RELAP5/SCDAPSIM/MOD3.5 analysis of the influence of water addition during a core isolation event in a BWR

This work analyses the influence of water addition in a boiling water reactor during a core isolation event in BWR. Injection of water is impacted by time as well as reactor vessel water level and it uses thermal hydraulic conditions representative of the reactor core isolation cooling (RCIC) system. A detailed RELAP/SCDAPSIM model of Laguna Verde BWR vessel and related reactor cooling system (provided by the Mexican Nuclear Regulatory Authority) has been used for this analysis. These calculations have been extended to the point of likely vessel failure or stable core cooling. They focus on initial heating and melting of the core where water addition is found to be most effective in limiting the extent of fuel melting. It also presents the results of a base case, a station blackout transient without water addition. These calculations have been carried out up to 5 h (after reactor scram) beyond the point of likely vessel failure. The maximum core surface temperature of 3042 K and hydrogen production of 367 kg is observed in this case. The importance of timing can be seen from 3500 s and 3700 s injection cases. One case leads to maximum core surface temperature of 1520 K with hydrogen production of 21 kg while the second case leads to temperature of 2940 K with hydrogen production of 193 kg. Temperatures (at the time of start of first injection) in both these cases are1371 K and 1590 K which explains this switching from stable core cooling to very high core surface temperature.