WARNING: This is a long answer. I find Chernobyl really fascinating.
Chernobyl was a nuclear power station in the former USSR, it’s now part of Ukraine. In 1986 they were testing the reactor and something went badly wrong. The bit of the power station with all the radioactive fuel in it exploded and caught fire. The fire spread radioactive particles a really long way. People found traces in Wales. A lot of people died at the time from the radiation, including the people in the nuclear power station and many firemen who tried to fight the fire.
Then the government had to make it safe, to avoid any more explosions and stop any more radioactive material getting out. So about 250,000 people built a massive concrete cap to seal it off. All of these people got big doses of radiation.
The concrete seal is too old now and work is underway to replace it.
Because of all the radioactive particles that were released in the accident, there is a 30km ring around the reactor that is an exclusion zone. This is because the radiation levels are so high there. Some scientists have estimated that it won’t be safe for humans for another 20,000 years!!! Even the people working there to repair it are only allowed to do short shifts and have long holidays.
Radiation has two effects. One is very quick that happens when you get a really big dose really quickly and is known as radiation sickness. It can cause burns to your skin and you die pretty quickly. It’s caused by the radiation killing the cells. Many of the people that worked on the Chernobyl disaster at the time died from this. One or two people have also had burns from x-ray procedures that have gone on too long for similar reasons (this is very rare!). The other health problem is caused by the radiation damaging but not killing the cells. When the damaged cells divide they can sometimes get out of control and become a cancer. It’s impossible to guess whether a person will get cancer from radiation exposure, but we know that the more radiation the more likely it is that you will get cancer from it. It’s very unlikely from a normal x-ray. The high radiation levels around Chernobyl make it more likely, especially when you stay there for a long time.
You can visit Chernobyl now, but you have to wear a radiation monitor and you can only stay for a short time. I have a friend who went, it sounded really fascinating, but really haunting as well because so many people died.