ML20133M646

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Forwards Final Draft Estimated Risk Due to Seismic Design Deficiencies. Estimates Suggest Overall Risk & Core Melt Probability Might Be Increased by Factor of Three to Four Relative to Estimates in Reactor Safety Study
ML20133M646
Person / Time
Site: Beaver Valley, 05000000
Issue date: 03/22/1979
From: Jerome Murphy, Taylor M
NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES)
To:
NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES)
Shared Package
ML20133M133 List:
References
FOIA-85-301 NUDOCS 8508130196
Download: ML20133M646 (6)
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SUBJECT:

ESTIMATES OF RISK ASSOCIATED WITILDIAVER' VALLEY SEISMIC DEFICIENCIES g

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On March 15, 1978, we provided D. Bunch, NRR, with a rough draft of an estimate of the risk associated with the Beaver Valley

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. - design deficiencies and briefed the Director of NRR on its contents.

The final version of the draft which is attached has also been provided to NRR. .

The estfimate suggests that the' overall risk'and the core melt probability might be increased by a factor of three to.four relative to the estimates presented in the Reactor Safety Study. Considering the error bounds associated with the RSS estimates, this increase is within the uncertainties of the analysis. Consistent with previous views using risk assessment (such as in NUREG-0460), factors of 3-4 probably should not be regarded as significant. ~

In addition, we believe these estimates are somewhat conservative, due 1argely to the applicability of the' Hsieh-Okrent estimates of seismic

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frequency for Eastern U. S. as we applied them to the Beaver Valley site.

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M. A. Taylof', PAS, RES

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J. A. Murphy, P , RES Attachment

'cc: Del Bunch Roger Mattsor ~- d 3

  • Beaver Valley is very simil,ar in design to Surry which was analyzed in the Reactor Safety Study.

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Estimated Risk Due to Seismic Design Deficiencies Given the seismic design deficiencies presently believed to exist at the Beaver Valley plant *, the staff has estimated the likelihood for various events occurring, including the possibility of a significant core damage accident occurring. These estimates are acknowledged to have significant uncertainties both upward and downward; nonetheless, they provide insight regarding the possible reactor accident risks involved in this matter.

The likelihood estimates given below have relied on seism 4e margin insights developed by Newmark and on Eastern U.S. earth. quake frequency estimates by r

. Hsieh and Okrent. Additionally, the recent and extensive work performed in conne'ction with the Diablo Canyon design was used to obtain insight as to the likelihood of failure of important ESF's at seismic l'evels significantly above the Beaver Valley Nuclear Plant design levels (e.g.,1.5 x SSE).

The Beaver V. alley plant at present appears to have exceeded the recommended code allowable stress levels that should have been used as design bases (DBE and other loads % 1.8 S H

, OBE plus other loads. Si,1.2 HS ) for the smaller piping connected to the reactor coolant system. The more serious over-stress levels are encountered in small piping that provides coolant to the reactor coolant pump seals. Further, some of the small piping in the ECCS might experience stresses somewhat beyond recommended code levels under seismic excitation.

  • Beaver. Valley, DBE % 0.12 g. "

OBE % 0.06 g.

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Considering the RCP seal injection piping which might be severely overstressed, likelihoods of exceeding allowable stresses and of piping failures are estimated to be as follows:

Exceeding OBE ii,1.2 S H Requirements %5 x 10-2/ year

. 9 .016 g.

DBE 11.8 SH Requirements -

N1.4 x 10-2/ year 9 .043 g.

.Ex_ceeding yltimate (%3.6 SH )

44.5 x 10-3/ year 9 .086 g.

.(]' (These estimates suggest that relative to WASH-1400, the likelihood

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of a small LOCA occurring (e.g.,1/2 - 2" dia. range) could be roughly increased by about a factor of 4.)

For small safety injection piping, the likelihood of exceeding allowable stresses given an earthquake are presented below:

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Exceedina OBE-(1.2SH Requirements %2 x 10-2/ year 9 %.033 g.

08E. % 1.8 SH Requirements N4.3 x 10-3/ year 9 %0.09 g.

Exceeding ultimate (%3.6 SH ) -

N1.2 x 10-3/ year 9 %0.18 g.

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3-Core Damage Estimates (Rough)

Using insights from Diablo Canyon seismic risk analyses and the estimated t

unavailabilit,ies of various ESFs over a range of accelerations through approximately 1.5 x DBE (where the RCP seal piping and the more severely stressed SI piping would be expected to fail), the loss of either ECCS or electric power is roughly:

BV Core Melt Estimates (Seismic)

RCP Lines:

PRCP Small Loca P x Independent ECCS failure

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9 0,086 g. s4 x 10-3 x 10-2

%4 x 10-5/RY --

BV Core Melt Estimates (Seismic)

Seismic-Induced Failure of RCP line and SI line:

BV-SIS (higher stressed (Some ECCS dependence ingle pipe) and LOCA assumed due to 1 train 0 0.18 g. loss) 1 x 10-3 x 10-1 (10-4/RY e  %

High Risk Estimate (Seismic)

High Risk Accident Sequence

' (All ESFs) 9 0.18 g. 10-3 %2 x 10-2 %2 x 10-5/RY

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4 Relative to WASH-1400 Results Estimated Factor of WASH-1400 BV Seismic + WASH-1400 Increase PWR Core Melt 46 x 10-5/RY 42 x 10-4/RY 3-4x PhRhighRisk48x10-6/RY N3 x 10-5/RY 3-4x O 4 D

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