The Importance of Criticality in the Safety Analysis of the Spent-Fuel Waste Container
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THE IMPORTANCE OF CRITICALITY IN THE SAFETY ANALYSIS OF THE SPENT-FUEL WASTE CONTAINER WILLIAM G. CULBRETH* AND PAIGE ZIELINSKI** *Dept. of Civil and Environ. Engineering, University of Nevada, Las Vegas, NV 89154 "**Graduate Research Assistant
ABSTRACT The storage of high-level spent reactor fuel in a proposed national geologic repository will require the construction of containers to be placed in boreholes drilled into the host rock. Federal regulations require that the fuel be maintained subcritical under normal or accident conditions. This is determined through the calculation of a neutron multiplication factor, kfr, that must remain below 0.95. Criticality will play an important role in the container design, the internal configuration of the fuel, and the selection of neutron poisons. An analysis of k0n should be a normal step in the conceptualization of new waste container designs. Unlike thermal effects in a proposed repository, criticality will remain a problem long after the 10,000 year lifetime of the facility. In this study, nuclear criticality has been determined for the proposed spent fuel container in various situations that include varying fuel enrichment and partial air gap flooding. Results will be presented to demonstrate the impact of these variables on the design of a safe spent fuel container.
INTRODUCTION Site characterization is underway to select a suitable location for a national high-level nuclear waste repository. The repository will be required to isolate the nuclear waste generated by commercial power plants from the accessible environment for a period of 10,000 years. The current site characterization plan' calls for spent nuclear fuel assemblies to be loaded into containers approximately four meters in height and 0.70 meter in diameter. Each container will accommodate three intact pressurized water reactor (PWVR) fuel assemblies or six intact boiling water reactor (BWR) assemblies. Consolidation of the spent fuel assemblies is also under consideration. The interior will be filled with an inert gas. These containers will generate a significant amount of radioactive decay heat that will decrease
Mat. Res. Soc. Symp. Proc. Vol. 294.
Canster Wall(10
10
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Figure 1 Spent Fuel Canister and Air Gap
1993 Materials Research Society
74
with time, causing the surface of the container to stay above boiling for hundreds of years. An air gap with a thickness of 10 mm will surround each container to prevent the accumulation of condensed water that may lead to corrosion of the container wall. The spent fuel to be&Xxlaced in these containers will contain enough fissionable material to sustain a chain reaction, or criticality, under the proper circumstances and precautions must be taken to prevent this from occurring. Neutron criticality is quantified through knf the effective neutron multiplication factor. A sustained chain reaction will occur if klf exceeds one and regulation 10 CFR 60.131 requires that the multiplication factor remain below 0.95 in any normal
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