Speaker
Description
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 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 of high carbon steel. Using Monte Carlo modelling and the results from the spectrometry 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 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. We also performed high-energy gamma spectral measurements of an RWC using a calibrated LaBr3 spectrometer. The gamma spectra revealed high-energy gamma rays in the ~5–9 MeV range, consistent with prompt fission emissions, providing additional evidence that spontaneous fission is the dominant neutron production mechanism. These findings have significant impact on radiation protection, dosimetry, waste management and radiation protection regulations.