Calculation analysis of the processes of boric acid mass transfer in the WWER core in boiling emergency mode

Abstract

In this paper the processes of boric acid mass transfer in a WWER-TOI nuclear reactor core in case of the accidents with main circulation pipeline rupture and loss of all AC power supply are considered. The heat removal from the WWER-TOI reactor in case of an emergency process is determined by the use of the passive heat removal and passive core cooling systems. Passive core cooling system of WWER-TOI consists of three stages of hydro accumulators. These systems provide reactor cooling by feed of boric acid solution with concentration of 16 g H3BO3/kg H2O. In view of length of accident process, the boiling of the coolant with high content of boron and considering low concentration of boric acid in the steam leaving the core, conditions may arise for the possible accumulation and subsequent crystallization of boric acid in the reactor that can lead to a deterioration of the process of core heat removal. To assess the chance of accumulation and subsequent crystallization of H3BO3 in the core of the WWER reactor, a hand calculation analysis of the change of concentration of boric acid in the reactor in case of emergency mode was carried out. In accordance with the calculation results, if a boric acid concentration in hydro accumulators of passive core cooling systems is 16 g / kg H2O, than a large excess of the final boric acid concentration in the core is observed in accidents after approximately 43 hours of an emergency process. The options of reducing the concentration of H3BO3 to 8, 4, 2 and 1 g/kg H2O are considered in the calculation process. The obtained results allow us to conclude that a decrease in the concentration of boric acid in the system of hydro accumulators of the third stage would allow to avoid the crystallization of boric acid in the core in the event of an accident.