SAFETY-RELIEF VALVE SYSTEM

This system is commonly used in most existing BWR plants. However, the system used in the SWR 1000 features some differences to previous standard designs:

■ Three of the six valves work on the pressurization principle and the other three on the depressurization principle, so a common mode failure is not possible due to diversity;

■ The quenchers are located in the flooding pool (instead of the condensation pool) at a comparably high elevational position (about 2.5 m below the water level);

■ The main valves are activated passively for automatic depressurization by means of diaphragm pilot valves, which in turn are activated by passive pressure pulse transmitters.

On the primary side a forced-flow condition prevails as long as the RPV pressure is more than 0.25 bar higher than the pressure in the drywell. In the pool there would normally be a forced — flow condition near the quenchers and natural circulation at distances greater than 1 m away from the quenchers. With an arrangement such as that in existing BWR plants, only the pool water above the quenchers (or only about 40% of the water inventory) could be used as a heat sink due to the stratification of the warm water above the cold. In the SWR 1000, the quenchers have been modified to prevent this limitation. The quenchers are mounted to the wall of the flooding pool and the more than 2000 quencher holes are directed solely towards the center of the pool. This gives a maximum pulse of about 50 kN for each quencher directed into the pool. These enormous forces lead to forced convection in the pool and to a complete mixture of the water inventory. Restrictions due to natural convection are thus eliminated entirely.