Estimate of radiation release from MIT reactor with low enriched uranium (LEU) core during maximum hypothetical accident

Abstract

In accordance with a 1986 NRC ruling, the MIT Research Reactor (MITR) is planning on converting from the use of highly enriched uranium (HEU) to low enriched uranium (LEU) for fuel. A component of the conversion analysis includes calculating the maximum hypothetical accident (MHA) dose implications for the two types of fuel. In this work, the dose levels at the site exclusion area boundary were calculated for the MITR MHA using both the HEU and LEU models of the MITR core. The core inventories from the reactor were calculated using the ORIGEN-S point-depletion code linked to the MITR spectrum. The MITR spectrum was used from an MCODE simulation of the equilibrium LEU and HEU versions of the core. Release fractions from the melted fuel to containment were established using melt test data from plate-type fuel as well as modified release fractions from NRC Regulatory Guides. The dose paths considered were the same paths used in the previous work, consisting of atmospheric release through potential containment leakage as well as direct and scattered gamma dose from the containment source term. The Total Effective Dose Equivalent (TEDE) values were calculated in addition to the whole body and thyroid doses. For dose comparison the LEU thermal power was 17% higher than the HEU thermal power in order to provide equivalent total flux levels to the experimental ports. The results showed that the LEU core operating at 7 MW will yield TEDE levels 22% higher than the HEU core operating at 6 MW for equivalent release fractions. The two-hour dose at the exclusion area boundary from the LEU core operating at 7 MW using the plate-type fuel release fractions was 0.440 rem at 21 m and 0.344 rem at 8 m, while the dose from the HEU core operating at 6 MW was 0.361 rem at 21 m and 0.281 rem at 8 m. These doses are within the public dose NRC regulatory limit of 0.500 rem TEDE.