The Mox fuel plant at Sellafield has provoked outrage
The protesters, who were joined by Irish MPs, were outraged by the government's recent decision to allow the start of operations at the new mixed oxide (Mox) plant at the Cumbrian site.
The leader of the Irish Green party Trevor Sargent said the protest was "designed to promote dialogue, not confrontation".
British Nuclear Fuels Limited said it had no objection to the anti-nuclear activitists marching to the main gates of its site.
A spokesperson for BNFL said the protesters' main complaint had "been dealt with in the courts".
Environmental groups have already failed in a legal challenge to the decision, which they say will increase pollution and provide a new target for terrorists.
The Irish Government has launched its own legal challenge to the mox decision, going to the UN's International Tribunal for the Law of the Sea in Hamburg for arbitration.
Campaigners have argued that sea pollution from Sellafield is the cause of above average cancer rates in some parts of the east of Ireland.
Irish Prime Minister Bertie Ahern has attacked the decision, while environmentalists have pointed out Sellafield is closer to Dublin than it is to London.
Plutonium stocks
A decision is expected from the Hamburg tribunal on 9 December. The Norwegian Government is also understood to be considering legal action over the issue.
The UK Government says the Mox plant will help recycle the growing stocks of plutonium which are a by-product of nuclear reprocessing.
The plutonium can be combined with uranium and turned into a new fuel source.
British nuclear officials argue that far from increasing the terrorist threat, the Mox plant will reduce the security dangers by reducing plutonium stockpiles.