Increasing transcurium production efficiency through directed resonance shielding

The Radiochemical Engineering Development Center at Oak Ridge National Laboratory is the world's leader in production of ²⁵²Cf. This and other heavy actinides are produced by irradiation of mixed curium/americium targets in the High Flux Isotope Reactor. Due to the strong dependence of isotopic cross-sections upon incoming neutron energy, the efficiency with which an isotope is transmuted is highly dependent upon the energy spectrum and intensity of the neutron flux. There are certain energy ranges in which the rate of fission absorptions in feedstock materials is reduced relative to the rate of (n, γ) captures. Using a variety of computational models it is shown that by perturbing the flux spectrum, it is possible to alter the net consumption of curium feedstock, as well as the yields of key isotopes for the heavy element research program, such as ²⁴⁹Bk and ²⁵²Cf. This flux spectrum perturbation is accomplished by means of focused resonance shielding through the use of filter materials. It is further shown that these perturbations can alter the target yields in a significant way, increasing the amount of ²⁵²Cf produced per unit curium consumption by over 40%.