Speaker
Description
Nuclear Restoration Services (NRS) operates the Dounreay site in the north of Scotland, which was built in 1955 as the centre of fast reactor research in the United Kingdom. Today, our challenge is to decommission and remediate the site, including our research reactor, our two fast reactors, our fuel cycle area, and our legacy underground waste stores which are no longer suitable for that purpose.
Our Remote-Handled Intermediate Level Waste (RHILW) processing facility uses a Passive/Active Neutron Interrogation System to determine the total mass of Plutonium and Uranium in variously sized cans. The system dates back to the 1980s; our lifetime plan and waste management strategies have identified the need for this measurement capability for several decades to come.
The neutron interrogation source for the RHILW Active Assay system was upgraded from a Californium Shuffler to a SODERN MEN 16NG Neutron Generator in 2021. Neutrons are detected using coincidence counting by an array of 16 He3 detector tubes which surround the measurement chamber. During refurbishment a design study was carried out by the upgrade manufacturer using MCNP modelling techniques to optimise the design of cans, waste matrices and to produce Active and Passive calibrations for each type of waste matrix. For both Passive and Active assay, 4 calibrations were developed using MCNP dependent on the Hydrogen density of the waste matrix.
Passive calibrations have performed well throughout the working life of the system since November 2022, however, a downward drift was observed in year-on-year calibration check measurements and in the daily active QC measurement. To prevent the system failing its active QC check and allow the site to continue to process RHILW, in May 2025 the Radiometrics Team at Dounreay developed new Active calibration curves based on empirical measurements made using four new certified Uranium standards, which we produced for the task using existing material, with U235eff mass ranging between 1g and 20g. In the event the calibration continues to drift due to deterioration of neutron generator output, NRS Dounreay has gained the internal ability to use these standards for system recalibration without the need for MCNP modelling.
In 2026, NRS Dounreay engaged a supply chain partner to undertake a technical and strategic review into the issues faced with the current configuration of the system and make recommendations for the future. On that basis, we are now considering a new configuration for fast neutron detection using Am-Li interrogation sources and a fast neutron collar of scintillator detectors. We have approximately 5 years to reach sufficient technical and operational readiness for this transformation, due to the lifetime of the neutron generator in the current configuration.