Page 196 - Dynamics and Control of Nuclear Reactors

194 CHAPTER 14 Pressurized heavy water reactors

Insulation on the fuel tubes reduces heat transfer between coolant in the tubes and
surrounding heavy water moderator in the calandria. In addition, a cooling system
keeps the calandria heavy water at a low temperature (60–65°C) and pressure
slightly above atmospheric.
Several considerations influenced the design decisions for PHWR reactors:
First was the choice of tubes and pipes to contain the high-pressure coolant. High-
pressure tubes and pipes require less advanced fabrication facilities than used in
manufacturing large vessels as those used in light water reactors. Countries without
large vessel manufacturing capability can produce pressure tubes and pipes.
Second was the decision to use natural Uranium fuel. This decision eliminated the
need for Uranium enrichment and the need for large, complex and expensive facil-
ities. This decision also led to the need for heavy water as the moderator and coolant.
Heavy water enables operation with natural Uranium. Of course, producing heavy
water involves isotopic separation of D 2 O from ordinary water, but this is easier than
separation of Uranium isotopes.
The design decisions facilitated implementation in developing countries as well
as in Canada.

14.3 Neutronic features [3]
A PHWR is large in a neutronic sense. Consequently, it is susceptible to spatial
instabilities. The control system addresses power shape control as well as global vari-
ables. PHWR reactors are over-moderated. That is, reduction of moderator or coolant
causes reactivity increases. See Section 7.3.
Since new fuel replaces some of the depleted fuel every day, there is a small
reactivity “bump” and a small change in power distribution every day.

14.4 Temperature feedback in heavy water reactors
The temperature feedback effect on reactivity in PHWRs has three components: fuel,
heavy water moderator, and heavy water coolant. PHWRs have strong negative fuel
temperature coefficients (Doppler effect), positive moderator and coolant tempera-
ture coefficients of reactivity, and positive void coefficient of reactivity.
PHWRs are over-moderated. That is, a decrease in moderator increases reactiv-
ity. Since PHWRs are over-moderated, the moderator and coolant temperature coef-
ficients of reactivity are positive. The large inventory of heavy water moderator in
the calandria is insulated and independently cooled, so its temperature does not
change much during a power transient.
The Doppler coefficient in PHWR reactors is always negative and is the dominant
temperature effect on reactivity.
Typical feedback coefficients for PHWRs are as follows [3]:
6
Fuel: 8 10 Δρ/°C