Modelling of a high enriched uranium fission chamber with the code MCNPX

Fission chambers loaded with high enriched uranium are used for spent fuel measurement in the socalled Fork detector. The Fork-detector is one of the work-horses during safeguards inspection and is also being considered for a verification of operator declared data in Go-noGo measurement station on spent fuel element in an encapsulation plant.
Having an accurate and validated model of the measurement equipment is beneficial for the investigation of this type of application. SCK•CEN is carrying out a significant effort to model the Fork detector with the MCNP code; however scarce information is known about the fission chambers.
This work describes the impact of the design information of the fission chamber on the calculated detector sensitivity and on the overall Fork response using Monte Carlo simulations. The heavy ions transport in the active layer of the fission chamber was also studied and the resulting fission products energy spectra are compared with the available experimental data.