Section 1.4 showed that reactivity changes arise through perturbations to Mission, and through changes in leakage. The reactivity variations are input into the calculational model through keff in the kinetic equations.

Such cross-section and leakage changes can arise from: (a) feedback effects from temperature changes, pressure changes, and structural move-

ment; and (b) external influences from control rod movement, the addition of poisons or moderator due to component failure, or core voiding in the sodium system. The magnitude of the reactivity effects from each cause is the concern of the physicist who evaluates them, using steady-state codes, as if the changes were a set of pseudo-steady states. These reactivity changes are then used on the presumption that the changes in reactivity occur more rapidly than the initiating mechanisms.

Thus the reactivity input is:

Л*(0=ЛО + МГі(0 (2.6)

where f(t) represents external influences and ST^t) symbolizes temperature and pressure changes which give rise to feedback effects. The latter are linked to relevant model equations which produce those temperatures and pressures.