This branch of nuclear engineering is of the highest importance in reactor safety, since it explains the question:
Why can’t we just turn off nuclear reactors?
Why do reactors have to be cooled for long periods, while the chain reaction can be stopped almost immediately?
There is an extra energy source known as decay heat, that contributes to thermal power in power operation, but dominates in shutdown mode. Decay heat is the heat released as a result of radioactive decay of fission products accumulated in the fuel. To understand the processes that are after reactor shutdown, we have to explain the term “residual heat”.
At power operation, the fission reaction is responsible for the power generated in a nuclear reactor, and the fission reaction rate is proportional to the neutron flux, so you might expect that thermal power output is proportional to nuclear power. The relationship between flux and thermal power, however, is not linear. In other words, after reactor trip, the nuclear power drops almost immediately, since the chain reaction is stopped almost immediately. On the other hand the thermal power decreases more slowly, for example, due to decay heat production. In this case the thermal power output is not, by no means, proportional to nuclear power.
See also: Thermal Power vs. Nuclear Power
In general, sources of these non-linearities that contribute to the residual heat are usually classified into four categories:
- Accumulated Heat
- Residual Nuclear Power
- Decay Heat
- Pump Heat
In power operation, the fuel and all structural materials contain huge amounts of accumulated heat. Note that, central region of fuel pellets can reach up to 1000°C. After reactor shutdown, also this heat must be transferred into the reactor coolant. This component of residual heat dominates in first seconds after shutdown, but disappears after about 30 seconds.
Generally, reactor coolant pumps are very powerful, they can consume up to 6 MWe each. About two-thirds of its power input appears as heat, known as pump heat, in the reactor coolant (about 16 MWth) as long as the reactor coolant pumps are operated. Heat is generated by fluid friction in the turbulent flow and depends on the fluid viscosity and the flow rate. This flow is forced just by reactor coolant pumps.