Simulated Response of a High Efficiency Neutron Counter (HENC) to Zirconolite-Based Plutonium Waste

One of the potential routes for the UK’s Pu stockpile proposed by the Nuclear Decommissioning Authority is to immobilise it within a ceramic matrix suitable for disposal in a Geological Disposal Facility. The primary ceramic candidate for this immobilisation is zirconolite, CaZrTi2O7, ­due to its excellent chemical durability and radiation tolerance. As part of these efforts, the National Nuclear Laboratory will produce Pu-containing zirconolite samples using the Hot Isostatic Pressing process. In order to assess the Pu inventory of these samples, they will need to be measured using suitable Pu assay equipment, such as a High Efficiency Neutron Counter (HENC). This paper examines the influence of this novel Pu-containing material on the performance of a HENC using its existing Pu mass calibrations. This evaluation was performed using neutron emission calculations and Monte Carlo simulations. The effect of different zirconolite-based materials was also investigated, including the impact of additional neutron poisons and different Pu isotopic compositions. It was found that the 240Pueff mass extracted from the simulated Totals rate was overestimated by 46 to 161 %, depending on the composition and age of the constituent Pu. This was related to the fact that Pu assay system mass calibrations are typically derived from PuO2 sources. The 240Pueff mass extracted from the simulated Reals rate was consistently overestimated by approximately 28 %. This reduced variation was related to the fact the Reals rate was less sensitive to the increased ? factor (ratio of spontaneous fission and (?,n) neutrons) of the zirconolite materials. It should also be noted that a comparable overestimation was observed in the 240Pueff mass extracted from the simulated Reals rate during benchmarking tests using PuO2 ­standards, indicating that the overestimation is related to the HENC model rather than the zirconolite samples.