Chernobyl 1986 disaster

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Last week, one strange observation reminded of this accident. It happened two decades ago and also affected Finland. I will write about that observation in my next post but I did lot of information mining about this accident over the weekend and thought to put all this in my blog.
(click on the individual pictures to enlarge)

Introduction :

Chernobyl disaster, which could be termed as one of the worst civilian nuclear accident happened between 1am to 2 am on April 26th, 1986. This was worse than the accident at 'Three-Mile Island' power station in USA in 1979. Then no one was killed.

Location:



(source : Google Earth, Red dot is Chernobyl)

Chernobyl power station in Ukraine is located 7km from the border of Belarus, 120km north of capital Kiev. It is situated on the bank of the river Pripyat which is a tributary of river Dnieper, a major river of Europe. Ukraine and Belarus were the two westernmost states of erstwhile USSR.

Type:

Chernobyl power station was one of Soviet styled graphite moderated nuclear reactor. Using graphite as a moderator means that natural Uranium (99.3%U-238 and 0.7% U-235) could be used as critical mass. One of the other benefits of using this type of reactor is that weapon grade plutonium is generated as by-product.

The accident:

The reactor 4 was to undergo maintenance shutdown and it was thought to use this shutdown for one of the test. The test was to see if the inertial rotation of the turbo-generator could be used to run the water pumps during the failure of the power supply (if it happens due to some reason) and start of diesel engine generators. The test was delayed and started on 26th April at 1am.

The accident happened due to the design quirk of such reactors and bypassing several of the safety mechanism mandated for such conditions. All the manual control rods needed to control the reaction were pulled out, the emergency core cooling system was turned off. The steam supply to generator was stopped and automatic shut down mechanism was disabled.

Series of steps followed, all in serious violation of the safety regulations. This resulted in a vicious cycle of 'reaction' and 'steam generation'. Both went on unhindered. At 1.22'.30" seconds a computer printed out that it is extremely unsafe to operate the reactor and it should be shut down. The operators ignored the warning.

Shortly therafter two explosions took place. The 1000 ton roof of the reactor was blown off by the stream explosion and the graphite started to burn due to inward flow of oxygen. This fire spewed up volumes of radio active dust in atmosphere.

The aftermath

Local fire fighters and rescue workers came in almost immediately and all the fire in the building was extinguished by 5 am, but the core of the reactor continued to burn. 31 people died, 28 of them due to radioactive exposure. Almost all of them were the fire fighters and rescue workers. The explosion was not nuclear explosion but steam explosion much like other boiler explosions.

The news was not made public. In the next two days technicians in Finland and Sweden started to notice increased radioactivity over their atmosphere. In Sweden they thought some accident had occurred in their own facility and alert was sounded. They evacuated 600 workers, working in their plant, only to find out that source of this radioactivity was USSR. It was then on evening of 28th April, 2006, USSR radio broke this devastating news.


(source: TIME )

Helicopters dropped 5000 ton of Boron, lead, dolomite sand and clay but graphite continued to burn for 9 days. Most of the helicopter pilots were exposed to radioactivity as well.

A mass evacuation drive was undertaken and some 116,000 people living within 30km radius were shifted to different places. The town was empty and all houses were locked by soldiers. The most hazardous isotopes released in the accident were Cs-137, I-131 and Sr-90. The death due to direct exposure due to radioactivity was limited but the radioactive dust deposited by rain or otherwise caused and is still causing chronic disease. Iodine in particular caused lot of thyroid cancer cases.

The reactor 4 was enclosed in concrete tomb known as ‘Sacrophagus’ to stop further spread of radioactivity. The whole area was declared as ‘forbidden zone’ with check points established. The other reactors continued working.

The present

The town today looks much like haunted city of movies with an eerie silence. The houses, market places and everything else is empty. Despite being forbidden zone some 1000 people have returned, most of them are old people who could not adapt to living in cities and came back resigned to their fate. Soon after the accident all the pine forests died, cattles grew three heads and number of wild animals declined.

The Chernobyl power plant was finally shutdown for good on 15th December, 2000 by the government of Ukraine.


(source: Google Earth)

The above is google map showing the present condition of Chernobyl plant. The plant area is shown by the red rectangle. I could not find detailed map of the area and this is the maximum resolution. You can see flat tan green fields which indicate the natural vegetation grown over abandoned agricultural farms. The radio active particles have settled deep in this soil and those nucliedes having low half life have decayed. Consequently wild animals have increased in number now, though still feeding on contaminated water and food.