2.4.3.1 LMFBR

The tertiary loop in a power-producing LMFBR is the steam cycle, but in an experimental reactor such as FFTF (6), a bank of blowers constitutes the tertiary heat dump.

In a power-producing LMFBR there may be only a single steam cycle and feed loop even though the primary loop may be split into three. In this case a tertiary failure (feed-valve closure or failure of feed supply for example) would affect all the primary loops in the same way. However in an experi­mental system each of the primary and secondary loop systems can have its own heat dump air blast heat exchangers and then a failure in one of these would only affect a single primary and secondary loop combination.

In any case there are considerable delays (two cold leg circuit times) of the order of 20-30 sec before any tertiary loop disturbance is felt by the LMFBR core. The only requirement is that one fault should not remove all heat removal capability from the system, since even when the core is shut down successfully, the decay heat must still be removed.

The reactor can best be tripped from an intermediate heat-exchanger high primary outlet signal, although signals would available during the transient from the following sources: (a) tertiary component signals such as electrical supply circuits for loss of electrical supply initiators or pump speed indicators for a failure of the feed pump; (b) IHX primary outlet temperature; and (c) core outlet temperature.

Figure 2.22 shows assumed flows in primary, secondary, and tertiary loops which might arise if a LMFBR lost feed supply. Figure 2.23 shows typical steam generator sodium outlet temperatures that might arise from this failure together with IHX secondary temperatures and the IHX primary outlet temperature which eventually initiates a trip at about 8 sec. Figure 2.24 shows the reaction of primary and core temperatures. In fact the first indication in the core that a failure has occurred is that the system shuts down; no excess temperatures are recorded.