ML19291C030
| ML19291C030 | |
| Person / Time | |
|---|---|
| Site: | Saint Lucie  |
| Issue date: | 11/07/1978 |
| From: | Baranowsky P NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | |
| References | |
| REF-GTECI-A-44, REF-GTECI-EL, TASK-A-44, TASK-OR NUDOCS 8001110055 | |
| Download: ML19291C030 (8) | |
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Draft Testimony of P.-W. Baranowsky p' 7~
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Nuclear Power Plant The purpose of this testimony is to provide background information on the unresolved safety issue, " Station Blackout" (A-44), which has been raised as a concern in the St. Lucie 2 hearing.
The matter of a station blackout and associated consequences was thrust into prominance by the findings of the Reactor Safety Study (RSS).
The accident sequence associated with a station blackout, or loss of all AC power sources, was found to represent a significant fraction of the total risk for the PWR analyzed.
Some time after the publica-tion of the RSS, and not necessarily as a result of the RSS, the Office of Nuclear Reactor Regulation's generic safety activity B-57 identified station blackout as an issue to be addressed.
With the,,Jblication of SECY 601, the station blackout concern was raised to the higher priority of an unresolved safety issue and designated A-44.
The responsibility for developing and implementing a program to resolve this issue was transferred to the Probabilistic Analysis Staff in the Office of Nuclear Regulatory Research in August 1979.
This was partly due to NRR manpower limitations and the recognition that this issue is strongly probabilistic in nature.
1734 292 In October 1979 a simple survey analysis was begun to make a rough estimate of the failure probability for all AC power and the loss of decay heat removal capability at currently operating LWRs.
The intent of this work was to provide a screening mechanism to identify operating plants highly susceptible to station blackout consequences at the outset of the program and to identify appropriate short tenn actions which could be taken to improve station blackout vulnerability while a more extensive program is undertaken.
The second and more extensive g () O 1 110 0 9 5
effort will be incorporated in the Integrated Reliability Evaluation Program (IREP) which will be conducted through PAS to provide safety reliability and accident probability estimates at all operating nuclear power plants during the next three years.
The early and somewhat incomplete results of the screening analysis have shown a large variation in the estimated accident sequence probabilities for station blackout. The results for some plants are higher than the comparable sequence results obtained from the RSS.
However, the uncertainty in these calculations is much larger than that associated with the RSS results and indi-cations are that the uncertainties predominate in the conservative direction.
On the other hand, it should be noted that this work has been primarily devoted to the analysis of one of many possible accident scenarios initiated by a station blackout. The enclosed representative event tree shows several possible pathways to success or failure (in terms of core cooling and damage) for this event.
Note that the heavied-in pathway is the scenario studied in the plant survey analysis. This scenario is expected to be a good measure of the plant's This susceptibility to a serious accident resulting from a station blackout.
scenario involves the independent failure of offsite and onsite (emergency)
AC power and the dependent or independent failure of the AFWS.
For plants in which the AFWS is highly reliable under station blackout conditior.>, other sequences involving reactor coolant system integrity or longer term cooling requirements could be limiting; and the overall core damage / melt frequency estimates could very likely be within the range of the RSS results.
For reference, the station blackout sequence addressed by the applicant and NRR staff which does not include independent failures is shown as a dashed line on the event tree.
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The survey analysis results indicate that the frequency of a station blackout lasting about one hour may be 10-5 per year or higher at some plants.
Diesel generator reliability appears to have the largest effect on this estimate.
The RSS predicted a station blackout frequency for a one hour duration of between 4x10-4 and 4x10-5 per year.
The larger value corresponds to a common mode failure of the emergency diesel generators.
The RSS also showed that the cumulative probability of a core melt following station blackout was approximately 6x10-6 per year.
The sum of all core melt probabilities for the PWR analyzed in WASH-1400 was approximately 5x10-5 per year.
Roughly speaking, the consequences of any of these core melt accidents would equal or exceed the dose limits associated with 10 CFR 100.
Although the uncertainty estimates and accuracy of the absolute values predicted in WASH-1400 have had some criticism, the acceptability of plants operating with such core melt frequency predictions has not been rejected within the NRC; and note that these evaluations have been made public since August 1974.
Another result stemming from the survey analysis is the identification of several design and procedural improvements which have the potential of mirl.imizing the accident probability for station blackout sequences.
These are identified as follows:
1.
Conformance with Regulatory Guide 1.108, reliability goals and associated minimum testing frequencies should be demonstrated with the limiting condition for power operation with one diesel out for repair established as a function of diesel reliability; 1734 294
2.
A decay heat removal system (AFWS) should be provided with at least one train independent of AC power supplies for activation, motive power, and control; 3.
The limiting conditions of operation should be amended to limit the time power generation may continue for combinations of start-up transformer (s),
AC independent shutdown cooling train (s) and emergency AC power supplies out of service; and 4.
Emergency procedures should be made available to operators, plant mainte-nance personnel, and offsite personnel (e.g., grid dispatchers) identifying the functions for coping with a station blackout and restoring cffsite emergency AC power supplies.
Incorporation of these aspects of design and operation should provide improved safety reliability for station blackout while a longer range and more detailed program is implemented to deal with the more subtle and smaller probability risks associated with a station blackout.
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LWu 155 3 pay.ps3 The purpose of this testimony is to provide background information on generic Task A-44, " Station Blackout." The issue addressed by this task is whether or not the loss of all AC power should be considered in the design bases of nuclear power plants. The issue of loss of all AC power has also been raised by the appeal board in the specific case of St. Lucie Unit 2.
The Office of Nuclear Reactor Regulation established the issue of Station Blackout as a generic task in 1977.
It was originally designated as generic Task B-57.
Currently, NRC requirements regarding Auxiliary Feedwater System (AFWS) power diversity and onsite and offsite power sources are relied on for new plants to provide sufficient protection against potential unaccep-table consequences resulting from a Station Blackout event. However, in late 1978, the staff's concern regarding the potential risk posed by Station Blackout events in older plants not reviewed against current re-quirements caused the staff to propose (and the Commission agreed) to report this issue to the Congress as an " Unresolved Safety Issue" pursuant to Section 210 of the Energy Reorganization Act of 1974, as amended. Accord-ingly, Task B-57 was elevated in priority and redesignated'as Task A-44.
The responsibility for developing and implementing a program to resolve this issue was transferred to the Probabilistic Analysis Staff in the Office of Nuclear Regulatory Research in August 1979. This was partly due to NRR manpower limitations and partly in recognition that the approach to resolving this issue would necessarily have to depend strongly on pro-babilistic analysis techniques.
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W'iwspp. ken 29 pup The longer term Task A-44 effort has not as yet been fully scoped. One approach being considered is to incorporate all or part of this effort ie_
the Integrated Reliability Evaluation Program (IREP) which will be con-ducted through PAS over the next three years to provide safety reliability and accident probability estimates at all operating nuclear power plants.
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