Chernobyl reactor 4 control room. Inside the control room of reactor number 4 at the Chernobyl nuclear power station, Ukraine. On 26th April 1986, reactor number 4 went out of control, leading to meltdown and an explosion. Around 1000 square kilometres of land were directly contaminated by radioactive fall-out. A concrete sarcophagus was built to contain the radioactive materials, but within the restricted area the radiation is still lethal to humans. The material inside the sarcophagus will remain dangerous for thousands of years.