IMPROVED ASSAY OF HIGHLY ACTIVE REPROCESSING WASTE BY THE USE OF A LINEAR ACCELERATOR BASED NEUTRON SOURCE

Dissolution of spent fuels in the THORP reprocessing plant results in large batches of spent fuel cladding which must be comprehensively monitored prior to its export from the plant. The Hulls Monitor has operated successfully since THORP went on stream in 1994, providing vital data for process control, criticality safety, nuclear material accountancy/safeguards, and inventory determination prior to ultimate disposal. The system utilizes a combination of three measurement techniques: active neutron Differential Die-Away, passive neutron counting and high resolution gamma spectrometry. The Differential Die-Away measurement on fuel residues in an industrial environment demands a commercially available, robust, pulsed high output neutron source. The original D-T generator delivered a detection limit of 2g235U equivalent in 600kg of zircalloy cladding. However current operational issues and future trends in the fuel to be reprocessed have provided the stimulus to develop alternatives. This paper describes the incorporation of a compact AccSys DL-1 linear accelerator neutron source in the Hulls Monitor and its successful commissioning and operation in THORP. The next generation of Hulls Monitor systems under development is also described. This will address the measurement challenges posed by the reprocessing of mixed plutonium/uranium-oxide fuels and very high burnup uranium oxide fuels.