Twelve years on and the Chernobyl contamination, which rained down in particular on Ukraine and Belarus, is still there and still a grave danger to those living nearby.
People close to the power plant and its wrecked Reactor No 4 are beginning to lose their fear of what the 30 km Exclusion Zone holds, however, according to Dr Barbara Rafferty, head of the radioecology research unit in the Radiological Protection Institute of Ireland.
No one is living there officially, but unofficially fences can always be climbed. "The attitude has become much more relaxed," she said, but this could be bad for locals who relish the wild mushrooms that grow plentifully in the forests around Chernobyl. The mushrooms, Dr Rafferty said, were "like sponges" for the radioactive contamination.
Dr Rafferty has been studying Chernobyl contamination since 1989. Her work is part of a much wider EU undertaking that seeks to learn what the radioactive materials are doing. "It is trying to understand how the environment is dealing with the contamination," she explained. "It is all leading towards a decontamination management strategy."
The RPII is the main Irish participant in this activity, but UCD has also been involved in some aspects. Dr Rafferty was most recently there last June and will travel to Ukraine again next month. She will stay near the edge of the zone and travel into it to carry out experiments with research colleagues.
She has studied transport and recycling of the main radionuclides, strontium 90 and caesium 134 and 137. Most of the caesium (Cs) is 137 because it decays and disappears much more slowly than Cs 134.
The work has examined various soil types, including bog and farm lands and movement of radionuclides in forests. Of particular interest at this stage is the fact that dispersal, and hence the radioactive exposure to people living in the area, has levelled off. It fell rapidly for the first three or four years then fell more slowly and stabilised, with no significant decline for some years.
The question, Dr Rafferty said, is why this has happened. Theories proposed immediately after the accident predicted that eventually the radioactivity would just go away, diluting into the environment, but this has not happened.
Each soil type under study handles the radionuclides in a different way, Dr Rafferty explained. The upland bogs have mainly organic soils and these don't hold Cs very well. For this reason the Cs penetrated fairly rapidly down into these soils and has already reached a depth of up to 1.2 metres in some areas.
The agricultural soils are much different because they tend to have a much higher clay mineral content. Clay minerals include materials such as mica, vermiculite and bentonite. "The main soil elements that hold on to the radio-contamination are the clay minerals," she said.
"Clay minerals have a lattice structure. The caesium ions go in between the layers and the layers trap them inside." For this reason the Cs has only penetrated to about 10cm after 12 years.
The forests, mainly coniferous, are similar to the bogs in terms of soil structure and the radionuclides have reached to about 20cm. There is a significant amount of Cs recycling here however. "The organic layer represents the major food source for the vegetation. Trees and forests are good recyclers." Fungi and bacteria are also at work, she said. "They are mobilising the caesium in the soil, making it available" for uptake back into the trees.
Knowing where the radionuclides go is important as a way to help bring about reductions in people's exposure to radiation. "There are things that can be done," she said.
Knowing how trees recycle radionuclides, for example, has led to special recommendations for forestry workers to help reduce exposures by protecting against dust inhalation. Ploughing is the simple suggestion for agricultural lands. Turning the soil over helps to mix the radionuclides further down into the ground. The deeper they are, the lower the exposure levels.
Potassium reacts in soils in a similar way to Cs. Applying potassium to lands helps to block Cs uptake and this measure is also under test for controlling contamination in crops.