Local Disturbance of the Water-Chemical Regime as a Cause of Increased Oxidation of Cladding of VVER-1000 Fuel Elements

It is shown that a sharp local decrease in the hydrogen concentration in a coolant and a corresponding increase in the radiolytic oxygen concentration can cause an increase of the oxide layer on the surface of the cladding of VVER fuel rods. This is due to two successive phase transitions in the reactor coolant—the decomposition of a solution of hydrogen in the coolant near the saturation temperature and subsequent liquid–vapor phase transformation. In this case, the coolant in some areas can leave the hard water-chemical regime of radiolysis suppression. A detailed study of the process of hydrogen release into gas-vapor bubbles near the saturation temperature of the coolant has been performed. The distributions of hydrogen concentration in the coolant in the presence of bubbles under VVER core conditions are obtained. The processes of dissolution and diffusion are taken into account. It is shown that the hydrogen concentration in the coolant can decrease by a few orders of magnitude in a short time. In this case, the partial pressure of hydrogen in the bubbles remains constant.