ML19318G575
| ML19318G575 | |
| Person / Time | |
|---|---|
| Issue date: | 09/14/1988 |
| From: | Kerr W Advisory Committee on Reactor Safeguards |
| To: | Zech L NRC COMMISSION (OCM) |
| References | |
| REF-GTECI-099, REF-GTECI-NI, TASK-099, TASK-99, TASK-OR ACRS-R-1320, NUDOCS 8809220114 | |
| Download: ML19318G575 (3) | |
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e,E ADVIsORY COMMITTEE ON REACTOR SAFEGUARDS wAssivace.n ' 20sss Hoyle Murley, NRR September 14, 1980 Thompsen, NMSS Jordan, AEOD Murray, OGC The Honorable Lando W. Zech, Jr.
, Fil_es Chairman U.S. Nuclear Regulatory Commission Washington, D.C.
20555
Dear Chaiman Zech:
SUBJECT:
PROPOSED RESOLUTION OF GENERIC ISSUE 99, "IMPROVED RELI-ABILITY OF RHR CAPABILITY IN PWRs" During the 341st meeting of the Advisory Committee on Reactor Safe-guards, September 8-10, 1988, we considered a proposal by the NRC staff that would serve as part of the resolution of Generic Issue 99, "Improved Reliability of RHR Capability in PWRs."
Our subcommittee on Decay Heat Removal Systems previously discussed this issue during a meeting on July 27, 1988. We also had the benefit of presentations by the NRC steff and of the documents referenced.
i Generic Issue 99, as originally posed, addressed concerns about the possible failure of core cooling that could result during shutdown operations of PWR plents from the inadvertent closing of suction valves for residual heat removal (RHR) pumps.
Later, the issue was broadened to consider the possible loss of core heat removal during periods of so-called "mid-loop" operation.
In the latter status, inventory of coolant in the reactor primary system is purposely reduced, for example to permit steam generator maintenance.
In such circumstances there have been incidents in a number of PWR plants in which lapses in control of water level have caused loss of suction head and simultaneous failure of pumping from all RHR pumps.
In some of these incidents reactor coolant has heated to boiling.
Boiling fer relatively brief periods, tens of minutes to hours, could cause enough less of coolant inventory to uncover part of the core and overheat the fuel..
Risk analyses indicate the probability of core~ damage from loss of RHR during shutdown to be of about the same magnitude as the prob-ability of core damage from all causes during power operation.
However, the risk to public health would seem to be substantially smaller during shutdown. Core decay power would be quite low because shutdown operations of such nature as to put the core at risk are normally conducted no sooner than about two days after shutdown.
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M( hq Also, the temperature and pressure of reactor coolant would be low.
l, Guidance to PWR licensees from the NRC staff in the form of a generic pi letter issued in 1987 seems to have accomplished little in terms of
()ft reducing the rate of occurrence of such events. As a result of this
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4 The Honorable Lando W. Zech, Jr. September 14, 1988
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unsatisfactory experience, the staff has proposed new guidance in a This letter makes a number of rec-draft of another Generic Letter.
ommendations and requests that PWR licensees inform the 'staff of their action in response to each recommendation.
Most of the recommendations concern improvements in procedures and instrumentation designed to make it less likely that RHR flow will be We are in agreement with interrupted during mid-loop operations.Another proposes procedures to insure t these recommendations.
the containment will be closed in the event there is a threat of core We damage caused by loss of RHR cooling during mid-loop operation.
have some concerns about this recomendation.
Most occasions for mid-loop operation will occur during periods of reactor shutdown when the containment building is open for mainte-nance activities. The staff proposal calls for licensees to maintain equipment in readiness and have procedures and personnel available so that hatch covers and seals can be quickly installed if there is a threatened loss of RHR cooling.
The proposal would not require containnient to be sealed as in normal operation; the argument is that with the depressurized condition of the reactor coolant system in any such circumstance, there is little source of energy to pressbrize the The NRC staff has judged that evea partially bolted containment. closures would successfully contain fission products containacnt released from a damaged core and mitigate consequences of such an accident.
We recognize that, in the circumstances envisioned, decay power and the quantities of short-lived and volatile fission products in the core will be small.
We also recognize that the other steps recom-mended by the staff can be expected to reduce the likelihood of a core damage incident to a very small valne.
Therefore, the proposed method of centainment closure may be a safficient measure of protec-However, we question whether tie proposal has been suffi-tion.
ciently analyzed in light of the many )arieties of cor.tainments and containment closures in service.
We ielieve further study of the practicality and efficacy of the closure proposal is needed.
If it cannot be shown that the proposed method is adequate, then con-sideration should be civen to a requirement that would prohibit mid-loop operation except when the containment is fully closed.
Sincerely, William Kerr Chainnan
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The Honorable Lando W. Zech, Jr. September 14, 1988
References:
- 1. -
Memorandum dated August 10, 1988 from J.. Sniezek, Office. of Nuclear Reactor Regulation, NRC, to E.
Jordan, Office for Analysis and Evaluation. of Operational
- Data, transmitting
.proposed Generic Letter, "Loss of Decay Heat Removal" 2.
U.5, Nuclear Regulatory Comission, NUREG/CR-5015 "Improved i
Reliability of Residual Heat Removal Capability in PWRs as-Related to Resolution of Generic Issue'99," dated May 1988 3.
U.S. Nuclear Regulatory Commission Generic Letter dated July 9.-
1987, "Loss. of Residual Heat Removal (RHR) While the Reactor
.l Coolant System (RCS) is Partially Filled (Generic Letter 87-12)"
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