As Japan deals with the after-effects of its strongest earthquake on record and an even more devastating tsunami, the world’s attention is focussed on the Fukushima nuclear power plant. The plant houses a number of nuclear reactors, which, despite being shut down during the earthquake, need to be carefully managed to ensure that the nuclear fuel is kept cool. This has not happened, as back up diesel generators failed in the tsunami, leaving pumps unable to circulate water through the reactor to stop the fuel from heating up.
How does a nuclear reactor generate electricity?
Nuclear reactors generate electricity by heating a coolant via a fission reaction. Fission is the process whereby unstable heavy elements, such as uranium, decay into lighter elements. For example, uranium-238, the most commonly occurring natural form of uranium, will decay to lead over a period of several billion years. As the uranium decays, energy is released in the form of gamma rays and fast moving neutrons.
While an element, such as uranium, is defined by the number of protons in the atomic nucleus, many elements come in different isotopes which differ by the number of neutrons in the nucleus. Uranium always has 92 protons, but uranium-238 has 146 neutrons while uranium-234 has 142 neutrons. As uranium-238 decays to lead naturally by emitting neutrons and other forms of radiation, it will spend several hundred thousand years as uranium-234.
However, this natural decay process is not fast enough to be a very useful source of power for a plant. Instead, nuclear power plants are designed so that the decay is sped up in a controlled way, through what is known as a chain reaction. A chain reaction occurs when a neutron that has been released by one uranium decay event collides with the nucleus of another uranium atom, causing it in turn to decay, this time prematurely. This in turn generates more neutrons, when then collide with more nuclei, and so on.
This chain reaction is what enables nuclear reactors to produce useful amounts of power.
So how do you shut down a nuclear reactor?
To shut down a nuclear reactor, you need to stop the chain reaction. To get a chain reaction in the first place, enough of the neutrons that are being produced have to reach enough of the nuclei that are waiting to decay. To shut the Fukushima reactors down, the designers installed neutron absorbing control rods that can be inserted between fuel rods to halt the chain reaction. These were activated when the earthquake occured, absorbing enough neutrons so that the chain reaction was quenched.
However, once the chain reaction stops, ordinary nuclear reactions take over and will continue to generate heat albeit at a lower rate. In fact, even once nuclear fuel is used up, it still needs to be kept cool for a number of years until these natural decay processes eventually fizzle out. This is also the case for a working reactor that has been shut down or fuel that has reached the end of its life – cooling is required to take away the heat generated by regular decay processes.
Thus in order to shut down a nuclear power plant safely, the fuel needs to be kept cool for an extended period of time even once the chain reaction is quenched.
When the Fukushima plant lost its back up power in the tsunami, it lost the ability to pump water around the nuclear fuel. This prevents the water from doing it’s job as a coolant and has resulted in a pressure build up, forcing the operators to periodically vent steam and other gases to keep this pressure in check.
Unfortunately, as the the fuel and coolant heats up, the fuel casings will start to corrode. The casings, which are made of a zirconium alloy, start to grab oxygen from the water, producing hydrogen in the process. If this hydrogen escapes into the atmosphere when steam is vented it will explode, just as we have seen at Fukushima. Furthermore, both these explosions and the corrosion of the fuel rod casing can lead to the release of fuel into the coolant.
This is why the Japanese government has decided to use seawater to cool the fuel. In a worst case scenario, if the temperature is not controlled, leaked fuel can melt and then aggregate, potentially allowing the chain reaction to restart in an uncontrolled way in what is known as a meltdown. Using seawater in this way effectively ends the chance of ever using the plant again but should prevent the prospect of a meltdown.