Criticality Study for Large Masses of Low Enriched Uranium Samples in an Active Well Neutron Coincidence Counter

A criticality study has been conducted while verifying large masses of Low Enriched Uranium (LEU) samples in an Active Well Neutron Coincidence Counter (AWCC). Fissile material mass limits were determined for some setup conditions to assure safe operation of the counter. The work was performed at the Department of Safeguards and Physical Protection, Nuclear and Radiological Regulatory Authority (ENRRA), during the period from February to December 2015.

The AWCC device was assumed to be employed in verification activities including measurements of different Nuclear Material (NM) samples in different setup configurations, forms and conditions. The MCNP5 code was used to estimate keff of relatively large masses of LEU in different forms including uranium oxide powder, compacts and fuel rods. All calculations were performed assuming the operation of the AWCC in active thermal mode at maximum capacity of its cavity. The uranium powder samples were modeled as dry and with different values of water contents. For compacts and fuel rods, the calculations were performed with and without the existence of moderating materials in the cavity of the device.

All studied cases were found to be subcritical except for a few cases of uranium oxide powder containing water. Criticality was reached for samples containing 235U masses ranged between 1.5 to 8.0 kg with corresponding percent water content from 67 to 25. The estimated mass limits of LEU samples with certain characteristics that could be safely verified in the device are presented.