Reactor physics recommissioning of Pickering NGS Units 1 and 2

Investigations following the rupture of Pickering Unit 1 pressure tube G16 in 1983, led to the shutdown of Units 1 and 2 for pressure tube replacement and numerous other upgrades. They were recommissioned in 1987 and 1988 respectively. This paper surveys the procedures used during the reactor physics recommissioning of these two reactors and presents the results of these measurements. Special note is made of the differences between this recommissioning work, and the initial commissioning of new CANDU reactors. From a physics point of view, the restarted units differed substantially from the original design. The main difference in the core configuration involved the conversion of 10 of the original adjuster rods into shutoff rods. The reactivities of the remaining adjusters were increased. These substantial changes to the core, together with the full core of fresh fuel, necessitated a complete set of reactor physics recommissioning experiments. Some of our procedures differed from those used to commission a new reactor. This was due mainly to the high levels of tritium in the moderator D2O and to radiological hazards on the reactivity deck. Also, the high residual activities in the rebuilt cores lead to increased difficulties in neutron monitoring and higher subcritical neutron count rates (hence a higher than usual reactor power at first criticality). In general the results of our recommissioning measurements closely matched the results of presimulations using the OHRFSP and SMOKIN computer codes. Results for Unit 2 were generally better than those for Unit 1. This was due to improved procedures which resulted from our experiences with Unit 1. (author). 4 tabs., 9 figs