The Advanced Experimental Fuel Counter (AEFC) is a nondestructive assay (NDA) instrument used for active neutron coincidence measurements of spent research reactor fuel. The instrument performs active and passive scans of spent fuel assemblies to establish a relationship between the measured net active Doubles rate and the fissile mass (235U, 239Pu, and 241Pu) present in the assembly. A calibration curve was previously established between net active Doubles and assembly fissile mass in highly enriched uranium (HEU) materials test reactor (MTR) fuel assemblies. However, many MTR-type reactors use low enriched uranium (LEU) instead of HEU. Simulations using the Monte Carlo N-Particle (MCNP) code were performed to determine if the calibration curve relationship for these HEU assemblies could be applied to LEU MTR assemblies. Fresh HEU and LEU MTR fuel assemblies were modeled in the fuel funnel of a benchmarked MCNP model of the AEFC. The results indicate that the HEU calibration curve will require a correction to accurately measure the fissile mass in the LEU assemblies. This result arises from changes in the system moderation due to the different geometries of the HEU and LEU assemblies as well as increased self-shielding in the LEU fuel.