Alexa Chernobyl: World’s greatest nuclear disaster

Chernobyl: World’s greatest nuclear disaster

Published: 20:04 9 September 2019   Updated: 10:16 12 September 2019



The world’s worst-ever nuclear disaster occurred on April 26, 1986, at Chernobyl, in Ukraine, near its border with Belarus, then the Soviet Union. There were four reactors at the site, each giving 1,000 megawatts of electricity. The fault was both human error and poor design.

31 people died within a few weeks of the accident from the initial steam explosion, exposure to radiation and thermal burns. More than 30 years on, scientists estimate the zone around the former plant will not be habitable for up to 20,000 years.

What was Chernobyl?

The Chernobyl Nuclear Power Plant was built between 1970 and 1983 and located in then-Soviet controlled Ukraine, 68 miles north of Kiev. The plant contained four nuclear reactors by the time of the disaster, while the construction of two more reactors was planned. Nuclear power plants do the same job as other power plants generating energy used to create electrical power but they do so by harnessing nuclear fission, which occurs when the nuclei of atoms of elements like uranium are split, releasing large amounts of energy.

Among the communities nearest the plant was the city that gave it its name, Chernobyl, home to around 14,000. But there were so many people employed by the plant that a community called Pripyat was founded in 1970 specifically to house the nuclear plants workers and their families. Home to 50,000 people, Pripyat, like any city, contained schools, factories, movie theaters, and hospitals. An amusement park was scheduled to open just days after the disaster.

Chernobyl plant and location

The Chernobyl nuclear power station is located in 130 km north of Kiev, the capital of Ukraine lies near Belarus beside the river Pripyat. During the accident, the total population within a 30-km radius of the power station was between 115,000 and 135,000. Of this, about 49,000 of plant employees and their families inhibited in the town of Pripyat, which is about 3 km away from the reactor, and a population of about 12,500 in the town of Chernobyl, about 15 km to the south-east of the plant.

The nuclear station consisted of four graphite-moderated, light-water-cooled pressure tube reactors (RBMK-1000), each capable of producing 1000 megawatt (MW) of electric power. The four reactors together produced about 10 percent of Ukraine’s electricity at the time of the accident. 

Construction of the plant began in 1970, with Unit 1 and 2 commissioned in 1977, followed by Unit 3 and 4 in 1983. Two more RBMK reactors were under construction at the site, but the construction was halted after the accident.

What is RBMK?

The Soviet-designed RBMK (Reaktor Bolshoy Moshchnosty Kanalny, high-power channel reactor) is a water-cooled reactor with individual fuel channels and using graphite as its moderator. It is also known as the light water graphite reactor (LWGR). As with a boiling water reactor (BWR), water boils in the fuel channels and steam is separated above them in a single circuit. 

It was designed in 1964-66 and is very different from most other power reactors. Its precursors were an experimental 30 MWt (5 MWe) LWGR at Obninsk which started up in 1954, and two small prototypes LWGR (AMB-100 & 200) units Beloyarsk 1 and 2, which ran from 1964 and 1968 respectively.

The ADE reactors at Zheleznogorsk and Seversk used for plutonium production are similar to the RBMK but with the much lower power density and smaller fuel elements.

The combination of graphite moderator and water coolant is found in no other power reactors in the world. As the Chernobyl accident showed, several of the RBMK’s design characteristics, in particular, the control rod design and a positive void coefficient were unsafe. A number of significant design changes were made after the Chernobyl accident to address these problems.

Design flaws and misuse of Reactor 4

Chernobyl had four reactors and each was capable of generating 1,000 megawatts of electric power. Chernobyl’s four reactors were different than most other power plants. The Soviet-designed RBMK reactor meaning “high-power channel reactor” was water-pressurized and intended to produce both plutonium and electric power and as such, using a rare combination of water coolant and graphite moderators that made them fairly unstable at low power.

If the reactors lose cooling water, they would dramatically decrease power output which would rapidly facilitate nuclear chain reactions. What’s more —the RBMK design didn’t have a containment structure which is exactly what it sounds like: a concrete and steel dome over the reactor itself meant to keep radiation inside the plant even if the reactor fails, leaks, or explodes.

These design flaws compounded with the staff of untrained operators made for the perfect storm of nuclear failures.

The rather inadequately trained personnel working on the Reactor 4 late that night on April 25 decided to complicate a routine safety test and conduct an electrical-engineering experiment of their own. Their curiosity of whether or not the reactor’s turbine could operate emergency water pumps on inertial power, unfortunately, got a hold of their judgment.

First, the team disconnected the reactor’s emergency safety systems as well as its essential power-regulating system. Things quickly worsened when they set the reactor at a power level so low that it became unstable and removed too many of its control rods in an effort to regain some control.

At this point, the reactor’s output reached over 200 megawatts. At that fateful hour of 1:23 am, the engineers shut the turbine engine off completely to confirm whether or not its inertial spinning would force the reactor’s water pumps to kick in. Tragically, it did not. Without the requisite water-coolant to maintain temperatures, the reactor’s power level spiked to unmanageable levels. 

How Chernobyl accident happened 

On April 25, prior to a routine shutdown, the reactor crew at Chernobyl 4 began preparing for a test to determine how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply. This test had been carried out at Chernobyl the previous year, but the power from the turbine ran down too rapidly, so new voltage regulator designs were to be tested.

A series of operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted test early on April 26. By the time that the operator moved to shut down the reactor, the reactor was in an extremely unstable condition. A peculiarity of the design of the control rods caused a dramatic power surge as they were inserted into the reactor. 

The interaction of very hot fuel with the cooling water led to fuel fragmentation along with rapid steam production and an increase in pressure. The design characteristics of the reactor were such that substantial damage to even three or four fuel assemblies would and did result in the destruction of the reactor. 

The overpressure caused the 1000t cover plate of the reactor to become partially detached, rupturing the fuel channels and jamming all the control rods, which by that time were only halfway down. Intense steam generation then spread throughout the whole core causing a steam explosion and releasing fission products to the atmosphere. 

About two to three seconds later, a second explosion threw out fragments from the fuel channels and hot graphite. There is some dispute among experts about the character of this second explosion, but it is likely to have been caused by the production of hydrogen from zirconium-steam reactions.

Two workers died as a result of these explosions. The graphite and fuel became incandescent and started a number of fires, causing the main release of radioactivity into the environment. A total of 14 EBq of radioactivity was released, over half of it being from biologically-inert noble gases.

About 200-300 tonnes of water per hour were injected into the intact half of the reactor using the auxiliary feedwater pumps but this was stopped after half a day owing to the danger of it flowing into and flooding units 1 and 2. From the second to 10th day after the accident, some 5000 tonnes of boron, dolomite, sand, clay, and lead were dropped on to the burning core by helicopter in an effort to extinguish the blaze and limit the release of radioactive particles.

The 1991 report by the State Committee on the Supervision of Safety in Industry and Nuclear Power investigated the cause of the accident and looked past the operator actions. It said — while it was certainly true that the operators placed their reactor in a dangerously unstable condition, it was also true that in doing so they had not, in fact, violated a number of vital operating policies and principles, since no such policies and principles had been articulated. 

Additionally, the operating organization had not been made aware either of the specific vital safety significance of maintaining a minimum operating reactivity margin, or the general reactivity characteristics of the RBMK which made low power operation extremely hazardous.

Immediate impact of Chernobyl accident

The effects of radiation exposure fall into two main classes — deterministic effects, where the effect is certain to occur under given conditions-individuals exposed to several grays over a short period of time will definitely suffer Acute Radiation Syndrome 

And stochastic effects, where the effect may or may not occur-an increase in radiation exposure may or may not induce cancer in a particular individual but if a sufficiently large population receive a radiation exposure above a certain level, an increase in the incidence of cancer may become detectable in that population.

The accident caused the largest uncontrolled radioactive release into the environment ever recorded for any civilian operation, and large quantities of radioactive substances were released into the air for about 10 days.

This caused serious social and economic disruption for large populations in Belarus, Russia, and Ukraine. Two radionuclides, the short-lived iodine-131, and the long-lived cesium-137 were particularly significant for the radiation dose they delivered to members of the public.

Initial radiation exposure in contaminated areas was due to short-lived iodine-131; later cesium-137 was the main hazard. About five million people lived in areas of Belarus, Russia and Ukraine affected and about 400,000 more affected in the areas of strict control by authorities. A total of 29,400 km/sq were contaminated above 180 kBq/m2.

It is estimated that all of the xenon gas, about half of the iodine and cesium, and at least 5 percent of the remaining radioactive material in the Chernobyl reactor 4 core (which had 192 tonnes of fuel) was released in the accident. 

Most of the released material was deposited close by as dust and debris, but the lighter material was carried by wind over Ukraine, Belarus, Russia, and to some extent over Scandinavia and Europe. The casualties included firefighters who attended the initial fires on the roof of the turbine building. All these were put out in a few hours, but radiation doses on the first day were estimated to range up to 20,000 millisieverts (mSv), causing 28 deaths, six of which were firemen by the end of July 1986.

Environmental and health effects

The 115,000 members of the public who had to be evacuated from the area around the plant received an average effective radiation dose of 30 mSv. The Chernobyl fallout had a major impact on both agricultural and natural ecosystems in Belarus, Russia, and Ukraine, as well as in many other European countries. 

In 2018, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) reported that the accident also was responsible for nearly 20,000 documented cases of thyroid cancer among individuals who were under 18 years of age at the time of the accident in the three affected countries including Belarus, Ukraine, and the Russian Federation.

This was due to the high levels of radioactive iodine released from the Chernobyl reactor in the early days after the accident. Radioactive iodine was deposited in pastures eaten by cows who then concentrated it in their milk which was subsequently ingested by children. This was further exacerbated by a general iodine deficiency in the local diet causing more of the radioactive iodine to be accumulated in the thyroid.

Radionuclides were taken up by plants and later by animals. In some areas, they were subsequently found in meat, forest food products, freshwater fish and wood. Environmental impacts vary according to location and ecosystem. Forests and freshwater bodies have been among the most affected ecosystems. 

The impacts on wildlife in the vicinity of the Chernobyl plant are disputed. The impacts on human health have been extensively studied, although experts are not unanimous in their views. Official assessments by United Nations agencies have been challenged. The major population groups exposed were clean-up workers, evacuees, and residents of contaminated areas of Belarus, Russia, and Ukraine. 

There has been no clear evidence of any measurable increase in radiation-induced adverse health effects in other European countries. The immediate and short-term effects resulting from heavy fallout exposure include radiation sickness and cataracts. Late effects are thyroid cancer, especially in children and adolescents, and leukemia among exposed workers. The accident has also had important psychosocial effects.

Psychological or mental health problems

According to several international studies, people exposed to radiation from Chernobyl have high anxiety levels and are more likely to report unexplained physical symptoms and poor health.

Concerns about fertility and birth defects

There is no evidence of decreased fertility in men or women in the affected regions. Because doses to the general population were low, it is unlikely that there would be an increase in stillbirths, adverse pregnancy outcomes, delivery complications, or negative impacts on children's overall health. Regardless, monitoring remains important and is ongoing.

Long-term impacts

The impact of the disaster on the surrounding forest and wildlife also remains an area of active research. In the immediate aftermath of the accident, an area of about four square miles became known as the “Red Forest” because so many trees turned reddish-brown and died after absorbing high levels of radiation.

Today, the exclusion zone is eerily quiet, yet full of life. Though many trees have re-grown, scientists have found evidence of elevated levels of cataracts and albinism, and lower rates of beneficial bacteria, among some wildlife species in the area in recent years. Yet, due to the exclusion of human activity around the shuttered power plant, the numbers of some wildlife, from lynxes to elk, have increased. In 2015, scientists estimated there were seven times more wolves in the exclusion zone than in nearby comparable reserves, thanks to humans’ absence.

The Chernobyl disaster had other fallout such as the economic and political toll hastened the end of the USSR and fueled a global anti-nuclear movement. The disaster has been estimated to cost some $235 billion in damages. What is now Belarus, which saw 23 percent of its territory contaminated by the accident, lost about a fifth of its agricultural land. At the height of disaster response efforts, in 1991, Belarus spent 22 percent of its total budget dealing with Chernobyl.

Today, Chernobyl beckons to tourists who are intrigued by its history and its danger. But though Chernobyl symbolizes the potential devastation of nuclear power, Russia never quite moved beyond its legacy—or its technology. As of 2019, there are still 11 operational RBMK reactors in Russia.

Chernobyl Exclusion Zone Today

A re-evaluation in 1991 had the zone extended to encompass some 1,600 miles which is how it remains today. These days, Chernobyl continues to serve as a site of scientific interest. NASA, for instance, has taken to study the organisms that survived within Chernobyl's exclusion zone in the hopes of developing a radiation blocker for astronauts. Studying these fungi and other organisms, NASA says, could eventually help scientists learn to grow crops on other planets as well.

Meanwhile, some reports have circulated that Chernobyl may be transformed into a solar farm. In political decision-making circles, critics still point to the Chernobyl disaster when questions of nuclear power are brought to the fore as a way to provide cheap energy to a consistently growing global population. 

Lessons from Chernobyl 

We learn from mistakes, however, and nuclear power is now safer because of these incidents. Although the other reactors at Chernobyl continued to operate, plans to construct a 5th and a 6th reactor were abandoned, and a process of decommissioning eventually led to the whole site being shut down. There is now, strangely, a tourist trip around the area.

No more Chernobyl-type reactors will be built, and careful consideration is given to site safety in determining the location of new reactors. Sites at risk to natural disasters are avoided. The big breakthrough is in design technology. The pebble bed designs use fuel ‘pebbles’ the size of tennis balls, and are designed to operate at, and cope with, high temperatures. Significantly, they are gas-cooled, not water-cooled, and the gas does not absorb neutrons. They are rated “far safer in every way” than the older type of reactor.

Production of energy by practically every method is in some degree dangerous. Hundreds have died in oil rig explosions. Hundreds die every year as hydro-electric dams collapse and flood villages. Coal miners are still being killed in underground fires and tunnel collapses. All have their environmental fallout, too. 

The way forward is not to abandon energy use and return to Stone Age technology; it is to make energy production as safe as we can, learning from disasters like Chernobyl what not to do. Compared with other energy sources, nuclear power is relatively safe and relatively clean, and will be part of energy production for the foreseeable future.