Hi, I'm Ben Arrona, here for About.com. I'm a historian with a Master's degree in American History, and today, we will be discussing the Chernobyl disaster.Cheap, pollution free, and virtually endless supplies of energy have long been touted as the benefits of nuclear power plants. While they have always carried with them a certain controversy, hundreds of nuclear power plants were built over the years, beginning in the late 1950's. It wasn't until the Chernobyl Nuclear Power Plant disaster on April 26, 1986, that much of the world's opinions soured on using nuclear reaction for energy. Here's a closer look.The Chernobyl power plant was comprised of four reactors. The first was built in 1977, and the remaining three came on incrementally over the next six years. The town of Pripyat, home of nearly 50,000 people, sat just three kilometers away from the plant. Chernobyl was comprised of what are called RBMK reactors. These types of nuclear reactors are not built with a concrete or steel containment structure over the reactor. As a result of this, radiation from the meltdown spread to a larger area.On April 25th technicians shut down reactor number four to do some routine maintenance, as well as to run a test on the cooling system. As part of this test, they decided to turn off the reactors' safety systems, a decision that would come back to haunt them as these systems would have prevented the disaster. Shortly after 1:00 a.m. on the 26th, reactor number four at Chernobyl experienced a sudden and dangerous drop in power. As technicians attempted to address the issue, the reactor went out of control, and at 1:23 a.m., it exploded.Lacking a containment structure, radioactive elements spilled from the destroyed reactor and into the surrounding environment. It wouldn't be until April 28th, however, that the Soviets would announce the meltdown to the rest of the world, only doing so after strong radiation readings were picked up in their surrounding countries. After the fires from the explosion were extinguished, the damaged reactor was sealed with concrete and a 30 kilometer exclusion zone around the plant was established. Ultimately, people had to be permanently relocated from around the reactor site.Thanks for watching. To learn more, visit us on the web at About.com.