A flow disturbance can result from either a loss of power to the pump motors causing the pump to coast down or a mechanical failure of the pump. The former coast-downs can occur in one or more pumps simul­taneously depending only on how the pump motor electrical supply lines are interconnected. However, a mechanical failure of a pump is expected to occur in only one pump at any time since it is a very unlikely fault.

The loss of electrical supply may occur in two or three primary pumps or in one primary and a secondary pump, just what happens to the core flow depends on a detailed flow balance in the system. Flow conservation equations are used together with assumed pump characteristics to derive core flow as a function of time t. Such flow coast-down curves take the form:

relative flow = 1 — [t/(t + ?j)] (2.3)

In a typical pump failure in an LMFBR due to a loss of electrical power the time constant is of the order of 2 sec, while a pump seizure would be somewhat faster with a tx of the order of 1 sec.

The core flow due to a mechanical failure of a pump in one loop of a two-loop plant might typically take the form:

relative flow = a + b tan(c — dt) (2.4)

where a, b, c, and d are constants.

This core flow behavior is input to a calculational model to predict changes in coolant and fuel temperatures with and without a reactor trip in the event of either of these flow perturbations. Figure 2.4a shows a typical flow rundown due to a loss of all electrical power to the pumps of a LMFBR, while Fig. 2.4b shows the resulting core temperatures.

It will be noted in Fig. 2.4b that the initial temperature rise is cut back as the reactor is scrammed, but it rises again as the flow drops rapidly to 5% or less than the decay power level. However the temperatures again decrease as the power decays still further and becomes less than the flow level relative value.

Notice that the monitored temperatures are the coolant and cladding temperatures as the former first changes due to the flow decrease and the failure of fuel pins would be the result of excessive cladding tem­peratures.

2.2.2.1 Trips

In the event of a flow perturbation the following trip signals would be available: (a) primary cause signals, electrical supply board power loss, or pump speed indication; (b) flow (say 85%) and power-to-flow rqtio; and (c) outlet temperature. These trips are in order of occurrence. Figure 2.4b assumes a flow trip.