Although control rod malfunctions cannot give rise to large step changes of reactivity, during any survey of acceptable reactivity changes, it is useful

to include a parametric survey of the effects of large step changes. As will be seen, it is possible to envelope many other reactivity effects within an acceptable step value.

Figures 2.17 and 2.18 show power rises and temperature rises for a range of step additions which show that approximately 600 would be acceptable for the particular control system considered. It is important to realize that this acceptable value is dependent on the failure criterion chosen (here it is incipient fuel melting) and on the response of the protective system.

Many of the other postulated mechanisms for adding reactivity that follow will actually result in total reactivity changes that are less than the accep­table step value. Thus all can be shown to be acceptable without performing separate reactivity transients in each separate case.