A senior Ukrainian Government scientist, Dr Georgiy Lisichenco, says some of the effects of the Chernobyl nuclear disaster will not peak until the second half of the next century.

Alex Kirby: The greatest problems are likely to be in southern Ukraine

Dr Lisichenco, of the State Centre for Environmental Radiochemistry in Kiev, says a study he has made of radioactivity in Ukraine's water courses shows that it will be at its highest in the river Dnieper in 60 to 90 years from now.

The Dnieper, Ukraine's principal river, supplies Kiev and much of the country with drinking water.

Dr Lisichenco said, in an interview for the BBC's Costing the Earth programme, that the contamination could be a particular problem for areas in the south of the country which rely on river water for irrigating crops.

Reactor number four at Chernobyl exploded early on 26 April 1986, killing 31 people in the immediate aftermath. The subsequent number of deaths the released radioactivity has caused are unknown.

Missed deadline

The level of thyroid cancers among children living nearby has risen substantially - a 20-fold increase in parts of Belarus, and smaller increases in Ukraine itself and in parts of south-western Russia.

Unit 3 is the one reactor still operating

Ukraine has agreed to close the one remaining working reactor at Chernobyl, unit 3, by the end of this year. But the head of the state-owned nuclear energy company Energoatom, Dr Mykola Dudchenko, told the programme he is certain the plant will not close by the deadline.

Nor does he think the West will honour its pledge to fund the completion of two new reactors at Khmelnitsky and Rovno in western Ukraine, to compensate for the closure of Chernobyl.

Dr Dudchenko says Ukraine will itself pay for the Soviet-designed reactors, which are 85% complete, to be finished.

Energy needs

The only way of bringing them near to Western safety standards would be a hugely expensive retro-fit, which is far beyond Ukraine's ability to fund.

The dark turbine hall for the now-closed units 1 & 2

An independent study for the European Bank for Reconstruction and Development, which will decide in July whether to fund their completion, concluded that the reactors were not the least-cost option for meeting Ukraine's energy needs.

The study group chairman, Professor John Surrey of the University of Sussex, says it came under high-level political pressure to think again.