Ground Motion Issues at Los Alamos National Laboratory
Purpose: This report documents Defense Nuclear Facilities Safety Board (DNFSB)
staff and outside experts evaluation of the July 18 and 19, 1994 final peer review meeting
on the "Seismic Hazards Evaluation of Los Alamos National Laboratory" by
Woodward-Clyde Federal Services (WCFS). The meeting was held at Los Alamos, New Mexico.
DNFSB staff members A. Hadjian and A. Jordan, and outside experts Paul C. Rizzo and Peter
J. Hutchinson, attended this meeting. Also attending the meeting, in addition to WCFS
project staff with their consultants and the external peer review panel, were
representatives of the Department of Energy (DOE), Los Alamos National Laboratory (LANL)
and Sandia National Laboratory.
Summary: The LANL seismic hazard studies are planned to be completed by the end
of September 1994. However, significant modifications made to the initial logic trees, the
work that is needed to complete the report and the intense discussions during the peer
review meeting of July 18 and 19, 1994, raise serious doubts whether the seismic hazard at
LANL facilities could be adequately performed by the end of September. The main
difficulties are: the logic trees have become extremely cumbersome; the seismogenic
characteristics of the Pajarito fault are not well understood; fault rupture displacements
need to be estimated close to or under critical facilities; and the effect of topography
on ground motion at the top of the mesas requires a more appropriate treatment.
Background: The objective of the WCFS Seismic Hazard studies was to develop
hazard curves for several of the Technical Areas at LANL using state-of-the-art assessment
methodologies. It should be noted that there are no DOE Standards to develop hazard
curves; and the profession is just beginning to sort out the several ground motion
assessment methodologies. Following issuance of the Draft Final Report "Seismic
Hazards Evaluation of the Los Alamos National Laboratory" Vols. I & II on June 1,
1993, two peer review panels held separate discussions on August 31 and September 1, 1993;
and September 28 and 29, 1993, on seismic source and ground motion characterizations,
respectively. Significant deficiencies and shortcomings in the WCFS report were
highlighted during these meetings. Subsequently, WCFS was tasked to finalize its report by
the end of September 1994. The primary purpose, therefore, of the July 18 and 19, 1994,
peer review meeting was to discuss the latest modifications made to the logic trees before
the final computer computations are made or the generation of the LANL seismic hazard
curves.
Discussion/Observations: Compared to the earlier version, the latest logic trees
are extremely complicated and hence cumbersome and confusing to work with. Moreover, the
weights assigned to the different branches do not reflect the best judgment of the
analysts. Throughout the meeting, WCFS was looking to the review panel members to provide
this information instead of presenting their preferences of the alternatives and defending
those positions. For example, given the tectonic environment, assigning the same weights
to nonintersecting and intersecting models of the Pajarito fault with the Rentage Canyon
and Guaje Mountain faults indicates indecision. Additionally, WCFS was unable to document
the slip rate, timing, and magnitude of the Pajarito fault, which would dominate the
Design Basis Earthquake.
Even though another meeting with the peer review panel was scheduled to be held before the
report is finalized, DNFSB staff and outside experts seriously doubt that the work that is
needed to complete the report could be adequately performed by the end of September to
characterize the seismic hazard at LANL facilities. These difficulties are primarily due
to the lack of standards in this field and the fact that the state-of-the-art in ground
motion assessment is in the developmental stage. Therefore, the following observations are
based on the professional judgment of the DNFSB staff and the Board's outside experts. In
addition to the logic tree complexity, the following technical issues need to be
adequately resolved:
No detailed Quaternary geologic map exists for the entire LANL site. Such a map
would serve as the primary basis for identifying and characterizing potentially capable
faults.
The seismogenic characteristics of the Pajarito fault may have to be better
understood in order that arguments regarding the level of conservatism that would
otherwise be required in the design ground motions would be minimal.
The mesas on which LANL facilities are located tend to amplify ground motions.
The work done to-date on the estimation of the topographic effects on ground motion at the
mesas is inadequate. Simple available solutions have been discarded in favor of untested
new solutions.
No work has been performed regarding the potential for fault rupture
displacements under or near critical facilities, considering that both the Rendija Canyon
and Guaje Mountain faults lie within the LANL boundaries. These surface ruptures could be
very detrimental to the integrity of structures.
To complete the seismic hazard evaluation at LANL, volcanism related issues,
such as evidence of late Pleistocene to recent volcanism, need to be addressed.
The present method of approximating the probabilities of continuous variables
by probabilities of discrete variables, which are then used in the logic tree, is an issue
that would impact adversely the accuracy of the results. The distributions of the
continuous variables must first be determined consensually, then discretized in a manner
which preserves information on the relevant tail of the distributions.
A technical reconciliation between the probabilistic and deterministic ground
motion estimates is deemed desirable before the design/evaluation basis ground motion
criteria for facilities is finalized.
Design basis ground motion and site soil properties need to be uniformly
correlated among the several facilities now being considered for potential structural
upgrades.
Future Staff Actions: Significant changes were introduced in the parameters of
the seismic hazard estimation model, which were not all endorsed at this meeting by the
peer review panel and DOE. The staff and outside experts of DNFSB will review the final
report, scheduled for the end of September 1994.