Page 108 - Dynamics and Control of Nuclear Reactors

102 CHAPTER 8 Reactor control

The total power production in all of the power plants on a grid must exactly match
the power demand by consumers on the grid. A mismatch between power production
and power demand causes a change in the frequency of the alternating current. This
change in frequency causes load following plants on the grid to change power pro-
duction until the frequency returns to its set point (60Hz in the U.S.). Note that an
operator induced change in the power production of a base-load plant (like most
nuclear plants) causes a grid frequency change and, consequently a change in power
production in load following plants on the grid.

8.8 The role of stored energy

Power reactors can supply added steam upon demand before the reactor power
changes. This is through use of energy stored in reactor fluid and metal compo-
nents. Fluids boil and metal components cool down to provide energy. For exam-
ple, stored energy in a typical PWR can provide around 10 full power seconds per
psi of pressure drop. Even in a BWR, stored energy can be used temporarily. Even
though opening the BWR main steam valve causes pressure reduction, increased
boiling, reduced reactivity and reduced power, this can be tolerated if control
action inserts reactivity to cancel the temporary reactivity decrease. The steam
delivered to the turbine during this temporary episode is provided mostly by energy
stored in saturated water.

8.9 Steady-state power distribution control
It is desirable for the power distribution to be uniform throughout the reactor core. A
uniform power distribution would cause equal fuel consumption throughout the reac-
tor, and consequently, better economic performance. But neutron leakage at the
periphery, moderator density variations in some reactors and position of control rods
used to suppress available reactivity all cause non-uniform power distributions.
There are naturally occurring effects, refueling procedures, and control actions
that can make the power distribution more uniform. The naturally-occurring effect
is higher fuel burnup and fission production in regions with higher power densities.
This causes a power density reduction in those regions as the reactor operates.
The refueling procedure in light water reactors involves shuffling old fuel
towards the center of the core and adding fresh fuel at the periphery. This causes
a flattening of the power distribution.
Control actions involve positioning control rods where they can aid in flattening
the power distribution. Control rods include full-length full-strength rods, full-length
part-strength rods, part-length rods, and fixed-position burnable poison rods. Full-
length full-strength control rods have a strong neutron poison throughout the length
of the rod. Therefore, they cause a localized reduction in power density along their
whole insertion length.