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November 12, 1999
Dr. William D. Travers
Executive Director for Operations
U.S. Nuclear Regulatory Commission
Washington, D.C. 20555-0001
Dear Dr. Travers:
SUBJECT: SPENT FUEL FIRES ASSOCIATED WITH DECOMMISSIONING
During the 467th meeting of the Advisory Committee on Reactor Safeguards, November 4-6,
1999, we reviewed a draft report of a technical study prepared by the NRC staff on the spent
fuel pool accident risk at decommissioning plants. During our review, we had the benefit of
discussions with representatives of the NRC staff, the Nuclear Energy Institute (NEI), and two
members of the public. We also had the benefit of the documents referenced.
Background
The staff discussed with us the status of its ongoing work on this issue. We appreciate the
opportunity to provide our views on the direction of this effort at this interim stage.
The staff has formed a Technical Working Group with the objective of assessing the risks
associated with spent fuel pools for decommissioning plants. The intent is to assist the Office
of Nuclear Reactor Regulation in developing an integrated rule for decommissioning, to provide
guidance for interim exemption requirements, and to identify areas where additional work is
needed.
Fuel removed from a reactor must be covered with water for cooling until its decay heat
generation rate falls below a critical value. Risks posed by fuel stored in a pool arise from
the possibility that this water cooling may be lost. The staff has a two-fold approach to
evaluating the issues of spent fuel storage: (1) develop estimates of the decay time required
to avoid runaway oxidation of spent fuel clad in the event of accidental uncovery, and (2) develop
a risk assessment using a broad set of initiating events and using the end-state consequence of
uncovery to the top of the fuel.
NEI has interacted with the staff on this effort and has provided a review of the draft report
entitled, "Technical Study of Spent Fuel Pool Accidents for Decommissioning Plants." NEI
provided us with its assessments. Our understanding of the more substantive issues raised by
NEI is:
Conservatism, especially in human error rates, has skewed the preliminary risk insights.
The choice of uncovery to the top of the fuel as the endpoint is difficult to relate to
public risk. NEI believes that the analyses should be carried all the way to postulated
runaway oxidation.
The cladding temperature used as the threshold for onset of runaway oxidation is too
low.
We also had benefit of the remarks by a member of the public who expressed concern about
the:
- Degree of public participation in this effort
- Acceptability to the public of PRA (probabilistic risk assessment) based
regulations
- Lack of sufficient margins and defense-in-depth
- Severity of the consequences
- Vulnerability to terrorism
- Applicability of the database used for equipment failures
- Potential for recriticality
Conclusions and Recommendations
We agree with the general approach for determining the decay time beyond which
runaway oxidation cannot occur. However, an uncertainty analysis related to the
oxidation kinetics and the heat rejection mechanisms is needed. The present analysis
is limited to relatively low-burnup levels and associated clad hydriding and oxidation.
There are no experimental data on the behavior of realistic fuel and cladding under
representative conditions. Either very conservative choices will have to be made for
decay times or additional experimental research will have to be conducted.
We support the staff's approach to developing a decay heat critical temperature for the
onset of runaway oxidation. Uncertainties in these analyses need to be quantified and
factored into any decisions regarding the required decay time.
PRAs should be as realistic as possible. The staff should reevaluate the basis for its
choices particularly for human error rates. We agree with the staff's proposal to use
expert opinion to validate or modify the human reliability analyses to ensure that the
analyses are not overly conservative.
Arguments about conservative versus realistic values are aggravated when point
estimates are used for the input parameters to the risk assessments. As stated in our
December 16, 1997 report, we believe that uncertainties can be best addressed by
expressing the inputs as probability distributions rather than point estimates. Such
distributions are easier to defend. In addition, the insights to be gained from the risk
analysis would greatly benefit if the results were presented as distributions.
We agree with the choice of uncovery to the top of the fuel as being an appropriate end
state for the PRA consequence analysis. The database on air oxidation kinetics for high-
burnup fuel, subsequent fuel damage behavior, and fission product release is too sparse
and the uncertainties too great to provide confidence in carrying the analyses any
farther. The acceptable frequency of this end point can be based on consideration of the
health consequences resulting from postulated fuel failures. Because prompt fatalities
cannot be ruled out, we recommend that the acceptable frequency for this end point be
the same as that for large, early release frequency in Regulatory Guide 1.174, which is a
surrogate for the prompt fatality Safety Goal.
With the choice of uncovery as the end state of the analysis, the uncertainties due to model
inadequacies associated with fire risk assessment are not large. We believe that the spent fuel
fire issue would be a good candidate for testing the development of a rationalist regulatory
approach, as discussed in our May 19, 1999 report.
We look forward to reviewing the staff's progress in this area.
Sincerely,
/s/
Dana A. Powers
Chairman
References:
Draft report entitled, "Technical Study of Spent Fuel Pool Accidents for Decommissioning
Plants," prepared by NRC Technical Working Group, June 1999.
A Review of Draft NRC Staff Report: "Draft Technical Study of Spent Fuel Pool
Accidents for Decommissioning Plants," prepared by ERIN Engineering and Research,
Inc., for Nuclear Energy Institute, dated August 27, 1999.
Draft (undated) EPRI Technical Report, "Evaluation of Spent Fuel Pool Seismic Failure
Frequency in Support of Risk Informed Decommissioning Emergency Planning,"
prepared by Duke Engineering & Services.
Letter dated September 3, 1999, from Mr. David A. Lochbaum, Union of Concerned
Scientists, to NRC Commissioners, Subject: Inadequately Monitored Spent Fuel Pool
Temperature and Operator Response Times at Permanently Closed Plants.
U.S. Nuclear Regulatory Commission, Regulatory Guide 1.174, "An Approach for Using
Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to
the Licensing Basis," July 1998.
ACRS report dated December 16, 1997, from R. L. Seale, Chairman, ACRS, to Shirley
Ann Jackson, Chairman, NRC, Subject: Treatment of Uncertainties Versus Point Values
in the PRA-Related Decisionmaking Process."
ACRS report dated May 19, 1999, from Dana A. Powers, Chairman, ACRS to Shirley
Ann Jackson, Chairman, NRC, Subject: The Role of Defense in Depth in a Risk-
Informed Regulatory System.
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