Page 197 - Dynamics and Control of Nuclear Reactors

14.6 Reactivity control mechanisms 195

Coolant +40 10 6 Δρ/°C
Moderator +80 10 6 Δρ/°C
In the U.S., the power coefficient is defined as the reactivity change per per-cent
change in power and the power defect is defined as the total reactivity change when
going from zero power to full power. In Canada, the power coefficient definition is
the same as the U.S. definition of the power defect. This confuses the issue a bit. Here
the U.S. definition of the power defect is used.
In PHWR reactors, the power defect is around Δρ¼ 0.003. The negative fuel
effect dominates over positive coolant and moderator effects because fuel temperature
changes much more than coolant or moderator temperature when power changes.

14.5 The void coefficient
Likethe RBMK,PHWR reactorshaveapositivevoidcoefficientofreactivity.Decreas-
ing in-core moderator, as by boiling a normally liquid moderator, in over-moderated
reactors causes a reactivity increase. The coolant in PHWRs contributes to neutron
thermalization in addition to the heavy water moderator in the calandria. Because of
insulation from fuel channels, independent cooling, and the large volume, moderator
boiling is not plausible. So, the issue is boiling in the coolant. Complete voiding of
the coolant channels would cause a large reactivity increase (Δρ¼0.007 to 0.013).
The positive reactivity for complete voiding exceeds prompt criticality and is unaccept-
able. Fortunately, complete voiding is unlikely, and should it occur in an abnormal
event, it progresses slowly enough to permit countermeasures (reactor scram).

14.6 Reactivity control mechanisms
Heavy water reactors have four reactivity control mechanisms.
The calandria contains chambers into which light water can be added or removed.
Since light water is a much stronger neutron absorber than heavy water, introduction
of light water reduces reactivity. The light water chambers constitute the main reac-
tivity control system. The light water chambers also serve for controlling the flux
shape in the reactor core. The water level in individual chambers can be adjusted
to influence the neutron flux in the region around the chamber.
“Adjuster rods” are absorber rods that serve to flatten the flux distribution and are
usually fully inserted. They can also provide positive reactivity when adding light
water to the in-core light water chambers is inadequate. They can also be removed
to help with Xenon override.
“Control absorber rods” are normally positioned outside of the core and are
inserted vertically into the core. They can be used to insert negative reactivity when
the light water chambers are inadequate.
Dissolved poison, typically gadolinium or boron, may be added or removed from
the moderator in the calandria to reduce or increase reactivity.