Page 153 - Dynamics and Control of Nuclear Reactors

150 CHAPTER 12 Pressurized water reactors

coefficient of reactivity. Since fuel temperature changes much more rapidly than
changes in coolant temperature following a power change, the fuel temperature reac-
tivity dominates prompt reactivity feedback following a reactor power change.
The feedback reactivity due to coolant temperature changes occurs because tem-
perature changes cause density changes and because temperature changes cause
changes in the thermal neutron spectrum (see Section 7.3).
Coolant density changes affect the quantity of coolant in the core, and, conse-
quently, parasitic absorptions in the coolant and thermalization of fission neutrons.
Increased temperature decreases the rate of parasitic absorptions, increases reactivity
and causes a positive component of the coolant temperature coefficient of reactivity.
Increased temperature reduces the quantity of moderator per unit volume in the core,
reduces neutron thermalization, decreases reactivity and causes a negative compo-
nent of the coolant temperature coefficient of reactivity. The thermalization effect
dominates when dissolved poison concentration is low or zero. In other words,
the reactor is under-moderated.
Dissolved neutron absorber (boric acid) is used to reduce available reactivity in
PWRs. Early in core life, available reactivity is large. Using dissolved neutron
absorber has two advantages: it reduces the need for control rod reactivity strength
and it controls reactivity without causing local power spikes or dips.
However, the presence of a strong absorber in the coolant/moderator affects the
coolant temperature coefficient of reactivity. A coolant temperature increase reduces
coolant density, thereby forcing coolant and contained boric acid out of the core.
Consequently, the coolant temperature coefficient of reactivity is positive when
the boric acid concentration is high. This is the case early in the life of a fuel loading
when a large boric acid concentration is used to offset a large available reactivity.
This affects transient behavior, but the fuel temperature coefficient of reactivity
(always negative) ensures satisfactory transient behavior.
Increases in moderator temperature cause hardening of the thermal neutron spec-
trum. This causes a negative component of the reactivity change due to changes in
U-235 absorptions and a positive component due to changes in Pu-239 absorptions.
See Section 7.3.
As in all thermal spectrum power reactors, burnup and production of Xe-135
causes a reactivity feedback effect. See Section 6.2 for the Xe-135 effect on power
variations.
The power coefficient of a PWR is always negative. The Doppler coefficient is
dominant, including when the moderator/coolant reactivity coefficient is positive.
Consequently, the response to an external reactivity change is a transition to a
new steady state power level.

12.7 Power maneuvering

Power may be changed by operator action or, in load following plants, by changes
specified by changes in the demand from the electrical grid. A logical place to
start in designing power maneuvering capability is to consider the way a plant