Characterization of neutron fields around refurbishment waste at Bruce Power and OPG using neutron spectrometry and Monte Carlo modelling

Using neutron spectrometry, we characterized the neutron energy spectra coming from irradiated components of a refurbished CANDU reactor. The measured neutron energy spectra were from refurbishment waste from calandria tubes (CTs) and pressure tubes (PTs). The measurements were conducted using a Nested Neutron Spectrometer (NNS), manufactured by Canadian company DETEC Inc. They were performed at Bruce Power, around a single isolated waste container, filled with approximately 2000 kg of PTs or CTs that were volume reduced into small coupons and stored in retube waste containers (RWCs), made almost entirely of high carbon steel. Using Monte Carlo modelling and the results from this spectroscopy campaign it was concluded that neutron fields coming from the waste containers are due to presence of Cf-252 spontaneous fission neutrons. It was found that the origin of Cf-252 in CTs and PTs is a trace amount of U-238 in Zr alloys of CTs and PTs. This U-238 is converted into Cf-252 after over thirty years of continuous neutron bombardment in high neutron fluence rate environment of a CANDU reactor core. In fact, the presence of Cf-252 in the refurbishment waste is a result of a multiple neutron captures and several beta decays. Another set of spectroscopy measurements, after a period of six months, measured the decay rate of the neutron source term within the RWCs and confirmed Cf-252 half life of 2.65 years. This work is a first of a kind, identifying a presence of spontaneous fission neutrons in CANDU reactor waste material. These findings have significant impact on radiation protection, dosimetry, waste management and radiation protection regulations.