ML20195E381
| ML20195E381 | |
| Person / Time | |
|---|---|
| Issue date: | 04/01/1988 |
| From: | Chilk S NRC OFFICE OF THE SECRETARY (SECY) |
| To: | |
| References | |
| FRN-53FR11311, RULE-PR-50 PR-880401, NUDOCS 8806240033 | |
| Download: ML20195E381 (6) | |
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[7590-0$f[yhcEO NUCLEAR REGULATORY COMMISSION CF' u 10 CFR Part 50 0004 9 ;i,-
an:.w Leak-Before-Break Technology; Solicitation of Public Coment on Additional Applications AGENCY: Nuclear Regulatory Comission.
ACTION:
Request for comments.
SUW.ARY :
The Nuclear Regulatory Comission (NRC) is proposing to investigate the safety benefits associated with using leak-before break technology to modify functional and performance requirements for emergency core cooling systems (ECCS) and environrental qualification (EQ) of safety related electrical and mechanical equipment.
bl 5, bh The comnent period expires on [ insert 90 days after publication].
Com-DATE:
ments received after this date will be considered if it is practical to do so, but assurance of consideration can only be given to coments received on or be-fore this date.
ADDRESSES:
Send written coments to the Secretary of the Comission, Washington, DC, 20555, Attention:
Docketing and Service Branch.
Copies of l
commerts received by the Comission may be examined at the NRC Public Document Room, 1717 H Street NW., Washington, DC 20555.
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FOR FURTHER INFORMATION CONTACT: John A. O'Brien, Office of Nuclear Regula-tory Research, U.S. Nuclear Regulatory Comission Washington, DC 20555. Tele-phone (301)492-3928.
SUPPLEMENTARY INFORMATION:
Existing Applications of Leak-Before-Break Technology On October 27,1987 (52 FR 41288), the NRC published a final rule which modified General Design Criterion 4 (GDC-4) in 10 CFR Part 50, Appendix A, by allowing the use of leak-before-break technology to eliminate from design con-sideration the dynamic effects of postulated ruptures in all piping in all reactor types that satisfy rigorous acceptance criteria.
The supplementary information to this rule stated, however, that containments, ECCS, and EQ of safety related electrical and mechanical equipment are not affected by leak-before-break technology. This introduced an inconsistency into the regulations which is addressed by this request for cornent, khile not emphasized in the final GDC-4 modification, when leak-before-break technology was disallowed for ECCS, EQ, and containment design, functional and performance requirements cited in different portions of 10 CFR Part 50 were maintained.
However, limited case-by-case modifications of EQ functional and performance requirements were allowed in the GDC-4 amendment using the exemption process.
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The specific functional and perfomance requirements retained when leak-before-break is accepted under the recent modification to GDC-4 are as follows:
- 1. For Containments. Global loads and environments associated with postulated pipe
- ruptures, including pressuriza-tion, internal flooding, and elevated temperature.
- 2. For ECCS.
Heat removal and mass replacement capacity needed because of postulated pipe ruptures.
- 3. For EQ.
Pressure, temperature, flooding level, humidity, chemical environment, and radiation resulting from postulated pipe ruptures.
However, under the recent modification to GDC-4 local dynamic effects uniquely associated with pipe rupturo may be deleted from the design basis of containment systems, structures and boundaries, from the design basis of ECCS hardware (such as pumps, valves, accumulators, and instrumentation), and from the design basis of safety related electrical and mechanical equipment when l
leak-before-break is accepted.
"Local dynamic effects uniquely associated with pipe rupture" means dynamic effects due to pipe whipping, jet impingement, missiles, local pressurizations, pipe break reaction forces, and decompression l
waves in the intact portions of piping postulated to rupture.
Gl,bal j
pressurizations, temperature transients, and fluoding transients on l
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centainment systems and structures are not local dynamic effects and may not be uniquely related to pipe rupture, and therefore are retained for containment design.
Thus, while functional and perfonnance requirements for containments, ECCS, and EQ remain unchanged under the now effective modification of GDC-4, the design bases for these aspects of facility design have been modified in that local dynamic effects uniquely associated with ruptures in piping which qualifies for leak-before-break may be excluded from consideration.
This present notice exa:aines the potential additional application of leak-before-break technology to modifying functional and performance requirements for emergency core cooling systems and for environmental qualification of safety related electrical and mechanical equipment.
Modification of func-tional and performance requirements for ccntainments is explicitly excluded from consideration at this time.
Invitation To Comment To meet its statutory obligation to assure an adequate level of safety, the NRC uses the "defense-in-depth" concept which is codified in the General Design Criteria in 10 CFR Part 50, Appendix A.
Stated in simple terns, and with some notable exceptions, defense-in-depth is implemented by utilizing high standards of design, fabrication, and inspection, and then postulating severe failure in structures, systems, and components.
It must be demonstrated that these severe failures will not lead to undue risk to public health and 4
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safety.
Risk is generally kept low by employing redundancy and diversity in design.
When severe fsilures are unacceptable (as for example, in reactor pressure vessels), extraordinarily high standards are required.
In the case of piping, different standards of design, fabrication, and inspection are imposed depending on the safety significance of the piping.
Until recently, severe failure for piping has been defined as the instantaneous double-ended guil-lotine break regardless of the standards applied to piping.
Under leak-before-break technology, it has become possible to exclude the double-ended I
guillotine break from the dynamic structural design basis because it is un-realistic and overly conservative in certain situations.
Piping which meets NRC's acceptance criteria now need only postulate stipulated "leakage cracks" as severe failure.
This relaxation in requirements under the final GDC-4 amendment actually improves safety bec6ust it allows the removal of counter-productive hardware which impedes inservice inspection, could restrain thermal growth of piping (leading to unforeseen stresses and cracking), and could de-grade seismic performance of piping due to impacting between piping and pipe whip restraints during earthquakes.
Worker occupational radiation dosages are reduced substantially.
When the Comission published the proposed broad scope amendment to GDC-4, coment was invited on the decision te limit impacts of this modifi-cation to only dynamic effects associated with pipe rupture.
In response to this request, a number of comentors stated that the use of the leak-before-5
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break technology should be extended to modify the requirements for EQ and ECCS, Safety benefits for EQ and ECCS were suggested wherein protection against the effects and consequences of postulated pipe ruptures causes less reliable overall performance. Because the NRC is primarily concerned with fulfilling its safety mission, documented evidence describing safety degradations and safety enhancements due to postulated pipe rupture requirements on EQ and ECCS is requested.
Specifically, actual citations from operating experience are requested;
- however, conclusions based on testing and deterministic or probabilistic evaluations would also be useful.
The priority which the NRC assigns to modifying functional and performance requirements for EQ and ECCS will be determined in large measure from the balance between accrued safety benefits and detrinents believed to result (including impacts on severe accident performance).
If it can be shown that net safety benefits outweigh the detriments, then modification to the existing design bases may be permitted.
Dated at Washington, DC, this
$ r-day of. nul 1988.
I For the Nuclear Regulatory Commission.
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. Chilk, Secretary of the Comission.
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