Reference document : http://chernobylgallery.com/chernobyl-disaster/cause/
It was a chilly night in the Ukrainian village of Chernobyl on 26th April 1986. Chernobyl name comes from the peculiar variety of grass/plant called Artensia vulgaris. It means the place of this grass/plant. The nuclear power plant there came into operation in the early eighties as one of the nine places shortlisted by the erstwhile USSR. The four 1000 MW units were installed one by one.
Unlike a thermal power plant, power shutdown in a nuclear power plant can take dangerous propositions. That is because in case of thermal power plants the boiler is heated by coal and you just need to stop the coal feeding and run the circulation water till the boiler cools down. But in case of Nuclear plants the residual heat is generated too fast if the absorbing medium is cut off. Since a sizeable nuclear fuel is already fed into the reactor, the only way is to control the reaction through control rods and the absorbing medium like heavy water/ circulation water as the case may be.
The circulation water was being pumped by three pumps of roughly 1.7 MW each. Once the power supply goes off, the DG sets were standing in as standby, and they would take 45 seconds on an average to supply this power of 5.5 MW to the circulating pumps. Till that time if the flow is not maintained it would create bubbles in the water due to heat accumulation. These bubbles were called voids. Due to these voids, the temperature further shoots up. Unfortunately the designed fission reaction of Chernobyl was a positive void reaction, i.e. if the temperature increases, the rate of reaction also would increase.
But the plant designers had a plan to fill this gap of 45 seconds. The 1000 MW Turbine and generator are mammoth machinery and it would be running at a huge momentum unless it is at a load of say less than 70%. Hence the system requirement was that the turbine and generator should be able to supply this miniscule 5.5 MW of power for a short duration of 45 seconds with their mammoth momentum. But this failed thrice during testing and the operators were looming large at an acute safety lacuna in the system. They got an ultimatum from the regulatory authorities to test it once more and if the problem persists, then go for urgent remedial measures.
On that fateful night of 26th April, the unit operator took a decision to test it for a fourth time, but without taking formal consent from the authorities and therefore without proper planning. The unit was running at more than 60% load and the power was shut off. The Turbine and Generator as usual did not deliver 5.5 MW of power till the DG sets took over. In the meantime disrupted flow caused void formation and the temperature shot up, increasing the reaction rate. There was a panic button to be pressed in case of such an emergency which would lower additional cadmium rods into reactor which would have slowed down the reaction at once. The panic button would bring down the rods into the reactor, but while doing so displaces the cooling water. To make things worse the rods had a tip of graphite instead of cadmuim, hence during this displacement time neither water nor cadmium absorbed the neutrons.
First there was an explosion due to high pressure built up in the water circuit which lifted the 2000 ton upper lid of the reactor dome. This explosion made all the water in the circulation system to escape and the reactor was running dry now. This was followed up by a built up of immense heat enough to generate 33 GW (33000 MW) of power for a unit designed to generate just 1000 MW. No need to furhter explain what would have happened if such high levels of energy are built up within a few seconds. Most of the victims were hapless rescue team members from the army who never knew what they were up to. 4000 people was the official estimate of casuality.
The iron curtain (euphemism for the tight veil of communism), made sure nobody in the free world knew about what exactly happened. Worse was the fact that no real culprit underwent any scrutiny not to speak about any conviction. Other than a few operator engineers every one went scot-free. As per my opinion the following real culprits should have been taken to task:-
- The Turbine-Generator manufacturers who could not deliver as per design the requisite momentum to feed Cooling water pumps for just 45 seconds.
- The regulatory authority who allowed the plant to run with such a gruesome safety lapse.
- The authorities who asked hapless armymen to do the rescue act without adequate safety gears.
- The chief engineer who took the decision to do the test without informing the regulatory authority.
Chernobyl gives a great lesson to human kind on laxity could create a debacle. If Fukushima was a technical failure, Chernobyl was a human failure. If we could perenially put Union Carbide on the grill for Bhopal gas tragedy, why shouldn’t we grill the companies responsible for Chernobyl?
These are some of the children who died without knowing who took their lives. It could have been our children as well, are we really bothered? Are our nuclear plants safe? How do we think so when the real culprits are roaming free? I recently did my little bit to prevent these culprits enter India again. Now it is your turn!
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