Year
1994
Abstract
Accurate assays of the fissile contents in waste drums are needed to ensure the most proper and economical handling and disposal of the waste. An improvement of accuracy will mean fewer drums disposed as transuranic waste when they really contain low-level waste, saving both money and burial sites. Shufflers are used for assaying waste drums and are very accurate with nonmoderating matrices (such as iron). In the active mode they count delayed neutrons released after fissions are induced by irradiation neutrons from a ^^Cf source. However, as the hydrogen density from matrices such as paper or gloves increases, the accuracy can suffer without proper attention. The neutron transport and fission probabilities change with the^hydrogen density, causing the neutron count rate to vary with the position of the fissile material within the drum. The magnitude of this variation grows with the hydrogen density. For many common moderating matrices, a simple hardware addition to reduce the average energy of the irradiating neutrons eliminates this problem. But this has the potential of creating another loss of accuracy by increasing self-shielding. Three other techniques are being investigated that maintain the high average neutron energy. These are based on (a) the variance among detector bank counts, (b) a mediumresolution imaging technique, and (c) neural network analysis of detector bank counts. The present states of all four techniques are summarized and compared.