The Primary Heat Transport System Vacuum Drying of a CANDU Power Plant

Abstract

Existing CANDU reactors were designed for a service life of up to 55 years. After about 25 years of operation, a CANDU power plant needs to be refurbished to extend its service life for another 25 to 30 years. As part of the Bruce Power Units 1 and 2 Restart Project, one major work scope is the fuel channels replacement. To facilitate this refurbishment initiative, the Primary Heat Transport (PHT) system is required to be drained of all heavy water (D2O) and a functional dry state must be achieved. However, after the PHT system is drained, a significant quantity of D2O still remains in the fuel channels, pre-heaters, and other low-lying portions of the PHT and Feed and Bleed (F&B) system. The residual D2O in the PHT and F&B systems is required to be removed and recovered to allow for adequate drying and to minimize the potential for radiological releases.The purpose of this paper is to describe the method used to dry the PHT system at Bruce A Nuclear Generating Station, Unit 1. The method used for the drying process is termed “vacuum drying” in which trapped D2O is boiled off through the combined application of heat addition to the PHT system under near vacuum conditions, such that the boiling point of D2O is reduced by lowering the pressure within the isolated PHT system. Proper isolation of the PHT system is critical for preventing air ingress and maintaining the vacuum conditions. Sufficient heat is required to be added to various locations suspected of containing significant quantities of D2O, in order to raise the D2O temperatures to allow an increased boiling rate of D2O within the PHT system. A vacuum drying process system was designed to evacuate the isolated PHT system and draw D2O vapour, re-condense the vapour, collect the condensate, transfer the collected condensate to the station D2O collection system, and purge non-condensed vapour and gases to the station vapour recovery system.