In a LMFBR the effect of a loss of primary system integrity is a loss of coolant and a loss of core flow, whereas in the gas-cooled and the steam — cooled systems the effect can better be represented as a depressurization.

The calculation of core flow in a LMFBR after a pipe rupture is again a hydraulic balance. The effects are very similar to a flow coast-down but more severe. Figure 2.5 shows a comparison of the flow reduction due to a coast-down and a pipe rupture. The problems are the same; the time scales are different.

However, because in a pipe rupture the system is also losing coolant, it is necessary to make sure that there is sufficient sodium to maintain a cooling circuit. In a loop system this means that the sodium must not drain down to a level such as to cut the main cooling circuit, but in a pool system, despite a primary line break (say between the intermediate heat exchanger and the core inlet), adequate sodium is always provided although the flow rate is reduced.

2.2.3 Depressurization Effects

These effects are very different in each of the three reactor types.

2.2.4.1 LMFBR

In the sodium-cooled system the main effect of depressurization (which may arise from a pipe rupture or a loss of cover gas pressure) is to reduce the pump suction head.

The normal pump characteristic curve (Fig. 2.6) shows the relationship between the mass flow M and the pump head as a function of pump speed w:

pump head = aw2 + bwM + cM2 (2.5)

The figure also shows the system resistance and the normal operating point for the primary system. At this operating point the available net positive

suction head (ANPSH) must be larger than the required net positive suction head (RNPSH) for that pump.

The behavior is different in the case when system pressure has been reduced, for the pump may now be cavitating. The pump characteristic now varies considerably (5).

Figure 2.7 shows predicted behavior following a large pipe rupture. The pipe rupture is first seen as a loss of system resistance and this curve rapidly falls to a new lower position B. The pump attempts to move to a new operating position C by increasing flow and moving down its characteristic

at constant pump speed. However as soon as it moves to that flow where the ANPSH falls below the RNPSH, then the pump cavitates at D. Mean­while, due to the loss of system pressure, the ANPSH curve has been dec­reasing from E to F to G. In order for the pump to operate to balance the system resistance and at the same time always maintain a RNPSH less than or equal to the available head, the behavior of the pump follows the curve A to D to H to J to K. The final flow is very low, satisfying the new system pressure conditions. Figure 2.8 shows transient conditions.

This effect and its representation complicates the prediction of core flows in the analysis of a pipe rupture in this fast reactor system (Section 2.2.3).