.the IPE follow-up program, the staff will evaluate PWRs which appear to have a high potential for core damage. sequences that can challenge SG tubes. Since the staff is already committed to follow-up related issues identified by the IPE program (e.g.,
station blackout leading.to high pressure core melt sequences), this effort will fit easily into the IPE follow-up framework as discussed ,
in the PRA Implementation Plan. Any additional requirements would be imposed consistent with the backfit requirements of 10 CFR 50.109. ,
Conclusions l
On the basis of this information, the staff has concluded that a revised ,
approach utilizing a compliance-based generic letter and a risk-informed RG is j a more appropriate means to accomplish the original objectives established ft :
rulemaking. To the extent practical, the revised approach is both I performance-bared and risk-informed, and it offers an adaptable framework to It should also be noted that-the support degradation-specific management. staff is continuing to address
" steam generator tube integrity and will provide a report documenting its conclusions as part of the issuance of the regulatory package for public comment.
The staff estimates that the revised approach (i.e., issuance of a final l generic letter with the associated regulatory guide:) can be completed withinT the schedule estimated for issuance of a final SG rule.
compatible with the current schedule for completion of the final regulatoryIn the in guides on risk-informed, performance-based regulation. staff ex Commission.
The delay results from a need to revise the already developed documentation to support the alternative generic letter approach rather than rulemaking, and from scheduling constraints associated with the Currently, the Advisory staff is Committee on Reactor Safeguards (ACRS) review.
scheduled to meet with ACRS in June arid expects to meet again with ACRS l
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in the July time frame.- Assuming a successful ACRS review, the staff can 4
provide the proposed regulatory package to the Commission in the September 1997 time frame. .
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