Photoneutron Source Strength Studies in Pickering Reactors

Abstract

Reactor power measurements and reactivity change measurements taken during the execution of approach to critical procedures following long outages (> 100 days) on Pickering reactors revealed the presence of a neutron source of greater strength than predicted by the 15 group delayed and photoneutron precursor model. The apparent additional neutron source has been characterized by modelling it as a sixteenth group in the Ontario Hydro code SPARK (Simulation of Photoneutrons and Reactor Kinetics). Data are available from three Pickering-A outages and two Pickering-B outages. The duration of each Pickering-A outage was greater than 250 days. The Pickering-B outages were of 117 days and 154 days duration. Two interpretations of the available data are possible: (i) after about 150 days, neutron source power is relatively constant, or; (ii) owing to differences in reactor structural materials, the source power is quite different on PNGS-A from PNGS-B. Characterization of the source for the stations is of practical benefit because it allows engineering staff to predict when to expect significant milestones in the approach to critical procedure. Examples of such milestones are: (i) the core reactivity at which regulating system ion chamber signals come on scale, and (ii) the critical power level.