Chernobyl Disaster

The Chernobyl disaster occured on April 26, 1986 when a series of explosions in Energy Block #4 of the Chernobyl Nuclear Power Station in Soviet Ukraine resulted in a core meltdown that spread radioactivity over a wide area, including contaminating about a quarter of the country of Belarus. Initially the accident was attributed to operator error, but subsequent investigations revealed major design and construction flaws.

Accident Cause

While the meltdown was initially attributed to operator error, subsequent investigation by the International Atomic Energy Agency (IAEA) blamed design flaws and shoddy construction of the nuclear power plant itself.

In the first early hour of April 26, began the events stemming from a scheduled shutdown and subsequent test of the reactor scheduled the previous day. The test was designed to determine how long turbines would spin and supply power if the plant lost main electrical power.

At approximately 12:28 AM, the power level had dropped to 500 MW(t) in accordance with the procedure, when control was transferred from the local to the automatic regulalation system. At this point, a signal to hold the power at level failed due either to operator or regulatory system response. Power produced by the plant rapidly fell to 30 MW(t).

Responding to this power drop, the operator retracts a number of control rods in attempt to correct the situation. Fewer than the effective equivalent of 26 control rods were remaining in the reactor, a situation that should have happened only with the station safety procedure approval of the chief engineer. 1:00 AM, the reactor power rises to 200 MW(t). Two pumps were switched into their opposite-handed cooling circuits in order to increase the water flow to the core. This operation of the additional pumps although removed heat from the core at a quicker rate, reduced the water level in the steam seperator.

Fifteen minutes later, automatic trip systems to the steam separator were disabled by the operator in order for the operation of the reactor to continue. The operator increased the water flow in attempt to address the cooling system problem. In order to increase power, the operator withdrew some of the manual control rods in an attempt to raise the temperature and pressure in the steam seperator, against an operating policy requirement that a minimum effective equivalent of 15 manual control rods were to be inserted into the reactor at all times. It is estimated that at this time the number of rods in the system was about half of the requirement.

Twenty minutes into the total process, the operator reduces the water flow rate to below normal in an attempt to stabilize the steam seperator water level, decreasing heat removal capability from the core. The increased amount of heat spontaneously generates steam in the core, abnormally giving the operator the appearance that the reactor was stable.

At 1:23 AM the actual shutdown process began. Turbine feed valves were closed off, and automatic control rods were withdrawn from the core. A 10-second withdrawl of the rodes from was the normal reponse to compensate for a decrease in the reactivity following the closing of the turbine feeds. Typically, a decrease in reactivity is the result of an increase in pressure in the cooling system and a consequent decrease in the quantity of steam in the core. This expected decrease did not happen due to the reduced feedwater to the core (the operator manually reduced the water flow rate).

Steam generation in the core increased, and because of the reactor's positive void coefficient, any further amount of steam would lead to a rapid increase in power output. Unfortunately, seconds later steam in the core begins to increase uncontrollably. The emergency button is pressed by the operator which sends control rods into the core. It is too late, and the insertion of the rods from the top concentrated all of the reactivity to the bottom of the core, causing the reactor power to rise 100 times the alotted design value. Fuel pellets shattered, reacting with the water to produce high pressure in the fuel channels. The channels rupture, causing two explosions: one a steam explosion and the other a result of expansion of fuel vapor. The two explosions cause a loss of integrity of the pile cap, allowing the entry of air that reacted with the graphite moderator blocks to form carbon monoxide gas. The gas ignites, causing a reactor fire.

The major design flaw in the Chernobyl-4 reactor was the ability for the nuclear chain reaction and power output to occur in the loss of cooling water or the conversion of that water into steam. This flaw was the key factor that instigated the power surge which led to its destruction.

Immediate Effects

Following the second explosion and subsequent reactor fire, approximately 14 EBq(10¹⁸ Bq) of radioactivity was released into the environment through the continuous nine-day burning of the graphite moderator, half of the radioactive material being biologically-inert noble gases.

In response, 5000 tons of boron, dolomite, sand, clay and lead were dropped on the burning core in an attempt to extinguish the blaze and limit the release of radiation.

It is estimated that all of the xenon gas, about half of the iodine and caesium, and at least 5% of the remaining material was released and desposited as dust and debris. Unfortunately the lighter material carried through the wind over Ukraine, Belarus, Russia, and parts of Scandinavia and Europe.

Emergency Response

Primarily, the casualities were limited to the firefighters; those that worked to extinguish the flames on the roof of the turbine building on the first day absorbed an estimated range of up to 20,000 millisieverts (mSv).

The next task was performed by recruiting liquidators (200,000) all throughout the Soviet Union to assist in cleaning up the radioactivity at the site, which took course over two years, during 1986 and 1987. They were exposed to high levels of radiation, averaging 100 mSv. About 20,000 of them were exposed to 250 mSv and a smaller few exposed to 500. Over these two years the amount of liquidators increased to 600,000.

Overall, the highest doses were received by those immediately to the scene: dispatched policemen, firemen, soldiers, and liquidators (approximately 1000 people).

Radiation Poisoning

Acute radiation poisoning takes about two weeks to kill. Symptoms include --

• skin lesions, sores, and burns
• vomiting, diarrhea, and suppuration
• loss of hair, teeth, and nails
• pulmonary edema and tissue swelling
• massive internal organ failure

Human Casualties

All totalled the amount of people dead from those immediately exposed to the Chernobyl accident is currently at 49; an initial 30 died from the accident itself (28 from radiation exposure). 19 of the 209 treated for acute radiation poisoning have died as a result of side effects of exposure, although out of those only 134 cases have been confirmed.

Reference Source

Interviews with survivors are documented in the book "Voices From Chernobyl" by Svetlana Alexievich, 2006, Picador St. Martin's Press $14 ISBN 0312425848.