The main difference between discharges through the safety valve and a pipe break from the steam plenum is the position of the discharge orifice. The pipe from the steam plenum is located lower than the safety valve. In normal regimes it opens into the steam volume. But during the discharge process, it may some times be covered by water or two phase liquid. In this case the total drain quantity of water depends on the opening is covered or not.

Discharge from different heights ( distance to the initial water level) has been tested.

In the case of small heights at the beginning of the discharge process the level of two phase liquid ( cause by the vaporization ) may be higher than the discharge orifice. It causes a mixed discharge of water and steam, and increased the total drain quantity. Specially for the bigger orifice diameter the discharge process is stronger, the total drain quantity is increased notable. This can be seen in Fig. 4. For the case of mixed discharge, the total drain quantity may be increased from 19 % to 25 % of initial inventory ( orifice diameters are increased from 4. 3mm to 8. 5mm respectively ) . The less the height difference between the initial level and discharge orifice, the more the total drain water quantity.

Tests showed that when the water inventory increased from 71% to 84% of total volume the total drain water quantity increased from 17.1 % to 20.3% of the total water inventory.