Year
2010
File Attachment
Abstract
The improvements in the fuel designs and fuel performance in the operating nuclear power plants have led to an increase in the discharge burnup of the used fuel assemblies. This increase in burnup also has resulted in the design of used fuel storage and transportation systems to accommodate these discharged fuel assemblies. The impact of higher burnups on the design of transportation packages offers unique challenges from a dose rate and radiation shielding perspective. The transportation cask radiological design is characterized by the applicable dose rate limits on and around the package surface under Normal Conditions of Transport (NCT) and Hypothetical Accident Conditions (HAC). For most transport cask designs, the 10 mrem/hour at 2m from the vehicle edge at NCT is the controlling dose rate limit. Used fuel assemblies with acceptable combinations of burnup, enrichment and cooling time (BECT) are considered eligible for loading in a transportation cask. Fuel qualification is the method of selecting these acceptable BECT combinations for loading in a transportation cask. The increase in fuel assembly burnup results in an increase in the decay heat of the fuel assembly and also results in an increase in the neutron source term of the fuel assembly. Therefore, cask designs to accommodate fuel assemblies with higher burnups need to be modified to enhance neutron shielding. Zone loading of fuel assemblies to effect self-shielding within the basket is also necessary for this purpose. Finally, the geometry and material modeling uncertainties in the shielding models need to be reduced to provide additional margins in the dose rate calculations. This paper examines the important considerations for qualification of high burnup fuel assemblies from a radiation protection perspective. This paper also evaluates the impact of higher burnups on the source terms and discusses the enhancements to the dose rate calculation methods. The qualification of fuel with higher burnups is truly an optimization problem that balances shielding material design, fuel qualification and shielding analysis methodologies.