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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6 Loss-of-Coolant Events in Spent Fuel Pools
6 Loss-of-Coolant Events in Spent Fuel Pools
Pages 132-154
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From page 132... ...
These technical studies also illustrate the importance of maintaining water coolant levels in spent fuel pools so that fuel assemblies do not become uncovered. FINDING 4.6: Additional analyses and physical experiments carried out by the U.S.
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From page 133... ...
sponsor an end-to-end validation of the MELCOR code for use in modeling coolant loss in spent fuel pools, and (2) validate key submodels in the code with particular attention paid to • Modeling the thermal and chemical behavior of spent fuel assemblies in partially drained pools.
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From page 134... ...
6.1 RESPONSE OF SPENT FUEL POOLS TO LOSS-OF-COOLANT EVENTS The U.S. Nuclear Regulatory Commission (USNRC)
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From page 135... ...
and computational fluid dynamics (CFD) models have been used to examine spent fuel pool behavior in water-filled, partially drained, and fully drained pools.
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From page 136... ...
Some of the recent work at Sandia National Laboratories validates some portions of the MELCOR application to spent fuel pools, but validation does not appear to have been carried out for aspects involving natural convection, coupled pool-fuel assembly behavior, or spray cooling. In the absence of three- dimensional calculations, it is not possible to quantify how the omission of a momentum equation and multidimensional buoyancy-driven flow affects model results for a variety of spent fuel pool scenarios.
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From page 137... ...
of spent fuel assemblies in the pool, even if the fuel were damaged in an attack? • In the event of a localized zirconium cladding fire, will such rearrange ent prevent its spread to the rest of the pool?
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From page 138... ...
Some details of the analyses, for example, the particular conditions under which fuel damage and zirconium cladding fires can occur, as well as the timing of such occurrences, are not provided in this report because they are security sensitive. 6.1.1 Physical Experiments In the late 1990s, USNRC staff evaluated spent fuel pool accident risks at decommissioned nuclear plants in the United States.
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From page 139... ...
The integral-effects tests used five one-third-length zirconium fuel assemblies arranged in a 1 × 4 configuration (i.e., a center assembly and four face assemblies; see Figure 7.2 in Chapter 7) in a 3 × 3 pool rack.
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From page 140... ...
Nevertheless, the MELCOR model still has several limitations: • It cannot model stratified flow or buoyancy-driven flow (i.e., for mation of plumes and circulatory flow patterns in large spent fuel pools or above-pool environments) or open-rack configurations in which fuel assemblies are not contained in solid-wall boxes as is current practice in dense-packed pools.
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From page 141... ...
. 6.1.2.2 Fuel Configuration in the Pool Sandia analysts used MELCOR to examine five configurations of fuel storage in spent fuel pools: 1.
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From page 142... ...
Sandia analysts used MELCOR to investigate the effect of leak size and location on air coolability limits of BWR and PWR assemblies stored in spent fuel pools. Two scenarios were considered: • Complete loss of coolant, where the pool was assumed to drain below the base plate of the spent fuel racks, allowing for natural convection of air through the assemblies, and • Partial loss of coolant, where the pool was not drained completely, so water covered the lower portions of the fuel assemblies.
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From page 143... ...
6.1.2.5 Radial Thermal Coupling Heat transfer from hot fuel assemblies to adjacent cold assemblies in a spent fuel pool plays an important role in the propagation of zirconium fires and the consequent release of radioactive material from the fuel. Radiation is the dominant mode of heat transfer at high temperatures, so accuracy in the calculation of radiative heat transfer is important for estimating assembly temperatures.
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From page 144... ...
To the committee's knowledge, this scenario has not been evaluated by the USNRC or Sandia. Sandia has carried out hand calculations to assess the coolability of debris beds formed from the relocation of degraded fuel rods, cladding, and structural material to the bottom of a spent fuel pool.
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From page 145... ...
However, prior to the onset of convective air flow, fuel cladding temperatures can exceed the threshold for oxidation, and fuel damage and radioactive material release can occur. The time to damage and release depends on pool water depth relative to the stored fuel assemblies.
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Sandia used MELCOR to determine the desired flow rate of makeup water under different accident scenarios. In the case where a spent fuel pool drains completely, adding makeup water will cover the lower portions of the fuel assemblies and block air convection.
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From page 147... ...
.9 Section B.5.b of the order directed nuclear plant licensees to develop and implement strategies to maintain or restore core, containment, and spent fuel pool cooling capabilities following large explosions or fires that damaged large areas of the plant. The order 9 The order is designated as Safeguards Information and has not been released to the public, but its requirements have been codified in 10 CFR 50.54(hh)
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From page 148... ...
of makeup water to the plant's spent fuel pools for 12 hours.11 The balance of the strategy involves the use of a portable spent fuel pool makeup capability as well as a 200 gpm spray capability from that same water source to enhance the robustness and flexibility of site responses. The Fukushima Daiichi accident renewed and heightened interest in the potential vulnerability of spent fuel pools to extreme natural events (see Chapter 2 of this report)
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From page 149... ...
The portable equipment would be staged on site and could also be brought in from regional staging facilities. If pool water levels cannot be maintained above the tops of the fuel assemblies, then portable pumps and nozzles would be used to spray water on the uncovered fuel assemblies.
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. The spent fuel pools in Units 1-4 of the Fukushima Daiichi plant were not accessible after the hydrogen explosions because of debris and high radiation levels.
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Simulations were carried out to examine mitigation strategies for representative spent fuel pools in PWR and BWR reference plants. The following mitigation strategies were examined: makeup water, pool leak repair, fuel dispersion, emergency sprays, building ventilation, and pool configuration.
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From page 152... ...
Experimental measurements of pressure drops in a p rototypic BWR fuel assembly were used to provide realistic correlations for flow resistance within the fuel bundle. Additional MELCOR Analyses of BWR Spent Fuel Pool Assembly Accident Response K.C.
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From page 153... ...
MELCOR was used to examine the effects of varying the following parameters on peak cladding temperatures within fuel assemblies of a single PWR spent fuel pool: decay heat, gas speed and temperature, oxide-layer thickness, flow resistance, rod ballooning, oxidation kinetics, rack configuration, and water level. For multiple fuel assemblies, the effects of fuel dispersion, including 1 × 4 and checkerboard (see Figure 7.2 in Chapter 7)
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From page 154... ...
Investigations of Zirconium Fires during Spent Fuel Pool LOCAs: PWR Assemblies Division of System Analysis, Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission; June 24, 2013; presentation to committee; 23 pages.
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