new steel columns but experts working on the project accept that the structure is barely adequate and needs replacing as quickly as possible.

The barrier is sitting over the top of what remains of a deeply radioactive reactor core, but also covers the vast bulk of the fuel from the plant which has seeped into the concrete floor.

A new permanent shield would prevent rain from getting into the old plant but also allow more work to be carried out on dismantling the remains of the equipment and the fuel without creating more escapes of hazardous dust and particles.

Fallout over UK

Nuclear workers in Sweden were the first to detect the radioactive material that was thrown into the atmosphere from Chernobyl. The fallout from the accident in the early hours of26 April 1986 crossed over Europe, and deposited the radioactive isotope caesium-137 in mainly upland areas of Wales, Scotland and England.

The disaster, which released at least 100 times more radiation than the atom bombs dropped on Nagasaki and Hiroshima, forced the government to put 9,700 farms and 4.2 million sheep under restriction across the UK.

Restrictions were lifted in Northern Ireland in 2000. The Department for Environment, Food and Rural Affairs confirmed yesterday that the last time the government updated the public on the restrictions was in 2009 when the then health minister Dawn Primarolo revealed 369 farms and 190,000 sheep were still affected.

Of those farms, 355 are in north Wales, with nine in Cumbria and seven in Scotland. Farmers in these areas must have their livestock scanned before