Page 84 - Dynamics and Control of Nuclear Reactors

7.3 Moderator temperature feedback in thermal reactors 77

to travel to core boundaries and leak from the core and the opposite occurs with
decreases in mean free path. The effect of a temperature increase is to decrease mod-
erator density, increase neutron mean-free path, increase neutron leakage and reduce
reactivity. The scattering component of the moderator temperature coefficient of
reactivity is always negative.
Temperature increases in coolant/moderator cause a density increase and,
consequently, fewer coolant/moderator atoms in the reactor core. Since coolant/
moderators have non-zero absorption cross sections, temperature increase decrease
neutron absorptions. The absorption component of the coolant/moderator coeffi-
cient of reactivity is always positive.
Reactor design includes evaluation of the effect of moderator population on
available reactivity. Fig. 7.4 illustrates the effect.
Too little moderator is sub-optimal in producing available reactivity by slowing
neutrons to thermal energy. A reactor with too little moderator is said to be under-
moderated. Too much moderator is sub-optimal in producing available reactivity
because neutron absorptions override the effect of additional slowing down in the
moderator. A reactor with too much moderator is said to be over-moderated.
Light water reactors are designed to be under-moderated. That is, moderator
removal causes a reactivity decrease. Thus, a temperature increase in a liquid
moderator-coolant causes a decrease in density, a decrease in neutron slowing down

under-moderated over-moderated
k eff

REACTIVITY Resonance escape probability (p)

Thermal utilization factor (f)

MODERATOR-TO-FUEL RATIO (N m /N U )
AS TEMPERATURE INCREASES
FIG. 7.4
Effect of moderator-to-fuel ratio on reactivity in under-moderated and over-moderated
reactors.
Adapted from www.nuclear-power.net.