Some reactor conditions require a reduction in power. This can be ac­complished safely through the scram function but it may be not necessary or desirable to scram in every case. A series of partial power reductions is sometimes more appropriate. In sequence: (a) warning, (b) setback in power, (c) controlled shut-down, and (d) scram. Such a scheme would avoid thermal shocks to the reactor components in many instances.

Table 3.9 shows the standard earthquake scale (11,12). Because of the possibility of structural movement bringing about core reactivity changes, the reactor may require power reductions when a serious earthquake occurs. Generally a scram is initiated at a modified Mercalli intensity of V. However, it is more consistent with modern operating practice and modern knowl­edge of seismology to provide a power setback system on the following scheme (11).

At a modified Mercalli intensity of V a warning is provided, at an inten­sity of VII a controlled shut-down would be instituted and then at an inten­sity of VIII a scram would be initiated.

This philosophy can be applied to many secondary warning indications in the plant without sacrificing any of the overall safety of the system. Usually, however, the warning and setback functions are part of the control system rather than the protective system, and it is necessary to coordinate the two systems most carefully to ensure overall safety under this scheme.

3.4.3 Other Safety Features

It is appropriate to discuss safety features in relation to the accidents which they were designed to mitigate.