THE APPLICATION OF ADDITIONAL, OFF-LINE, ANALYSIS TECHNIQUES TO PCM MONITOR RESULTS TO AID THE EFFICIENT AND COST EFFECTIVE REPACKAGING OF LEGACY PCM WASTES CONTAINING CALCIUM AND POTENTIALLY PLUTONIUM FLUORIDE

摘要

There are a small number of legacy, orphan waste, PCM (Plutonium Contaminated Material) drums at Sellafield Site containing calcium metal potentially contaminated by plutonium (Pu), some of which may be in the form of plutonium fluoride (PuF_4). These drums were measured on a TRU-D PCM Drum Monitor to give a Nuclear Safety value for the Pu mass based on Neutron Coincidence Counting (NCC) and the Pu isotopic composition measured for each drum using a germanium detector based High Resolution Gamma Spectrometry system. In some circumstances the presence of Pu in the form of PuF_4 can cause a significant overestimate of the measured Pu mass. This is as a result of alphas emitted by the spontaneous decay of Pu isotopes interacting with light elements such as fluorine, resulting in the emission of "random" (alpha, n) neutrons. The potential overestimate may be very large for total neutron counting based systems if the presence of PuF_4 is not accounted for in the system calibration. However, significant quantities of PuF_4 may also result in overestimates for NCC systems due to potentially large statistical uncertainties in the measurement results caused by accidental coincidences involving the random (alpha,n) neutrons. Therefore in some circumstances, less pessimistic measurements may be obtained from the total neutron count, corrected using the measured "PuF_4 ratio". Standard TRU-D Drum Monitor measurements of the calcium containing drums were carried out by plant operators. Relevant data was then downloaded from the plant instrument allowing additional off-line techniques to be applied to the high resolution gamma spectra associated with each drum. These spectra are routinely generated as part of the standard TRU-D PCM Drum Monitor measurement. This analysis was based on the patented PuF_4 analysis technique developed by VT Nuclear Services personnel to determine the mass ratio of Pu in the form of PuF_4 to total Pu mass using the PuF_4 reaction gammas and standard Pu gammas observed in the gamma spectrum. This additional, off-line analysis reduces the potential overestimate in the Pu Nuclear Safety Mass associated with each drum aiding the repackaging of the legacy material into Bull Pit cans. Following this, similar measurements and off-line analysis was carried out for the filled Bull Pit cans using a TRU-D PCM Piece Monitor set up and commissioned specifically for this task. The further analysis results allowed the Bull Pit cans to be efficiently and cost effectively packed in 200 litre drums. The resulting new 200 litre drums were then measured using a standard, routinely operated TRU-D PCM Drum Monitor for final sentencing (again taking into account off-line PuF_4 analysis) to allow safe and secure storage. This paper describes the work carried out and the additional off-line PuF_4 analysis techniques and how they have been applied within the exacting demands of Nuclear Safety in support of legacy material treatment and ultimate safe storage.