Page 81 - Dynamics and Control of Nuclear Reactors

74 CHAPTER 7 Reactivity feedbacks

UNBROADENED
ABSORPTION CROSS SECTION s a DOPPLER

BROADENED

E 0
ENERGY
FIG. 7.1
Doppler broadening at a resonance cross section due to increasing temperature, such as in
U-238 and Pu-240 isotopes.

U-238 and Pu-239. The fuel temperature coefficient of reactivity is always negative
in a thermal reactor.
Because fuel temperature changes before changes in any other process variable
following a power change, fuel temperature reactivity feedback occurs quickly after
a power change. It is sometimes called the prompt reactivity feedback. This negative
reactivity helps to limit power changes.
The fuel temperature feedback following a power change depends on the mag-
nitude of the fuel temperature change as well as the magnitude of the feedback coef-
ficient. Fuel temperature changes per unit change in power depend on the heat
capacity of the fuel and the heat transfer resistance between the fuel and the coolant.
See Chapter 10.

7.3 Moderator temperature feedback in thermal reactors
The neutron moderator in a reactor may be a liquid as in a moderator/coolant in
light water reactors, as liquid separate from the coolant as in CANDU reactors, as
a solid as in gas-cooled thermal reactors, fluid-fuel thermal reactors and Russian
RBMK reactors, or absent as in fast reactors. Moderator temperature reactivity
feedback is always slower to respond to a power change than fuel temperature feed-
back. But moderator/coolant temperature feedback changes first for a reactor inlet
temperature change in reactors with liquid that serves both as the coolant and the