Speaker
Description
This study presents a detailed characterization of neutron emissions from irradiated components within Canada Deuterium Uranium (CANDU) reactors, specifically investigating the mechanisms underlying neutron production in pressure and calandria tubes after operational periods of up to 30 years. Given the unique neutron flux profile and heavy water moderation in CANDU reactors, these components undergo substantial exposure to thermal neutrons, leading to the formation of spontaneously neutron-emitting isotopes.
In this study, we conducted over 30 neutron spectroscopy measurements on nine shielded containers holding irradiated reactor components, with approximately half containing pressure tubes and the remainder calandria tubes. Neutron fields were analyzed outside each container to examine spectral characteristics and gain insights into potential production mechanisms within. Measurements were performed using a Rotating Spectrometer (ROSPEC) and Nested Neutron Spectrometer (NNS), with results validated through Geant4 and MCNP Monte Carlo simulations to ensure correlation with expected neutron emission profiles for various neutron-emitting mechanisms.
Our findings indicated that spontaneous fission from Cf-252, produced via neutron activation of trace uranium impurities in the structural materials, was a primary contributor to neutron emissions. Gamma spectroscopy of individual and bulk container samples detected high-energy gamma rays in the 6-9 MeV range, consistent with ongoing fission and supporting our hypothesis regarding Cf-252. Additionally, simulations and experimental data provided compatible estimates for neutron emission rates, correlating closely with components' exposure history and positioning within the reactor core, as components nearer the core center experienced higher neutron flux and thus higher activation rates. Furthermore, we performed a longitudinal study on one container to measure the half-life of the mechanism, which aligned with the 2.647-year half-life of Cf-252.
This work documents one of the first direct identifications of Cf-252 in irradiated CANDU reactor materials, enhancing our understanding of neutron emission sources during refurbishment activities. These findings hold significant implications for dosimetry, radiation protection protocols, and waste management for long-term storage, emphasizing the need to account for neutron emissions in radiation field assessments. This research provides a foundation for refining safety protocols in CANDU reactors, highlighting considerations for future component management and disposal strategies.
