Chernobyl is a long way from the UK, but some of the radioactive pollution was carried here by the wind.
That means that even 20 years later, sheep on hundreds of farms in hilly areas have to be tested for radiation before their meat can be eaten.
After the explosions at the power plant, heat and smoke from the fires carried radioactive material 1km up into the sky. A north-westerly wind blew the pollution across Europe.
In Sweden, 1,000 miles from Chernobyl, some scientists noticed that radiation levels had doubled but they didn't know why this was happening.
That was because the Soviet Union, which controlled Ukraine at the time, was very secretive and had not yet admitted to the world that there had been an accident.
The wind carried the radioactive cloud past Sweden to Holland, Belgium and the UK.
By now the Soviets had been pushed into telling the world what had happened at Chernobyl.
The radiation that escaped from the reactor's core was eventually detected in countries all over the northern hemisphere.