Abstract
Placing spent fuel into dry cask storage as soon as possible allows decommissioning plants to lower site risk, reduce costs associated with management and maintenance of the spent fuel pool, and accelerate decommissioning. The SENTRY™ Spent Fuel Management System1 is a dual-purpose system satisfying requirements for both storage and eventual transportation of the spent fuel. This paper describes the features and characteristics of the SENTRY system and explains the key areas where the product family represents a step change from existing industry offerings.
Fuel being loaded into casks within times as low as 1.5 to 2 years presents substantial new challenges to electric utilities and cask vendors. Decay heat falls off as a sum of exponentials and is much higher at early times. By the 1.5 to 2 year timeframe, heat loads are decreasing on the order of 10% per month. Radiological source terms are not only falling in strength, but are also changing in energy intensity and type. Harder neutron and photon spectra at these earlier times must also be addressed.
Thermal and radiological source term data for the SENTRY system have been established through the use of an NRC-approved Westinghouse data set characterizing fuel over a wide range of burnup, enrichment, and decay time values. This data is used to construct cooling tables which provide loading rules in the form of the minimum cooling time required to load the assembly in a particular zone of a SENTRY canister. No further plant-specific calculations or demonstrations are required by the utility.
Unique to Westinghouse is the use of adjoint methods to develop shielding results, which are de-coupled from the source term associated with specific assemblies. As the industry’s leading performer of Reactor Vessel Surveillance Programs, Westinghouse makes routine use of a combined forward/adjoint approach to radiation transport. The adjoint shielding methodology allows for the explicit calculation of dose rates over a very large range of source term values with essentially no additional computation. As a result, Westinghouse can make use of its ADSORB computer code to develop shielding results for the full set of enrichment, burnup, and decay times described above. ADSORB also uses an equivalent process to determine the earliest time that ensures the fuel’s decay heat will fall below the analyzed values used to demonstrate acceptable thermal performance. The use of this methodology allows Westinghouse to establish the shortest overall time for which fuel can safely be placed into defined regions of a spent fuel canister.
