Active neutron interrogation for the verification of irradiated HEU residues

Production of medical radioisotopes at the Petten Research Reactor (NL) generates residues packed in cylindrical containers involving around 450 g of irradiated HEU each. Because the chemical process selectively removes the fission products, the verification can no longer rely on observation of conventional attributes such as gamma emissions of Cs-137. The International Atomic Energy Agency conducted a feasibility study to determine the most suitable verification method able to provide evidence of presence of HEU in the sealed containers prior to loading in dry storage casks. Recognizing that HEU does not provide any observable passive signal, detection of neutrons from induced fissions was found to be the best method to detect the presence of HEU. However utilizing a coincidence neutron instrument to differentiate between interrogation and induced fission neutrons would be impractical, expansive and bulky. Therefore, extensive Monte Carlo simulations were run to determine a design supporting differential transmission active neutron measurement. The optimized design reduces the detector yield to the interrogation source while the induced neutrons have higher detection probability. The differential transmission technique enhances the signal to noise ratio and allows implementing a simple total neutron counting technique. Rather than designing a new counter the existing Safeguards MOx PYthon (SMOPY) device was selected as it supports both total neutron counting and gamma spectrometry. A special mechanical arrangement was prepared to safely accommodate both an americium-beryllium and the SMOPY measurement device into an integrated measurement station meant to be submerged in the spent fuel pond at Petten. The verification was smoothly carried out during the storage cask loading without significant impact on the loading schedule. Presence of induced neutrons was certified on each container selected for verification and semi quantitative evaluation of the data was even possible. The measurement method is applicable to any similar cases where HEU residues are generated from the production of medical radioisotopes.