The Accident at Chernobyl: What Caused the Explosion?

On April 26th, 1986, at 1:23 am, Alexander Akimov did what he and thousands of other nuclear plant operators have been trained to do. When confronted with confusing reactor indications, he initiated an emergency shutdown of Unit 4 of the large electricity generating station near Pripyat in Ukraine.

By doing so, he unwittingly initiated an explosion whose effects continue to be felt throughout the world. On the occasion of the tenth anniversary of the Chernobyl accident, it is appropriate to understand what went wrong and what has been done to prevent it from happening again.

Initial Conditions

Before pressing the AZ button – used to initiate an emergency shutdown – Akimov and his fellow operators were immersed in the conduct of a special test. The procedure was designed to prove that the reactor would be provided with sufficient cooling water even if a complete loss of power to the large electric generating complex occured while the emergency cooling system was inoperable.

According to engineering calculations, the inertia of the plant’s big 500 MW electric turbines would allow them to generate enough electricity to keep cooling water pumps operating during the 30 to 50 second delay required to start the emergency diesel generators.

The engineers who designed the test were specialists in electric generators, not in nuclear reactors. The historical record indicates that there was little consultation with nuclear reactor specialists during the procedure preparation.

The test was planned for a time when the plant was to be shut down for routine maintenance and its power output was not needed for the national electrical grid.

Establishing the initial conditions for the test proved difficult and more time consuming than initially planned. The first problem was that the grid needed the power longer than expected. It was after midnight when the plant was finally allowed to begin the test, and a new shift of operating personnel had just taken over. The new shift was not very familiar with the test and did not get a complete briefing by the off-going shift operators.

The actions of the off-going shift operators had put the plant into an unusual situation because the power history and the resulting concentration of fission product poisons was different than any situation considered during the design of the control system.

The man in charge of the test, Anatoli Dyatlov, the deputy chief engineer of the plant, had been involved in the test preparations and in setting the initial conditions. The new operators deferred to him for decisions, because of his experience, his official position and his familiarity with the specific test protocol.

Pressure to Succeed

There were several times during the period immediately prior to the test when the plant technicians considered action that could have averted the catastrophe. They did not like the way that the plant was responding to their control inputs.

If they had taken appropriate action, however, there is no guarantee that they would have been rewarded for their decision. In fact, there is every likelihood that they would have been punished for delaying the test and the subsequent maintenance period. Dyatlov had a reputation as an irritable taskmaster; apparently he was especially impatient on the night of the accident.

Though Soviet reactor plant operators were not under pressure from their bosses to maximize financial profit, the Communist political system provided considerable incentive to maximize production for the benefit of the the state and the party. Failures or perceived weakness were often severely criticized or punished by demotion or reassignment. During the night of April 26th, all the Chernobyl operators had to offer as a reason to discontinue the test was a sense of confusion over the plant indications.

Apparently, Akimov must have comforted himself with the knowledge that he knew exactly where the AZ button was and readied himself to push it if it became necessary. There is no way he could have known that pushing the button could lead to a dangerous insertion of positive reactivity.

Positive SCRAM Reactivity

Much has been made of the fact that RBMK reactors can develop what is known as a positive void coefficient of reactivity. What that long phrase means is that increasing boiling caused by increasing core temperature can lead to an increase in core reactivity, an increase in core power and even more boiling. This positive feedback mechanism is assiduously avoided in most reactor plant designs.

What has not been so well understood is that the shutdown button of an RBMK could, under very special initial conditions, initiate a positive insertion of reactivity that could increase core temperature rapidly enough to cause a steam explosion. No nuclear reactor plant can explode in a manner even remotely similar to an atomic bomb, but, as boiler operators have known for well over a hundred years, a steam explosion can pack quite a punch.

Though much has been made of the lack of a “safety culture,” lack of containment, and violations of procedures by operators, the specific cause of the Chernobyl explosion and subsequent release of radioactive material from the Chernobyl reactor was a shutdown system that initiated a positive reactivity accident. For those readers who have never operated a nuclear reactor, it might be helpful to think of the cause as a brake pedal that – without the driver’s knowledge – transformed itself into an accelerator.

It took far longer than it should have to find the technical cause of the accident, largely because efforts were made to protect the designers and their powerful bosses and to fix the blame on the less politically connected operators.

The design fault that caused the explosion required several straightforward technical modifications that have now been completed on all operating RBMK reactors. There is essentially no chance that the accident could happen again.

However, the fix has not satisfied the calls for action from those that do not understand the production of electricity. The fix has – rationally enough – not satisfied the desires of those that want to use the accident as an excuse to get rid of nuclear power for political or competitive reasons.