Sit down, my children, and I'll tell you a story. It's a heartbreaking story, and a chilling one. It's a tale of the nuclear power, the Soviet regime, an enormous catastrophe, lives that will never be the same, and lives that are no longer. It's a tale of incompetence and bravery, of secrets and lies, of misinformation and lack of information, of illness and ill-use. It is the story of Chernobyl--specifically, the story of the April 25-26 explosion in the #4 reactor at the Chernobyl power plant.
Our story takes place near the town of Chernobyl. At various times in Chernobyl's history, it was a crown village of the Grand Duchy of Lithuania; part of the Kingdom of Poland; annexed by the Russian Empire; and incorporated into the Ukranian SSR (Soviet Socialist Republic). Chernobyl's religious history was rich--in the past, it was made up of Jewish, Eastern Orthodox, Greek Catholic, Russian Orthodox, and Dominican (Catholic) communities. Chernobyl survived the Polish-Cossack War of 1648-54, and an uprising in 1768-69. It survived the Russian Civil War in 1918-22, and the Polish-Soviet War of 1919-20. It survived the Frontier Clearances of 1936, though it suffered the deportation of its Polish citizens. It survived the German Occupation during World War II, from 1941-44, though its Jewish community was destroyed by the Nazis.
Chernobyl survived a great deal throughout its nearly millenium-long recorded history. What it could not survive was the Soviet regime. Twenty years after World War II ended, Chernobyl was chosen as the site of one of the first Soviet nuclear power stations. Twenty years after this fateful decision, on April 25-26, 1986, for all practical purposes, Chernobyl was no more.
Chernobyl, April 25-26, 1986: What actually happened
Chernobyl nuclear power station is a vast, four-reactor plant at Pripyat, in present-day northern Ukraine, some 80 miles north of Kiev. Actual construction of the plant began in the 1970s, and the four reactors were commissioned in 1977 (#1), 1978 (#2), 1981 (#3), and 1983 (#4). All four reactors are RMBK-1000 reactors. I'll get into the importance of that later.
Reactor #4 at Chernobyl was scheduled, on April 25, 1986, to be shut down for routine maintenance. Plant operators decided to use this shutdown as an opportunity to test the reactor's turbine generator. At Chernobyl, the safety systems were powered by external electricity. The plant had a pair of diesel generators as a backup, to power the safety systems in the case of a loss of external electric power. Those generators, however, did not start immediately upon a loss of external power.
Essentially, the test being run at Chernobyl was this: the plant operators intended to use reactor power to start the reactor's turbines spinning. In this test, once the turbines were spinning, they were disconnected from the reactor and allowed to spin on their own. The point of the test was to determine whether the turbines, spinning under their own inertia, could produce enough power to run the reactor's safety systems while the generators were being started.
It sounds simple enough, kids, and in another reactor, it may well have been a simple test. At Chernobyl #4, it was a disaster waiting to happen, for two reasons: the reactor's design, and bad training of and performance by the plant's operators.
So. There was a planned shutdown, and the operators of the plant decided to perform a test while the shutdown was underway. The test was safer to perform at a lower level of power, so the decision was made to drop the reactor power level.
The start of the test was delayed. In making up for lost time, plant operators dropped the power level too quickly, and concentrations of xenon-135 gas, a nuclear poison, grew inside the reactor. Xenon-135 is a gas usually consumed in a reactor under higher power output. Because of the power drop, the gas concentration was not decreasing as it normally would have. In order to aid the neutron absorption of the Xenon-135, the control rods were pulled further out of the reactor than was allowed under normal safety regulations.
The water pumps which were to be driven by the reactor's turbines in the upcoming experiment were turned on slightly after 1am on April 26th. This resulted in an increased water flow to the reactor. Because water absorbs neutrons and slows the nuclear reaction, plant operators decided to completely remove the manual control rods, which created a dangerously unstable situation inside the reactor.
According to the most commonly agreed-upon sequence of events on April 26, the turbine test began at 1:23:04am. Electricity to the water pumps was shut off, and as the turbines were disconnected from the reactor and began to spin under their own inertia, the rater of water flow decreased. As the rate of water flow decreased, the temperature of the coolant increased, and pockets of steam began to form in the coolant. The increase in steam and correspondent decrease in water caused instability in the reaction. This situation continued, making the reactor increasingly more unstable and more dangerous. In the reactor, the power output increased dramatically and unexpectedly.
At 1:23:40am, the reactor operators pressed the SCRAM button, to manually insert all the control rods and shut down the reactor. The chief engineer on Chernobyl power station at the time of the accident says that control systems showed absolutely no reason the SCRAM was ordered, and that "the reactor was simply being shut down upon completion of the experiment". Others have speculated that the SCRAM button was pressed due to the dramatic, unexpected jump in power output. Because the person who actually pressed the button died 2 weeks later from radiation poisoning, it is unlikely we will ever know the reason for the SCRAM.
Due to factors explained below (in the "Cause" section), pushing the SCRAM button actually caused the reaction rate to increase for a few crucial seconds. The rods became stuck after being inserted only 1/3 of the way, and were useless to stop the reaction.
At 1:23:47am, the power output of the reactor was 30GW, ten times its normal outpout. The control rods began to melt. Steam pressure rapidly increased and caused a large steam explosion that destroyed the reactor lid and knocked it out of place, ruptured coolant tubes, and blew a hole in the roof of the plant.
cross-section diagram taken from the thesis of R. Alexander Sich at MIT. used for educational purposes only.
This is a picture of the hole in the reactor building, after the fire was put out. Copyright status unknown, used for educational purposes only
Chernobyl: What Was Released?
The intial steam explosion, my friends, immediately released all the noble gases contained within the reactor into the atmosphere, gases like Krypton and Xenon. It released slightly more than half of the radioactive iodine in the reactor, in vapor, solid particles, and organic iodine compounds. Cesium and tellurium were also released, in aerosol form. Particles of zirconium, niobium, lanthanum, and cerium were released (these are nonvolatile radioisotopes), as well as neptunium, plutonium, and minor actinides, which "will be responsible for the bulk of radiotoxicity of used nuclear fuel".
Estimates vary on how much fuel was released from the core. Original estimates suggested that something in the area of 5-30% of the core material had escaped.
This is a graph of contributions made by the different isotopes to the dose (in air) around Chernobyl shortly after the accident. It's taken from the wikipedia article (graph copyrighted and rights irrevocably released).
Chernobyl: How Did They Try to Fix It?
The first thing you need to know about the Chernobyl explosion is that even the people on the scene didn't understand the scope of the disaster. They had the wrong equipment and made bad assumptions, which they acted on. This only exacerbated the problems in the days immediately following the explosion.
First, the equipment. All but two dosimeters (radiation-measuring equipment) in the #4 reactor had limits of 1 milliroentgen/second. Of those two remaining dosimeters, which had limits of 1000 roentgen/second, access to one was blocked by the explosion. The second broke when turned on. The reactor operators could only ascertain that levels in much of the reactor building were above 4 roentgen/hour.
What's a roentgen? Damned if I know. But a lethal dose of radiation is 400 roentgen over 5 hours. The true radiation levels at Chernobyl #4 were estimated at up to 20,000 roentgen/hour, or (if I'm doing the math right) 100,000 roentgen/5 hours.
Because the crew couldn't get accurate dosimeter readings, they assumed the reactor was intact. Another dosimeter was brought into the reactor building at 4:30am, but its readings were ignored. Apparently, they were so high that the crew assumed the dosimeter was defective. Though particles of graphite and reactor fuel were lying around the reactor building, no one seemed to believe that the reactor had been breached. The crew stayed inside the building to pump water into the reactor in an attempt to cool it down. Firefighters were also brought in to put out the fire. This was accomplished by 5am.
The pumping of water into the reactor building had created a major potential radioactive hazard. All this water had flowed beneath the reactor floor. The burning core material was starting to burn its way through the reactor floor. If the core material had come into contact with the water, it would have created a thermal explosion, ejecting even more radioactive material, including core material, into the air. The Soviet government sent liquidators in wetsuits to open the sluice gates in the bottom of the reactor, to vent the radioactive water.
By the evening of Saturday April 27, the government committee tasked to investigate the situation at Chernobyl had undebiable evidence of extremely high radiation levels. By that evening two people had died of radiation poisoning and 52 were hospitalized. It was only at this point, more than 24 hours after the explosion, that the decision to evacuate the surrounding residents was made.
In the days, weeks, and months following the explosion, an estimated 300,000 - 600,000 "liquidators" were sent by the Soviet government to assist in collecting radioactive debris and help contain the damage. They dropped thousands of tons of sand, lead, and boric acid from helicopters to cover the reactor. Liquidators were also used in building a concrete sarcophagus to seal off the reactor and the radioactive material it still held.
This is Chernobyl #4 after the sarcophagus was finished. Image is copyright Sergei Supinski/AFP; used for educational purposes only.
Chernobyl: What Caused It?
The first problem with the design at Chernobyl #4 is the reactor type itself. Chernobyl #4, and in fact all 4 Chernobyl reactors, were RMBK reactors, which use light water for cooling, and graphite for moderation (control) of the reaction. Using water for coolant and graphite for moderation allows the plant to use low-enriched uranium for fuel. This makes the process cheaper and easier.
RBMK reactors are "positive void coefficient" reactors. Chernobyl #4 had a dangerously high positive void coefficient. "Voids" are steam bubbles that form in reactor coolant. In a "positive void coefficient" reactor, the more steam that forms in the coolant, the greater the thermal power output. As the thermal power output increases, more steam will form in the coolant. If the void coefficient is large enough and nothing is done to control this heat-makes-steam-makes-heat-makes-steam reaction, all of the coolant will boil out of the reactor. This is what happened at Chernobyl. Because of the possibility of this sort of out-of-control reaction, called a "positive feedback loop," the RBMK positive-void coefficient reactor type is considered unstable. They are especially unstable at low power.
The design of the reactor was a problem in another way. The Chernobyl #4 reactor had a very large void coefficient. The larger the void coefficient, the faster reactor power will increase in the absence of neutron-absorbing coolant or control rods. Because of the large void coefficient at Chernobyl #4, which makes a runaway reaction a distinct possibility, a fast-acting control system, and one designed to properly control was necessary. The control rods specified in the RBMK reactor design were too slow-acting to properly control the reaction--they took 18-20 seconds to complete insertion. Their design was also insufficient to properly control the reaction.
A second, and equally disastrous, design flaw in the Chernobyl reactor was that it was designed with only partial containment. This was done both to cut costs and because the reactor was so big, and in the end only magnified the scope of the disaster.
The basic idea is this: control rods are inserted into a nuclear reactor to slow down the reaction. Thus, they need to be made of neutron-absorbing material, not neutron-moderating material.
The control rods of RBMK-type reactors were made with three sections: first, graphite tips; next, 1m-long extenders that were hollow and water-filled; the balance of the control rod was made of boron carbide. The control rods are inserted into the reactor in order to control, or slow down, the reaction. Thus, the RBMK-style control rods would enter the coolant like this: graphite, then hollow, then boron carbide.
Let's break for a little science. Neutron absorbers, well, absorb neutrons, which is how the reaction is slowed down. Neutron moderators enables the nuclear reaction (the reverse of slowing it down). Coolant water is a neutron absorber. Graphite is a neutron moderator. Boron carbide is a neutron absorber.
What this all means, in terms of Chernobyl, is that when the control rods are inserted into the reactor to slow the reaction, the coolant water (which slows the reaction) is displaced by the graphite rod tips (which accelerate the reaction). This means that the first few seconds of controlling, or slowing down, the reaction actually accelerate it instead.
In addition, as you might have noticed if you've read this far, most of the above reactions (putting in control rods first increases the reaction rather than decreases it, reactor is most dangerous at low power, etc.) is rather counterintuitive. It's clear from a review of what was done during the test and the safety measures that were ignored in the test preparation that these were either the worst-trained nuclear operators in the world, or the most incompetent. Reactor crew error was a definite contributing cause to the Chernobyl explosion, and might have been the but-for cause.
In sum, Chernobyl was a nuclear power plant designed to fail. The way it was operated only ensured that failure.
Update [2006-4-25 2:46:1 by celticshel]: Part II: The Effects of Chernobyl, and What Has Changed in the Nuclear Power Industry.
(Disclaimer: I'm not a scientist, and I hate math. And broccoli.)
Link to Chernobyl Revisited, part 2