Lessons Learned from the Fukushima Nuclear Accident for Improving Safety and Security of U.S. Nuclear Plants Phase 2 (2016) / Chapter Skim
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7 Expedited Transfer of Spent Fuel from Pools to Dry Casks
7 Expedited Transfer of Spent Fuel from Pools to Dry Casks
Pages 155-190
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From page 155... ...
FINDING 4.11: The U.S. Nuclear Regulatory Commission has com pleted a technical analysis of spent fuel pool accident consequences to inform a regulatory decision on expedited transfer of spent fuel from pool to dry cask storage.
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From page 156... ...
. A few months after the Fukushima accident, USNRC staff initiated the Spent Fuel Pool Study, which examined the consequences of a beyonddesign-basis earthquake on a spent fuel pool that is similar in design to some of the pools at the Fukushima Daiichi plant (USNRC, 2014a)
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From page 157... ...
. The Spent Fuel Pool Study and Expedited Transfer Regulatory Analysis are described in Sections 7.2 and 7.3 below.
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From page 158... ...
7.2 SPENT FUEL POOL STUDY The Spent Fuel Pool Study analyzed the consequences of a beyonddesign-basis earthquake on a spent fuel pool at a reference plant4 containing a General Electric Type 4 boiling water reactor (BWR) with a Mark I containment.5 The USNRC describes this study as one in a continuing series of examinations of postulated spent fuel pool accidents (see Sidebar 7.1)
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From page 159... ...
No cost-benefit analyses were performed. The report concluded that "Deterministic evaluations in the staff's preliminary draft risk assessment indicated that zirconium cladding fires could not be ruled out for loss of SFP [spent fuel pool]
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From page 160... ...
. FIGURE 7.1 Scenarios analyzed in the Spent Fuel Pool Study.
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From page 161... ...
Reactor operating cycle phases:7 • OCP1: 2-8 days; reactor is being defueled. • OCP2: 8-25 days; reactor is being refueled.
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From page 162... ...
Low Density Spent Fuel Pool Fuel Loading (Regulatory Baseline) (Proposed Alternative)
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From page 163... ...
. Assuming no complications from other spent fuel pools/reactors or shortage of available equipment/staff, there is a good chance to mitigate the small leak event.
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From page 164... ...
notes that the human-error probabilities were estimated under the assumption that mitigation equipment was available, there was no simultaneous core damage or primary containment failure that caused inaccessibility of the refueling floor, and there was sufficient staff to deploy the spent fuel pool mitigation strategy. If the earthquake caused damage in multiple reactors and spent fuel pools, such as occurred at the Fukushima Daiichi plant (see Chapter 2 of this report)
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From page 165... ...
The Spent Fuel Pool Study (USNRC, 2014a, Appendix D) also included a regulatory analysis for the reference plant.
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From page 166... ...
. The Expedited Transfer Regulatory Analysis described in this chapter is an example of a regulatory action that is subject to safety goal screening.
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From page 167... ...
. The USNRC identified three benefits of this alternative: less long-lived radionuclide inventory in the spent fuel pool, lower heat load in the pool, and a small increase in the initial water inventory in the pool (because water would displace the fuel assemblies that were moved from the pool to dry cask storage)
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From page 168... ...
. The analysis considered eight types of initiating events that were judged to have the potential to lead to the loss of cooling in spent fuel pools: seismic events, drops of casks and other heavy loads on pool walls, loss of offsite power, internal fire, loss of pool cooling or water inventory, inadvertent aircraft impacts, wind-driven missiles,15 and failures of pneumatic seals on the gates in the spent fuel pools (USNRC, 2013, Table 43)
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From page 169... ...
10) A regulatory analysis would normally be terminated once USNRC staff determined that the alternative action (in this case expedited transfer)
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From page 170... ...
A key constraint for assessing the consequences of severe spent fuel pool accidents is the guidance for assessing offsite impacts: "In the case of nuclear power plants, changes in public health and safety from radiation exposure and offsite property impacts should be examined over a 50-mile distance from the plant site." (USNRC, 2004, p.
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From page 171... ...
$2,000 per person-rem for the base case and $4,000 per person-rem for the sensitivity analysis; 18 The cost of replacement power is the difference between the cost of electricity from the shutdown nuclear reactor and the next least-costly available generating source. The Spent Fuel Pool Study assumed that only the nuclear plant where the accident occurred would be taken out of operation.
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From page 172... ...
7.4 DISCUSSION The USNRC staff put a great deal of thought and effort into the development of the Spent Fuel Pool Study and Expedited Transfer Regulatory Analysis and the explication of their results. The staff also spent a good deal of time presenting the results of these analyses to the committee and responding to follow-up questions.
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From page 173... ...
7.4.1 Sabotage Risks The Expedited Transfer Regulatory Analysis considered a large number of initiators for spent fuel pool accidents (see Section 7.3) ; the analysis did not include initiators for spent fuel pool sabotage.
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From page 174... ...
, even though these requirements are a condition of every nuclear plant operating license in the United States. The Spent Fuel Pool Study and Expedited Transfer Regulatory Analysis considered beyond-design-basis severe accidents where required mitigation actions failed to be completed successfully.
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From page 175... ...
. Representative results for the frequencies and probabilities for the "no-mitigation" scenario in the Spent Fuel Pool Study are shown in the second column in Table S7.1.
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From page 176... ...
The following three conditional probabilities could have correlated and high numerical values if knowledgeable and determined saboteurs attack the plant in certain ways during certain parts of its operating cycle: • P(loss of offsite power | sabotage) , • P(operating cycle vulnerability | loss of offsite power & sabotage)
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From page 177... ...
254-255) The Expedited Transfer Regulatory Analysis did not examine the safety or sabotage risks of dry cask storage.
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From page 178... ...
9) In fact, a spent fuel pool accident can result in large radioactive material releases, extensive land contamination, and large-scale population dislocations.
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From page 179... ...
EXPEDITED TRANSFER OF SPENT FUEL FROM POOLS TO DRY CASKS 179 B C FIGURE 7.3 Continued
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From page 180... ...
A spent fuel pool accident can result in large releases of radioac tive material, extensive land interdiction, and large population displacements.
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From page 181... ...
.28 In fact removing the 50-mile limit and raising the value of the averted dose conversion factor to $4,000 per person-rem increased the base-case average estimated benefits of expedited transfer by a factor of 5.9, that is, from about 13 percent of the estimated costs of expedited transfer to about 80 percent.29 Moreover, for the 20 reactors with shared spent fuel pools and the four AP1000 reactors currently under construction (see Section 7.3) , the base-case benefits were found to exceed the costs of expedited transfer (i.e., expedited transfer would have been cost beneficial)
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From page 182... ...
701 (170-1,802) NOTE: These results are averaged over the four spent fuel pool groups, weighted by the number of pools in each group, and have not been weighted by release frequencies.
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From page 183... ...
7.4.4 Bounding Assumptions The USNRC staff used numerous bounding assumptions in the Expedited Transfer Regulatory Analysis to "ensure that design, operational, and other site variations among the new and operating reactor fleet were addressed and to generally increase the calculated benefits from the proposed action" (USNRC, 2013, p. 7 in Memorandum to Commissioners)
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From page 184... ...
Table 7.2 shows selected accident consequences and cost estimates for a base-case scenario for the Expedited Transfer Regulatory Analysis. Also shown are the ranges of low and high estimates from the analyses.
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From page 185... ...
2. The safety and sabotage risks for dry cask storage.
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From page 186... ...
The decision to expedite the transfer of spent fuel from pools to dry casks is a policy decision for the USNRC, not the task of this study. The committee's critiques of the Spent Fuel Pool Study and Expedited Transfer Regulatory Analysis are intended to strengthen the quality of any future analyses of spent fuel pool storage risks to support sound decision making by the USNRC and nuclear industry.
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From page 187... ...
31) In response to a question from the public about whether the results of the Expedited Transfer Regulatory Analysis (USNRC, 2013)
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From page 188... ...
The control-volume approach of MELCOR, which was used in the Spent Fuel Pool Study (USNRC, 2014a) , is poorly suited for modeling these types of flows.
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From page 189... ...
The assumptions and methodology used in the regulatory analysis for Generic Issue 82 are similar to those used in USNRC (2014a) : A seismic event is considered the most likely initiator of the accident and spent fuel pool damage frequency is taken to be about 2 × 10–6 events per reactoryear.
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From page 190... ...
took more credit for the operating cycle in reducing the risks of zirconium cladding fires. A more pessimistic view on the uncertainties of modeling spent fuel pool loss-of-coolant accidents was expressed by Collins and Hubbard (2001)
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