ML19240C081
| ML19240C081 | |
| Person / Time | |
|---|---|
| Issue date: | 04/10/1981 |
| From: | Harold Denton Office of Nuclear Reactor Regulation |
| To: | Bradford P, Gilinsky V, Hendrie J NRC COMMISSION (OCM) |
| Shared Package | |
| ML19240C082 | List: |
| References | |
| FRN-45FR18045, REF-10CFR9.7, RULE-PR-50 NUDOCS 8104200189 | |
| Download: ML19240C081 (16) | |
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8 MEMORANDUM FOR: Chairman Hendrie
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Conmissioner G111nsky Commissioner Bradford Conmissioner Ahearne 4
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Harold R. Der, ton, Director Of fice of Nuclear Reactor Regulation THRU:
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I Executive Director for Operat !one SUBJ ECT:
REQUIREMENTS FOR A MANUF/.CTURING LICENSE FOR THE FLOATING NUCLEAR PLAMT The staff initially proposed that the containment for the Floating Nuclear Plant (FMP) be strencthened to neet a design pressure of 60 psig, in addition to inplementing effective hydrogen control, to significantly reduce the risks of severe accidents in this ice condenser plant. The choice of 60 psig for the containment design limit was based on the belief that it could be accom-rodated by modifications which would not alter the ice condenser desian concept, and which could be accomrodated by design, natvial, and construction technioues in use in the nuclear ind9stry. Discussions with the applicant, Offshore Powe-Systems, have indicated that increasing the containment desian strength to 60 psig would recuire najor nodifications to the conceptual design because of limitations of tyina the cylindrical containment shell to the rectangular structural nenbers of the barge. However, the containment could be nodified by using a henispherical head, increasing the wall thickness to a mininun of 1 inch, and making other modifications which would increase the design pressure to 25 psig.
At this design level, the ultimate pressure capability was estimated to be 80 psia.
Based on an evaluation of severe accidents at the Seouoyah plant (ice condenser) by Sandia and the inclusion of an Pc0 core catcher and effective hydrogen control in the FUP, the applicant contends that additional protection afforded by a containment having a 60 psia desion pressure is not warranted.
We have reviewed the accident secuences resulting in containrent failure from overpressure identified by the Sandia staff (under contract to RES) for the Seouoyah plant. These secuences are small LOCAs with loss of core cooling and core and containment cooling, and total loss of feedwater (nain and auxiliary). These secuences are based on a containnent failure probability of 1.0 assumina hydronen conflacration. An effactive and reliable hydrocen control systen should substantially reduce this mode of containnent failure; however, containnent failure could still occur because of steam pressures, cither in the form of " spikes" or slower pressure buildups.
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Comnissioners Une replacement of the concrete pad below the reactor vessel with a magnesium oxide (M7D) ladle (i.e., a passive core catcher) will provide substantial resistance to a core melt through. Based on a best-estimate analysis, the Mgo core ladle will retain the nolten core debris for approximately 1 week before nelt through of the FNP barge hull would occur (see NUREG-0054, Feb. 1980).
In addition, the Mg0 pad eliminates the generation of large volumes of noncondensible and/or combustible cases (e.g., CO, CO2, H2) which would otherwise be generated from the interaction of the nolten core with the concrete (before its replacement with the Mg0); the elimination of these gases and the acctmpanying energy concurrently eliminates the associated increased loadings on the containment. Thus, as was said earlier, the major remaining threat to the containment (assuming the hydrogen threat has been elinineted) would come fron steam released to the containment as the core degrades and proceeds to melt through the vessel and into the cavity below.
The containment loadings would come either from steam pressure spikes or slower pressure buildup from the steam generation as the multen core comes into contact with water. The probability of eliminating this later threat can be substantially increased by the addition of dedicated core and/or containment heat removal systen(s); the precise nature of such system (s)
(which could be a passive system) would derive from a more detailed risk study which the ML applicant has committed to do.
It shculd finally be pointed out that the staff in its SER of the FNP noted the potential threat to containment from core meltdown sequences and conmitted the applicant and the staff to " perform more detailed, integrated studies that will involve modeling the containment and core ladle as a system to ascertain the system response to various core meltdown sequences" (NUREG-0054, pp 44-46).
The ACPS in its letter of April 16, 1980 to the EDO on the subject of the
" Floating Nuclear Plant (FUP) Core Ladle" agreed with the staff's evaluation and also noted the need for more detailed, integrated containment response studies which both the applicant and the staff have underway.
As a result of our evaluation, we believe that the use of effective and reliable hydrogen control, post-accident containment cooling, core catcher systens, and modified containment in a floating nuclear plant will reduce the likelihood of containment failure sufficiently and that increasing the containment desion pressure in FNP to 60 psic may yield little risk reduction.
We believe that the containment loading and mitigation system studies being performed at Brookhaven National Laboratory over the next year will confirm this conclusion.
These findings have been discussed with Connissioner Gilinsky during the week of March 2,1981, and we want to apprise all the Commissioners of our belief that there is a viable alternative for reducing the risk in the FNP.
mi riped by H. R. Centett Harold R. Denton, Director Office of Nuclear Feactor Reculation "M
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