A cross-party delegation of councillors was being given an extensive tour of the Cumbrian plant by British Nuclear Fuels.

The government of the Irish Republic and politicians in Northern Ireland have been pressing for Sellafield to be closed on environmental and health grounds.

Politicians from the Northern Ireland Local Government Association (NILGA) are visiting the Mox site to "ascertain facts" before deciding on a future course of action.

There has to be an end to the discharge of radioactive and toxic waste into the Irish Sea Margaret Ritchie, SDLP

Heather Moorhead, chief executive of NILGA said: "This is primarily a fact-finding mission.

"We appreciate there is much unrest concerning the plant, our members and a staff officer are visiting the site with a view to ascertaining the facts.

"When we consider these it will inform any further action we may take."

Terrorist action

Margaret Ritchie, of the SDLP, said: "For years now the people of Ireland have been concerned about the dangers to the environment and their health that Sellafield is causing."

"There has to be an end to the discharge of radioactive and toxic waste into the Irish Sea.

"But principally, it is essential that Sellafield is safely decommissioned to allow for its eventual closure."

A spokesman for BNFL said it was pleased to welcome members of the Northern Ireland Local Government Association on a fact-finding visit to the site.

The Mox plant turns nuclear fuel already "burnt" in a reactor into a new fuel source, but creates new waste.

There are also fears that the Mox plant could be the target of a terrorist action.