In the NHR-5 some pipes are connected under the water level, such as the boron injection system pipe, the pipe for the hydraulic driving facility for control rods etc. A pipe break of this type is more dangerous. In the case all the water above the inlet of the pipe will be discharged as well as some water lower than the inlet because of siphonage.

A discharge test has been made to model a pipe breaking in the boron injection system. The boron injection pipe is opened under water at a level equivalent to the middle of the chimney.

In the test steam content, drain water quantity, liquid level, pressure and their transient characters have been measured.

The typical pressure change during the test is shown in Fig. 5. The change of drain water quantity is shown in Fig. 6.

From these figures it can be seen that at the beginning of the drain process discreasing of the pressure wasn’t very fast, but increasing of the drain water quantity was quite considerable. This can be understood, because at the beginning water was discharged, vaporized steam compensated for the change of water volume, so the pressure changed slowly. When the liquid level was lower than th. e opening of the discharge pipe, steam and gas began to drain. The pressure decreased faster, but the drain water quantity changed reasonably slowly. The decrease of the liquid level from linear changed to slower. Eventually draining stopped because of losing heat and pressure.

When the liquid level was lower than the opening of the pipe, some water was drained because of siphonage. The greater the distance between the pipe opening and the liquid level, the less the drain water quantity. Consequently there was a smaller the decrease of the level.

The relationship between the drain water quantity and the height (to the liquid level) of the opening is shown in Fig. 7. From this can be seen that when the height is more than 5cm, the height almost doesn’t effect on the drain water quantity.