Reactor Theory (Reactor Operations)
Reactor Operation Summary (Cont.)
Three methods are used to shape or flatten the core power distribution.
Use of reflectors
Installation of neutron poisons
Axial or radial variation of fuel enrichment
Power tilt is a non-symmetrical variation of core power in one quadrant of the
core relative to the other quadrants.
Shutdown margin is the instantaneous amount of reactivity by which a reactor
is subcritical or would be subcritical from its present condition assuming all
control rods are fully inserted except for the single rod with the highest integral
worth, which is assumed to be fully withdrawn.
The stuck rod criterion is applied to the shutdown margin to ensure that the
failure of a single control rod will not prevent the control rod system from
shutting down the reactor.
Several factors may change during and after the shutdown of the reactor that
affect the reactivity of the core.
Control rod position
Soluble neutron poison concentration
Temperature of the fuel and coolant
Decay heat is always present following reactor operation due to energy resulting
from the decay of fission products.
The amount of decay heat present in the reactor is dependent on three factors.
The pre-shutdown power level
How long the reactor operated
The amount of time since reactor shutdown
Decay heat immediately after shutdown is approximately 5-6% of the pre-
shutdown power level. Decay heat will decrease to approximately 1% of the
pre-shutdown power level within one hour of reactor shutdown.