Loading Optimization Algorithm for Solving Assembly Assignment Problems with Uncertainties in Decay Heat Powers in the Final Disposal of Spent Nuclear Fuel in Finland

In Finland, spent nuclear fuel assemblies are placed in copper-iron canisters that are disposed of in a deep underground disposal facility in Olkiluoto. The decisions on which fuel assemblies to assign to which disposal canisters can be determined by formulating and solving a mathematical optimization problem, which is called an assembly assignment problem. New requirements have emerged, implying new objectives, constraints, and data for the problem.

In this paper, the heuristic disposal canister loading optimization algorithm is developed further to solve the refined problem. One major advancement in the new version of the algorithm is that it takes into account the uncertainty of the decay heat power of a disposal canister. This uncertainty is due to the uncertainties in the decay heat powers of the fuel assemblies assigned to the disposal canister and possible dependencies between these uncertainties.

The other major advancement is that the new version of the algorithm manages the spent nuclear fuel interim storage efficiently according to new requirements. To get a prediction of how the disposal of all fuel assemblies for a certain fuel type can succeed, the new version of the algorithm is capable of simulating the loading of all disposal canisters with the current data by assigning fuel assemblies to the disposal canisters iteratively one batch of the encapsulation plant at a time. In Finland, the safe and cost-efficient assignment of fuel assemblies to disposal canisters is facilitated by the developed loading optimization algorithm.