July 24, 2013
—The NRC’s independent Advisory Committee on Reactor Safeguards is recommending for publication a recent staff study on the effects of severe earthquakes on used fuel storage pools. The study based its analysis on a representative Mark I boiling water reactor and concluded that a severe earthquake has an extremely low (“approximately one-in-10-million-years”) likelihood of causing enough damage to the used fuel pool to release radioactive material to the environment.
The ACRS review of the staff’s “Consequence Study of a Beyond-Design-Basis Earthquake Affecting the Spent Fuel Pool for a U.S. Mark I Boiling Water Reactor
” concurs with the study’s findings that health effects from extremely severe earthquake scenarios are very low, even for high-density loadings of used fuel in the pool. The committee also agrees with the staff’s conclusion that expediting the transfer of used fuel to dry cask storage systems does not provide a substantial safety benefit.
A July 18 letter from the ACRS
to the commission says its review demonstrates that the study was “thorough and systematic” and provides a state-of-the-art assessment of the accident consequences of a beyond-design-basis earthquake on the used fuel pool of a reference General Electric Mark I boiling water reactor of the type operated at Fukushima Daiichi.
The agency intends to incorporate the results of this study into a broader regulatory analysis of severe accident consequences to used fuel pools at all U.S. nuclear reactors.
The ACRS letter concludes that the study provides “sound approaches, tools and insights” to extrapolate severe seismic event consequences to used fuel pools of different designs and will be “valuable” in determining whether expedited transfers to dry cask storage systems produce safety benefits to other U.S. boiling water and pressurized water reactors.
The letter says an “important insight” is that reducing the density of fuel loading should be explored as an additional defense-in-depth measure. The ACRS notes that although the high-density loading scenario used in the study—in which one “hot,” recently discharged fuel assembly is surrounded by four cooler, low-power assemblies (the 1x4 configuration)—projects no pool leaks or radiation releases, the reference reactor actually uses a more favorable 1x8 configuration, which would significantly reduce any consequences of seismic damage.
“Checker-boarding used fuel in the pool—alternating newer and hotter assemblies with older and cooler ones—is one of many methods the industry already uses to enhance the ability to keep fuel cool and recover quickly from adverse conditions,” said Steven Kraft, NEI’s senior technical adviser.
“A combination of sturdy plant design and construction, ongoing monitoring and inspections, and the augmentation of already-redundant safety systems following the Fukushima accident ensures the continued safety and security of used fuel management,” he added.
Kraft said he welcomed ACRS’s concurrence with the study’s conclusion as further support for the environmental impact statement being prepared for the NRC’s new “waste confidence” determination, which assesses the environmental effects of storing used fuel at nuclear energy facilities between the end of a plant’s license term and the time the fuel is shipped off site for storage or disposal (see Nuclear Energy Overview, June 27
The NRC’s draft used fuel pool study, conducted as part of its Fukushima lessons-learned activities, is available for public comment. Instructions for submitting comments, which will be accepted until Aug. 1, are available in a July 2 Federal Register notice
—Nuclear Energy Overview