30 Years Since Chernobyl – How Nuclear Reactors Work

Early in the morning on 26 April 1986, a safety system test at the Chernobyl power plant in Pripyat, now part of Northern Ukraine, ended in a nuclear disaster with catastrophic consequences for both those working at the plant and those living in the surrounding area. 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.

Though I’m a chemistry teacher by trade, the physics behind nuclear power has always held something of a fascination for me. 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. Chernobyl’s legacy is a perhaps understandable wariness and distrust in the safety of nuclear power from a significant proportion of the public, to many of whom it stands as an example of a dangerous series of events that could befall any nuclear plant. 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.

Let’s start with the basics: how do nuclear plants generate electricity? How they do this is not too dissimilar from how it is produced in coal or gas power plants, with the key difference 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 to produce electricity. The steam that drives the turbine is cooled and condensed back to water, which can then be recycled back through the reactor continuously.

In terms of types of reactors, there are two main variations on the above theme for Western reactors. The variations are related to the water that’s heated to produce the steam that drives the turbine. In boiling water reactors (BWR), the 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 (PWR), the 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 Chernobyl reactor was of a different type, the RBMK reactor. This reactor differs from the two described above – but to understand how we first need to know a little more about what’s going on inside the reactor itself.

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. The majority of uranium in the fuel is uranium-238, but a small percentage (3-5%) is uranium-235. Uranium-235 can undergo nuclear fission, and is, therefore, the portion of the fuel we’re interested in.

So what is nuclear fission? Put simply, it’s when an atom splits into two smaller atoms, releasing a lot of energy as it does so. Essentially nuclear fission accomplishes what alchemists spent centuries trying to achieve: turning one element into another. 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-235 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-235 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.

Of course, a chain reaction can quickly get out of control; after all, this is what’s allowed to happen in nuclear weapons! 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 carbon), though 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 similarities with 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.

The RBMK reactor uses graphite as a moderator, whereas water in the reactor acts as what’s known as a ‘poison’ – capturing neutrons much like control rods and slowing the fission reaction. This is a major difference: whereas the loss of water in a normal BWR would stop the reaction, in the RBMK, reactor power will increase if water is lost. Referred to as a ‘positive void coefficient’, this is a significant design flaw that was one of the contributing factors to the accident at Chernobyl. So what 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 01:23:40 by Andrew Leatherbarrow.

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 continue, so it would still need a supply of cooling water from pumps. The test was to see if the power produced as the reactor’s fission slowed was enough to power the water pumps until the backup generators at the plant could come online.

This ‘run down’ feature should have been part of unit 4’s reactor from its initial switch-on, but had been skipped over in a bid to open the plant ahead of schedule. 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, the first mistake was made – the rods were inserted further than planned, dropping the power to a lower level than had been anticipated. When the fusion reaction is shut down, atoms of a particular fission product, xenon-135, increase in number. Xenon-135 is what’s known as a ‘neutron poison’ – it absorbs neutrons and slows the fission reaction down. Xe-135 built up in the Chernobyl reactor, slowing the reaction and reducing the reactivity of the fuel rods.

This was another missed opportunity to halt the test. However, after a heated discussion, the engineers tried to bring the reactor back up to full power so they could attempt the test again. They did this by pulling out a large number of the control rods, but getting the power back up to the original levels was next to impossible due to the xenon poisoning that had occurred, from which the reactor took a significant time to recover.

Undeterred, more control rods were raised from the core until as few as six remained. This was far fewer than the number considered safe, and a higher volume of water than usually permitted was also being pumped into the reactor as a coolant. Computer warnings of reactor instability flashed up and were ignored. The turbine was released from the reactor, as had been intended as part of the test, but this increased the amount of steam in the reactor core, causing dangerous steam voids which could lead to a drastic power increase.

With the reactor’s power clearly skyrocketing, the emergency safety button was pressed to lower the raised control rods back into the core and slow the fission reaction. Here, another flaw reared its head – the control rods may have been made of boron, but their ends were made from graphite, a moderator that caused a further surge in the nuclear reactions, fracturing the fuel rod assemblies and making it impossible to lower the control rods any further. The surge in power, pushing well over safe operating limits, blew the top off the reactor, exposing the core, and a second explosion led to an estimated 50 tons of nuclear fuel being vaporised.

This is, as far as we know, the most likely course of events as they unfolded. We don’t know for definite because of the secrecy of the Soviet authorities in the aftermath of the disaster. They tried to play down the disaster’s severity; the residents of the nearby city of Pripyat weren’t evacuated until over 24 hours after the explosions at the reactor. The Soviet account of the accident blamed the plant operators entirely; though their perseverance with the test was undoubtedly a contributor, more significant were the reactor’s design faults that led to the disaster.

Could Chernobyl happen again? Well, a small number of RBMK reactors are still in use, all eleven in Russia. These reactors have had major modifications made to them since Chernobyl to make them safer and prevent the same chain of events from occurring. As we’ve already mentioned, other modern nuclear reactors are of a different type, which pose a much lower risk.

The areas surrounding Chernobyl are still radioactive today, though much lower than thirty years ago. Chillingly atmospheric drone videos show the abandoned city of Pripyat, but it’s not quite a complete ghost town – around 7000 workers still visit the plant every day to help with decommissioning work. The other three reactors at the site were also still running until 2000, requiring workers to visit the plant every day, and supposedly some elderly residents have resettled within the exclusion zone.

When will the Chernobyl area be safe for people to live in again? As mentioned above, people do live in the exclusion zone right now, but it’ll take thousands of years for the radiation levels to return to normal background levels. In a sense, the area is one big experiment – we don’t know a great deal about how long-term exposure to higher levels of radiation than background affects the health of humans or animals. One thing seems apparent – the absence of human activity in the area has turned it into a veritable nature reserve.

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References & Further Reading

• Pressurised water reactors – Nuclear Tourist

• How boiling water reactors work – UCS USA

• RBMK reactors – World Nuclear.org

• The Chernobyl reactor design features & reason for accident – M V Malko

• Physics of uranium and nuclear energy – World Nuclear.org

• What is a moderator? – N Touran

• Control rods – Nuclear Power.net