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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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