The Passive Neutron Albedo Reactivity (PNAR) Ratio is proportional to the net neutron multiplication of a spent fuel assembly. In the planned integrated non-destructive assay instrument for Finnish encapsulation safeguards, a PNAR instrument is used to confirm the presence of fissile material. In this study, the sensitivity of fuel-type-specific PNAR Ratio measurements to the size of the water channel of the instrument is determined using MCNP5 Monte Carlo simulations. Based on the study results, use of the smallest possible water channel is recommended to maximize the dynamic range of the instrument. In the Finnish fuel encapsulation context, this means using water gap sizes of 5 mm and 3 mm for measurements of boiling water reactor (BWR) and water-water energetic reactor (VVER-440) fuel, respectively. Based on the neutron emission rates of the Finnish spent fuel inventory, we recommend maximizing count rates by having detectors all around the fuel assembly, i.e., 4 detectors for BWR fuel and 6 detectors for VVER-440 fuel. With these water gap sizes, and neutron detectors all around the fuel assembly, the variation of the PNAR Ratio measurement caused by the uncertainty on the position of the fuel in the instrument is estimated to be 0.06% for BWR fuel and 0.13% for VVER-440 fuel.
Publication Date
Volume
48
Issue
1
Start Page
22
File Attachment
V-48_1.pdf3.1 MB
Abstract