Monitors for the Prediction of Alternate Nuclear Material Concentrations for Pressurized Water Reactor Spent Fuel

A methodology for determining alternate nuclear material (²³⁷Np, ²⁴¹Am, and ²⁴³Am) concentrations in spent nuclear fuel based on the use of various monitors was developed and validated for use with several pressurized water reactor fuels. The monitors studied included the fuel burnup, the total plutonium concentration, the ²⁴⁰Pu/²³⁹Pu isotopic ratio, the ¹⁴⁸Nd/²³⁸U isotopic ratio, and the ¹³⁷Cs activity. Calculations were performed using the HELIOS-1.4 lattice physics code for spent fuel from the Mihama Unit 3, Genkai Unit 2, and Calvert Cliffs Unit 1 reactors. These calculations were compared to measured values for the fuel. It was determined that the ²⁴⁰Pu/²³⁹Pu isotopic ratio and the ¹³⁷Cs activity were the most useful and accurate for use in predicting alternate nuclear material concentrations at reprocessing facilities for safeguards purposes. Based on these comparisons, it was determined that measurements of these monitors would allow for determination of ²³⁷Np, ²⁴¹Am, and ²⁴³Am concentrations to within ±4, ±6, and ±15%, respectively. It is suggested that these uncertainties may be decreased through improvements in measurement techniques and additional benchmarking. These monitors may be used to provide an accurate prediction of the concentrations of the alternate nuclear materials while decreasing the need for direct measurement of these isotopes. This will translate into a monetary savings for reprocessing facility safeguards.