Monday, March 20, 2006

Chernobyl (26th April 1986).

Although the event has received little press attention so far, the days are inching toward the 20th anniversary of the disaster at the Chernobyl nuclear power plant, which occurred on the 26th of April, 1986. Since the result was widespread radioactive contamination, Chernobyl was not a single event, but is an ongoing process. I remember the initial event well, and its aftermath, since I was working in Russia at the time. There was, as I recall, very little information made available within the U.S.S.R., and my Russian colleagues learned most about what had happened from their colleagues in the West. I have mentioned "Chernobyl" to a few of my friends and acquaintances recently, and from this small survey it seems that no-one under the age of about forty is aware of even the name of the place, let alone what happened there.
It was thought initially that there had been a release of radioactive material from the Forsmark nuclear power station in Sweden. The story at the time was that one man, having spent the weekend out hiking, had set-off the radiation monitors on his way "into" work, rather than on the way out of the plant. Once it was established that there was no leak at Forsmark, a search was inaugurated for the real source, which led to the first suggestion of a major nuclear incident in the western Soviet Union. As I recall, this was immediately denied, then played-down, and finally the extent of the problem was admitted, of a magnitude which nobody before or since has had to deal with. Let's hope it stays that way.
The upshot was an international cooperation involving health specialists, physicists, nuclear engineers, radiation biologists and many other kinds of scientists from all over the world, working together toward the common goal of stabilising the reactor and decontaminating land, animals and humans, to minimise further calamity. These efforts continue.
Contoversy remains as to the precise cause of the events at Chernobyl, but there appears to have been an inopportune convergence of circumstances. There is some speculation that the design of the RBMK-1000 reactor is inherently unsafe, in particular the control rods. Dangerous operating procedures were blamed too, in that many of the reactor safety systems had been turned off in order to conduct an experiment during which all but seven of the total 211 control rods were removed from the reactor. On the night of April the 25th, the unit 4 reactor was due to be shut down for routine maintenance, and the decision was taken to test whether the reactor turbine generator could provide enough electricity to run the reactor's safety systems in the event of a loss in external electric power. Ironically in the interests of safety, the power output was reduced to an intended level of 700 MW from the normal level of 3.2 GW (thermal power = 1 GW electrical power); however, it actually fell to just 30 MW. It was decided to power-up the reactor by pulling out the control rods, but because the water flow ran much higher than normal, and water absorbs neutrons hence reducing the reactor power, the control rods were removed manually, thus rendering the reactor in a very unstable condition. The power of the reactor suddenly increased, particularly as bubbles of steam formed in the primary coolant and rose further, whereupon a "scram" was ordered, meaning that all the control rods are inserted, which should shut-down the reactor.
However, due to the hollow tips of the rods and the temporary displacement of coolant, this actually caused the power to rise further still and the consequent high temperature distorted the control rod channels, with the result that the rods became stuck on the way down, and were thus unable to slow down the reactor. However, it was recently pointed put to me by an expert on nuclear power who has worked at Sellafield for many years that "the Chernobyl accident was not due to sticking control rods: once their full withdrawal (in defiance of standing rules and after deliberate disablement of safety systems) had succeeded in starting a rise in power level, inherent positive feedback effects rendered it unstoppable with a rapid rise to an estimated hundred times nominal maximum." The reactor power jumped to about ten times its rated output, the fuel rods began to melt and the water in the primary coolant circuit flashed to steam causing a "steam explosion" of sufficient force to rupture the cooling pipes, blow the 1000 tonne concrete containment lid off the top of the reactor, and blast a hole through the roof of the building. The person who pressed the "scram" button died of radiation poisoning two weeks after the incident.
Remarkably, the Chernobyl disaster could have been far worse, and tremendous efforts were made to "bury" the reactor by dropping thousands of tonnes of sand, lead, boric acid and other materials onto it from helicopters. Counter-productively, the effect of these materials was like lining the walls of a furnace with firebrick and increased the temperature of the molten fuel which was already melting its way through the reactor floor. Water that had been pumped into the building in a vain effort to extinguish the fire from the burning graphite moderator began to collect beneath the reactor and without the self sacrifice of two men ("liquidators") sent in wearing only divers' "wet suits" to release the valve and vent the radioactive water from the building, a thermal explosion would have ensued when the molten fuel contacted it, rendering an area of land around the plant occupying hundreds of thousands of square kilometers uninhabitable for hundreds of years. There are various estimated figures, but undoubtedly many lives were saved by these two men, who gave their own in so doing.
It is estimated ( that the unit 4 reactor contained around 180-190 tonnes of uranium oxide fuel and its fission products. With the reactor core exposed, oxygen came into contact with the very hot fuel and graphite moderator, causing the latter to catch fire. This resulted in a plume of radioactive smoke being borne upwards into the atmosphere, from where it was transported by weather currents over much of the western U.S.S.R. In Bulgaria, children were fed biscuits baked with zeolites in them - see my earlier listing "Zeolites - the stones that boil" - in an effort to remove any radioactive caesium and strontium they may have ingested. The adults were advised to drink a bottle of red-wine every day, in the belief or hope that the antioxidants in it would protect them against the effects of radiation. The fallout was further carried over western Europe, and as far as the western United States. 375 farms remain contaminated in the U.K. which hold 200,000 sheep - the sheep need to be moved to uncontaminated ground and grazed there for a few weeks before they are fit for market, and then only following an inspection by an appropriately accredited vet.
The human costs are immense. About 50 people died in the immediate stages; 28 from acute radiation exposure. Some children living in heavily contaminated areas received very high radiation doses of up to 50 Grays (Gy) from the absorption of iodine-131, a radioactive isotope with a half life of 8 days (far shorter than the biological half life for residence in the thyroid of 120 days). This, they ingested from locally produced milk which was heavily contaminated, and nine of them died from thyroid cancer. In total it is estimated that another 3,940 people are likely to die from cancer as a consequence of radiation exposure from Chernobyl. Over 300,000 people were rapidly displaced to safer areas after the disaster, 50,000 of them from the town of Pripyat where the nuclear power plant actually is (it lies 11 miles to the north west of the city of Chernobyl). In the intervening time many, mostly the old, have returned to their former homes.
The political impact of the Chernobyl disaster has been highly significant. It has been said that the event itself and the complications of dealing with it which were compounded by the prevailing Soviet secrecy of the time, helped to bring about the dissolution of the Soviet Union. It's initial influence was to turn nation populations and their leaders more steadfastly away from nuclear power. Ramping up the arms-race too, would have been unthinkable. Time and circumstances have moved on since then and the U.K. looks set to promulgate its nuclear capability, although the government is receiving somewhat conflicting advice. It's Chief Scientific Advisor advocates nuclear power on grounds of its low carbon emissions while a number of highly credible objections have been raised in a recent report by the Sustainable Development Committee. Assuming we do "go nuclear" I recall reading a while back that the government had planned an extra £10 billion for the purpose, and I see that £2 billion is planned to overhaul the Atomic Weapons Establishment at Aldermaston ( The U.S. and Russia too, are each refurbishing their own nuclear defenses, though within guidelines agreed between them.
20 years on, I see that the nuclear issue, while lying dormant until recently, never entirely went away.

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