From Platinum to Mercury: The Alchemist's Nightmare

Abstract

In CANDU reactors, in-core platinum, Pt, detectors are used for bulk and spatial control and also for safety purposes. In the latter case, the detectors provide a local over-power protection to prevent dry-out in fuel channels. For these purposes, the detector response to power transients should be known accurately. The majority of the platinum detector signal is prompt and results from gamma-ray interactions, (gamma,e) and (n,gamma,e). Although the detector's response to gamma rays is prompt, a fraction of the fission product gamma rays is delayed. This latter effect combined with (n.beta) interactions in the detector assembly, results in a delayed fraction of the detector's response to changes in neutron flux. The relative natural isotopic content of the Pt emitter changes with irradiation resulting in the ratio of prompt to delayed response of the detector changing with time. This paper describes two long- term delayed-response characteristics that have been observed for Pt detectors in the 600 MW(e) CANDU PHWR reactor. Gentilly-2. The first effect is a dip in the signal that occurs two to three hours after a shutdown. This effect is due to the (n.beta) interaction& of Mn-55 and Ni-64 which are present as impurities in the detector sheath and lead cable. The effect is expected to decrease with detector irradiation and be insignificant after about ten years. The second major effect is characterized by the increase in the amplitude of the long-term delayed fraction of the detector signal. The low neutron absorption cross-section of Pt-196 combined with the more rapid burn-out and conversion of the Pt isotopes of lower atomic number results in build-up of the concentration of Pt-196 to a maximum after approximately ten years of irradiation. Pt-196 is transformed by (n,gamma) interactions and beta-decay to Hg-199 or Hg-198. The long half-lives associated with the beta-emissions give rise to the observed long-term delayed response. It has bean observed that the Pt detector signal at 70 hours after shutdown increased from 0,67. full power to 0.9Y. full power and to 1.2% full power following detector irradiation of 0.25, 1.9 and 2.9 full power years of operation.