USE OF GAMMA RAY IMAGING INSTRUMENTATION IN SUPPORT OF TRU WASTE CHARACTERIZATION CHALLENGES

Over the last ten years or so the field of gamma ray imaging has developed and become established within the nuclear industry. This has played a useful role in the radiological characterization of redundant nuclear facilities and specific plant items. These instruments, operated remotely with little or no operator dose uptake, are able to offer systematic monitoring of extended areas of plant, and are becoming an established technique in situations where dose rates are high or in which there are physical access limitations. Gamma ray imaging can unambiguously identify, visualize and record permanently the origins of radioactivity within contaminated environments. The standard output from these devices is a picture showing the location, distribution and intensity of gamma radiation, represented by color, superimposed on to a still video image of the scene under investigation. More recently, the data that such instrumentation produces has begun to be used in more novel ways and in a complementary manner with other established measurement and characterization technologies. This combination of established technologies, when used with the quantitative output from gamma imaging instrumentation, has provided the opportunity to solve specific plant characterization challenges in new and different ways from that previously possible. A step change in the rate of progress of a variety of waste characterization challenges has been achieved. Two examples are described in this paper. The first example has addressed the challenge of determining the plutonium inventory of a number of discrete items of waste stored in crates and drums. These items have been stored outside for over thirty years on the Sellafield site in north-west England, UK, and are scheduled to be transferred to an engineered waste store for interim safe storage. The plutonium mass within each item is required, with sufficient accuracy and confidence, to guarantee that subsequent handling and any processing is performed safely. A complementary combination of measurements and analysis techniques has been used to accomplish this challenge. Secondly, a gamma imaging survey, followed by neutron coincidence counting measurements, were performed within a redundant cell in the reprocessing area of UKAEA’s Dounreay site in northern Scotland, UK. These data were used in conjunction with computer modelling techniques, plant-provided engineering drawings and plant-provided plutonium isotopic data to determine the plutonium hold-up within the cell process structures. The work has provided sufficient confidence in the total plutonium mass present to enable further, practical decommissioning plans to commence safely.