[DNFSB
LETTERHEAD]
June 1, 2005
Mr.
Charles E.
Anderson
Principal Deputy Assistant
Secretary for
Environmental Management
U.S. Department of Energy
1000 Independence Avenue, SW
Washington, DC 20585-0113
Dear Mr. Anderson:
The Defense Nuclear Facilities
Safety Board (Board) has been following closely the effectiveness
of the high-level waste tank integrity program for
double-shell tanks (DSTs)
at the Hanford Site. In letters dated August 29, 2000, November 15,
2002, and January
18, 2005, the Board noted problems associated with preserving both tank leak
integrity and tank
structural integrity.
The Board was
encouraged that the
contractor at the Hanford tank farms
convened
an expert panel,
composed o f nationally known chemistry and
corrosion experts, to establish initiatives for the chemistry optimization of DSTs. The
Board’s staff reviewed
the experimental
test plan for corrosion studies recommended by the panel―Test Plan for Evaluating the Corrosivity of Double Shell Tank 241-AN-107 Waste. As
a result of the experimental
tests defined in
his test plan, tank chemistry limits will likely be revised. The staff concluded that the test plan
is adequate for optimizing
chemistry in the supernate and sludge of high-level waste tanks to minimize corrosion that
could lead to leaks.
Recently, the Board’s staff
learned o f new data on corrosion in the vapor
space regions in DSTs
(reported at the National Association of Corrosion Engineers [NACE]
Corrosion 2005 Meeting
in Houston, Texas, in a paper entitled
“The Application of Electrochemical Noise Based Corrosion Monitoring to Nuclear
Waste Tanks Vapor Space
Environments at the
Hanford Site”).
These data revealed localized crevice
corrosion that occurred between 1 and 5 months after insertion of
electrochemical noise probes into the vapor space
of Tanks AN-104, AN-105, and AN-107 at Hanford. A sample
removed from the
vapor space of Tank AN-107 showed crevice corrosion had occurred
on the electrochemical noise probes. The Board is concerned that these new
findings indicate a potential for vapor space corrosion in DSTs.
Additionally, the current experimental test plan recommended
by the expert panel does not include tests that simulate vapor space conditions.
Tank AN-107 will be the first tank whose waste chemistry limits
may be revised on
the basis of results from the current experimental test plan. The Board is aware that multilevel, multisensor
corrosion monitors are to be placed
in Tank AN-107 in the sludge, in
the supernate,
at the liquid-air
interface, as well as in
the vapor space regions of the tank. However,
the Board believes it prudent to have a better
understanding of any impact on vapor space corrosion
before implementing revised waste chemistry limits.
The Board encourages DOE to
continue the laboratory studies identified in the experimental test plan on an expedited schedule. However, DOE should ensure that the panel is aware of these recent findings and provide panel
members the opportunity
to suggest additional tests in
the experimental program to help understand vapor space corrosion. The Board also requests that DOE provide
timely updates to
the Board’s staff regarding
laboratory test results and any changes to the test plan incorporating vapor
space corrosion
experiments.
There are risks
and benefits in
revising waste chemistry limits. The
benefits of reducing inhibitor additions to the tanks include saving the costs of making
the inhibitor additions, saving tank space, reducing processing costs at the Waste Treatment Plant, and possibly
reducing the number
of canister shipments
to a deep
geologic repository. However, the risk is
that one
or more tanks may
leak prematurely. Although emergency
tank space is maintained as an
immediate contingency should a tank leak
occur, there are
longer-term questions
of risk versus benefit that should be addressed before
revising waste chemistry limits. These include the safety impacts of
any leaks, what would be done with the supernate removed from one or several tanks that leaked, and
what would be the impact
of leaking tanks
on overall
operations of the high-level waste system. These are
questions that should be addressed before revising waste chemistry limits.
Pursuant to 42 U.S.C.
§ 2286b(d), the Board requests that DOE provide a briefing to the Board
within 90 days of the receipt of this letter on (1) the incorporation of vapor space data into the ongoing test programs for DSTs, and
(2) the risk
versus benefits of
revising waste chemistry limits.
Sincerely,
A. J.
Eggenberger
Acting
Chairman
c: Mr. Roy J. Schepens
Mr. Mark
B. Whitaker, Jr.