COATI. Optimized Cask Loading and Storage Planning of an ISFSI tool


The management of the spent fuel stored in the nuclear power plants fuel pools is a complex activity, since it involves the inventory of spent fuel assemblies, the capacity of the pool that houses them, the characteristics of the casks in which these assemblies must be loaded and the storage system of these casks. In addition, the acceptance criteria established in the casks Safety Analysis Reports and the dose limits established in the Safety Instruction IS-29 for temporary fuel storage facilities must be met.

In order to enable this activity, the COATI tool (Optimized Cask Loading and Storage Planning of an ISFSI) has been developed, a computer system that helps to carry out the aforementioned spent fuel management and minimize the plant’s operating costs after the final shutdown of the plant.

For COATI to function, a number of basic site-specific data are required to be initially loaded into the system. These data are: the full inventory of spent fuel assemblies with their characteristics (fuel assembly type, date of discharge, burnup, initial enrichment and physical integrity) and source terms, the design of the casks to be loaded, the authorized loading patterns, the characteristics of the ISFSI and the location of the point of interest.
It is also necessary to import into COATI the neutron and gamma response functions at the rack positions during loading and storage of the cask to be considered. These functions represent the dose rate per particle from the source at the point or area of interest and are calculated externally for a given location using a method based on the attached flux calculated with Denovo and MAVRIC from the SCALE 6.2 system.

Once the basic data are available, the system allows the user to design casks loading maps by optimizing the selection of the spent fuel assemblies to be loaded, so as to comply not only with the selected loading pattern, but also with a radiological criterion to be chosen by the user. This radiological criterion may be to maximize or minimize the dose rate in the surroundings of the cask or at a point of interest.

In addition, once the most suitable spent fuel assemblies have been selected, the system identifies the optimum location for each assembly among the cask positions, so as to minimize the operational doses received by the operators during loading works.

With all this, the system makes it possible to optimize the selection of those spent fuel assemblies that, once loaded into their respective casks, generate dose rates that meet the radiological requirements applicable to the storage facility, allowing the emptying of the spent fuel storage pool.

Finally, the system has an integrated viewer with a 3D model of the ISFSI that allows configuring the distribution of the casks designed in an ISFSI, as well as making casks loading schedules over time grouped in batches.

For a given cask arrangement, COATI allows the user to perform instantaneous and approximate calculations of the dose rate at the point of interest, choosing the date (present or future) at which the calculation is to be performed, making use of the previously generated response functions.

COATI also has a function to automatically generate the MAVRIC code input files for the execution of an accurate calculation, but with a longer execution time.

The results obtained with the approximate calculation show differences with respect to the accurate calculation of less than 5% in all cases, with the advantage that they can be executed instantaneously.

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