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UNITED STATES NUCLEAR REGULATORY COMMISSION a
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e M f hA h MEMORANDUM FOR: d.Hulman, Chief Accident Evaluation Branch FROM:
Robert E. Jackson, Chief Geosciences Branch, DE
SUBJECT:
INPUT TO LIMERICK FES Attached are proposed revisions to the Limerick Draft Environmental Statement and Geosciences Branch answers to comments submitted by PECO in their letter of February 6,1984.
In the first suggested revision we discuss interim results from the Lawrence Livermore National Laboratory (LLNL) Seismic Hazard Characterization Program. These results further emphasize the uncertainty associated with calculations of seismic hazard used in the LGS-SARA. They diverge fgom the LGS-SARA results particularly at frequencies ~less-than 10 per year. We do not anticipate better agreement between commercial PRAs and the LLNL calculations at other sites.
At this-time we do not'pecessarily believe that one is wrong and the other is right. We are attempting to evaluate and determine to what extend this divergence is the result of inherent uncertainties in state-of-the-art hazard estimates or systematic errors in input assumptions. Substantial advances in our understanding of earthquake causality and ground motion may be. needed to significantly improve the picture. As a result we would like to reemphasize the off-stated preference (see for example NUREG-1050) of not placing much confidence in bottom line numbers produced in seismic PRAs. Central estimates (means or medians) are vulnerable to the highly judgmental choice of input parameters.
In addition uncertainty bands cannot be rigorously defined. As it appears that environmental statements require the generation of these numbers, the following (in whole or in part) may be used to put these calculations in the proper context.
1.
Central estimates should be deemphasized and greater weight given to uncertainty bands stressing, however, that these bands may not be accurately defined and should be viewed as " representative ranges. "
2.
Displays of any calculations should avoid more than one significant figure since the uncertainty is certainly several orders of magnitude.
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Calculations may be put in context thru the use of qualitative descriptier and qualifiers (see for example the second proposed
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4.
One approach to dealing with the uncertainty may be to define the hazard : levels (or range) above which qualitative conclusions with respect to a particular nuclear power. plant would change.
Qualitative and/or quantitative discussions could then be used to describe why-reasonable estimates of the hazard would lie above or below that level (or range).
5..
Further study should be given to answer the question as to whether the presence of a nuclear power plant significantly increases the risk to the public during a severe earthquake (the kind needed to cause a core melt).
If the argument can be made that this incremental risk is negligible then the need for reliable estimates of rare earthquake occurrence may be greatly diminished for the purpose of the DES. An initial examination of this issue in response to PECO-24 indicates that this statement cannot be made at this time.
These counts were prepared by Leon Reiter and Abou-Bakr K.
Ibrahim of the Geosciences Branch.
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Ro ert E. J p n, Chief Geosciences/EWanch Division of" Engineering
Enclosure:
As stated cc: w/ enclosure J. Knight T. Sullivan A. Thadani F. Coffman E. Chelliah S. Brocoum S. Acharya
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Revisions of DES 1.
The following should be inserted after the words "... uncertainties with such events" in the next to last paragraph on p 5-48 of the DES.
"This statement reflects the staff's view that the rigorous definition of seismic hazard a.nd its uncertainty at low probabilities is beyond the state-of-the-art at this time and should be recognized as such. Different studies would not necessarily yield equivalent results.
For example an interim
' report " Seismic Hazard Characterization of the Eastern U.S." of an ongoing. study being carried out by Lawrence Livermore National Laboratory (LLNL) for the NRC shows seismic hazard calculations for the Limerick site which overlap, but are not necessarily coincident with, the range of seismic hazard assumed in LGS-SARA.
The median (50%) hazard calculated in the interim LLNL report is within, but near the high end of, the range of hazard curves utilized in_ LGS-SARA. Additional studies of seismic hazard in the eastern U. S. are being carried out by such groups as the Electric Power Research Institute. Given the highly judgmental nature of seismic hazard calculations, there is no reason to believe that these studies or the final LLNL report would not show differences in estimated seismic hazard and uncertainty between themselves and _.
the LGS-SARA, particularly at the low probabilities being calculated for Limerick."
2.
The following should be inserted at.the end of the first paragraph after the words "...and M-la and b" on p. 5-49 of the DES.
"We do not mean to imply that higher risk estimates are more appropriate than the median, mean or lower estimates.
Indeed the most significant earthquake damage anywhere within the vicinity of the Limerick 3ite, in the two to three hundred years during which we have records, are fallen chimneys 50 kilometers away during an earthquake at Wilmington, Delaware in 1871 whose magnitude can be estimated to have been less than 5.0, We certainly cannot exclude from the range of reasonable assumptions the judgment that there essentially is no risk to the public resulting from earthquake-induced damage at the seismically-engineered nuclear power plant at Limerick during its operating life."
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- ..i-Responses to PECO Consnents on DES Following are responses to comments on the DES as stated in their letter of February 6,1984. The numbering scheme is that which has been forwarded to us along with the letter.
-PECO-10.
The applicant in the LGS-SARA has indicated that Modified Mercalli Intensity (MMI) IX is the boundary level below which evacuation would not be impeded by earthquakes. The applicant assumed that MMI IX corresponds to an effective peak acceleration (EPA) of 0.61. The staff 9
agreed that 11MI IX is a reasonable dividing boundary, but found that an EPA of 0.61g is not an appropriate value to be used.
While there is a wide scatter in the correlation between intensity and acceleration, a reasonable choice can be made providing that it is consistent with the assumptions made in the LGS-SARA itself, and in recent investigations.
In the LGSTSARA reference is made to Kennedy'(1981) to justify the use of an upper bound EPA.
In that work Kennedy assumed that MMI IX is associated with an EPA of 0.4g-0.5g or less. Recent investigations also indicate a lower EPA than 0.61g for MMI IX. For example, Krinitzsky and
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Marcuson (1983) propose a mean acceleration of 0.48g (0.38g EPA) fur PMI=IX at a near-field, hard rock site. Similarly, in an LLNL (1984) report, a survey of various intensity acceleration correlations resulted in a median acceleration of 0.35g (0.28g EPA) to be associated with epicentral intensity MMI=IX.
In accord with the assumptions in the LGS-SARA, the EPA is derived from the peak acceleration by multiplying it by a factor of 0.8.
Based on the above we recommended that an EPA of approximately 0.4g be used to characterize MMI IX, we believe that this value is more consistent with both the assumptions used in the LGS-SARA itself and recent investigations.
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References Krinitzsky, E.L. and W. I. Marcuson III,1983, Principle for Selecting Earthquake Motions in Engineering Design, Assoc. Of Eng. Geol. Bull.
V.xx, pp. 253-265.
Lawrence Livermore National Laboratory,1984, An Empirical Assessment of Near-Source Strong Ground Motion for a 5.5 g Earthquake fo the Eastern
-U. S.
Technical report submitted -to NRC by K. Campbell.
Kennedy, R.,1981, Comments on Effective Ground Accelerati.n Estimates-for the Indian Point Site, Structural Mechanics Associates, Inc.
PECO-19.
See response to PECO-10.
PECO-24
- Detailed studies of non-nuclear related earthquake risk for the Limerick site and most of the Eastern U. S. have not been carried out.
Extrapolations of estimates of fatalities due to severe earthquakes in California and South Carolina indicate that these fatalities may be on the order of thousands of individuals. These are of the same order as -
those calculated from fatalities resulting from a severe earthquake-induced core melt as presented in the DES. They are gross extrapolations that do.not take into account the specific of the Limerick site and its surroundings. At this point, however, it appears premature to support the intuitive argument that the earthquake-related risk resulting from the presence of a nuclear power pl. ant at Limerick is negligible compared to the general earthquake. risk.
FECO See revisions to p. 5-48 of DES to be incorporated into the FES.
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