Experimental and analytical studies of boric acid concentrations in a VVER-440 reactor during the long-term cooling period of loss-of-coolant accidents

Concentrating and mixing of boric acid (H₃BO₃) during the long-term cooling 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 specific facility was built to simulate boron mixing in the lower plenum of the reactor. The experiments with the VEERA facility showed that in the VVER-440 reactor fuel bundles the mixing is complete due to boiling and U-tube oscillations and, hence, concentration distribution of boric acid in the bundles is uniform. The U-tube oscillations proved to be an important mechanism in transferring concentrated boric acid from the core to the lower plenum. The experiments demonstrated that crystallization of boric acid in the reactor core simulator is possible, if a stable long-term cooling situation with water boiling in the core continues long enough. In the experiments the crystallization of boric acid in the core simulator led to a flow blockage of the fuel rod bundle and overheating of the rod simulators when the flow through the core ceased.

Experimental results were used to develop a computational model for calculations of boric acid concentrations in the reactor during LOCAs. The development work was supported with a series of RELAP5/MOD3 small-break LOCA analyses. The results of the RELAP5/MOD3 calculations were used to determine the boundary conditions under which concentrating of boric acid might occur. Reactor analysis showed that the crystallization of boric acid in the reactor is not possible during the long-term cooling period of LOCAs. This is mainly due to the fact that the ice-condenser in the Loviisa plant contains sodium tetraborate Na₂B₄O₇10H₂O (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 crystallization.