Nuclear Accidents

Three Mile Island
Chernobyl

At 4:00 AM on March 28, 1979, a reactor at the Three Mile Island unit two (TMI-2) nuclear power facility near Harrisburg, Pennsylvania suddenly overheated, releasing radioactive gases. During the ensuing tension-packed week, scientists scrambled to prevent the nightmare of a meltdown, officials rushed in to calm public fears, and thousands of residents fled to emergency shelters. Equipment failure, human error, and bad luck would work together to create America's worst nuclear accident.

The reactor's fuel core became uncovered and more than one third of the fuel melted.
Inadequate instrumentation and training programs at the time hampered operators' ability to respond to the accident.
The accident was accompanied by communications problems that led to conflicting information available to the public, contributing to the public's fears
Radiation was released from the plant. The releases were not serious and were not health hazards. This was confirmed by thousands of environmental and other samples and measurements taken during the accident.
The containment building worked as designed. Despite melting of about one-third of the fuel core, the reactor vessel itself maintained its integrity and contained the damaged fuel.

By 8:00 a.m., after cooling water was lost and temperatures soared above 5,000 degrees, the top portion of the reactor's 150-ton core collapsed and melted.

It was never supposed to happen. In the predawn hours of March 28, 1979, a pressure valve suddenly malfunctioned at the Three Mile Island nuclear power plant near Harrisburg, Pennsylvania. What occurred next--a combination of technical failure, human error, and bad luck--would result in the worst nuclear accident in American history. For five nerve-wracking days, engineers struggled to control a runaway reactor, government officials debated whether to evacuate the area, and residents contemplated the ultimate horror of a nuclear meltdown.

The accident began about 4:00 a.m. on March 28, 1979, when the plant experienced a failure in the secondary, non-nuclear section of the plant. The main feedwater pumps stopped running, caused by either a mechanical or electrical failure, which prevented the steam generators from removing heat. First the turbine, then the reactor, automatically shut down. Immediately, the pressure in the primary system (the nuclear portion of the plant) began to increase. In order to prevent that pressure from becoming excessive, the pilot-operated relief valve (a valve located at the top of the pressurizer) opened. The valve should have closed when the pressure decreased by a certain amount, but it did not. Signals available to the operator failed to show that the valve was still open. As a result, cooling water poured out of the stuck-open valve and caused the core of the reactor to overheat.

As coolant flowed from the core through the pressurizer, the instruments available to reactor operators provided confusing information. There was no instrument that showed the level of coolant in the core. Instead, the operators judged the level of water in the core by the level in the pressurizer, and since it was high, they assumed that the core was properly covered with coolant. In addition, there was no clear signal that the pilot-operated relief valve was open. As a result, as alarms rang and warning lights flashed, the operators did not realize that the plant was experiencing a loss-of-coolant accident. They took a series of actions that made conditions worse by simply reducing the flow of coolant through the core.

Because adequate cooling was not available, the nuclear fuel overheated to the point at which the zirconium cladding (the long metal tubes which hold the nuclear fuel pellets) ruptured and the fuel pellets began to melt. It was later found that about one-half of the core melted during the early stages of the accident. Although the TMI-2 plant suffered a severe core meltdown, the most dangerous kind of nuclear power accident, it did not produce the worst-case consequences that reactor experts had long feared. In a worst-case accident, the melting of nuclear fuel would lead to a breach of the walls of the containment building and release massive quantities of radiation to the environment. But this did not occur in the Three Mile Island incident.

Three Mile Island Unit 2 was too badly damaged and contaminated to resume operations.

Early in the cleanup, Unit 2 was completely severed from any connection to TMI Unit 1. Defueling the TMI-2 reactor vessel was the heart of the cleanup. The damaged fuel remained underwater throughout the defueling. In October, 1985, after nearly six years of preparations, workers standing on a platform atop the reactor and manipulating long-handled tools began lifting the fuel into canisters that hung beneath the platform. In all, 342 fuel canisters were shipped safely for long-term storage at the Idaho National Laboratory, a program that was completed in April, 1990.

Today, the TMI-2 reactor is permanently shut down and defueled, with the reactor coolant system drained, the radioactive water decontaminated and evaporated, radioactive waste shipped off-site to an appropriate disposal site, reactor fuel and core debris shipped off-site to a Department of Energy facility, and the remainder of the site being monitored. The owner says it will keep the facility in long-term, monitored storage until the operating license for the TMI-1 plant expires at which time both plants will be decommissioned. TMI-1's license expires in 2034.

Chernobyl Return to Top^

At 1:23 AM on April 26, 1986, two explosions ripped through the Unit 4 reactor of the Chernobyl Nuclear Power Plant in Chernobyl, Ukraine, of the former USSR. The reactor block and adjacent structure were wrecked by the initial explosion. Nearby buildings were ignited by burning graphite projectiles. Radioactive particles swept across the Ukraine, Belarus, the western portion of Russia and eventually spread across Europe and the whole Northern Hemisphere. The accident followed a safety experiment in which the plant was operated outside of its designed parameters at very low power and unfavorable cooling conditions.

The radioactivity released was estimated to be about two hundred times that of the combined releases in the bombing of Hiroshima and Nagasaki. Millions of people were exposed to the radiation in varying doses.

Prior to a routine shut-down, the reactor crew at Chernobyl-4 began preparing for a test to determine how long turbines would spin and supply power following a loss of main electrical power supply. Similar tests had already been carried out at Chernobyl and other plants, despite the fact that these reactors were known to be very unstable at low power settings.

A series of operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted test early on that fateful day. As flow of coolant water diminished, power output increased. When the operator moved to shut down the reactor from its unstable condition arising from previous errors, a peculiarity of the design caused a dramatic power surge.

On April 26th, 1986, at 1:23 am, Alexander Akimov did what he and thousands of other nuclear plant operators have been trained to do. When confronted with confusing reactor indications, he initiated an emergency shutdown of Unit 4 of the large electricity generating station near Pripyat in the Ukraine.

By doing so, he unwittingly initiated an explosion whose effects continue to be felt throughout the world.

Before pressing the AZ button - used to initiate an emergency shutdown - Akimov and his fellow operators were immersed in the conduct of a special test. The procedure was designed to prove that the reactor would be provided with sufficient cooling water even if a complete loss of power to the large electric generating complex occurred while the emergency cooling system was inoperable.

According to engineering calculations, the inertia of the plant's big 500 MW electric turbines would allow them to generate enough electricity to keep cooling water pumps operating during the 30 to 50 second delay required to start the emergency diesel generators.

The test was planned for a time when the plant was to be shut down for routine maintenance and its power output was not needed for the national electrical grid.

01:23:04 The actual test procedure begins with the turbine feed valves being closed to start the turbine coasting.

01:23:10 The automatic control rods were now withdrawn from the core. This was a normal occurrence and was designed to compensate for the drop in reactivity after the turbine feed valves were closed. The average withdrawal time was 10 seconds. This decrease is usually precipitated by a decrease in the quantity of steam in the core caused by an increase in cooling system pressure. This time the amount of steam did not decrease as there was a reduced feed water flow rate to the core.

01:23:21 The quantity of steam in the core was at a point that due to the positive void coefficient - a measurement of how the reactor responds to increased steam formation in the water coolant - a further increase in steam would create a rapid increase in power.

01:23:35 There was now an uncontrolled increase in the quantity of steam in the core.

01:23:40 The operator now pressed the emergency button (AZ-5). The control rods now started to enter the core from the top. This had the effect of concentrating the reactivity in the bottom of the core.

01:23:44 The reactor power had now peaked at approximately 100 times the reactors rated value.

01:23:45 Fuel pellets had now started to shatter and as they then reacted with the coolant, this produced a burst of high pressure in the fuel channels.

01:23:49 The fuel channels ruptured.

01:24:00 At this time there was thought to be two explosions, one a steam explosion and the other being a product of the fuel vapor expansion. These explosions lifted the pile cap of the reactor vessel which allowed air to enter. This resulted in the ignition of flammable gas and a reactor fire ensued.

The fuel elements ruptured and the resultant explosive force of steam lifted off the cover plate of the reactor, releasing fission products to the atmosphere. A second explosion threw out fragments of burning fuel and graphite from the core and allowed air to rush in, causing the graphite moderator to burst into flames. There is some dispute among experts about the character of this second explosion.

Over 1200 tons of graphite burned for nine days, despite dumping over 5000 tons of boron, dolomite, sand, clay and lead onto the burning core by helicopter in an effort to extinguish the blaze, causing the main release of radioactivity into the environment. The fire eventually extinguished itself when the core melted and flowed into the lower part of the building, then solidified, sealing off the entry. About 71% of the radioactive fuel in the core (about 135 metric tons) remained uncovered for about 10 days until cooling and solidification took place.

Within seven months of the accident the reactor was contained within the purpose built 'sarcophagus' which is approximately 180 feet high by 180 feet long and is supported on the remains of the old reactor building. Although this building was supposed to last for 30 years, it is already showing serious signs of deterioration because of poor standards maintained during construction. Another problem is the fact that the concrete is suffering from constant irradiation and a considerable temperature differential between the inner and outer faces.

The accident destroyed the Chernobyl-4 reactor and killed 30 people, including 28 from radiation exposure. A further 209 on site and involved with the clean-up were treated for acute radiation poisoning and among these, 134 cases were confirmed (all of whom apparently recovered). Nevertheless 19 of these subsequently died from effects attributable to the accident. Nobody off-site suffered from acute radiation effects. However, large areas of Belarus, Ukraine, Russia and beyond were contaminated in varying degrees.

Following the accident, questions arose on the future of the plant and its eventual fate. All work on the unfinished reactors 5 and 6 was halted three years later. However, the trouble at the Chernobyl plant did not end with the disaster in reactor 4. The damaged reactor was sealed off with concrete. The Ukrainian government continued to let the three remaining reactors operate because of an energy shortage in the country. A fire broke out in the turbine building of reactor 2 in 1991; the authorities subsequently declared the reactor damaged beyond repair and had it taken offline. Reactor 1 was decommissioned in November 1996 as part of a deal between the Ukrainian government and international organizations such as the IAEA to end operations at the plant. On December 15, 2000, then-President Leonid Kuchma personally turned off Reactor 3 in an official ceremony, effectively shutting down the entire plant transforming the Chernobyl plant from energy producer to energy consumer.