To get some perspective of plant and core level limitations, Table 3.5 shows a typical set of trip points for a LMFBR system. These levels also constitute a set of failure criteria, at least as far as continued operation of the plant is concerned. The operator is concerned with remaining inside these values, which necessarily means that he will also not approach the previously discussed fuel pin failure criteria because a considerable margin of safety is provided by the choice of the low trip points.

Other trips may be possible and desirable. However it must be remembered that too many trips constitute a safety hazard because they cause operator frustration in shutting down the reactor when the operator’s job is to keep it operating safely. This situation is worse on an experimental system, where the operators are under considerable pressure from experimentalists to maintain constant power conditions.

A power rate-of-change trip (period meter) is sometimes required as it gives a very early indication of things beginning to change. However the device is noisy [being a divider to give P(dPjdtY1] and is not particularly

in favor among control designers. A reactivity meter which derives the reactivity as a function of time from measurements of flux as a function of time is a requirement in modern LMFBR systems.