On the possibility to investigate irradiated fuel pins non-destructively by digital neutron radiography with a neutron-sensitive microchannel plate detector with Timepix readout

Abstract

Post irradiation examination (PIE) of nuclear fuels provides valuable information on their overall performance and is important for the assessment of safety and integrity of nuclear fuels. PIE is also very useful for the optimization of the spent fuel processing and for the development of future advanced fuels. Neutron radiography (NR) provides more comprehensive information about the internal condition of irradiated nuclear fuels than any other non-destructive PIE examination, and NR informs subsequent characterization by destructive techniques. At the present time most NR of irradiated fuel is conducted through image transfer methods using radiographic films or imaging plates. Digital neutron radiography of irradiated fuel poses significant challenges due to the extreme levels of radiation generated by the irradiated fuel itself. In this paper, we present the results of neutron imaging of irradiated fuel by a digital detector installed in a radial neutron beam with direct line of sight to the reactor core. The results of these experiments demonstrate the possibility to perform nearly real-time initial screening of the internal structure of an experimental transmutation fuel pin at the Neutron Radiography reactor at Idaho National Laboratory. The detector used in these experiments, based on neutron sensitive microchannel plates and Timepix readout. It sustained operation at high radiation environment with gamma dose rates of ∼5.5 Sv/hr from the sample itself and ∼2.0 Sv/hr in the neutron beam The influence of various experimental setup characteristics (beam collimation, gamma and neutron background fluxes, image integration time) on the quality of resulting neutron radiographic images was studied for the optimization of the future imaging experiments. These measurements also demonstrate the possibility to reduce the fraction of unwanted gamma events by lead filter installed between the detector and the irradiated fuel. At the optimal configuration the fraction of neuron counts was measured to exceed 90% of all registered events in the detector. The results indicate that non-destructive post irradiation examination of nuclear fuels can be extended beyond conventional neutron radiography to more advanced imaging techniques, which include tomographic, dark field, energy resolved neuron imaging and many others, enabling more detailed quantitative characterization beyond preliminary screening.