Year
2003
Abstract
An instrument pod has been designed and constructed at Pacific Northwest National Laboratory (PNNL) to estimate the quantity of TRU isotopes present in hot cells in Hanford’s 327 Building. This estimate is required to assign the proper waste classification for ultimate disposal of the hot cell. The instrument pod consists of a cylindrical polyethylene moderator containing three neutron detectors. The pod diameter was chosen to allow the pod to be deployed through a 7-inch port in the hot cell wall. A polyethylene traverse pipe, long enough to extend through the interior of the cell, was designed for insertion through the hot cell port, allowing the pod to be positioned inside the cell. The neutron detectors chosen for the pod were fission chambers, which provides acceptable sensitivity to neutrons with excellent rejection of gammas even in high-contamination cells. The neutron pod was tested and calibrated in PNNL’s Low Scatter Room (LSR), using a 252Cf source in both unmoderated and D2O-moderated configurations. The calibration configurations were then modeled using MCNP to arrive at a calibration constant that relates the neutron flux incident on the pod to neutron count rate. The neutron pod was used in two hot cells in the 327 Building. These hot cells had been decommissioned and are slated for disposal. For each hot cell, the neutron pod was positioned at about 10 locations inside the cell and count rates were recorded. The recorded count rates were all very close to background levels, since substantial cleanup effort had been expended in each cell. MCNP modeling was then performed, with plutonium distributed inside the cell in several likely distributions. The models produced values of neutron count rate per gram Pu in the cell. These values were then applied to the measured count rates to determine the quantity of Pu that is in the cell. The comparison of measurements and modeling estimated a TRU inventory of approximately 1 g Pu for each cell.