Some Facts About Neutron and Reactor Physics - The Risks of Nuclear Energy Technology - page 22

Fig. 2.8 Masses for 1 ton of initial reactor fuel after a burnup of 60,000 MW th /t
2.9 Afterheat of the Fuel Elements After Reactor
Shut Down
For reactor shut down the absorber/control elements are inserted into the reactor
core and the coolant flow is drastically reduced. After reactor shut down the fuel
elements—having reached their maximum burnup—can be unloaded. However,
although the power is shut down, the gradually decaying fission products generate
heat in the reactor core, even if the neutron fission chain reaction has been
interrupted (after shut down of the reactor core). This afterheat, or decay heat, is
composed of the contributions by the decay chains of the fission products and of
contributions of radioactive decay by U-239, Np-239, and the higher actinides,
which are unstable. It is a function of the power history of the reactor core before
shutdown and is thus strongly influenced by the burnup of the fuel. Figure 2.9 shows
the relationship between the power of the fuel elements in the reactor core of a PWR
after shut down, P(t), and the power during operation, P 0 . The afterheat, P(t), drops
very sharply as a function of time. Shortly after shut down it is about 6 %, after 6 h it
is still about 1 %, after 1 week 0.3 %, after 3 months about 0.1 %, and after 1 year it
is 0.04 % of the nominal reactor power, P 0 , during operation.
After reactor shut down this afterheat must be transferred to the cooling towers
or a river by the normal cooling system. After unloading from the reactor core the
spent fuel elements are cooled in intermediate spent fuel storage pools [ 11 ].