In the early morning hours of April 26, 1986, the Chernobyl nuclear power plant in Ukraine (formerly part of the Soviet Union) exploded, creating what has been described as the worst nuclear disaster the world has ever seen.

Even after many years of scientific research and government investigation, there are still many unanswered questions about the Chernobyl accident — especially regarding the long-term health impacts that the massive radiation leak will have on those who were exposed. The nearest town to the power plant was the newly built city of Pripyat, which housed almost 50,000 people in 1986, according to the World Nuclear Association. A smaller town, Chernobyl, was home to about 12,000 residents. The remainder of the region was primarily farms and woodland. The Chernobyl plant used four Soviet-designed RBMK-1000 nuclear reactors — a design that's now universally recognized as inherently flawed. RBMK reactors use enriched U-235 uranium fuel to heat water, creating steam that drives the reactors' turbines and generates electricity. In most nuclear reactors, where water is used as a coolant and to moderate the reactivity of the nuclear core, as the core heats up and produces more steam, the increase in steam bubbles or "voids" in the water reduces the reactivity in the nuclear core. This is an important safety feature found in most reactors built in the United States and other Western nations. A few seconds later, a second explosion of even greater power than the first blew the reactor building apart and spewed burning graphite and other parts of the reactor core around the plant, starting a number of intense fires around the damaged reactor and reactor number 3, which was still operating at the time of the explosions. The explosions killed two plant workers, who were the first of several workers to die within hours of the accident. For the next several days, as emergency crews tried desperately to contain the fires and radiation leaks, the death toll climbed as plant workers succumbed to acute radiation sickness.

Most of the radiation released from the failed nuclear reactor was from iodine-131, cesium-134 and cesium-137. Iodine-131 has a relatively short half-life of eight days, according to UNSCEAR, but is rapidly ingested through the air and tends to localize in the thyroid gland. Cesium isotopes have longer half-lives (cesium-137 has a half-life of 30 years) and are a concern for years after their release into the environment.

On April 27, the residents of Pripyat were evacuated — about 36 hours after the accident had occurred. By that time, many were already complaining about vomiting, headaches and other signs of radiation sickness. Officials eventually closed off an 18-mile (30 km) area around the plant; residents were told they would be able to return after a few days, so many left their personal belongings and valuables behind.

Till this day the city remains quiet. Empty. But not forgotten.
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