Heat is produced in a nuclear reactor during operation, and in a nuclear power plant the heat is commonly carried away by a medium, usually water though possibly gas, to raise steam to turn a turbine and generate electricity.
However, unlike a fossil fuel-fired power plant, in which there is no heat generation after shutdown, a nuclear reactor will continue to produce heat after nuclear fission has stopped, owing to the decay heat that is being continuously released from the unstable products of nuclear fission. While the quantity of decay heat may not be large, which is at about 7% of the full reactor power at the moment of reactor shutdown, falling to about 0.5% after 24 hours, the heat needs to be continuously removed to keep reactor temperature within limits and therefore continuous cooling of the reactor is required.
In a nuclear power plant, a dedicated backup cooling system is installed using pumps and heat exchangers with sufficient duplication and partitioning so that the heat will be reliably extracted and discharged into the environment without any release of radioactive products.
Of greater interest or importance is the effective removal of reactor heat in an unlikely failure of the cooling system for the reactor, due for example to a major breach in the cooling system pipework. Consideration of such a postulated scenario, among many others, has contributed to the safety arguments for the conventional nuclear power plant, and for a water-cooled reactor in this particular situation. Backup cooling systems that may operate at different pressure levels will be activated in stages with their water pumps, heat exchangers and water in storage tanks to maintain prolonged and effective cooling of the reactor.
Some advanced nuclear power plants that are entering operations take a different approach. Instead of using backup cooling systems that contain pumps which require power input, these plants rely on the energy in the high temperature breached water from the reactor cooling system to drive natural convection of the air inside the containment vessel for the reactor cooling system where reactor heat may traverse the vessel wall and dissipate effectively into the outside.
This article is contributed by Ir Richard Fung with the coordination of the Nuclear Division.