Chernobyl 20 years on Print E-mail
Sunday, 16 April 2006 15:49

Twenty years ago this month, a test in Chernobyl's Unit 4 nuclear reactor went wrong... horribly wrong...

A typical shift in the Unit 4 reactor control room. 

Early on the morning of April 26th, Unit 4 nuclear reactor was scheduled to undergo a safety test - specifically to see how long the turbines could generate power to run the safety systems upon loss of external power.

Looking into the core of the reactor the morning after the explosion. 

Safety back-up generators are on stand-by incase of power loss, but do not automatically cut-in.  The test was designed to see how long the turbines would generate power when using their own inertia, a test that had been carried out before on one of the other reactors.

In order to conduct the test in a safer environment, the power output was to be reduced from the normal 3.2GW to 700MW, however the power level was reduced too rapidly, falling dangerously low, to 30MW.

The damage extended across a large area of the complex.

At the same time, there was an increase in the concentration of xenon-135 - a neutron absorbing element.  To combat the effects of this, the control rods were raised.  Shortly after, the coolant pumps were turned on, increasing the flow of water through the core.  As water is also a neutron absorbent, the control rods were again raised, this time to a point somewhat higher than normally considered safe.

As the test continued, power to the coolant pumps was turned-off and allowed to be driven by the turbine.  As the turbine slowed, so did the flow of coolant through the system.  As the coolant flow slowed, steam pockets formed.

One of the design faults of the RBMK (A Russian acronym for "reaktor bolshoy moshchnosti kanalniy" ) type reactors is a large positive void coefficient - meaning that in the absence of neutron absorbent material (such as water, or the graphite control rods) the power output will rise dramatically and the reactor operation will become increasingly unstable.

For some reason, the unstable operation of the reactor had not registered on the indicators in the control room - and it appears that none of the staff were fully aware of the danger.

The control room of Unit 4 now sits empty and deserted.

At the end of the test (around 1:30am), the order was given to lower the control rods fully back into the core, in order to shut the reactor down.  The reactor was scheduled to be shut down for routine maintenance.

As the rods were being lowered, the hollow graphite tips of the control rods displaced some of the remaining coolant, causing an increase in the power output of the reactor, and heat.  The excess heat deformed the control rod guide channels, causing the control rods to jam about 1/3 of the way in.

Since the control rods were only 1/3 of the way into the reactor, they were unable to stop the reaction.  As the rate of reaction increased, power output jumped to about 30GW - somewhere around ten times the normal output of the reactor.

The fuel rods began to melt and the steam pressure rapidly increased, resulting in a large explosion that destroyed the lid of the reactor and coolant lines, and blew a hole in the roof of the facility & displaced the lid of the reactor containment vessel.  The inrush of oxygen, combined with the high temperature of the core material, caused the graphite to ignite.  The graphite fire is attributed with causing the spread of the greater proportion of radioactive material throughout the region.  As there was no containment building, a large cloud of radioactive fallout drifted over parts of the western Soviet Union, Eastern Europe, Scandinavia, UK and the eastern United States.  Large areas of Ukraine, Belarus and Russia were badly contaminated.  It is estimated that 60% of the fallout landed in Belarus.  200,000 people were eventually evacuated from the contaminated areas - some residents refused to leave, some have moved back into the area.

The deserted town of Pripyat.

Shortly after the explosion, firefighters were called in to fight the fire, they were not told of the dangers of the radiation.  The fire was extinguished by 5am.

It took more than 24 hours for government investigators to assess the scene and acknowledge the destruction of the reactor.  They ordered the evacuation of Pripyat, a nearby town built to house the family and workers of the reactor.  The residents were told that the evacuation was temporary - none have returned in twenty years.  The area may be uninhabitable for 600 years.

The Liquidators did not know of the dangers of radiation.

In the days following the explosion, "liquidators" - comprising of army and other workers - attempted to contain the scene.  Helicopters were used to dumped sand, lead and boric acid into the reactor.  Radioactive core material was collected - mostly by hand - and dumped into what was left of the reactor.  Some of the liquidators were subjected to lethal doses of radiation with as little as two minutes exposure.

Army officers were given the choice - two minutes on the roof of the reactor
or two years on the front line in Afghanistan.  They didn't know the risks.

A large concrete sarcophagus was built to seal off the reactor - however this sarcophagus is now in urgent need of repair, with large cracks appearing, letting rain water enter, and radiation escape.

More than two hundred people were hospitalized immediately following the accident, with 28 dying from radiation exposure.  It is estimated that 700,000 liquidators were involved with the clean-up operations.

The news of the accident wasn't made public for 18 days.  On April 28th, technicians at the Forsmark nuclear power plant, 60 miles north of Stockholm, recorded abnormally high levels of radiation and began urgently inspecting their reactor, fearing a leak.

Soviet President Mikhail Gorbachev appeared on television, and said:


"Good evening comrades.  All of you know that there has been an incredible misfortune - the accident at the Chernobyl nuclear plant.  It has painfully affected the Soviet people, and shocked the international community.  For the first time, we confront the real force of nuclear energy, out of control."

It is estimated that more than 60,000 people will eventually die from the effects of radiation exposure. Health problems, birth defects and increases in cancer rates caused by the radiation are expected to be seen in the regions population for many generations to come.

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