Nuclear reactors are designed to have inherent power controllability, i. e. to have a negative power reactivity coefficient. Nuclear reactors are also designed to have excess reactivity for controlling the core against changes in the power as well as the burnup reactivity to allow its operation for the designed period. The excess reac­tivity is adequately designed so that the reactor is safely operated and shut down.

[1] Reactivity coefficients

The reactivity coefficients indicate the change in the reactivity against the

temperatures of fuel, structure and coolant, and the coolant void fraction etc.

They depend on the plutonium content, uranium content and burnup condition.

(a) Doppler coefficient: This is the ratio of the reactivity change and the change in the effective fuel temperature. The value is negative as long as the fissile enrichment is not too high. When the fuel temperature rises due to the increase in the power or other causes, thermal motions of nuclei become stronger and the apparent width of the resonance absorption cross section curve of U and Pu is expanded. This increases resonance absorption of neutrons by those nuclei. Since the Doppler effect is mainly provided by the resonance absorptions of U and Pu, it gets stronger for the core with more neutrons at the resonance energy. The Doppler effect dominates the reactivity feedback against the change in the reactor condi­tions. Thus, the power reactivity coefficient, which is obtained by combin­ing all the reactivity effects, is always kept negative for all the operating regions and hence the reactor has an inherent safety feature.

(b) Fuel temperature coefficient excluding the Doppler effect: This is the ratio of the reactivity change by thermal expansion of fuel elements mainly in the axial direction to the change in the fuel temperature causing the thermal expansion.

(c) Structure temperature coefficient: This is the ratio of the reactivity change by thermal expansion of structures to the change in the structure tempera­ture causing the thermal expansion.

(d) Coolant temperature coefficient: This is the ratio of the reactivity change by a decrease in the coolant density to the change in the coolant temperature causing the density change.

(e) Core support plate temperature coefficient: This is the ratio of the reactivity change by enlarging the fuel assembly gap caused by thermal expansion of the core support plate to the change in the temperature of the core support plate.

(f) Void reactivity: This is the reactivity change when a void is generated in the coolant. Fast reactors are designed so that the coolant does not evaporate at normal operation, anticipated abnormal occurrences and even design basis accidents, and that gas bubbles are not formed. For the purpose of defining and calculating the void reactivity, the following assumptions are made.

• Gas bubbles formed in the primary system due to a certain cause pass through the core.

• The coolant evaporates although it is technically not possible.