Neutron and gamma-rays sensitivity of new fiber-mounted scintillation neutron detectors for spent fuel pin diversion measurements

The verification of spent fuel represents a pressing safeguards challenge in view of the increasing amount of spent fuel and the commissioning of spent fuel repositories. In this framework, we are investigating a novel method to detect and identify missing/diverted fuel rods in a spent fuel assembly, which were replaced by dummy rods. The method is based on performing measurements with thin fiber-based scintillation neutron detectors, inserted into either the detector channel positions in the assembly, or in between the fuel pins. The neutron flux and its radial gradient are measured and compared to the ones expected to exist in an intact assembly. In this work we report about the measurements carried out at the BR1 research reactor and the Laboratory for Nuclear Calibrations (LNK) facilities of the Belgian Nuclear Research Centre SCK CEN. At BR1 we measured the detector response in a well-known Maxwellian thermal neutron flux and determined the detector sensitivity to thermal neutrons. These measurements allowed also to assess the impact of dead time as well as the impact of gamma-rays on the detector response. Additional measurements at LNK were instrumental to study the sensitivity of the detector to different exposures with 137Cs and 60Co calibrated sources. The detectors were insensitive to gamma-rays up to a dose rate of 70 Gy/h. The obtained neutron sensitivity was compared with estimations of a Monte Carlo model based on the nominal detector data. Building on previous work carried out at SCK CEN, the obtained sensitivity was used to estimate the expected count rates and measurement times with an intact 17.17 PWR fuel assembly in a spent fuel pool.