Few subjects raise as much controversy and depth of feeling as that of nuclear energy. On the one hand, it offers huge amounts of power, a fuel source that will last for thousands of years, and zero atmospheric emissions. On the other, it has the potential to be environmentally devastating, produces waste that has to be sealed off for thousands of years, and manufactures the raw materials for nuclear weapons. This debate is not split down the usual political lines however; a few greens actually support nuclear energy and those opposed to it come from all walks of life. It is not a well-understood concept amongst the public and is consistently misrepresented in the popular media, giving rise to frequent scare stories about accidents, terrorist attacks and health problems in nuclear workers and their families.

The accident that occurred at the Soviet Chernobyl plant on 26th April 1986 forever skewed the argument against nuclear power. It was without doubt the worst nuclear accident in history, and left the world painfully aware of what could happen if things went wrong. Despite worries over fossil fuel reserves and global warming that would appear to favour nuclear power, its contribution to the global grid is decreasing. At the time of writing, there are about 440 operational reactors in 32 countries, generating 16% of the world’s electricity. Only 27 new reactors are under construction, mainly in eastern Europe and Asia¹. Not one of the remaining 22 countries with nuclear power is currently building any new reactors, including the US, Canada and all of Western Europe. The western world has put its nuclear power programme on hold. This is arguably due at least in part to the Chernobyl accident and the ensuing perception that no matter how small the risk, it is just not worth it. The situation may change in future, but only because the pressure to deliver on commitments to reduce CO₂ emissions is even stronger. This entry offers a description of the events leading up and following the accident, and is intended to dispel a few myths, put a few minds at rest and clarify exactly what we should still be worried about.

The Chernobyl² power complex is located in Ukraine, former Soviet Union, about 100 km north of Kiev and only 7km south of the border with Belarus. It comprised four RBMK-1000 1000MWe³ reactors, unit 1 going operational in 1977, unit 2 in 1978, 3 in 1981 and 4 in 1983. Two more RBMK reactors were under construction at the site at the time of the accident. The town of Pripyat 3km away grew up to service the plant, and had 49,000 inhabitants prior to the accident. The old town of Chernobyl, which had a population of 12,500, is about 15 km southeast.

Nuclear power – the basics

To understand what happened, it is first necessary to understand in basic terms how a fission reactor works - please skip this section as required.

If a heavy atom is split into smaller products, the combined mass of the products will be slightly less than that of the original atom. The mass difference is converted to energy according to Einstein’s famous equation E=mc². A typical reactor core consists of some fuel, usually uranium-235 in the form of uranium dioxide, arranged into a number of pellets or rods for logistic purposes and clad with an unreactive metal to prevent the fuel reacting chemically with its surroundings. Uranium-235 is a heavy but relatively stable atom with a half-life⁴ of about 710,000,000 years. However, if it is converted to U-236 by adding a neutron, it will decay rapidly and split into smaller products, releasing energy. The most common products are barium-141, krypton-92 and three neutrons, although there are other combinations. The neutrons will fly off and may hit other uranium atoms, in turn causing these to decay, thereby initiating a chain reaction. In practice, most neutrons move too fast and just bounce off the heavier atoms rather than bind to form U-236, so a moderator is introduced, usually the coolant water or carbon, to slow them down. A set of control rods can be inserted into the core to absorb the neutrons and thus slow the reaction as required. In an emergency, these are dropped to stop the reaction completely, known as a “scram”. A coolant, generally water (although carbon dioxide or liquid metal such as sodium or potassium are also used) is circulated around the core to extract the heat. The coolant water also provides the moderating function in many reactors. This hot coolant is usually contained within a closed loop (the primary circuit) as it becomes highly radioactive. If the coolant is water, it is kept under high pressure to prevent it boiling, hence these are known as Pressurised Water Reactors (PWR). The primary coolant is passed through a heat exchanger, producing steam in a secondary circuit to drive a turbine and a generator. In some reactors, including the RBMK at Chernobyl, the primary coolant is boiled within the reactor to produce steam directly, known as a Boiling Water Reactor (BWR). The core is enclosed within an extremely rugged pressure vessel that contains the heat, pressure and radiation. The entire assembly is usually (though not always) housed in a thick concrete containment building designed to contain any radioactive debris in the event of an accident⁵.

The RBMK Reactor

The RBMK⁶ design is unlike any other fission reactor, as it was based on a design intended to produce plutonium for nuclear weapons. The RBMK itself is also designed to manufacture plutonium, albeit as a by-product of power generation. It is powered by slightly enriched (2% U-235) uranium dioxide fuel pellets, stacked into a 3.65m tube clad with zirconium-alloy. 18 fuel rods are arranged cylindrically to form a fuel assembly, and two fuel assemblies are stacked on top of each other and placed in their own individual pressure tube, about 7m high. This arrangement allows individual fuel assemblies to be removed separately, thus enabling the reactor to be refuelled while running. Graphite (carbon) blocks between pressure tubes act as moderator, and 211 boron carbide control rods, 179 of which can be inserted or removed from above, control the reaction. Water is pumped directly through each pressure tube and allowed to boil, driving the turbine directly. Although there is no secondary cooling circuit, there are two separate primary circuits with a further back-up. The core is concrete-lined and topped with a steel pile cap which also supports the fuel assemblies. There is no secure containment building as such. The features that make the RBMK unique, and flawed, are:

1. High availability due to ability to refuel without shutting down the reactor.
2. The combination of graphite moderator and water coolant is found in no other power reactors. This type of fuel would be unsuitable for a water-moderated reactor.
3. Low power density, allowing the core to survive without damage following loss of electrical power for up to an hour.
4. Lack of a proper containment building.
5. Inadequate accident mitigation and fire suppression systems, poor separation and redundancy of safety and electrical systems, complicated pipework.
6. Positive void coefficient.

This last feature is probably the most significant contributor to the accident. In the event of coolant loss, water in the pressure tubes turns to steam and steam pockets, or voids, are formed. Steam is less dense than water and provides less cooling, so the fuel gets hotter. However, where the water also provides the moderating function, the neutrons will speed up due to the lack of moderation and the reaction will slow down. This is called a negative void coefficient, and ensures that any uncontrolled increase in core temperature will slow down and ultimately stop the reaction. Most reactors are built this way and are thus inherently “safe”. Conversely, in the RBMK at lower power levels (less than 20% of maximum), the graphite moderator allowed the reaction to proceed in spite of the loss of coolant. The number of free neutrons would increase, as there is no water to absorb them. This positive void coefficient meant that an uncontrolled temperature increase could, in the right circumstances, lead to a runaway reaction.

The Accident

On the 25 April 1986, reactor four was to be shutdown for routine maintenance, so it was decided to take advantage of this to run a test. Ironically, the test was designed to improve safety. The reactor’s cooling pumps relied on electrical power, so the operators wanted to see how long the turbines could produce sufficient energy to keep the pumps running in the event of a loss of power. The reactor's emergency cooling system was deliberately disabled, as they didn’t want it cutting in when the main pumps slowed. To reduce cooling requirements, the reactor was to be run at low power, despite the fact that these reactors were known to be unstable at low power settings. The test had been attempted on two previous occasions but never completed.

There were two main cooling systems excluding the back-up, each with four main pumps. Four of these pumps were powered by the generator that was to “fail”. Prior to the experiment, with reactor power reduced and all eight pumps operating, water flow exceeded permitted levels. The amount of water in the steam-raising circuit reduced steam production. Additionally, the extra water was absorbing neutrons and causing power to fall. Power fell to less than 1% of capacity, so the operators manually removed control rods to compensate, switched off automatic regulators and eventually stabilised the reactor at the planned test power level. At one point only 6-8 control rods were being used. According to procedure at least 30 are required to maintain control, and if there are any less the reactor should have been shut down. They allowed the test to continue, despite knowing that about 20 seconds would be required to lower all the rods and shut down the reactor in the event of a power surge.

Then both generators were shut down to start the test. The cooling pumps slowed, reducing water flow in the core and producing more steam. The excess water had up till now been absorbing neutrons, so the formation of steam pockets caused neutron flux to increase (the positive void coefficient). At 01:23 hours on the 26th April, reactor power increased exponentially, up to an estimated 100 times nominal. The control rods could not be re-inserted in time; the fuel overheated and some of the rods ruptured.

The resulting explosion, thought to be caused mainly by steam pressure and chemical reaction with the exposed fuel, blew the 1000-tonne lid clear of the core. A second explosion threw out fragments of burning fuel and graphite from the core and allowed air to rush in, causing the graphite moderator to burst into flames. The exact cause of the second explosion remains unknown, but it is thought that hydrogen may have played a part.

The Immediate Aftermath

With the core now fully exposed, a plume of smoke, radioactive fission products and debris rose up to about 1 km into the air. The material was carried northwest by the wind – mostly to Belarus but some to other nearby areas, including Ukraine. Fires broke out all over the plant. About 250 firemen were called, many of whom were not equipped with measuring instruments to monitor the radiation dosages they were receiving. The operators and rescue workers are to be commended - many stayed on call in the area after having been relieved and many risked their lives to save others and bring the situation under control. Most of the fires had been extinguished by 05:00, but the graphite fire continued for another nine days. The main release of radioactivity into the environment was caused by the burning graphite.

Once the order was given, on the 27th April, the town of Pripyat was evacuated completely within 2.5 hours. The evacuees were never to return, and the town remains how it was left. While the fire raged, about 5,000 tonnes of different materials were dropped by helicopter onto the exposed core. Some were intended to smother the fire, some to absorb neutrons to prevent renewed chain reaction, some to act as a heat sink, lead to act as a radiation shield, and sand and clay to prevent further contaminants escaping. 1800 helicopter flights were made, their task severely hampered by not being able to hover over the core due to high radiation levels. Their efforts were relatively successful, although there was a further major release of contaminants when (it is thought) the core melted. On May 9th, work began digging a tunnel underneath the core to install a huge concrete slab and cooling system. The slab was intended to cool the core if necessary, and also act as a barrier to prevent radioactive material leaching into the groundwater. Finally, the core was entombed in a 300,000-tonne concrete and steel sarcophagus, and the surrounding land and buildings decontaminated.

It is estimated that about 3.5%, or 6 tonnes, of the uranium dioxide fuel and fission products escaped as well many other radionuclides, principally xenon, krypton, iodine, tellurium and caesium. A total of about 12 x 10¹⁸Bequerels of radioactivity was released, about 200 times that of the Hiroshima and Nagasaki bombs according to the World Health Organisation. The highest levels of contamination were within a 30km radius of the site; levels of Caesium-137 exceeded 1500kBq/m². Caesium-137 is used as it is easily measurable, and posed the greatest health risk after the Iodine-131 had decayed. ⁷. Levels of 40kBq/m² covered large parts of Northern Ukraine and Southern Belarus, with a number of extreme “hot-spots” occurring where it happened to be raining as the cloud passed over.

Predictably the Soviet government tried to deny that anything had happened, so the first time the plume was detected outside of the USSR by workers at a Swedish nuclear plant, they suspected another Swedish facility. It was only then that the Soviets, under extreme international pressure, owned up. The cloud was tracked thereafter, and passed over Scandinavia, Holland, Belgium and the UK carried by the northwesterly wind, and then went south, covering much of the rest of Europe after the wind changed. Contamination was detected in nearly every country in the northern hemisphere, as far as North America and Japan, although the southern hemisphere seems to have escaped. National authorities were surprised at the scale of the problem, and none had effective mitigation plans in place, particularly for a disaster that transcended national borders. Outside of the Soviet Union, countermeasures were pretty much limited to restrictions and prohibitions on the marketing, consumption and importing of certain foods thought to be contaminated or from contaminated areas.

The Casualties

The highest radiation doses were received by the 400-odd plant workers and firemen present on the 26th April. 31 died as an immediate consequence of the accident; one in the explosion itself, one from coronary thrombosis, one from thermal burns and 28 from acute radiation syndrome. A further 134 were treated for radiation poisoning. Everyone living within a 30km radius of the plant, some 116,000 people, were evacuated immediately following the accident, and a further 210,000 people were resettled between 1990 and 1995.

Some 226,000 further workers were involved in the clean-up operation within the 30km-zone between 1986-1987 and are thought to have received high doses. A further 400,000 were exposed during later work in the zone. Information on the individual received doses as well as the numbers of workers⁸ is very sketchy, but the average dose is thought to have ranged from 170 milliSieverts in 1986 to 15mSv in 1989. The maximum “safe” exposure level (if such a level exists) is subject to constant debate but a commonly used limit is 1mSv per person per year above natural background levels. For comparison, average natural background radiation levels in the UK are 2.2mSv per person per year, and a single dose above 10mSv poses a significant threat to the human body.

Many health effects have been observed that could be attributed to the accident. Most significantly, a notable rise in thyroid cancer⁹ in the most contaminated areas of the former Soviet Union is most likely a direct result of the accident. The cancer is most prevalent in children who were aged 0 to 5 years at the time, and is thought to be caused by radioactive iodine contamination in certain foods and milk collecting in the thyroid gland. Children, who drink more milk and have smaller thyroid glands, received a higher dose than adults. The disease is treatable and not often fatal. Although the risk has subsided as the iodine has decayed to low levels, the disease takes years to show up and the number of cancers diagnosed is still increasing. Mental health effects have also been reported in the region, such as anxiety, stress, apathy, despair and withdrawal. These effects may be a result of diverse factors such as resettlement, social degradation, poor economic conditions and the inevitable fear of health risks following the accident rather than radiation exposure itself.

Conversely, there have been no observed increases (to date) in rates of leukaemia, congenital defects, abnormal pregnancies or any other radiation induced sickness, even in heavily contaminated areas. Estimates of the total lifetime cancers expected as a result of the accident are 0.01% above the natural incidence in Europe, and only 0.004% in the Northern hemisphere overall. Contaminated areas are considered to be those with Cs-137 deposition levels greater than 37kBq/m². These areas are restricted to Ukraine, Belarus and Russia, but cover about 3% of the European part of the former Soviet Union and about 5 million people are still living there.

The exact casualty figures are disputed. It is estimated that five million people were exposed to dangerous levels of radiation. Ukraine's health ministry has estimated that 3.5 million people have suffered some illness as a result of the contamination, and the Ukraine Radiological Institute suggest that there were 2,500 deaths. A group representing those who worked in the relief operations following the accident estimate the number of deaths at 15,000. The Chernobyl Children's Project amongst other groups claim significant increases in birth deformities, cancer and leukemia. Nevertheless, the most comprehensive assessment of the health effects of the accident, issued by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) in September 2000, stated "(thyroid cancers aside) there is no evidence of a major public health impact related to ionizing radiation 14 years after the Chernobyl accident."

Environment and Agriculture

The cloud dropped fall out all over Europe. Caesium-137 was again the main problem; its 30-year half-life means that half the material released will still be in the environment in 2016. Caesium is chemically similar to the nutrient potassium, so tends to be taken up readily be plants and animals and quickly got into the food chain. As it rises up the food chain, its concentrations become higher. The main routes into the food chain are from consumption of contaminated berries, mushrooms, game and fish, and via grass and hay eaten by dairy cattle. It is estimated that concentrations in fish in Lake Kozhanovskoe, Russia, will remain above the recommended maximum limit for consumption for another 50 years or more. Although the Iodine-131 danger has now subsided, contaminated milk in the Soviet areas was responsible for the thyroid cancers mentioned above, and huge quanitites of milk in Poland, Hungary, Austria and Sweden were destroyed.

Many countries across Europe burned contaminated vegetation, and a ban on many agricultural goods was placed across Eastern Europe. Among the worst affected were Sweden's reindeer and sheep. The sale of milk, meat, many fruits and vegetables was banned in 1986 and 1987 in the Russian markets of Kiev, Chernigov, Minsk, and other smaller cities and towns. Having received 70% of the fall out, the worst affected area was Belarus, 21% of which is still contaminated. Large areas of Ukraine, Belarus and Russia remain off-limits for agriculture. Although banned, it is thought that locals continue to hunt and fish in these areas. In the UK, Ministry of Agriculture restrictions on sale and slaughter of sheep were expected to last for only a few months after the accident. In some farms in Cumbria, Scotland and Wales, restrictions are still in place now.

One of the main concerns immediately following the accident were the waters of the river Dnieper, on which some 30 million people, including the city of Kiev, depend. The Pripyat is a tributary of the Dnieper. Although the river did indeed distribute contamination throughout Ukraine, mitigation efforts have been fairly successful and drinking water was largely unaffected. Nevertheless, contamination has accumulated in other water basins, and there is a serious risk of groundwater contamination from Strontium and Americium, which has a half-life of 433 years.

Chernobyl Since

The initial clean-up operation was impressive, the sarcophagus was completed in only seven months in November 1986 and radiation levels on the site are now relatively low. However, the emphasis at the time was on rapid containment and it was never meant to be a permanent solution. The sarcophagus is deteriorating and several scenarios have been considered in which it may collapse or otherwise cause further severe contamination. The clean up operation itself created a large amount of waste, which is currently stored in various sites within the 30km exclusion zone. More permanent solutions are currently under discussion, and facilities are being constructed for processing and long term storage of fuel, liquid and solid waste.

With reactor 4 entombed, operation of the other 3 continued. A new town 50km to the east of Chernobyl, named Slavoutich, was created to house plant personnel. Reactor number 2 was irreparably damaged by fire in 1991. Unit 1 stopped in 1996 and was subsequently decommissioned. Unit 3 became increasingly unreliable, and was finally shut down on the 15th December 2000 to comply with an agreement made with the G7¹⁰ group of nations in 1995. For their part, the G7 group agreed to assist in upgrading other Soviet-era reactors to western safety standards, and supported the continued construction of two new reactors, Khmelnitskiy 2 and Rovno 4, to replace the Chernobyl plant. The two new reactors are VVER-1000 units, similar to the West's PWR designs, and are partially funded by the EU, Ukrainian and Russian governments.

As a result of lessons learned and further international assistance, all remaining operational RBMK reactors were heavily modified to eliminate the positive void coefficient problem¹¹, to improve scram performance and to generally improve safety and integrity. There are currently 13 operational RBMK reactors (11 in Russia and 2 in Lithuania) and one more under construction.

Lessons Learnt

A lot has changed since 1986. Disaster plans have been put in place in most nations, and the safety of nuclear plants continues to improve. We have invaluable data on the movement of radionuclides through the environment, and their effect on health. Although RBMK reactors are still in operation and minor accidents still occur in reactors all over the world, there have been no further major releases of radiation. It is abundantly clear that the events at Chernobyl were not an inevitable consequence of nuclear energy, they were a result of a flawed design coupled with serious mistakes made by poorly trained operators working at night when they may have been tired, and in a culture with little regard for safety. In contrast, the incident at Three Mile Island in Pennsylvania, USA on 28th March 1979, resulted in no injuries or adverse health effects, despite a Homer Simpsonesque catalogue of operator errors following a minor secondary cooling system failure. In this case, the reactor shut down automatically and although coolant was lost and a build up of hydrogen exposed the core, it was fully contained within the surrounding building. Chernobyl remains the only commercial reactor incident to result in radiation-related deaths.

While it may be obvious to most, it must also be pointed out that at no time was there a risk of a nuclear explosion. The spectre of a reactor accident leading to an explosion is one of the more disturbing and pointless Hollywood myths¹². Reactor fuel is enriched to only 2% - 5% pure uranium; weapons grade uranium is enriched to levels as high as 90%. Reactor fuel is simply not explosive.

That is not to say that nuclear power is completely safe. If Chernobyl has taught us anything, it is that the sheer volume of radioactive material present in commercial reactors poses a huge risk, no matter how well it is contained. The cost of the constantly-improving safety procedures and regulation surrounding the industry has ballooned to the point that nuclear power is no longer significantly cheaper than fossil fuels. The costs of decommissioning and waste disposal are only just coming to light, but are likely to be huge. The cost of effectively defending all nuclear plants against terrorist attack is prohibitive. On the other hand, the costs of global warming may be greater still.

Further Information