STEAM DRIVEN SCRAM TANKS

Figure 5 shows the basic design of the SWR 1000 scram systems. The control rods can be inserted into the core in about 70 s by electric drives, or rapid control rod insertion can be implemented within 3 s by means of the hydraulic scram system. In addition there is a boron system which pumps a pentaborate solution into the RPV over a period of some 30 minutes.

The SWR 1000 has four scram tanks operating at a pressure of about 140 bar. In the existing Siemens BWR design this high pressure is generated by nitrogen. The first of the two valves downstream of the tanks is normally in the closed position while the second valve is open. To actuate a scram, the first valve is opened in a very short time and the water is routed from the tanks to the piston drives of the control rods. Several seconds after scram initiation the second valve must be closed. Otherwise the entire water inventory of the tank would be discharged into the RPV, and after that the nitrogen would follow.

It is not allowable for large quantities of nitrogen to ingress the RPV of the SWR 1000, as the emergency condensers would become ineffective given a certain content of non-condensable gases. Therefore, steam serves as the high-pressure medium inside the scram tanks, generated by electrical heating of the upper part of the water inventory such that the upper part of the scram tank is filled with saturated steam and saturated water. Due to the stable stratification no convection occurs inside the tank, and the main inventory of the tank remains cold.

In the event of a scram, the first valve is opened. The cold water flows from the tank to the piston drives of the control rods and the pressure inside the tank decreases. But with decreasing pressure, new steam is generated out of the saturated water inventory such that the pressure reduction is comparably small. Normally the second valve closes after several seconds, as otherwise hot water and steam could ingress the scram system which would generate thermal stresses in the piping which is not designed for strong temperature gradients.

Boron shut down system

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FIG. 5. Conceptual arrangement of the SWR 1000 scram systems.

To avoid thermal stressing in the event that the second valve fails to close, a skirt is integrated into the scram tank. The thermally stratified water behind the skirt remains at its given elevational level after the water level inside the skirt drops due to the outflow of cold water. After a certain time the driving steam comes in contact with the cold parts of the skirt and condenses (see Fig. 6). Thus, the pressure inside the scram tank is reduced by condensation of steam. When the pressures in the RPV and in the scram tank are the same, flow within the scram system ceases despite the fact that both valves are still open.

This scram tank design was experimentally tested at a VVT laboratory in Finland. Results demonstrated that this principle of pressure reduction is effective. Prior to scram initiation, strongly stratified conditions prevailed both inside and behind the skirt. Subsequent to scram, natural circulation developed both behind and inside the skirt. Again, the small pressure differentials of several Pa in the natural circulation circuits resulted in pressure reductions in the scram tanks of some 60 bar and more.

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FIG. 6. Pressure reduction in the scram tank by steam condensation with falling water level.