Thirty three years ago there was an explosion at the Chernobyl nuclear power plant in Ukraine, then part of the Soviet Union. We knew hardly anything about it at the time – only that radiation levels were rising in Western Europe. Of the emergency workers sent to tackle the blast, 28 died within months 19 have died since - 134 got acute radiation sickness. But now tourist groups visit the exclusion zone all the time - and scientists are studying there because the whole place has become a massive laboratory what happens in the aftermath of a nuclear disaster. BBC science correspondent Victoria Gill went there earlier this year and tells us how to assess the risks of radiation.