Determining Optimal Neutron Spectra for Transmutation and Isotope Management

This work presents a methodology for determining the optimal neutron energy spectrum for meeting user-specified transmutation objectives. A simulated annealing routine is used to find the optimal neutron energy distribution by iteratively modifying the flux spectrum, performing depletion calculations, and computing the value of the cost function.

To demonstrate this methodology, we found optimal flux spectra for transmuting used nuclear fuel (UNF) to maximize proliferation resistance and to maximize repository capacity by minimizing decay heat. Multiple cost functions are evaluated for each of the two objectives. For maximizing proliferation resistance, we determined the optimal spectra for minimizing ²³⁹Pu mass, maximizing ²³⁸Pu mass, maximizing ²⁴⁰Pu mass, and minimizing the mass ratio of ²³⁹Pu to ²³⁸Pu and ²⁴⁰Pu. The results of this study show that while both fast and thermal neutrons are useful for reducing the amount of ²³⁹Pu, a thermal spectrum is best for rendering plutonium from UNF unusable as weapons material.

Optimal spectra for maximizing repository capacity are found for minimizing the time-integrated decay heat generated by the transmuted UNF. This study shows that optimal transmutation of the full UNF vector can reduce the amount of decay heat released over 10 000 yr by [approximately]39% and that even more substantial reductions are possible with transuranic element-only transmutation, which can decrease decay energy by >81%. Furthermore, it is shown that a thermal spectrum is substantially more effective than a fast spectrum for reducing decay heat released by UNF over 10 000 yr, thus increasing the capacity of heat-limited waste repositories. Results such as these provide powerful insight into the complicated energy dependence of transmutation and illustrate this methodology's effectiveness as a scoping tool.