ML20058K177
| ML20058K177 | |
| Person / Time | |
|---|---|
| Issue date: | 04/27/1990 |
| From: | Taylor J NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| To: | Carr NRC COMMISSION (OCM) |
| Shared Package | |
| ML20058K180 | List: |
| References | |
| ACRS-GENERAL, NUDOCS 9005100241 | |
| Download: ML20058K177 (8) | |
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APR 2 71990 t
hli MEMORANDUM FOR: Chairman Carr Commissioner Roberts-i Comissioner Rogers i
Comissioner Curtiss f
3 Comissioner Remick b
FROM:
James M.' Taylor 1
_ Executive Director for Operations i
SUBJECT:
STAFF RESPONSE TO ACRS CONCLUSIONS REGARDING-EVOLUTIONARY' LIGHT WATER REACTOR CERTIFICATION ISSUES a
-The ACRS sent a letter to the Comission addressing the subject issues.
In general -the ACRS supported the staff positions',Lwith' additional coments -
and recomendations. The staff has evaluated these ACRS comments and s
recomendations and its evaluation is provided in the enclosure.
Original SignW BW James M. Taylotr James M. Taylor.
Executive. Director
Enclosure:
for Operations-hl:8$N OGC ACRS Distribution [EVOLUTIONARYLIGHT]
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Enclosure A EVOLUTIONARY LIGHT WATER REACTOR CERTIFICATION ISSUES-AND THEIR RELATIONSHIP TO CURRENT REGULATORY REQUIREMENTS L
1.
Evolutionary LWR Public Safety Goals-The Advisory Committee on Reactor Safeguards.(ACRS) stated, "We regard it as not inappropriate that applicants should work to tighter standards when 3
it serves their purposes, but'we do'not believe it is appropriate that the i
NRC should require such standards."
The staff is accepting EPRI's proposed core damage frequency goal and
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their large release safety goal. We are not: proposing to push them for more conservative safety goals, but the staff expects ELWR applicants to:
demonstrate that they have taken reasonable, steps to achieve their stated goals.
2.
' Source Terr The ACRS stated that, "Since the issue of siting of these plants is not yet resolved, and since revisions to 10 CFR 200 are being considered, there may be no alternative to proceeding asithe staff proposes, however awkward it may seem. However, we can make no informed judgment concerning the appropriateness of the procedure until we know more about-the criteria to be used in the selection of a source term. and-the results of its application."
The staff will keep the ACRS informed of our source term approach for ALWRs as we develop the details of,its application during our reviews.
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3.
Anticipated Transients Without Scram 4ATWS)
The ACRS stated that "It appears to us that a design that.can ride out an ATWS-without serious damage is feasible for PWRs.and is preferable-to~a scram system with diverse logic, which has a reliability calculable, at best, with large-uncertainty. We recommend that the staff permit demon-'
s stration that the consequences of an ATWS are acceptable as an alternative j
to a diverse scram logic."
l The staff agrees with the ACRS coment that resolution of 'ATWS through a design that can' ride out an ATWS is preferable to reliance on a diverse scram logic.- This option _was offered in the staff Draft Safety Evaluation-Report for Chapter 5 of the EPRI Requirements' Document and has been pro-posed by EPRI in. Chapter 10.
Whether' the currently proposed diverse systems could be deleted would be based on the adequacy of this-design, Analysis'to demonstrate that an ATWS would not lead-to serious damage.
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p 4.
Mid-Loop Operation The ACRS stated that, "We agree with the staff's proposal, but recomend that more specific requirements be considered for mid-loop operation:
Design provisions to help ensure continuity of flow through the:
core and residual heat removal system with low liquid levels at the junction of the DHR system suction lines and the RCS.
1 Provisions to ensure availability of reliable systems for decay-heat removal.
-c Instrumentation for reliable measurements of liquid levels in the reactor vessel and at the junction of.the DHR system suction lines and the RCS.
Provitions for maintaining containment closure or for rapid closure of containment openings.
The staff agrees with the ACR$ coments.
In fact,'past staff actions on this issue for operating reactors focused on most of these items.
The staff plans to ensure that these factors are addressed in the ELWR designs.
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5.
Station Blackout The ACRS stated that the staff recommended that an alternate diverse ac power source capable of powering one complete safety train of safe shutdown loads be installed in the ELWRs-as the means used to meet the requirements of the Station Blackout Rule (10 CfR 50.63). The ACRS noted that the rule permits alternative methods for coping with station blackout, and cited.
10 CFR 50.2 as defining.the acceptable capability for _the alternate AC source.
The ACRS also stated, "Although taken by itself, this may seem to be desirable, it has not been demonstrated that it:is required to conform to the safety goal. Nevertheless, we endorse the staff's recomendation."
The alternate ac power source was identified as the preferred approach for meeting the Station Blackout Rule only in ELWRs because it offers several' advantages over other approaches.
It has the advantage of powering 3
a complete complement of safe shutdown equipment, it can.be utilized in other events in addition to station-blackout (such as during mid-loop operation when one of the normal lass IE standby power sources is out i
for maintenance or otherwise unavailable), it.is essentially not time limited while providing power during a station blackout, and-it provides forauniformhardwarebasedapproachrequiringlittleornocof'ing i
analyses and fewer specialized operating procedures.
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6.
Fire Protection j
The ACRS supports the enhanced fire protection requirements recommended by the staff. Additionally, they recomend further enhancements including;.
- 3) Required separation of Environmental Control System (HVAC systems) so that. independent fire areas containing redundant safe shutdown trains are not serviced by the same ventilation train;and,
- 2) That the 6 items identified in the' fire risk scoping study be fully addressed for ELWRs.
The staff proposed requirement to consider the effects of smoke, heat, and fire suppressant migration may result in separate HVAC systems.
However, other options may be available to the designer.
In reference to the ACRS recomendation that the 6 fire risk scoping study-issues be fully addressed for ELWRS, it is the staff opinion that this is being done through the proposed enhancements and current regulatory efforts as indicated below.
1.
Improved Analytical Fire Codes The enhanced guidance eliminates redundant safety systems in the same fire area so it is not necessary to have a. code to show that +,he redundant train-in the same area is protected. This issue'is moot for ELWRs with the enhanced criteria.
2.
- ontrol System Interactions l
Current regulations require an-independent safe shutdown capability-in the event of~1oss of the control room. This is reiterated in the= enhanced I
guidance for ELWRs.
3.
Total Environment Equipment Survival GDC-3 require; fire protection systems to be designed not to impair safe shutdown systems. Although in some instances fire system actuation has.
disabled a safety. system, in no case have redundant systems been disabled.
4.
Fire Barrier Qualification, and 5.
Manual Fire Fighting Effectiveness Bothoftheseissues(4and5)areaddressedinthecurrentregulations.
The ELWR designs must include information to demonstrate that appropriate maintenance exists for fire barriers and training exists for fire brigades.
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Seismic /rire Interactions l
The fire risk scoping study concluded that-this could be addressed by a walkdown.
The staff will evaluate this issue as part of the PRA review and expects that a walkdown will be: required as part of ELWR design t
certification, i
7.
Intersystem LOCA The ACRS stated that, "We recommend approval of the staff resolution, provided consideration is given to all elements of the low pressure system, (e.g., instrument lines, pump seals, heat exchanger tubes, and valvebonnets.)
The staff agrees with the ACRS comment and will consider all elen,ents of the low pressure system to ensure that the system, to the_ extent prac-ticable, will withstand full RCS pressure.
8.
Ifydrogen Generation and Control-The ACRS stated that the, " Staff recommends that the ELWR designs provide for hydrogen control that can safely accomodate hydrogen generated by the reaction of water with 100% of the fuel cladding surrounding the active fuel."
The ACRS stated that, "We support the staff's recommendation."
The ACRS also stated, "We are not aware of. any experimental cr analytical work that demonstrates that the detonation of hydrogen at the 105, 135, or some other level could damage the integrity of containment and essential components.
It is our impression that the effect, if any, is something that experts dealing with gas explosions can calculate with reasonable confidence. We suggest that the staff seek further technical information on possible effects, including' stratification, before establishing a ' limit for the average concentration."
This staff recomendation is-consistent with the reqirements set forth in 10CFR.50.34(f). Although this section of the regulations was originally written for a select group of plants, 10 CFR Part 52 established these requirements to be a minimum :;tandard for future plants.
In this: context, the requirement of 100% has beceme a surrogate to. address both in-vessel -
and ex-vessel hydrogen production.
It was not. intended to be mechanistically based, but rather a design goal that appears to be reasonable in view of all the analyses performed to date. - The staff has not uncovered any.new evidence in this regard that would cause' them to alter their view.
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While it is true that there are no analyses that demonstrate that detonation loads are a threat to the containment or components within the containment, the staff believes that such an endeavor would be extremely complex and coupled with large degrees of-uncertainty. The question of wave reflection and whether they could possibly reinforce within the containment is but one unknown affect. Therefore, the staff believes a prudent course of action is to assure that the likelihood of both global and local detonations is minimized.
To this end the staff has reviewed the available detonation test results and concludes that 10% represents a reasonable limit to assure non-detonable
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mixtures.
It is felt that the true lower limit lies somewhere between 10%
and 13%.
Therefore, establishing a 10% criteria provides a degree of margin t
which is to address the issue of local detonations and the potential for stratification resulting in concentration gradients within the containment.
9.
Core-Concrete Interaction--Ability to Cool Core Debris The ACRS agreed with the staff's recommendation that each ELWR design provide sufficient reactor. cavity floor space to enhance debris spreading and provide for quenching of the debris in the reactor cavity. The staff had indicated that considerable uncertainty remains on debris coolability and that they would continue to evaluate the issue as more experimental data and information became available.
ACRS concurred in this evaluation and stated that "... resolution of this issue will require engineering judgment as many of the physical processes are not fully understood."
Experiments to assess coolability of water covered debris are underway at Argonne National Laboratory under, joint EPRI/ Industry /NRC support.
The results of these experiments are expected to be available within one year.
- 10. High-Fressure Core Melt Ejection The ACRS stated, "This is an extremely improbable event, and we see no need to require two modes of coping with the possibility.
Either depressurization or cavity design provisions alone should be adequate.
Because of possible safety benefits for other events, reliable depres-surization is probably the preferred approach."
High pressure melt scenarios are dominant for some designs. The staff proposal is to provide a design concept with a degree of mitigation along with a certain amount of prevention.
The RCS depressurization' capability retains a degree of uncertainty. Such questions as the-rate of depres-surization and the cut-off pressure need to be addressed. As a result, we believe that a design can be developed with little or no added expense thatcanbeeffectiveinpotentiallyeliminatingthedirectpath'make to the containment. Since there are no major down sides, it appears to sense to pursue this parallel approach of both depressurization and cavity design.
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-11. -Containment Performance The ACRS stated that "We believe that the staff proposal will be adequate for ELWR review if it'is supported by an appropriate regulatory guide developed on a timely scheoile,.and if it can be reasonably demonstrated L
that a containment that meet t this guidance has a CCFP of not more than 0.1."-
The staff believes that a cor. ainment performance guideline, either a.
. conditional containment fai?ut probability of 0.1 or a deterministic performance goal that: offers e>mparable protection is necessary to give:
the ALWR designers guidance on what is acceptable for containment r
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performance under severe accident conditions. The alternative.is to have a containment designed for loss-of-coolant accident (LOCA) and other design basis accident. (DBA) s.onditions' and then to evaluate its. severe
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accident failure probability after it-is designed.
The staff will keep the ACRS infot;'ed of our development of a regulatory guide in this area:and of our.progres; in evaluating ALWR containments using the containment performance guideline as our review progresses.
- 12. ABWR Containment Vent Design The ACR$ stated that "We recommend that use of a containment overprotection system be approved subject to the results of the regulatory review."
The staff's review of the vent. system design 1s underway. The purpose of-this review is to insure that any downside of. venting is minimized and-a that capability is provided to maintain control over the venting process.
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- 13. Equipment Survivability The ACRS states that, "We endorse the staff's position. We note, however, that in this instance the staff's position includes much more t
than.the underlined portions of the enclosure-to SECY-90-016."
The ACRS supports the. staff. position that equipment for severe accident-mitigation-does not need to meet the sane quality standards as equipment for design basis accidents.
Instead of 10 CFR 50.49, 10 CFR 50, Appendix A, J
and 10 CFR Appendix B, severe accident equipment need only be high quality industrial grade, selected and cualified by analysis or test for the intended '
service.
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- 14. Operating Basis Earthquake (OBE)/ Safe Shutdown Earthquake ($$E)
ACRS has no objection to.the staff considering exemptions to the= require-ment that OBE be at least one half the SSE, where this~ can be justified, j
ACRS suggests:
- 1) other bases for-justification may have to be developed-j l
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for unsited standard plant designs, 2) design be based only on the SSE with appropriate load factors and limit states, and 3) a smaller but more likely earthquake be established as a threshold for plant shutdown and inspection.
The staff accepts the ACRS suggestion which is very close to the design specific approach proposed by the staff.
15.
Inservice Testing of Pumps and Valves The ACRS endorses the staff's proposed approach while emphasizing the need to apply the requirements of Generic Letter 89-10 to ELWR plants.
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The ACRS also recommends that the staff resolve the check valve testing and surveillance issue and indicate how it is to be applied to ELWRs
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including consideration of industry proposed alternative ways of meeting inservice and surveillance requirements.
The staff agrees with the ACRS recommendations and will keep the ACRS informed of any new developments in this area.
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