Study of release of nitrogen gas dissolved in water during depressurisation--application to primary circuit of PWR

The potential release of the nitrogen (N2) gas dissolved in the water of the accumulators of the emergency core coolant system of the Loviisa Nuclear Power Plant (PWR of VVER-440 type) has been investigated. A model of the dissolution and release of N2 gas has been implemented in the thermal-hydraulic code CATHARE for nuclear safety. In collaboration with VTT, an analytical experiment has been performed with some components of the PACTEL facility to determine, in particular, the value of the release time constant of the nitrogen gas in the depressurization conditions representative of the small and intermediate break transients postulated for the Loviisa Nuclear Power Plant. Such transients, with simplified operating procedures, were calculated using the modified CATHARE code. In comparison with the cases calculated without taking into account the release of nitrogen gas, the start of the LPIS is delayed by between 1 and 1.75 h. Applicability of the obtained results to the real safety conditions must take into account the real operating procedures used in the Loviisa Nuclear Power Plant