Speaker
Description
Characterisation of fissile material inventories in legacy nuclear facilities remains a significant challenge for successful decommissioning. To do so effectively requires portable neutron techniques, which face several difficulties – constrained access, complex geometries, elevated dose rates that interfere with neutron measurements, and often a need for rapid operational decision-making. Over the past year, several neutron detection developments have been undertaken at Sellafield to address these obstacles and improve the capability of portable neutron assay in active plant environments.
Central to this work has been the production of a localisation and quantification method of fissile material based on an array of in-situ measurements with Arktis S670 fast neutron detectors called FINDER. The data are combined with computational optimisation approaches to infer source position and strength from the measured distributions. The solution is obtained using detector response matrices generated in the Monte Carlo N-Particle (MCNP) package, followed by a novel adaptive clustering method that groups similarly detectable source terms. This acts as a dimensionality reduction step, producing a stable and physically plausible initialisation for the Maximum Likelihood Expectation Maximisation (MLEM) solve. The methodology enables localisation in complex geometries while simultaneously providing estimates of fissile material inventory and has been designed to deliver robust, physically realistic solutions in challenging measurement environments. The effectiveness of the approach has been evaluated using representative deployment scenarios and is currently undergoing benchmarking prior to use in nuclear facilities.
Alongside this development, a Kromek TN-15 deployment package has been engineered with the supply chain to facilitate operation in high-dose environments, enabling thermal neutron measurements within channels beneath legacy storage tanks. This package is an alternative neutron measurement technique for carrying out material localisation and quantification activities and has been deployed successfully. In parallel, an Arktis total neutron counting system is being developed to coarsely quantify fissile material in waste packages for clearance against acceptance criteria, facilitating appropriate disposal.
This presentation will describe the technologies, benchmarking, methods, deployment challenges, and applications that have been explored over the past two years. It is expected that these capabilities will shortly transition from development to deployment to support hazard reduction and waste management across Sellafield.