Estimate of radiation release for MIT research reactor during design basis accident

Abstract

During a postulated design basis accident at the MIT Research Reactor (MITR), radioactive fission products may be released from melted fuel plates into the con­tainment. To comply with regulations, the whole-body dose and thyroid dose at the boundary of the exclusion area as a result of this accident are determined. The fractions of the fission products contained in the fuel that are released through the reactor coolant system (RCS) into the containment are determined based on current regulations, experimental tests, and results from TMI-2 accident. After the fission products are released into the containment, a portion may be released to the outside through a containment crack or the stack. Also, the por­tion retained in the containment would contribute to the external gamma dose. The calculated dose due to atmospheric release depends on the source strength, the me­teorological conditions, and the dispersion model. For containment crack release and stack release, different dispersion models are used according to pertinent regulatory guides. The gamma dose through penetration or scattering depends on the struc­ture of the containment shielding and is determined analytically under appropriate approximations. Because the MITR is considering upgrading its power level, results at power levels from 5 to 10 MW are determined. At 5, 6, 7, 8, 9, 10 MW, the whole body doses at the back fence (8 meters away from the MITR) are 0.644, 0.764, 0.885, 1.00, 1.13, 1.25 rem respectively; the thyroid doses at the back fence are 0.112, 0.135, 0.157, 0.179, 0.202, 0.225 rem respectively; the whole body doses at the front fence (21 meters away from the MITR) are 0.887, 1.06, 1.22, 1.39, 1.56, 1. 72 rem respectively; and the thyroid doses at the front fence are 0.112, 0.134, 0.156, 0.179, 0.201, 0.224 rem respectively. The results show that even under conservative assumptions, the released doses for power levels from 5 MW to 10 MW are well below the regulatory limit - 25 rem for whole body and 300 rem for thyroid.