Radiolysis of the Coolant during the Decomposition of a Hydrogen Solution near the Saturation Temperature in VVER Reactors

Abstract: Radiolysis of the VVER coolant during the transition of hydrogen from the liquid phase into gas-vapor bubbles is calculated. It is found that the decrease in the concentration of hydrogen in the liquid phase occurs until the hydrogen flow into bubbles becomes comparable to the hydrogen production due to radiolysis. In this case, a jump in hydrogen concentration leads to a sharp restructuring of the entire pattern of time dependences of the concentrations of radiolysis products. This is especially true for the behavior of concentrations of O₂ and H₂O₂, i.e., the main oxidants of the zirconium cladding of fuel elements. It is shown that the local oxygen concentration relative to the permissible value can increase by more than 1000 times when the reactor is operated at >50% N_nom power. Similar behavior is demonstrated by hydrogen peroxide H₂O₂. However, the increase in its relative concentration by the same time is slightly more than one order of magnitude. A significant local increase in the concentration of oxidizing products of the radiolysis of O₂ and H₂O₂ in the coolant seems to be the main reason for the appearance of a white deposit on the cladding of VVER fuel elements near the 12th spacer grid.