Monte Carlo simulation of a scintillation detector for spent fuel characterization in a hot cell

Spent fuel characterization describes the overall study of fuel rods that are extracted from a nuclear power plant at the end of their useful cycle in energy production or in case of a failure that needs to be analyzed. The knowledge of the radioactive source term of spent fuel allows, to some extent, the improvement of nuclear fuel design and the optimization of the core configuration during the different burning cycles. The same knowledge is essential for the correct handling of the fuel after discharge. The Paul Scherrer Institute (PSI) is one of the few research centers in Europe that are equipped with the appropriate facilities to perform a full-length spent fuel rod characterization. In particular, for the source term of spent fuel, different kinds of detectors are used depending on the type of radiation that is measured. The goal of this work is to investigate the possibility to build a measurement station based on two new fast neutron scintillators, the EJ-299 by Eljen Technologies and a composite ZnS(Ag) scintillator with embedded wave-length shifting optic fibers developed in-house, to measure the axial neutron profile of spent fuel rods received at the PSI Hot-Lab. To design the measurement station, we used a well-characterized UOX spent fuel sample of 40 cm with ∼70 MWd/kg and more than 10 years cooling time as neutron and gamma-ray sources and modeled the fast neutron efficiency of the detectors with MCNP6.1.1 in test conditions and in realistic configurations encountered in the hot-cell. Two methods, based on the usage of a blinding cone and of a collimator, are studied for the measurement of the fast neutron axial profile. The effect of the photon source term of the spent fuel rod on the measurement is also studied. First results show that, thanks to its gamma blindness, the composite ZnS(Ag) scintillators could be used in a specific design to determine the neutron spent-fuel axial distributions of the studied sample with a 2 cm resolution.