Page 252 - Dynamics and Control of Nuclear Reactors

APPENDIX B Advanced reactors 253

depend on fuel temperature, coolant temperature, moderator temperature (in thermal
reactors), pressure (primarily in reactors with boiling coolant and temperature of
structural components (feedback due to dimensional changes and primarily in fast
reactors).
All of the advanced reactors have a negative fuel temperature coefficient of reac-
tivity. That is, a fuel temperature increase causes a decrease in fission rate. Most, but
not all, of the other feedbacks in the various advanced reactors are also characterized
by negative coefficients of reactivity. For example, the pressure coefficient is pos-
itive in reactors with boiling coolant and the temperature coefficient of the coolant is
positive in reactors with significant concentrations of a neutron poison in the coolant.
The rates of change of the conditions (temperature, boiling, etc.) of reactor com-
ponents determine the speed of associated reactivity feedbacks. These rates of
change depend on heat transfer coefficients and the heat capacity of the component
and is characterized by the time constant. Long time constants mean that the asso-
ciated response lags behind the initiator of a transient. Time constants of reactivity
feedbacks vary significantly among the advanced reactors. The time response of
reactor components has two impacts on system dynamics: reactivity feedbacks
are delayed and reactor component temperatures change more slowly if time con-
stants are large. For example, graphite-moderated, gas-cooled reactors have large
heat capacities and they respond slowly to a disturbance. It should be noted that neg-
ative feedbacks can be destabilizing if lagged behind the transient initiator.

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