United
States
Nuclear Regulatory Commission
Washington, DC
November 14, 2000
Dr.
Edwin S. Lyman, PhD
Scientific
Director
Nuclear
Control Institute
Washington,
D. C. 20036
Dear
Dr. Lyman:
I
am responding to your October 19, 2000, letter to Chairman Richard A. Meserve,
concerning the susceptibility of ice condenser containments (ICCs) to early
failure from hydrogen combustion during a severe accident. I want to emphasize
that plants with ICCs meet NRC design-basis requirements. Severe accident
studies evaluate low probability hypothetical accidents beyond the design basis
to gain additional insights into a specific plant's design features. In your
letter you discussed a severe accident study released by Sandia National Laboratories,
"Assessment of the DCH [Direct Containment Heating] Issue for Plants with
Ice Condenser Containments" (NUREG/CR-6427). The study was performed, as
part of the Office of Nuclear Regulatory Research's program, to resolve certain
severe accident phenomena that were identified in risk analyses as potentially
significant. The study concluded that ICC plants are more vulnerable to early
containment failure than large dry containments, but that this vulnerability is
not due to DCH; rather, it is attributable to hydrogen combustion phenomena
during station blackout events.
Even
though the vulnerability of ICC plants was judged to be higher for particular
severe accident sequences, the overall safety of the plants remains adequate
considering the probabilities of these events in the context of the
Commission's safety goals. The key finding of the report was that early
containment failure in ICC plants is dominated by hydrogen combustion which
largely depends on plant-specific probabilities for station blackout. As you
stated, ICC plants have igniter systems for hydrogen control and these systems
are not operable during station blackout events. The NRC staff shares your
thoughts regarding the need to evaluate the functionality of hydrogen igniters
during station blackout at ICC plants through the generic safety issue program.
The NRC staff informed the Commission of our intention to perform such an
evaluation consistent with the policy discussion on backfit considerations in
SECY -00-0198, dated September 14, 2000.
Your
letter also discussed the use of nuclear power plants with ICCs in the
Department of Energy's (DOE's) Material Disposition Program for excess
weapons-grade plutonium. In public interviews, the NRC staff reviewing the
Sandia report has stated that an accident in a plant using mixed
plutonium-uranium oxide (MOX) fuel would not challenge the containment
significantly more than an accident in a plant using conventional fuel. But
this is only a preliminary conclusion, and we have begun a research program to
corroborate it and prepare a technical basis for reviewing license amendments
associated with the use of MOX fuel. The research program is 'described in a
February 11, 2000, memorandum from the Executive Director for Operations to
Chairman Meserve and the Commissioners (enclosed). The research program plans
to obtain international fission product release data through the NRCs
cooperative severe accident research program to confirm the similarity of MOX
and uranium-based fuel with regard to source terms.
At
this time, the NRC staff cannot respond to the technical issues you raise about
MOX fuels because those parties developing plant-specific designs and analyses,
including plant-specific consequence analyses, have not finished their work. In
fact, it is our understanding that the industry is not likely to submit the
application for lead test assemblies to the NRC before August 2001 and the
application for batch irradiation of MOX fuel before late 2003. Issues, such as
you have raised, will be considered when we review the issue of using ICC
plants to irradiate MOX fuel.
You
note in your letter that you have asked the Department of Energy (DOE) to
consider the findings of NUREG/CR-6427 in its November 1999 Surplus Plutonium
Disposition Final Environmental Impact Statement. We have no comment on your
request to DOE at this time.
Sincerely,
Samuel
J. Collins, Director
Office
of Nuclear Reactor Regulation
Enclosure: As stated