• pressurized water reactor
• pwr
• sbo
• station blackout

A station blackout is a plant condition with complete loss of the alternating current power from offsite sources, from the main generator, and from standby alternating current power sources. The most recent station blackout accident happened in the Fukushima Daiichi power station in 2011. The aftermath of such accident brought back the awareness over the importance of an effective coping strategy that makes good use of the system water inventory for this type of scenario.
The present study investigates
1. the evolution of a station blackout in a typical Westinghouse 4 loop pressurized water reactor nuclear power plant;
2. the effect of the auxiliary feedwater system feeding one of the steam generators on the beginning of the core damage time;
3. the effect of the steam generator depressurization procedure on the beginning of the core damage time;
4. the effect of the auxiliary feedwater system feeding one of the steam generators combined with the steam generator depressurization on the beginning of the core damage time.
In station blackout accidents, the amount of water available in the steam generators has a paramount influence on the core damage time for the plant. For this reason, most of the power plants rely on auxiliary feedwater systems that are available for several hours even during station blackouts. Maximizing the amount of the water pumped into the steam generators trough the auxiliary feedwater delays the beginning of the core damage time considerably.
The analysis was performed by simulating with RELAP5-3D the following four transients:
1. Station blackout in the absence of the auxiliary feedwater system;
2. Station blackout with one of the steam generators fed by its auxiliary feedwater system for 4 hours;
3. Station blackout with one of the steam generators fed by its auxiliary feedwater system for 4 hours and depressurized by the operators;
4. Station blackout in the absence of the auxiliary feedwater system and with one of the steam generators depressurized by the operators.
In all the station blackout transients, the main feed water systems, the auxiliary feed water systems, the reactor coolant pumps, and the emergency core coolant systems are not powered, while the safety valves on both primary and secondary sides are available.
The power plant response was analyzed and several figures of merits were illustrated for each of the analyzed cases. The key ones are:
1. the peak cladding temperature and the beginning of the core damage time, defined as the time at which the peak cladding temperature reaches 1477 K (2200 F);
2. the total mass of the secondary side released through the valves;
3. the primary system inventory and collapsed liquid level;
4. the secondary system inventory and collapsed liquid level.
The investigation revealed the synergistic effect of the steam generator depressurization procedure combined with the availability of the auxiliary feedwater on the same steam generator.
During a station blackout, the decay power produced within the primary system is initially removed through the steam generators and carried out as latent heat through the valves. For this reason, a lower pressure in the secondary side of the steam generators is desirable as the specific latent heat of steam increases as the pressure decreases and less mass flow should be required to remove the same amount of power. The analysis shows that such effect is always negligible and that the delaying in the beginning of core damage is mainly due to maximizing the amount of the water pumped into the steam generators trough the auxiliary feedwater.
Two sensitivities were also performed to investigate further such results:
1. the effect of the beginning of the auxiliary feedwater injection time and of the beginning of the secondary side depressurization time on the beginning of core damage time in relation to the mass discharged from the secondary side;
2. the effect of the duration of the auxiliary feedwater injection on the beginning of core damage time in relation to the mass discharged from the secondary side.
The results of such sensitivities are aligned with the results of the four previous cases.