Nuclear power stations work by starting a chain reaction in the uranium fuel. This reaction causes the splitting (or "fission") of uranium atoms into other radioactive elements, and gives off very large amounts of heat. This heat needs to be continually transferred away from the reactor.
It is not physically possible to convert all the heat generated in a nuclear power reactor into electricity. The safe operation of a nuclear power station depends on its cooling systems, which remove the heat from the reactor during normal operation and when the reactor has been shut down.
Nine out of the UK's ten existing nuclear power stations use carbon dioxide gas (CO2) to cool the reactor core. Sizewell B, the UK's only pressurised water reactor (PWR), instead uses pressurised water as the coolant. In each case the coolant picks up heat from the reactor core and carries it away. In a PWR, the coolant leaves the reactor at about 300°C.
Multiple cooling systems
Once it has picked up enough heat from the reactor, the coolant itself needs to be cooled. This is the job of the secondary cooling system, sometimes known as the steam circuit, where water is the coolant. The secondary coolant picks up heat from the primary coolant – the CO2 or pressurised water – in heat exchangers or boilers. The primary and secondary coolants do not mix directly in the boilers. The secondary coolant exits the boilers as high pressure steam, and the primary coolant circulates back through the reactor core . This steam is used to drive the power station’s turbines. The turbines convert much of the steam's heat energy into mechanical energy, which is then converted into electrical energy by a generator.
Before it is pumped back around the steam circuit to the boilers, the steam needs to be cooled further and condensed back into water. To achieve this, the steam from the turbine passes through another heat exchanger, the condenser, where it comes into contact with cold water-filled pipes.
The majority of the UK's nuclear power stations are situated on the coast, so the water for the condenser can be pumped in directly from the sea . It passes through the condenser just once before being discharged back into the sea, at a slightly higher temperature than when it entered . None of the water discharged into the sea from the condenser has ever entered the reactor, come into direct contact with the radioactive fuel, or mixed with any coolant that has.
Even after the reactor is shut down, the radioactive by-products of fission continue to give off some residual heat (known as decay heat). Back-up cooling systems are provided to remove this heat from reactor to prevent it overheating.
Cooling in the Fukushima nuclear accident
In March 2011, an earthquake and tsunami hit the Fukushima Dai-Ichi nuclear power station in Japan. The combined effect of the earthquake and subsequent tsunami knocked out the power station’s connection to Japan’s electricity grid and flooded its backup diesel generators . Deprived of power, the power station’s cooling systems failed, and without cooling, the reactors overheated.
Fukushima Dai-Ichi used boiling water reactors (BWRs) , a nuclear reactor design that has never been used in the UK. The UK's existing nuclear fleet includes some Magnox reactors, some advanced gas-cooled reactors (AGRs) and the PWR at Sizewell. The way Magnox reactors and AGRs are designed means that even without power the CO2 coolant continues to circulate naturally. This means these power stations would take much longer than Fukushima to overheat following a loss of power, giving their operators more time to take remedial action.
Sizewell B has one of the most robust cooling systems of any PWR in the world. In addition to its main cooling system, it has two auxiliary systems – one driven by back-up diesel generators and one driven by the heat from the reactor itself – and an emergency system.
In the Fukushima accident, some of the overheating reactors boiled away enough of their coolant water to expose the fuel rods. The hot metal cladding of the fuel rods reacted with steam to produce hydrogen, which caused explosions when it was vented and mixed with oxygen. This could never happen in the UK's Magnox or AGR power stations, as these designs are cooled using CO2, not water. Sizewell B is cooled using water, but has safety systems specifically designed to prevent hydrogen forming.
The reactor designs currently being considered for the UK's next generation of nuclear power stations have multiple back up safety and cooling systems to ensure that adequate cooling is maintained under all credible circumstances.