EXPERIMENTAL AND ANALYTICAL STUDIES OF BORIC-ACID CONCENTRATIONS IN AVVER-440 REACTOR DURING THE LONG-TERM COOLING PERIOD OF LOSS-OF-COOLANT ACCIDENTS

Concentrating and mixing of boric acid (H3BO3) during the long-term co oling period of loss-of-coolant accidents (LOCAs) in the Loviisa VVER- 440 reactors has been studied with the REWET-II and VEERA facilities. To get more detailed information on boric acid mass transfer, a specif ic facility was built to simulate boron mixing in the lower plenum of the reactor. The experiments with the VEERA facility showed that in th e VVER-440 reactor fuel bundles the mixing is complete due to boiling and U-tube oscillations and, hence, concentration distribution of bori c acid in the bundles is uniform. The U-tube oscillations proved to be an important mechanism in transferring concentrated boric acid from t he core to the lower plenum. The experiments demonstrated that crystal lization of boric acid in the reactor core simulator is possible, if a stable long-term cooling situation with water boiling in the core con tinues long enough. In the. experiments the crystallization of boric a cid in the core simulator led to a flow blockage of the fuel rod bundl e and overheating of the rod simulators when the flow through the core ceased. Experimental results were used to develop a computational mod el for calculations of boric acid concentrations in the reactor during LOCAs. The development work was supported with a series of RELAP5/MOD 3 small-break LOCA analyses. The results of the RELAP5/MOD3 calculatio ns were used to determine the boundary conditions under which concentr ating of boric acid might occur. Reactor analysis showed that the crys tallization of boric acid in the reactor is not possible during the lo ng-term cooling period of LOCAs. This is mainly due to the fact that t he ice-condenser in the Loviisa plant contains sodium tetraborate Na2B 4O710H2O (borax), which enters the reactor when emergency core cooling water is taken from the sump. Borax increases greatly the solubility of boric acid in water and, hence, decreases the risk of crystallizati on.