This week, Ukrainian authorities restarted the last working reactor at the Chernobyl power plant, site of the world’s worst nuclear disaster 13 years ago. Officials say the reactor is completely safe and free of any potential Y2K bugs.

Considering that everybody living in the northern hemisphere 13 years ago was the unwilling recipient of at least a few radioactive particles from Chernobyl, we all have good reason to hope they’re correct.

Nuclear reactors split uranium atoms by bombarding them with neutrons. A small portion of the atoms’ mass becomes energy, and they release more neutrons, which in turn bombard other nearby atoms, splitting them and beginning a chain reaction. If the material is packed together tightly enough, this chain reaction is uncontrolled, and you get an atomic explosion. In nuclear reactors, a material called the moderator controls the chain reaction by slowing the nuetrons.

In Chernobyl-style reactors, the reaction is moderated by graphite. Ordinary water is piped through the core of the reactor and heated into steam. This both cools the reactor core and drives the power turbines. Chernobyl-style reactors don’t have a containment vessel–the steel-and-concrete tower familiar from North American reactors–and become unstable at low power, liable to sudden surges of power.

On April 25, 1986, the Number 4 reactor at Chernobyl was to be shut down for routine maintenance, and it was decided to take advantage of that to run a test of emergency power systems. Due to poor communication, a series of actions was taken that led to a dangerous situation: the reactor’s power output fell to the point where it became unstable, certain safety systems were disabled, and most of the control rods, used to damp neutron output and thus shut down the reactor in a hurry, had been withdrawn.

At 1:23 a.m. on Saturday, April 26, 1986, the unstable reactor suffered a power surge estimated to be 100 times greater than normal. The fuel ruptured. Hot fuel particles hit the water system, causing a steam explosion that destroyed the reactor core. A second explosion ripped the roof off the reactor building, exposing the reactor core and sending a shower of hot, highly radioactive debris into the air.

The building caught fire, giving rise to more clouds of radioactive steam and dust. More than 100 fire-fighters fought the blaze, many of them suffering fatal radiation doses; the building fires were extinguished within a few hours, but by then the reactor’s graphite had caught fire. It burned for 10 days, hurling a constant stream of radioactive material high into the atmosphere. Radioactive emissions continued in total for 20 days.

After the accident, the reactor was encased in a steel-and-concrete sarcophagus that is currently being re-fortified. Two of the other four reactors were permanently shut down. Ukraine was supposed to close all of them by 2000, but because the government can’t afford to build the two new reactors it needs to replace the power it draws from Chernobyl, reactor Number 3 is now up and running again.

Immediately after the accident, 134 people showed signs of acute radiation sickness, of whom 28 died. 135,000 people were evacuated from the area; most received significant doses of radiation. Approximately 800,000 workers were brought in to try to decontaminate the area, and received varying doses of radiation. Around 270,000 people continue to live in contaminated areas.

Large portions of agricultural land were contaminated, including almost a quarter of the agricultural land in Belarus. A swath of forest near the site received so much radiation the trees died and had to be destroyed as radioactive waste. Interestingly, wildlife now abounds in the area, and although rodents are so contaminated you wouldn’t want to handle them, researchers have yet to find any malformed individuals.

There has been a substantial increase in reported cases of thyroid cancer in Belarus, Ukraine and some parts of Russia. An increase in leukemia was expected, but hasn’t shown up yet. Nor have there been perceived increases in other cancers–but that could be simply because enough time has not yet elapsed. Nevertheless, it’s estimated that, when all the health consequences are taken into account, 3,756 people have now died from the accident.

Harder to measure have been the psychosocial effects, caused by the fear of disease, the stress of being exiled from their homes, a distrust of authorities, and economic and social hardship.

Not to mention insomnia. With a nuclear reactor once again operating at Chernobyl, it’s a safe bet a lot of people aren’t sleeping well these days.

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