[DNFSB LETTERHEAD]
February 6, 2003
The Honorable Jessie Hill Roberson
Assistant Secretary for Environmental
Management
U.S. Department of Energy
1000 Independence Avenue, SW
Washington, DC 20585-0113
Dear Ms. Roberson:
The Defense Nuclear Facilities Safety Board
(Board) continues to follow efforts of the Department of Energy (DOE) and its
contractor to safely stabilize deteriorating spent nuclear fuel stored in the
K-East and K-West Basins at Hanford, in accordance with the Secretary’s
Implementation Plan for Recommendation 94-1, Improved Schedule for
Remediation in the Defense Nuclear Facilities Complex, and Recommendation
2000-1, Prioritization for Stabilizing Nuclear Materials.
Prompted by a leak test failure, the Board,
in its letter of February 15, 2002, discussed deficiencies in the process for
mechanically sealing the spent fuel containers, or Multi-Canister Overpacks
(MCOs). MCOs are mechanically sealed in
the K-West Basin, using closure bolts, to allow transport to the Cold Vacuum
Drying Facility for processing, followed by transport to the Canister Storage
Building for interim storage and final sealing via welding. MCOs 1 through 40 have been removed from the
K-West Basin without a torque check of the closure bolts and, even having
passed a leak check, the continued adequacy of the mechanical seals cannot be
ensured. An inadequate mechanical seal
could allow the inert atmosphere of the MCO to be lost prior to welding,
leading to the introduction of oxygen and possibly violent reactions and/or
explosions.
The Board’s letter requested information
regarding accelerated welding of the 40 MCOs with suspect seals. The DOE response letter, dated April 18,
2002, committed to accelerate welding of the first 40 MCOs, after checking
closure bolt torque and ensuring that any MCOs which failed the torque check
were properly sealed and inerted. In
addition, DOE added a requirement to check the closure bolt torque for
subsequent MCOs at the K-West Basin.
In May 2002, MCO 63 failed the leak test at
the Cold Vacuum Drying Facility after having passed the required torque check
at the K-West Basin. A subsequent torque check of MCO 63 at the Cold Vacuum
Drying Facility indicated significant torque relaxation from the values
measured during the torque check at the K-West Basin. It should be noted that there will be lifting stresses imposed on
the mechanical seal during movements at the Canister Storage Building that
might contribute to torque relaxation.
In addition to leak test failures, a small
population of MCOs that passed the leak test exhibited leakage rates several
orders of magnitude higher than most other MCOs. These higher than normal leak rates raise questions concerning
the adequacy of the mechanical seal.
In discussions with DOE and its contractor,
the Board’s staff has communicated its concern with further leak test failures
and higher than normal leak rates, and has suggested that all MCOs be
retorque-checked prior to welding.
However, the current welding procedure requires verification of torque
only for MCOs that did not receive an initial torque check in the K-West Basin.
Because of the continuing uncertainty regarding the adequacy of the mechanical
seals, the Board believes that this is not a conservative approach.
The Board believes it would be appropriate
for DOE to require a final torque check of closure bolts for all MCOs prior to
welding, as well as appropriate corrective actions to ensure that MCOs that
fail the torque check are properly sealed and inerted. Therefore, pursuant to 42 U.S.C. §2286b(d), the Board requests that DOE
provide a report within 15 days of receipt of this letter that provides
assurance that MCOs are properly sealed and inerted prior to welding, and the
proposed disposition of any MCOs that have already been welded.
Sincerely,
John T. Conway
Chairman
c:
Mr. Keith A. Klein
Mr. Mark B. Whitaker, Jr.