NUCLEAR TRANSMUTATIONS IN HFIR'S BERYLLIUM REFLECTOR AND THEIR IMPACT ON REACTOR OPERATION AND REFLECTOR DISPOSAL

摘要

The High Flux Isotope Reactor (HFIR) located at the Oak Ridge National Laboratory utilizes a large annular beryllium reflector that is subdivided into three concentric regions and encompasses the compact reactor core. Nuclear transmutations caused by neutron activation occur in the beryllium reflector regions, which leads to unwanted neutron-absorbing and radiation-emitting isotopes. During the past year, two topics related to the HFIR beryllium reflector were reviewed. The first topic included studying the neutron poison (~3He and ~6Li) buildup in the reflector regions and its effect on beginning-of-cycle reactivity. A new methodology was developed to predict the reactivity impact and estimated symmetrical critical control element positions as a function of outage time between cycles due to ~3He buildup and was shown to be in better agreement with actual symmetrical critical control element position data than the current methodology. The second topic included studying the composi- tion of the beryllium reflector regions at discharge and during postdischarge decay to assess the viability of transporting, storing, and ultimately disposing of the reflector regions currently stored in the spent-fuel pool. The post-irradiation curie inventories were used to determine whether, for disposal purposes, the reflector regions are discharged as transuranic (TRU) waste or become TRU waste during the decay period and to determine the nuclear hazard category, which may affect the controls invoked for transportation and temporary storage. Two of the reflector regions were determined to be TRU waste at discharge, and the other region was determined to become TRU waste < 2 yr after being discharged due to irradiation of the initial uranium impurity content (0.0044 wt% uranium). It was also concluded that all three of the reflector regions could be classified as nuclear hazard category 3 (potential for localized consequences only).