Experimental study of the features of crystallization of boric acid from a saturated solution during emergency cooling of the WWER core

The paper is devoted to the study of the features of boric acid crystallization in the core of pressurized water reactors. A review of domestic and foreign publications, which presented the results of studying this process that takes place in the event of emergency cooling of the reactor was carried out. The results of an experimental study of the crystallization of boric acid under parameters characteristic of a WWER reactor in the event of a large break loss of coolant accident and long-term (up to 72 hours) operation of passive safety systems are presented. The experiments were carried out at pressures of 0.2 and 0.3 MPa, the initial temperature of the boron solution corresponded to the values at the saturation line. The linear energy density on the surface of the fuel rod simulators was 70 W/m, which corresponds to the parameters in the reactor 72 hours after the start of the accident. A description of the experimental facility used for visual observation of the crystallization process of boric acid is given. The methodology for conducting experimental studies is described. The results of photographing the crystallization of boric acid on the surface of fuel element simulators, the rate of its deposition and distribution in the working volume with a decrease in the solution temperature and fixed initial values of acid concentration (371 and 466 g/kg H₂O) were obtained. It has been shown that boric acid deposits can occur in the coldest areas of the WWER reactor core. At the same time, the formation of deposits on the heated surfaces of the fuel rod simulators was not observed in the experiments.