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Peter Bunyard explains the Chernobyl Accident

On Saturday 26 April 1986, at 1.23 am the Number 4 reactor at Chernobyl blew aside the 1,000 tonne lid of a massive steel vessel and blasted through the surrounding concrete containment structure. Bits of graphite, chunks of uranium fuel, and pieces of control rods were strewn around the reactor building. The red-hot graphite still inside the reactor burst into flame as the air rushed in, and like coke in a blast furnace it began to burn vigorously.

Two men died in the first moments of the blast, one from falling masonry and one from burns. Over the following months some 30 others were to die, most of them firemen who battled heroically to prevent the fire from spreading to the reactor in the next building.

All died from radiation burns and radiation sickness following exposure to strong gamma and beta radiation from the broken reactor core. In addition to radiation from outside their bodies, some had also been exposed to large internal doses from breathing in radioactive particles.

Over the next few days Soviet authorities marshaled their resources to evacuate 135,000 people, all of whom lived within 35 km (19 miles) of the power station - an area of nearly 3,000 square miles. Livestock had to be moved and attempts were made to control the contamination that was settling out by covering soil and buildings alike.

Meanwhile, the fire in the reactor had to be put out, and the reactor itself smothered to prevent the escape of even more radioactive material. Helicopters, flying 24 hours s day, were used to dump 5,000 tonnes of material onto the burning core.

This included 800 tonnes of dolomite - a limestone rock - to generate carbon dioxide gas to quench the fire; boron carbide - a neutron absorber - to insure that the nuclear chain reaction remained shut down; 2,400 tonnes of clay and sand to help seal off the fire. The strategy worked and by May 6 the temperature of the core had fallen, and the release of radioactive materials had fallen sharply.

The Soviets believed that between 30 million and 50 million curies of radioactive substances escaped, amounting in all to a few percent of the total inventory in the core.

Moreover, it was the more volatile substances such as iodine and caesium that escaped in relatively large quantities, while inert gasses such as krypton and xenon escaped in their entirety.

The estimate is that 20 percent of the radio-iodine was lost from the core, and approximately 12 percent of the radio-caesium. These substances would later fall-out over much of Europe, contaminating food and water supplies.

Fallout was significantly high over the first couple of weeks after the explosion that many thousands of people received substantial radiation doses through gamma and beta radiation. The Soviets used a comparatively large dose as a criterion for evacuation.

The Soviets stated the official annual limit for individual exposure to radiation as 5 rems (50 millisieverts), which is ten times the ICRP limit and equivalent to more than 25 times the natural background radiation dose.

Even so, towards the end of May some villiages and small towns in Byelorussia several hundred km away from Chernobyl had to be evacuated, in addition to those in the 30-km zone around the stricken reactor.

The Chernobyl disaster was the result of a runaway chain reaction of the kind that creates an atomic bomb explosion. In addition, the fallout from Chernobyl was many times greater, possibly by a factor of a thousand, than would occur after a Hiroshima-sized atomic bomb blast.

Heavy rains over Scandanavia and Western Europe washed out considerable quantities of radioactive substances. Some of these area received several hundred times the fall-out compared with those areas remaining dry. Radio-iodine and radio-caesium were by far the most significant isotopes. Milk products and vegetables were quickly contaminated.

What needs to be known at this point is how a certain level of radionucleotide such as caesium-137 - in the air, on the ground, or in the water supply - is likely to behave in terms of its getting into the food chain and ultimately into human beings. Caesium on pasture translates into such-and-such a dose to infants, children, and adults. At a dose equivalent of 50 millisieverts to the thyroid gland, the authorities should seriously consider whether the public ought to be given stable iodine tablets to prevent the uptake into the thyroid of radio-iodine. By the time the dose has been put to 250 millisieverts action should have already been taken.

An estmated dose of 5 millisieverts to the whole body, or 50 millisieverts to the skin, thyroid, lungs, or other individual organs, ought to have alerted the authorities to the eventuality that the public should take shelter. Once those doses have reached 25 millisieverts to the whole body and 250 millisieverts to any one organ, shelter should have already been prescribed.

During 1-5 May 1986 the maximum radiation concentration in the air at Cadarache in France was found to be 18.5 becquerels per cubic metre. Meanwhile Cadarache received 10 milliliters (less than a half-inch) of rain and the fall-out levels on the ground were measured at 14,200 becquerels per square meter. At Cruas (63 miles from Cadarache) the maximum air concentration was 8.7 becquerels per cubic meter. It rained more than 30 milliliters (1.2 inches) during the same days and the deposition on the ground was 250,000 becquerels per square meter. This was twenty times more radioactive deposition than at Cadarache, demonstrating the importance of weather conditions. Also the Eastern part of Corsica was badly hit by fall-out from Chernobyl. Corsicans, Britons, Swedes and Lapplanders, measured high radioactivity in livestock which increased six months after the initial fall-out. Because of the initial weather conditions, the levels of radioactive caesium in livestock continued to increase during 1987.

Obviously, not all localities are measured for fall-out, and some areas will be entirely missed (especially in the countryside) and people who live there left in ignorance.

What we have learned is we should be on our guard if it rains heavily at a time when the radioactive cloud is somewhere in the vicinity.

If there is contaminated air in the vicinity, we should not go outside, nor drink rainwater, and not go for a swim. Animals must be sheltered and kept inside for many weeks. This reduces the exposure to short-lived radio-isotopes like iodine-131. Contamination of winter feed became a problem in Germany where fall-out contaminated the summer supply of wheat and feed.

Wild vegetation tends to be more efficient at concentrating radioactive material than well-fertilized domestic crops. Game animals that live on these plants are likely to have higher levels of radioactivity compared with domestic ones.

Some buildings are more effective in shielding us from radiation than are others. The best protection is found in an air-tight building, concrete blocks or apartments. The traditional wooden structures are less protective.

Filters in an air cleaning system will have to be replaced following a radioactive event, and care must be taken when changing and disposing of the contaminated filters.

The main task after a major radiation release should not be to hose down vehicles, roads, buildings, because that only disperses the particles into the soil; the main task is to confine the contamination and dispose of it at a safe site.

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