[DNFSB LETTERHEAD]
March 7, 2003
The Honorable Linton Brooks
Acting Administrator
of the National Nuclear Security
Administration
U.S. Department of Energy
1000 Independence Avenue, SW
Washington, DC 20585-0701
Dear Ambassador Brooks:
The staff of the Defense Nuclear Facilities
Safety Board (Board) has reviewed the status of flood mitigation measures
following the Cerro Grande wildfire at Los Alamos National Laboratory
(LANL). The wildfire, which occurred in
summer 2000, increased soil hydrophobicity and led to a much greater potential
for flooding at LANL. As discussed in
the enclosure to this letter, recovery of the severely burned areas has been
very slow because of drought conditions at Los Alamos. In addition, it has not been possible to
validate and refine hydrological modeling of the potential for flooding because
of limited rainfall, so the risk of flooding remains only partially
understood. It is the Board’s
understanding that Cerro Grande funding for hydrologic modeling expires in
fiscal year 2003.
Defense nuclear facilities at Technical Area
18 (TA- 18) are protected by a flood retention structure designed and built
under emergency conditions. This
structure apparently will be needed longer than expected because a return to
prefire conditions will take more than the typically observed 3 to 5
years. The structure will probably be
needed until the relocation of the TA- 18 mission is essentially complete. The Board understands that the Department of
Energy has not yet received a report on material and construction quality
assurance for the flood retention structure.
The need for test results on the properties of the concrete of the flood
retention structure was identified earlier in a Board letter dated November 5,
200l.
It is important that hydrologic modeling be
completed to confirm the adequacy of design assumptions for the TA-18 flood
retention structure, as well as to ensure that other flooding risks are no
greater than currently understood. It is
also important that the Department of Energy confirm the adequacy of the flood
retention structure. Therefore,
pursuant to 42 U.S.C. § 2286b(d),
the Board requests within 90 days of receipt of this letter, a report that
documents:
Sincerely,
John T. Conway
Chairman
c:
The Honorable Jessie Hill Roberson
Mr. Mark B. Whitaker, Jr.
Enclosure
DEFENSE
NUCLEAR FACILITIES SAFETY BOARD
Staff Issue Report
February 27,
2003
MEMORANDUM FOR: J.
K. Fortenberry, Technical Director
COPIES: Board
Members
FROM: D.
Ralston
SUBJECT: Flood
Mitigation at Los Alamos National Laboratory Following the Cerro Grande
Wildfire
The staff of the Defense Nuclear Facilities
Safety Board (Board) recently reviewed the status of flood mitigation measures
at Los Alamos National Laboratory (LANL).
Staff members A. Jordan, A. Hadjian, B. Jones, and D. Ralston and
outside expert P. Rizzo participated in the review.
The Cerro Grande wildfire in May 2000
severely burned thousands of acres of Department of Energy (DOE) property and
surrounding hill slopes. The fire
decreased vegetation and increased soil hydrophobicity in burned areas, and
greatly increased runoff potential and the risk of flooding. To protect Technical Area 18 (TA-18) from
increased storm flows in Pajarito Canyon, DOE contracted with the U.S. Army
Corps of Engineers to build a flood retention structure (FRS) upstream of
TA-18. The FRS was constructed of
roller-compacted concrete to a height of 72 ft above the streambed, and was
completed in August 2000. The FRS was
designed to retain the volume of a 6-hour, 100-year storm and drain within 96
hours.
After the fire, LANL modified existing
hydrologic models of the laboratory to account for changes in runoff
characteristics of the land surface, but runoff increased much more than the
initial estimates. On June 28, 2000, a
small rainfall of 0.69 inches in Pajarito Canyon, predicted by the models to
yield a peak flow of 11 cubic feet per second (cfs) at TA-18, actually produced
a peak flow of 150 ±30 cfs. The model input parameters were adjusted
again so that the results matched this observed flow, and these changes were
incorporated into the design of the FRS.
In the 2 years since the Cerro Grande fire,
Los Alamos has experienced severe drought conditions. During the drought, there have been no significant storms or
stream flows in Pajarito Canyon to augment the event of June 28, 2000, used for
model calibration. Without additional
data for model evaluation, the input parameters cannot be validated or refined. The hydrologic models are highly sensitive
to the input parameters describing the condition of the watershed, so without
additional field data, the estimates of storm flows at TA-18 remain highly
uncertain.
Another consequence of 2 years of drought is
that recovery of the severely burned areas has been very slow. Precipitation spurs growth of vegetation,
washes away ash and debris, and breaks down the hydrophobic layer of soil that
limits infiltration. Work crews have
raked, seeded, and watered burned areas on LANL property to aid recovery, but
the most severely burned watersheds are upstream on U.S. Forest Service
property and have seen little or no recovery work. Changes in infiltration characteristics can be monitored in the
field, but thus far the field investigations of recovery have been very
limited.
Because of the drought, runoff from severely
burned watersheds is unlikely to return to prefire conditions within the
typically observed 3 to 5 years. To
protect TA-18 from a continuing flood risk, the FRS will likely be needed for
longer than initially thought. An
extended life span for the FRS makes an upcoming Corps of Engineers report to
DOE on material and construction quality assurance for the FRS even more
critical. Although the FRS was
constructed in the summer of 2000 and data should have been available in late
2001, this report will not be completed until March 2003. The report is particularly critical since it
will provide data on the strength of the roller-compacted concrete used in the
FRS. The need for such a report was
discussed in a Board letter dated November 5, 2001. Test results for unconfined compressive strength at 1 year,
direct shear strength at 1 year, and density are particularly important to
understand the structural margin of safety for the FRS. Data on the potential for alkali reactivity
associated with the aggregate used in the FRS are also of interest,
particularly if the life of the FRS were to be as long as about 20 years.
Because of the slow recovery of the
watersheds and because removal of the FRS will be a major, expensive
undertaking, the FRS will be part of the safety basis for TA-18 for the
foreseeable future, perhaps until the relocation of the TA- 18 mission is
complete. Effective implementation of inspection
and maintenance plans will be an essential component of the TA-18 safety basis.
LANL continues to assess the risk of
flooding by updating the hydrologic models with the changing watershed
conditions. The modeling group is
currently funded as part of the Cerro Grande recovery efforts, but those funds
expire in fiscal year (FY) 2003. While
the plan is to shift the responsibility for flood modeling to the hydrology
group at LANL, DOE does not yet have a plan for funding of the work for FY 2004
and beyond. Substantial uncertainty
remains in the models because of the lack of field data and the slow watershed
recovery.