Emergency planning teams today gathered in Ireland's nuclear disaster nerve centre for an exercise in how to deal with a catastrophic radioactive accident in Britain.
Government officials joined emergency and security services at the Radiological Protection Institute of Ireland for a simulated incident based on a top-level emergency at the Wylfa nuclear power plant in Wales.
Within six hours of the shout going up this morning, the Emergency Response Co-ordination Committee estimated that the nuclear plume - supposedly released after an earthquake - would have reached the shores of Ireland.
The dry run comes at a time of heightened fears that terrorists could attack Sellafield, or radioactive cargo that will be transported through the Irish Sea when the planned mixed oxide fuel plant opens.
Minister with responsibility for nuclear safety, Joe Jacob, was at the Dublin nerve centre for the tests, which began with the simulated disaster at 5.15 a.m. this morning.
"The reason for this exercise is to make sure that the country is in a state of total readiness for any emergency that may occur," he said.
"Our most serious concern about nuclear installations in the UK is the possibility, however remote, of a serious incident or accident."
He said the simulation was the biggest of its kind ever carried out in Ireland.
"There are arrangements in place internationally whereby we get a warning when such an event like this occurs. That comes into the Garda network and to the round-the-clock team at the RPII.
"All the relevant bodies then assemble together to kick off the emergency plan. The basic information from our experts here would be to stay indoors. It might sound simple but that is the most important instruction," he said.
In the exercise they also planned how the message would be sent out to the public, which areas could be affected and how people should protect themselves, Mr Jacob said.
But he admitted that there was no nuclear bunker for the emergency planners to retreat to in the event of nuclear fallout in Ireland.
Fears have escalated after reports both by the New Scientist magazine and a body advising the European Commission, claimed the effects of a major incident at Sellafield could be many times more catastrophic than the Chernobyl accident.
A recent attempt by Mr Jacob to reassure the public ended in uproar when he was unable to answer basic questions in an interview about nuclear emergency responses.
The results of today will be made public in the coming days. They will be used in finalising a national plan of action which will be made public within weeks.
Sean O'Riordáin of the Environmental Management Group will report back to the Government and this will help in the drawing up of a national emergency strategy.
He was "quite happy" with the speed of the reaction by the emergency services to the simulated incident this morning.
"This incident could have happened anywhere at any one of the nuclear facilities along the west coast of Britain," he said.
PA