Nuclear Power Reactors Updated October Most nuclear electricity is generated using just two kinds of reactors which were developed in the s and improved since. New designs are coming forward and some are in operation as the first generation reactors come to the end of their operating lifetimes. This paper is about the main conventional types of nuclear reactor.
The narrative seems to be a classic cautionary tale against the utilisation of nuclear reactors to generate power, but the reality is more nuanced. Here, we look at how nuclear reactors work generally, what led to the accident at Chernobyl 30 years ago, and the differences between Chernobyl and modern reactors.
In part, it probably stems from a visit to the Hinckley Point nuclear power plant at the age of around eight part of a family holiday — you end up having some occasionally weird excursions when one of your parents works in nuclear safety.
I recall staring down into the reactor hall and being amazed at the thought of the invisible atomic processes occurring below, eventually resulting in the generation of electricity for hundreds of thousands of people.
An interest in the workings of nuclear reactors inevitably leads to an interest in Chernobyl, the one nuclear plant that likely anyone can name. As is often the case, however, the truth is slightly more complicated, and an understanding of how modern nuclear reactors work can help make sense of what happened 30 years ago today.
The manner in which they do this is actually not too dissimilar from how it is produced in coal or gas power plants, with the key different being that the fuel is in the form of heat-producing nuclear reactions instead of these fossil fuels.
The heat generated by these reactions is used to heat water and produce steam, which goes on to turn a turbine. This in turn drives a generator producing electricity. The steam that drive the turbine is cooled and condensed back to water, which can then be recycled back through the reactor continuously.
In terms of types of reactor, there are two main variations on the above theme for western reactors.
In boiling water reactors BWRthe source of the steam that drives the turbine is water in the reactor core; this means that short-lived radioactive substances pass through the turbines, so they must be shielded when the reactor is active.
In pressurised water reactors PWRthe water heated in the reactor is contained under pressure, and used to produce steam in a secondary loop of water which then goes on to turn the turbines. The majority of western nuclear reactors are PWRs. The fuel in the reactor core is contained in fuel rods.
These consist of pellets of uranium oxide, packed into pellets and sealed in a zirconium metal tube. Uranium comes in different forms, or isotopes — these are atoms of uranium that have the same number of protons in the nucleus, but a different number of neutrons.
So what is nuclear fission? Essentially nuclear fission accomplishes what alchemists spent centuries trying to achieve: Using nuclear fission can indeed turn lead into gold, although the expense and the minuscule amount of gold that would be obtained far outweigh any incentives for doing so.
In nuclear reactors, uranium atoms can split into smaller atoms when neutrons collide with them. Remember that neutrons are one of the three subatomic particles that make up atoms, and they can be ejected from atoms by natural radioactive processes. In nuclear reactors, the collision of neutrons with uranium atoms can produce a range of different fission products, along with the heat energy that helps heat the water to drive the turbines.
These processes are also self-sustaining, because when the uranium atom splits into smaller atoms, it also releases neutrons, which can go on and collide with other uranium atoms — therefore starting a self-sustaining chain reaction. For this reason, we need a way of controlling the process in the reactor.
This is where control rods come in.
These are rods made of materials such as boron or cadmium, and their job is to help control the reaction by absorbing neutrons. When the control rods are lowered fully into the reactor, they slow or even stop the chain reaction by absorbing neutrons that could otherwise trigger continued fission.
When they are raised from the reactor, more reactions can take place, and the chain reaction intensifies. As well as the control rods, nuclear reactors also contain substances known as moderators to help promote the chain reaction.
The uranium atoms can capture neutrons more easily if they are moving more slowly. For this reason, the moderator slows the neutrons to a speed where they are easily able to trigger fusion. Moderators can simply be the water present in the reactor, or can also be in the form of graphite a form of carbonthough this is now very rarely the case.
We now know enough about the workings of nuclear reactors to explain how the Chernobyl RBMK reactor works. It has some similarity with the boiling water reactors, in that water in the reactor is turned to steam to drive turbines.
However, the fuel is contained in individual pressurised tubes, rather than the single pressurised vessel that houses the fuel in BWRs. This is a major difference: So what actually happened?
There are already far better written accounts of the ins and outs of events at Chernobyl thirty years ago, so the explanation here will be kept relatively brief — though how the events unfolded is enthralling, and for a more in-depth account, I highly recommend Chernobyl The accident occurred in unit 4 of the power station, during planned maintenance.
During the maintenance, it was also planned to carry out a test to see how the reactor would run during a power failure. In the event of any power failure, of course the fission reaction would still continue, so it would still need a supply of cooling water from pumps. If it had been completed, as should have been required, the entire test would have been unnecessary, and the accident could have been avoided.
During the test, the control rods were intended to be lowered halfway into the reactor, to simulate a power cut.Here, we look at how nuclear reactors work generally, what led to the accident at Chernobyl 30 years ago, and the differences between Chernobyl and modern reactors.
Though I’m a chemistry teacher by trade, the physics behind nuclear power has always held something of a fascination for me.
Here, we look at how nuclear reactors work generally, what led to the accident at Chernobyl 30 years ago, and the differences between Chernobyl and modern reactors.
When they are raised from the reactor, more reactions can take place, and the chain reaction intensifies. A fission fragment reactor is a nuclear reactor that generates electricity by decelerating an ion beam of fission byproducts instead of using nuclear reactions to generate heat.
By doing so, it bypasses the Carnot cycle and can achieve efficiencies of up to 90% instead of 40–45% attainable by efficient turbine-driven thermal reactors.
In some nuclear power plants, the steam from the reactor goes through a secondary, intermediate heat exchanger to convert another loop of water to steam, which drives the turbine. The advantage to this design is that the radioactive water/steam never contacts the turbine. Nuclear reactors are designed to sustain an ongoing chain reaction of fission; they are filled with a specially designed, solid uranium fuel and surrounded by water, which facilitates the process.
When the reactor starts, uranium atoms will split, releasing neutrons and heat. A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a self-sustained nuclear chain reaction.
Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships.