The worst nuclear power reactor disaster that the world has known began in the late evening of April 25, as poorly trained workers at the Chernobyl nuclear power plant in northern Ukraine began an unauthorized test while they were doing a scheduled shutdown of unit 4. They wanted to see how long the slowing turbine could provide power after the reactor was shut down, and they shut off the emer­gency core-cooling system since it would draw power. This was the first of many major safety violations. The next major safety violation they made was to with­draw most of the control rods to increase power as it fell to dangerously low levels. The reactor was supposed to be running at 700-1,000 MW thermal (MWt) for the test, but it actually dropped to 30 MWt for unknown reasons. At 1:03 a. m. on April 26, they activated the cooling water pumps, but this, combined with the low power, required manual adjustments, so they turned off the emergency shutdown signals. At 1:22 a. m. they shut off the trip signal just as it was about to shut down the reactor. At 1:23 a. m. the test began, but the reactor was in a very unstable state, so that any increase in power could cause a rapid surge in power due to a design characteristic of the reactor called the positive void coefficient. As the power rose, water turned to steam, reducing the absorption of neutrons and causing a rapid increase in power.

The operators tried to insert control rods, but that actually increased the reac­tivity because the rods had graphite at their ends, which acted as a moderator to slow down neutrons and increased the rate of fission. Power surged to 100 times the operating capacity of the reactor, the uranium fuel disintegrated and caused a huge steam explosion that blew the 1,000-ton lid of the reactor aside. A second explosion a few seconds later, probably from hydrogen gas released by the Zircaloy cladding of the fuel rods, blew through the reactor walls and threw burning blocks of graphite and fuel into the compound. A plume of radioactive debris rose 10 kilometers into the atmosphere, and the reactor spewed radiation over the next 10 days as fires continued burning (12, 13). It is often stated that the pure graphite core itself burned, but that is somewhat controversial since “tests and calcula­tions show that it is virtually impossible to burn high-purity nuclear-grade graph­ites” (13). However, in 1957 the Windscale graphite-moderated nuclear reactor in Sellafield, England, caught fire and released more radiation than any other accident before Chernobyl (1). The United States had one graphite-moderated, helium-cooled reactor in Fort St. Vrain, Colorado, but that was closed down in 1989 and converted to a natural gas plant (1), so no US reactors could have the kind of fire that happened at Chernobyl.

While the operators violated a number of safety procedures, the design of the reactor was also at fault. This Soviet-made reactor was of a type called RBMK, which was unique in the world. It was designed both to produce power and to produce plutonium for making bombs. These reactors have a graphite core that serves as the moderator to slow down neutrons with channels for water to cool the core and to produce steam. The reactor was of a general type called a boiling water reactor (BWR), as contrasted to the PWR used at Three Mile Island. There is only one cooling loop, so the water that goes through the reactor core also goes through the turbines to turn the generator. This combination of a graphite mod­erator and water cooling is dangerous. Water actually absorbs some of the neu­trons and slows down the fission reaction, but if the water turns to steam, it cannot absorb as many neutrons so the fission reaction proceeds more rapidly. This is called the “positive void coefficient” and was instrumental in causing the accident. When the reactor started to get out of control, it turned the water into steam, which increased the reactivity, which turned more water to steam, and so on, in a positive feedback loop. The graphite on the ends of the control rods also made things worse since graphite is a moderator (it slows down neutrons rather than absorbing them). The initial effect of inserting the control rods was to actually increase reactivity before the control rods begin to absorb the neutrons and shut down the reaction. As a result of these factors, the reactor quickly became uncontrollable. The RBMK reactors were the only ones in the world designed like this (14).

Another major fault with the reactor design was that, in contrast with other BWR reactors, there was no massive concrete containment structure that could contain a core meltdown, such as at TMI. Instead, the reactor had a 1,000-ton lid that could be removed to change fuel while the reactor was actually operating. When the reactor went supercritical, the steam explosion blew off the lid and blew apart the relatively flimsy containment building. This was a steam explosion, not a nuclear explosion. It is impossible for a power reactor to explode like an atomic bomb because the concentration of 235U is not high enough for that to ever happen.

The fires continued for 10 days as the firefighters dumped sand, lead, clay, dolomite, and boron on the ruined reactor in quick bombing raids from helicop­ters and poured hundreds of tons of water per hour to quench the fires and radio­activity. People were evacuated from the nearby city of Pripyat, where 45,000 people lived, and from a 30-kilometer exclusion zone. By October, a temporary concrete sarcophagus was built to enclose the entire demolished reactor unit 4 so the other reactors (units 1-3) could continue to operate. Reactor 2 was shut down in 1991 after a fire, and reactors 1 and 3 were permanently shut down by the year 2000 on orders of Ukrainian president Leonid Kuchma. A new struc — ture—the New Safe Confinement—is now being built to cover the reactor and the temporary sarcophagus. It is scheduled for completion in 2015 (12, 13).