Page 85 - Dynamics and Control of Nuclear Reactors

78 CHAPTER 7 Reactivity feedbacks

and a reactivity decrease. The moderator temperature coefficient of reactivity is
always negative in under-moderated reactors with liquid moderator-coolant. How-
ever, poison (typically boric acid) is sometimes dissolved in a coolant/moderator.
Thermal expansion causes removal of the dissolved poison from the core along with
the coolant. This poison removal is a positive reactivity effect. Therefore, the coolant/
moderator temperature coefficient may be positive or negative, depending on the con-
centration of dissolved poison. This can happen, especially at the beginning of cycle
when the dissolved boron concentration is the highest, and an increase in the moder-
ator temperature decreases the boron nuclei per unit volume, thus have a positive
effect on reactivity.
CANDU reactors have pressurized coolant channels embedded in a large tank
(calandria) filled with heavy water. Heavy water also serves as a coolant for the fuel
in the channels. Both heavy water regions contribute to neutron thermalization.
CANDU reactors are over-moderated. Therefore, the moderator temperature coeffi-
cient of reactivity is positive.

7.4 Pressure and void coefficients in thermal reactors
Thermal reactors with liquid coolant experience reactivity changes when voids in the
coolant undergo concentration changes (even liquid-cooled reactors can have small
bubbles in the coolant). The quantity of coolant in the core decreases as liquid boils,
thereby reducing absorptions in the coolant. This is a positive reactivity feedback.
Also, the slowing down of neutrons to thermal energy decreases because of a reduced
moderator concentration. This is a negative reactivity feedback. In under-moderated
reactors the net effect is a negative void coefficient.
There are two types of existing power reactors that are over-moderated and have
positive void coefficients, The Soviet RBMK (see Chapter 11) and the Canadian
CANDU reactor (see Chapter 14). Both types have moderator (graphite in the RBMK
and heavy water in the CANDU) that is physically separated from the coolant chan-
nels. The coolant is boiling light water in the RBMK and liquid heavy water in
the CANDU.
In reactors with in-core boiling (BWRs) voids decrease when pressure increases.
Therefore, they have positive pressure coefficients of reactivity. Active, full-blown
boiling in liquid coolants is an abnormal occurrence. Voiding in this situation is a
factor in determining the response to the abnormal occurrence.

7.5 Fission product feedback
A change in reactor power causes a change in the fission product inventory. The
new equilibrium inventory after a power increase causes increased fission product
production, increased neutron absorptions and reduced reactivity, but effects on
reactivity vary during the transition to equilibrium.