Benchmark Critical Experiments and FP Worth Evaluation for a Heterogeneous System of Uranium Fuel Rods and Uranium Solution Poisoned with Pseudo-Fission-Product Elements

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A series of critical experiments were performed using heterogeneous cores at the Static Experiment Critical Facility (STACY) of Japan Atomic Energy Agency (JAEA) in order to obtain systematic benchmark data concerning the dissolving process in a reprocessing plant. Focusing on the introduction of the burn-up credit, critical mass measurement was conducted for a combination of uranium dioxide fuel rods (5 wt% 235U) and uranyl nitrate solution (6 wt% 235U) poisoned with pseudo-fission-product (FP) elements—samarium, cesium, rhodium, and europium. Fuel rods were arrayed at a 1.5-cm lattice interval in the poisoned fuel solution in a 60-cm-diameter cylindrical tank. The uranium concentration of the solution was roughly kept at about 320 gU/L, and the FP element concentrations were adjusted to be equivalent to that in a burn-up of about 30GWd/t. The result provides basic experimental data for validation of computational methods to evaluate the reactivity effect of each FP element, as well as benchmark criticality data for validation of neutron multiplication factor calculation for heterogeneous systems of spent fuel. In this report, the details of the experiments and benchmark models will be presented as well as the procedure and the result of separate reactivity worth evaluation for each FP element. The experimental results and the computational evaluation results will also be compared.