WHAT HAPPENED TO CAUSE THE ACCIDENT AT CHERNOBYL?

Chernobyl Focus | 27.6.2017

What were the causes of the disaster at the Chernobyl power plant – the seconds’ scenario will tell you more about that night.

The accident, which occurred in the early morning of April 26, 1986, resulted when operators took actions in violation of the plant’s technical specifications. Operators ran the plant at very low power, without adequate safety precautions and without properly coordinating or communicating the procedure with safety personnel.

However, it doesn’t mean that the blame is all on the staff of the reactor.

The four Chernobyl reactors were pressurized water reactors of the Soviet RBMK design, or Reactor BolshoMoshchnosty Kanalny, meaning “high-power channel reactor” with an output of 1000 MWe. Designed to produce both plutonium and electric power, they were very different from standard commercial designs, employing a unique combination of a graphite moderator and water coolant.

 

 

 

This type of the reactor is know to be unsafe, and very difficult to operate.

The reactors also were highly unstable at low power, primarily owing to control rod design and “positive void coefficient,” factors that accelerated nuclear chain reaction and power output if the reactors lost cooling water.

A standard maintenance stop for reactor N°4 was planned on April 25. To run the RBMK type plant requires the generation of electrical power, mainly for cooling. In the event of a power failure, emergency generators start up a few seconds later. Due to problems with the new emergency generators, it was decided to carry out a test on the cooling pumps, which required the bypassing of safety systems. The aim of the test was to check if the inertia of the turbines provided enough power to keep the cooling pumps operational during the time required to start the emergency generators.

Here is chronological run-down of the chain of events that took place in the days and hours that led up to the accident:

Friday April 25 1986:

  • 01.00 a.m.: the operators decrease the power of the reactor
  • 02.00 p.m.: the reactor runs at half power
  • 11.00 p.m.: decision to start the test. Due to an error in the regulation, the power is much lower than normal. Rather than stopping the reactor (and the test), the operators try to increase the power again by lifting many more control bars than allowed (6-8 rather than 30). The problem is that at low power, the reactor has a positive void coefficient

Saturday April 26 1986:

  • 01.22 a.m.: the test begins while the reactor continues operating under non-authorised conditions. The operators switch off the safety mechanism that should stop the reactor in case of loss of steam supply to the turbine.

  • 01.23.04 a.m.: the turbines shut down and the cooling pumps stop. This increases the steam content in the tubes and the reactor power increases rather than decreases due to the positive void coefficient.

  • 01.23.40 a.m.: an attempt is made to manually stop the reactor by releasing the control bars (211). The control bars take about 20 seconds to reach the core, and their design is such that reactivity increases during the initial seconds. Fuel elements start breaking up. A few seconds later, shocks are felt and explosions are heard. Steam explosions destroy the reactor core and blow the roof off the reactor building. Fires start all over the place. The worst civil nuclear accident in history has just occurred.

  • 01.28 a.m.: the first fire-fighters arrive on the scene

  • 02.30 a.m.: the largest fires are under control

  • 05.00 a.m.: the graphite fire starts

Today, the causes and the consequences of the accident have been thoroughly studied and many lessons have been learnt.

The main causes of the accident, as identified by Western experts are:

  • Unsafe and unstable reactor design: In addition to generating electricity, the RBMK reactors at Chernobyl were also designed and adapted for the production of plutonium for military purposes, as fuel can be loaded and unloaded during operation. This double function restricted the reactor’s built-in safety mechanisms. Consequently, the accident cannot be disassociated from the politico-military context of the former Soviet Union at that time, even if there are no indications that at any time plutonium was produced there for military purposes

  • The operators’ lack of theoretical training and knowledge: During the cold war, safety was clearly not a priority. There was a critical lack of safety culture at Chernobyl, which was amplified by an global lack of understanding and training

 

  • The culture of strict confidentiality that reigned in the former Soviet Union due to the strong interdependency of civil and military nuclear applications: Within the context of the 1980’s, operators were not supposed to think critically or take initiatives in case of emergency situations, which were never even officially considered.

The Chernobyl plant did not have the massive containment structure common to most nuclear power plants elsewhere in the world. Without this protection, radioactive material escaped into the environment.

Thus, according to some experts, there are no area in the whole world which was not affected by the Chernobyl accident.

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