SDLP leader Mr Mark Durkan has raised the issue of Sellafield with UK prime minister, Mr Tony Blair, in his first official meeting as party leader.
Mr Durkan said he had no doubt that Mr Blair knew the SDLP had always opposed Sellafield and the new MOX plant.
Meanwhile, Mr Durkan spoke tonight of the need for a change in approach by Northern Ireland politicians in talks with Prime Minister Tony Blair.
"I think we have to stop going in and out of meetings waving lists going in, and waving lists coming out. That is not how government works," he declared after his first Downing Street meeting since taking over from John Hume.
"It was a conversation between two people who are in government in different ways, two people in positions of political leadership, and it was not a lobbying exercise."
Mr Durkan has replaced his party colleague Mr Seamus Mallon as deputy First Minister at the Northern Ireland Assembly and will be with the First Minister Mr Trimble in Dublin on Friday when ministers from Belfast, the Republic and London meet up.
It will be the north-south ministerial council and British-Irish council meetings since the IRA began their process of disarmament last month.
With Mr Trimble due to resist demands to set a new decommissioning deadline when he faces his party's council at a special meeting in Belfast on Saturday, weapons was one of a number of issues raised at today's Downing Street talks.
But Mr Durkan refused to go into details.
He said: "We discussed all the issues within a framework approach. It is about consolidating the progress that has been made and concentrating on all the solutions which can be developed, rather than concentrating on the problems we've known."
PA