Chernobyl
It has been 20 years since the Chernobyl incident. This was one of the two defining disasters of my time – the other being Bhopal (Dec 3, 1984). It is hard to think of myself as old, but an entire generation has now grown up since those two accidents occurred. Nuclear power was already dying in this country and the Chernobyl disaster was just a nail in the coffin. But as bad as this disaster was, one must remember the extraordinary circumstances that led to this incident.
The RBMK Reactor used at Chernobyl had a bad design flaw. It was highly unstable at low power output. The reactor used water as a moderator – a neutron absorber that slows down a nuclear reaction. When water boils, it creates steam which is a very poor moderator. When excess steam pockets formed in the RBMK reactor, it led to increased power generation (known as a positive void coefficient). This excess power caused additional heating thus producing more steam and less neutron absorption causing the problem to escalate. All of this happens very rapidly and if not stopped quickly is almost impossible to stop, as was evidenced on April 25, 1986.
As Reactor #4 was to be shutdown for routine maintenance, it was decided to take advantage of this shutdown to run a test. The test was to check whether, in the event of a shutdown enough electrical power was available to operate the emergency equipment and core cooling pumps until the diesel power supply came online. For the test the reactor was to be stabilised at 1,000MW prior to shutdown, however, due to operational error the power fell to about 30MW where the positive void coefficient became a problem. The operators tried to raise the power by freeing all the control rods manually and at 01:00hr on 26 April the reactor stabilised at 200MW. At this point they still had a chance to ABORT the test and avoid disaster. But it was not to be.
The RBMK Reactor used at Chernobyl had a bad design flaw. It was highly unstable at low power output. The reactor used water as a moderator – a neutron absorber that slows down a nuclear reaction. When water boils, it creates steam which is a very poor moderator. When excess steam pockets formed in the RBMK reactor, it led to increased power generation (known as a positive void coefficient). This excess power caused additional heating thus producing more steam and less neutron absorption causing the problem to escalate. All of this happens very rapidly and if not stopped quickly is almost impossible to stop, as was evidenced on April 25, 1986.
As Reactor #4 was to be shutdown for routine maintenance, it was decided to take advantage of this shutdown to run a test. The test was to check whether, in the event of a shutdown enough electrical power was available to operate the emergency equipment and core cooling pumps until the diesel power supply came online. For the test the reactor was to be stabilised at 1,000MW prior to shutdown, however, due to operational error the power fell to about 30MW where the positive void coefficient became a problem. The operators tried to raise the power by freeing all the control rods manually and at 01:00hr on 26 April the reactor stabilised at 200MW. At this point they still had a chance to ABORT the test and avoid disaster. But it was not to be.
Shortly afterwards an increase in coolant flow and a drop in steam pressure occurred requiring the operators to withdraw nearly all the rods. The reactor then became very unstable and the operators had to make adjustments every few seconds to maintain a constant power. The operators then reduced the flow of feedwater to maintain steam pressure. The pumps that were powered by the slowing turbine now provided less and less cooling to the reactor. This created additional steam in the cooling channels (positive void coefficient) and the operators could not control a power surge estimated to 100 times the nominal power.
The sudden increase in temperature caused part of the fuel to rupture, fuel particles then reacted with the water creating a steam explosion which destroyed the reactor core. A second explosion added to the destruction two minutes later.
The sudden increase in temperature caused part of the fuel to rupture, fuel particles then reacted with the water creating a steam explosion which destroyed the reactor core. A second explosion added to the destruction two minutes later.
A number of safety features were violated that day including using only 6-8 control rods. The standard operating procedure required a MINIMUM of 30 rods to keep the reactor under control. Additionally the reactor’s emergency cooling system was disabled as that was part of what was being tested. The test was carried out without a proper exchange of information between the team in charge of the test and personnel responsible for the operation of the nuclear reactor.
The human error factor was huge compared to equipment failure in this incident. But people will still blame nuclear technology for this incident. In the end - as trivial as this sounds, this disaster was due to a communications failure - between the team in charge of the test and the people responsible for the operation of the reactor.
I work in the chemical industry and deal with risk on a daily basis. My generation has certainly learned from this tragedy and from Bhopal. And we apply the lessons learned on a daily basis. I hope this next generation will continue to do so and prevent another Chernobyl from ever occurring.
As for nuclear power in this country on the 20th anniversary of Chernobyl? Maybe someday the youngsters of today might realize this accident for what it was, and believe in a safe nuclear power plant again. It will certainly go a long way to alleviating our energy crisis. But I wouldn’t bet the farm on it.
I work in the chemical industry and deal with risk on a daily basis. My generation has certainly learned from this tragedy and from Bhopal. And we apply the lessons learned on a daily basis. I hope this next generation will continue to do so and prevent another Chernobyl from ever occurring.
As for nuclear power in this country on the 20th anniversary of Chernobyl? Maybe someday the youngsters of today might realize this accident for what it was, and believe in a safe nuclear power plant again. It will certainly go a long way to alleviating our energy crisis. But I wouldn’t bet the farm on it.
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