LONDON, UK, June 7, 2000 (ENS) -- Effects of the 1986 Chernobyl nuclear accident on the United Kingdom will last 100 times longer than originally estimated, according to new research.

However, scientists have discovered that contaminated sheep can rid themselves of radiocaesium when moved to unpolluted pastures.

Scientists from the UK and the Netherlands have revised estimates on how long it will take to eradicate radioactive contamination from the hills and mountains of Wales, Scotland, and the north of England.

As a result, more than 230,000 sheep on nearly 400 sheep farms are subject to restrictions for a further 10 to 15 years. This means they must undergo continued testing for radiation before slaughter.

Damaged reactor soon after the explosion and fire at the Chernobyl Charity Online)

Radiocaesium, referred to scientifically as Cesium 137, was part of a radioactive plume blown over Europe and deposited by rain after the accident in 1986. It was originally thought the natural absorption of radiocaesium into clay soil would render it harmless. Scientists believed radioactive atoms would lie dormant in the lattice of molecules that make up clay, only surfacing again after about 1,000 years.

New research indicates this process happens much more quickly, particularly once the concentration of radiocaesium in the soil becomes higher than the amount around it. Radiocaesium is thought to resurface in equalibrium, forming what Dr. Jim Smith of the Centre for Ecology and Hydrology in Dorchester, England, and his colleagues call a "reversible steady state."

"The Radiocaesium can diffuse out when the concentration outside the [clay] lattice is lowered," said Dr. Smith. It is also removed by weathering.

Over a longer time, because radiocaesium can diffuse out quickly, it enters a state of equilibrium. This means that the radioactive caesium is constantly being released into the soil and water, entering the food chain and polluting plants and animals.

No increase in cancer associated with the radiocaesium has been detected in the inhabitants of the UK, but heavy restrictions on agriculture in contaminated areas remain.

The UK limit on radiocaesium in meat is 1,000 Becquerels per kilogram. The rate of radioactive decay is measured in Becquerels -- one unit per second.

Some sheep in affected areas have levels of radiocaesium well above that considered safe for human consumption, yet the animals' health is unaffected. Contaminated sheep can be moved to unpolluted pastures where they rid themselves of radiocaesium by excretion.

"Radiocaesium has about a 30-day half life in sheep," said Dr Smith. Like uranium and plutonium, caesium emits radiation when its molecules break down. The term half life refers to the time it takes for half the element to decompose. Biological half life means the amount of time it takes for radiocaesium to leave the sheep.

According to Dr. Nick Beresford, also of the Centre for Ecology and Hydrology, at Merlewood in Cumbria, the biological half life of radiocaesium in sheep ranges between 10 and 30 days. The smaller the animal, the shorter the half life. The half life for lambs removed from contaminated upland fells, is about 10 days.

Sheep in northern England

Once relocated to lowland pastures, a lamb with 1,000 Bq/kg in its tissues halves its radiocaesium level every 10 days -- 500 Bq/kg after 10 days, 250 Bq/kg after 20 days.

The situation is far worse nearer to Chernobyl. Around 52,000 square kilometres (20,080 square miles) of agricultural land in Belarus, Ukraine and Russia are contaminated, and agricultural restrictions are expected to continue there for another 50 years.

Radioactive fumes and debris covered the region for 10 days after two explosions ripped through one of Chernobyl's four reactors during the night of April 26, 1986. Workers at the power plant, 120 kilometres (75 miles) north of Kiev, Ukraine, had been carrying out an experiment with its safety system when the accident blew the roof off the reactor building.

Radioactivity from the Chernobyl explosions was first detected outside the former Soviet Union at a nuclear power station in Sweden. Monitoring stations around the world began intensive sampling programs.

Chernobyl affected all living creatures. People who remained in the area were exposed to contaminated food, water and air. It became dangerous to keep cattle and grow fruit and vegetables. (Photo courtesy Chernobyl Charity Online)

The radioactive plume was initially blown northwest by the wind, with rainfall depositing contamination in Scandinavia, the Netherlands, Belgium and the UK. The wind changed direction, shifting the plume over South and Central Europe.

In the years since the disaster, more than 4,000 people in Ukraine have died as a result of acute radiation syndrome and thyroid cancer, and another 15,000 are unable to work. Scientists have discovered genetic mutations in children whose parents were exposed to the radioactivity, so the effects are being passed from one generation to the next.

Earlier this week Ukraine's President Leonid Kuchma announced that the units of the Chernobyl reactor that are still in operation will be closed permanently December 15, 2000.