The differential dieaway technique applied to the measurement of the fissile content of drums of cement encapsulated waste

This report describes an investigation of a novel form of the differential dieaway technique, designed specifically for the measurement of the fissile content of 500 ''litre'' drums of cement encapsulated intermediate level radioactive waste. The differences from the standard technique are that the thermalisation of the source neutrons occurs inside rather than outside the sample, and that the detectors are not symmetrically placed around the sample. The results show that the limit of detection is 10 grams of fissile material when 1011 source neutrons are used in the measurement. When the drum is rotated, the information from the main neutron detector results in a response which varies from 0.4 to 1.4 times the average response, depending on the position of the material in the drum. This can be improved by using the information obtained by a second detector at 90o to the first, although additional uncertainties will be introduced by the water content of the cement grout., (author)