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Chernobyl
The Chernobyl incident occurred on the 26th April 1986 at the Chernobyl Plant, Ukraine, in the former Soviet Union.
Chernobyl was an INES Level 7 accident which released a massive amount of radioactive material worldwide. The overall number of deaths that resulted from exposure to radiation as a result of the accident is unclear.
- 2 workers died in the initial explosion.
- 28 firefighters died of acute radiation syndrome within three months of the accident.
What happened?
On 25th-26th April 1986, an experiment was being done at one of the four RBMK reactors at the Chernobyl power plant. The aim was to test the ability to continuously supply back-up power to the turbines and emergency backup system in in the event of a main electrical power supply failure. The automatic shutdown mechanisms were disabled, and the experiment went ahead despite the existing knowledge that there were stability problems at low power settings.
RBMK Reactors
An RBMK (Reaktor Bolshoy Moshchnosty Kanalny) reactor is a Soviet designed reactor.
The reactor’s coolant is light water and the moderator is graphite.
The design of RBMK reactors is inherently unstable: instead of the safe negative thermal feedback seen on modern PWR reactors, RBMK reactors have positive thermal feedback. This means that as reactor temperature increases, the rate of nuclear fission increases. The result? A potentially uncontrollable chain reaction.
When reactor power was reduced for the test, the subsequent loss of stability caused a power surge so rapid that the control rods could not be lowered down in time to shut down the reactor and prevent a criticality accident occurring. The power surge was exacerbated by the RBMK’s control rod design – their graphite tips acted as a moderator, increasing criticality.
This power surge and resultant heat ruptured the fuel rods and vaporised the coolant water. The subsequent steam explosion blew off the reactor lid and smashed a hole in the roof. Hence, directly releasing radioactive fission products into the atmosphere. A second explosion then followed dispersing highly radioactive burning material from the core over surrounding buildings, whilst simultaneously, the contact with the air ignited the graphite core. The core burned for 9 days.
Aerial View of Chernobyl
An aerial view of Chernobyl power plant's reactor four in 2007, following the incident of 1986. The wreckage of the reactor is contained within the sarcophagus. This has since been enclosed within the new containment structure.
Photo Credit: Vadim Mouchkin / IAEA
What were the causes?
RBMK reactors have positive thermal feedback. This means that as reactor temperature increases, the rate of nuclear fission increases. The result? A potentially uncontrollable chain reaction.
There was enormous pressure, both organisationally and politically, to produce electricity at the expense of safety.
Staff were not sufficiently trained, competent or experienced and were ignorant of safety requirements.
At the time of the disaster many of the reactor’s key safety systems were shut down. Operating in these conditions was illegal, but the operators did not know why it was dangerous to do so.
Some of the operators did question the decision to do the test, but their concerns were ignored.
The culture was of secrecy and fear, rather than safety and a questioning attitude.
What were the impacts?
Short Term Impacts
Immediately after the accident, the secrecy imposed by authorities meant that no reports were released to the national or international community for 3 days after the explosion. The evacuation of residents from nearby Pripyat did not commence until several days after the accident, meaning many had already been exposed to airborne contamination, particularly iodine-131. Iodine-131 has a short half-life so an earlier evacuation would have drastically reduced the risk to the local area. As well as lessening the impact on the management of livestock and milk.
In total, approximately 200,000 people from all over the Soviet Union were exposed to on average 100 millisieverts. The highest doses were for the 1000 emergency workers on site during the first day of the accident. In total, there were 30 direct deaths from the accident.
Long Term Impacts
Long term, the total number of deaths from the worldwide spread of radiation is unclear. Best estimates range from 4,000 to 35,000 deaths worldwide over 50 years. Reports of cancer rates, particularly child thyroid cancer, have increased both in the immediate area and worldwide. Plus, it caused significant negative public perception, stroked by fear of the global impacts from nuclear accidents.
However, behaviour and regulation for the international nuclear industry has completely changed, particularly in former soviet countries. Following the incident, the INES scale was created, and the World Association of Nuclear Operators (WANO) was established. Moreover, it has led to increased international cooperation. In November 2016, the New Safe Confinement (NCS) structure was created – to prevent release of radioactive contaminants and protect the structure from external weather.
To find out more about the key events that shaped the development of the nuclear industry in the UK and across the globe, why not check out our dedicated page?
