Measurement of the axial distribution of radioactivity in the auxiliary charcoal bed of the Molten Salt Reactor Experiment at ORNL

The Molten Salt Reactor Experiment (MSRE) at Oak Ridge National Laboratory commenced operation in 1964 and was shut down in 1969. It was fueled with 233UF4 in a carrier salt of LiF-BeF2-ZrF4, and it operated at 1,200 F. After it was shut down, the fuel was heated annually to 200 C to recombine fluorine (with the fuel) released due to radiation-induced reactions in the fuel salt. However, a competing reaction oxidized uranium to UF6, which was released (along with F2) from the fuel and trapped in one of four charcoal filters in the auxiliary charcoal bed (ACB). One of the tasks for decommissioning of the MSRE requires that at least 90% of the estimated 3 kg of 233U, and radioactive decay products, in this filter be removed, and one of the proposed methods is to vacuum the charcoal above a specified axial position in the filter. This requires that the axial distribution of activity in the filter be measured in a 60 rad/h radiation field to determine where this penetration can be made. To accomplish this, the shielded detector with a pinhole collimator, and with a laser positioning capability, was remotely translated to various axial positions to accomplish these measurements. Activities in the steel screen, and various regions of the charcoal bed, are estimated, and uncertainties in these estimates are generally <1%. Results from this analysis are used for continued operational decisions for decommissioning of the MSRE