Some aspects of water chemistry in the candu primary coolant circuit

The phenomenon of radioactivity transport in all water-cooled nuclear power reactors is characterized by in-core deposition, activation, then release of mobile corrosion products formed out-core. Corrosion products chiefly deposit by precipitation and as particles. Water radiolysis in the reactor core may suppress in-core precipitation of magnetite from non-boiling water compared to boiling water. As summarized below, two hypotheses are proposed to account for the observed behaviour of corrosion products in a series of in-reactor loop experiments in the NRX reactor at Chalk River Laboratories. a. Radiolysis. Boiling on nuclear fuel sheaths results in water radiolysis to give H2, O2, and H2O2 in a thin water layer surrounding a steam bubble even when radiolysis is suppressed in the bulk coolant. When first formed, the bubble contains the same mass concentration of H2 as the bulk water and Henry's Law is assumed to apply at the steam/water interface. b. Precipitation. Reactor radiation is postulated to suppress the crystal growth rate constant for in-core magnetite precipitation from non-boiling water with reducing radiolytic species. However, if the water is boiling the observed precipitation rates in-core and out-core are similar. (author)