Seismic Working Group Meeting of March 11, 1991

DOE-STD-1024-92

Figure 7 discussed later in this Standard shows the original LLNL mean

seismic hazard results for the Savannah River Site with and without

LLNL-AE5 and preliminary draft results for LLNL study underway for

Savannah River. The most recent results show probability reductions in

the mean hazard of about 8 at a peak acceleration of about 0.20g.

These preliminary results will be used to help establish the appropriate

way of containing the LLNL and EPRI seismic hazard curves.

2.3

Seismic Working Group Meeting of March 11, 1991

In addition to the above information, the S WG held a meeting on March

11, 1991, with the specific purpose of obtaining from these three

investigators their input related to the causes of the differences between

the two studies. Based on the above, the S WG has evaluated these

studies and has reached the following conclusions:

There is a high degree of similarity between the LLNL and EPRI

seismic hazard studies ranging from overlap of experts used to

general overlap in parameter input from the experts. The key

difference between the two studies relates to the topic of

uncertainty assessment, particularly "modeling uncertainty"

assessment for all input variables. Identified issues relate to the

process of expert opinion elicitation, particularly the issues of how

and whether experts assess and understand uncertainty;

The S WG has concluded that the mean seismic hazard curves

from the LLNL and EPRI study should not be used as the seismic

hazard curve to implement UCRL-15910 (DOE-STD-1020) or as

the single seismic hazard curve for probabilistic risk assessments.

The work of McCann (1991) demonstrates that the LLNL mean is

sensitive to the upper tail of the hazard curve distribution (above

about 90th percentile) at the Savannah River Site. Given the

location of the mean hazard curve at the sites evaluated by LLNL

(LLNL 1989), i.e., generally above the 85th percentile, it is likely

that the above conclusion would hold at many locations. The

seismic hazard curves at fractiles above about the 85th percentile

may not represent realistic seismicity estimates. While the above

generic statement is controversial, it is clear that there is doubt

regarding these highest fractiles, and this doubt is serious enough

at the present time to support the judgement that fractiles above

the 85th percentile should not dominate the choice of seismic input;

The above conclusion may be generally extended to the EPRI

study. McGuire (1990a and 1990b) has recommended that, based

on comparison to historic seismicity, that some of the EPRI team

input could be questioned as underestimating the frequency of

earthquakes. Additionally, LLNL (1987) has noted that the