Revisiting Nuclear Power : Part 2 : The Danger

A couple of weeks back, I wrote a little about how governments are reconsidering their attitude to nuclear power, and talked about the mechanics of how a reactor works and how it can solve the problems of getting us to net zero carbon emissions and securing energy supply in the long term. 

But we can’t gloss over the bad reputation that nuclear power has. Can’t it blow up, like a nuclear bomb ? And haven’t there been a number of high profile nuclear accidents ? And even if we can prevent those problems, what do we do about all the waste ? Any debate about nuclear has to address these questions. In this piece I’ll address some of the major nuclear accidents that have occurred since the first nuclear reactors were built, in support of the contention that the safety issues and dangers need to be viewed in perspective relative to the safety of using fossil fuel and of other industrial processes in general.

There have been four serious nuclear accidents that most people have heard of : the Windscale fire in 1957, the Three Mile Island accident in 1979, Chernobyl in 1986 and Fukushima Daichi in 2009. 

Windscale Reactor
The piles at Windscale Picture: Sellafield Ltd

The events which led to the Windscale fire began in the nuclear arms race that marked the early period of the cold war. Shortly after the end of World War 2, the US Congress passed a law barring co-operation with foreign governments on nuclear weapons. The UK, smarting from what it felt was a betrayal, decided that it had to have nuclear weapons at all costs in order to retain the status of a great power. With growing talk of a global testing ban, they were anxious to get the bomb while they could while avoiding diplomatic entanglements.

The earliest nuclear bombs work on the same principle of nuclear fission within a nuclear power reactor. Uranium consists of fissile (able to release energy easily) and non-fissile variants, with the majority of it being non-fissile. The process of enrichment increases the proportion of the fissile variant – to build a bomb, it must be increased from 0.7% to over 80%. The first bomb dropped on Hiroshima was made with highly enriched uranium. However, enriching uranium to the extent required by a bomb is a very expensive process. Scientists discovered that they could more easily make a bomb based on another substance : plutonium, a material which no longer occurs in nature but is synthesised from regular uranium within a nuclear reactor. It is much easier to extract plutonium than to enrich uranium, and it produces a more powerful explosion – first deployed to devastating effect on Nagasaki at the end of the pacific theatre of WW2. 

If the plutonium is left in the reactor for too long, it is itself converted to a non-fissile variant. Therefore the reactor must facilitate easy removal of the uranium fuel at regular intervals to have the plutonium extracted. Earlier dedicated nuclear reactors were built solely for this purpose and discarded the heat produced by the reaction. Later nuclear power stations converted the heat into electricity but retained the ability to replace fuel easily. This had a commercial advantage in that the reactor did not have to be shut down to be refuelled; but secretly, many governments used these reactors for their weapons programmes. British and Soviet reactors designed in the 1960s and 1970s were all designed to facilitate easy extraction of plutonium. The American and French ones were not.

The Windscale reactors consisted of a huge assembly of graphite blocks with channels passing through. Cartridges containing nuclear fuel would be pushed into the channels and left to marinate inside the reactor for a while. After a time, the cartridges would be pushed all the way through the channels and out the back, falling into a cooling pond filled with water where they would be collected to extract the plutonium. The whole assembly was cooled by air; huge fans pulling air through the graphite, over the fuel cartridges and up a chimney. 

Windscale diagram
Diagram of the Windscale reactors

The reactors were constructed at breakneck speed with little thought given to safety. As they began operating, the scientists working at the plant began to discover radiation hotspots in the local area. It was found that the fuel cartridges did not always make it into the cooling pond in one piece. Some would get jammed at the back of the reactor. Others missed the pond and hit the concrete floor, splitting open and releasing highly radioactive dust. When these problems were discovered they were kept secret up by the British government.

The government subsequently decided to use the Windscale reactor to make the radioactive hydrogen isotope tritium, a key component of a hydrogen (fusion) bomb. The cartridges placed in the reactor were modified to run hotter (at this point, several of the senior staff at the site could no longer stand over the government’s reckless attitude and resigned in protest). At the same time, it was discovered that the graphite blocks were retaining so-called “Wigner energy” from the nuclear reactions, and releasing it spontaneously in flare-ups which threatened to set the reactor on fire. To remove it, the reactor operators had to intentionally overheat the reactor to allow a controlled release of the retained energy. During one of these heating cycles, the fuel within the reactor caught fire and began releasing large volumes of radioactive smoke and dust into the atmosphere. The reactor’s air cooled design kept the fire fed with oxygen and helped to force the material into the surrounding area.

By some miracle, the reactor operators were eventually able to put the fire out. It is through sheer luck that the reactor building did not collapse and irradiate the entire area. It’s estimated that around 300 people died indirectly from the radiation. The politicians who oversaw this dangerous scenario were never held to account. They came within a hair’s breadth of turning large parts of northern England into an uninhabitable nuclear wasteland. 

The Windscale fire provided important lessons around transparency, safety culture, and the need to have reactor designs that are not inherently dangerous. But they are not directly relevant to nuclear power today. No reactors in the world use forced-air cooling, and the use of graphite as a moderator is not common. All modern reactors have extensive containment so that in the event the reactor does catch fire, the damage is limited to within the reactor building. No nuclear reactor in use today can fail in the way that Windscale did. 

Most modern reactors account for the problems that occurred at Windscale. They consist of thick steel pressure vessels, are housed within robust containment buildings, and are airtight : cooled and moderated by water. Within these reactors, the nuclear chain reaction stops immediately if the water is removed, since this ends neutron moderation. This prevents a runaway chain reaction; they cannot explode, catch fire or fail provided that one important condition is met : the cooling water continues to flow through the reactor. This is because around 10% of a reactor’s output power comes not from fission, but from decay heat – radiation released within the fuel by the radioactive substances that build up during the reactor’s operation. If this extra heat is not removed via cooling, the reactor fuel will melt – a nuclear meltdown.

Pressurized Water Reactor

The accidents at Fukushima and Three Mile Island were nuclear meltdowns caused by a failure to circulate cooling water around the reactor. At Fukushima, this happened because the diesel backup pumps were flooded and then disabled by the tsunami; the water boiled off inside the reactor, exposing the fuel rods, which then melted through the reactor vessel into the concrete below. At Three Mile Island, a series of faults with the cooling system combined with inaccurate diagnostics and operator error caused the cooling water to be released and the fuel to melt, although it did not melt through the reactor vessel. In each case, relatively small amounts of radioactivity were released. At TMI, no deaths and no radiation-related cancers have been identified. The situation at Fukushima is worse, with one known death and several hundred suspected cancer cases traceable to the accident. This must be set against the number of deaths caused by the tsunami which triggered the nuclear accident. 

The schematic below illustrates the likely state of the failed Fukushima Daichi reactor, showing how melted fuel debris penetrated the pressure vessel and is retained within secondary containment.

Fukushima reactor following accident

In these most serious cases, the most dangerous materials released by the damaged reactor were largely retained within the containment structures. The TMI site was safe enough to allow the continued operation of the adjacent reactor, and it remained in service for 40 years, shutting down in 2019. 

The accident at Chernobyl is the subject of an excellent HBO dramatization which covers the events with reasonable accuracy. Unlike the water-moderated reactor designs which are common elsewhere, the Soviet RBMK reactor was designed to be easily and cheaply built, and run on unenriched uranium. To achieve this, it was based around a graphite moderated, water-cooled core. These characteristics meant that the core had to be very large, and any containment would have to be even larger. To economise, the Soviets decided that extra containment was not necessary.

The RBMK reactor had a secondary problem – a problem reported to the Soviet government by the KGB in the late 1970s and considered a state secret – due to its graphite moderator. Water and graphite both moderate neutrons; water also absorbs neutrons. This absorption was accounted for in the reactor design. However, if the water is allowed to boil and turn to steam, the absorption decreases, leaving more neutrons free to split more uranium atoms. This meant that if the reactor overheated, it would go into a feedback loop where the power would increase, heating the steam further, which further increased the power, and so on. 

This picture shows a defuelled RBMK reactor vessel, looking down at the thick steel reactor lid. You can clearly see the channels into which fuel rods were inserted. These reactors were often used for other purposes alongside generating power, eg irradiating other substances for scientific, engineering or medical purposes. Samples could be inserted into the channels in place of fuel rods.

RBMK Reactor from above

Like all nuclear reactors, the RBMK reactor must be kept cooled at all times, even when not operating. On the night of the accident, Chernobyl’s operators were performing a test to make sure that cooling water would continue to pump through the reactor in the event of an unexpected reactor shutdown. The reactor power was therefore reduced in order to perform the test. However, the operators on shift at the time were inexperienced and did a near-complete shutdown. They did not know that a shutdown reactor takes several days to restart safely, due to the presence of specific waste elements within the reactor which serve to inhibit the nuclear chain reaction at low power levels. When the reactor failed to properly restart, and fearing disciplinary action, they bypassed all the safety procedures and pulled out all the control rods in order to jump-start the reactor quickly. The reactor power levels increased, turning the cooling water into steam, triggering the runaway scenario described above. The reactor spiked at over ten times its specified power output before the overwhelming steam pressure blew off the thick steel lid. The graphite core and fuel, exposed to the air, then caught fire. The heroic actions of volunteers, firemen and soldiers eventually got the fire under control. They hurriedly erected a makeshift containment structure around the core to prevent any further release. However, serious damage was done. Around 30 people died as a direct result of the explosion, and a several hundred died from radiation-related disease.

The diagram below shows the end state of the reactor, with fuel melted through the floor of the building and the massive steel lid resting on its side. During the accident, this lid was blown up into the air by the steam pressure in the runaway reaction.

Post Chernobyl accident

Despite the serious accident at Chernobyl, the site itself was not evacuated (although the surrounding 30km area was depopulated). The other reactors on the same site were modified to prevent this runaway reaction from recurring and were kept in service for several years to come, the last one finally being shut down under international pressure 15 years after the accident. Even with the radiation release that occurred, the site remained a safe place to work and continued to produce electricity for Ukraine following the downfall of the Soviet Union.

The accident which happened at Chernobyl is impossible with all of the other reactor designs currently in use around the world. Water-moderated reactors will automatically shut down if the cooling water boils off or leaks out, limiting the scope of any meltdown. If meltdown occurs, the melted fuel is contained within the reactor vessel or secondary containment. There are no recorded instances of melted fuel breaching all of the containment measures.

Modern designs hold the promise of being even safer. The European Pressurized Reactor, an example of which is currently under construction at Hinckley Point C, has additional redundant cooling systems to keep water flowing through the core even in the event of a Fukushima-style disaster. Older BWR and PWR designs continue to be perfectly safe provided they are properly risk assessed and appropriate action taken. For example, the Fukushima Daini reactor, a short distance away from the reactor which melted down, had been retrofitted to withstand a tsunami. When the tsunami arrived, the reactor automatically shut down and the core was kept safely cooled. Had this retrofitting been carried out across the Japanese fleet, we would never have heard of Fukushima. 

Nuclear energy is difficult for people to understand. The presence of deadly radiation, capable of killing a healthy person within hours, and which cannot be seen, tasted or smelled is naturally unsettling. The secretive approach taken by governments during the cold war, coupled with the horrific possibilities of nuclear weapons, and no small amount of anti-nuclear propaganda, have served to encourage a degree of superstition about nuclear as an energy source.

It’s not my intention to downplay the deaths and suffering of the victims and their families caused by these historic nuclear accidents. One death or injury is one too many and the industry and regulators must constantly strive to reduce incidence of accidents to zero. However, I cannot help but observe the absence of even-handedness here. Tragedy of this kind is a feature of human industrial development generally. There are serious accidents all the time associated with the extracting and refining fuel oil and gas, yet nobody insists that all fossil fuel energy production and utilisation must cease – indeed, bizarrely, nuclear power in some parts of Europe has been displaced by increased reliance on coal, oil and gas, particularly in Germany. The process of installing and maintaining wind turbines, especially out to sea, must be fraught with danger, and that is before we consider the human safety and environmental impact of extracting the raw materials used to produce turbines, solar panels, and the cabling and machinery to interconnect them. These raw materials are often mined in second or third world countries under abysmal working conditions with scant regard for worker safety.

In Part 3, I will talk in more detail about the safety of modern nuclear plants and why they cannot explode like a nuclear bomb; and we will discuss the difficult problem of what to do with the waste. Comments and feedback are always welcome.



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