The fourth layer, where most particles end their lives, is made of caesium iodide crystals.

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They are also concerned this water may bring more dissolved caesium into rice fields in the future.

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And for those particles that make it past the caesium iodide, a huge electromagnet tracks their passage out of the detector.

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"Caesium stays mobile for longer in organic soils, hence why England and Wales still have some post-Chernobyl restrictions in upland areas, " he told BBC News.

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What is more, caesium particles tend to lurk in the grass, which means radiation is more of a risk at toddler height than for adults.

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Besides pressure- hosing urban areas, this would involve removing about 5cm of topsoil from local farms as well as all the dead leaves in caesium-laden forests.

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Radioecologist Nick Beresford from Centre of Ecology and Hydrology in Lancaster explained that once in soil, caesium will become bound to mineral components, which limits its uptake into plants.

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While the caesium is hard for plants and animals to take up when it is tightly bound to the soil, once in solution (dissolved in water) it can be more readily absorbed.

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The caesium finds its way into tiny pores on the surface, and the only way to remove it effectively is to grind off the surface, a difficult procedure which can create contaminated dust.

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But the Korea Institute of Nuclear Safety said that although small levels of radioactive iodine and caesium particles had been found in rain falling in part of the country, they were not enough to cause concern over public health.

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