The accident in one of its laboratories at noon yesterday brought immediate renewed calls from environment groups for the plant to close.
Fears about the presence of highly toxic plutonium at Sellafield and concern about the effects of waste on the environment have a led to a long-running campaign for its closure.
One worker was contaminated by the radioactive material and 72 evacuated from the laboratory which produces fuel using plutonium and uranium.
BNFL, which operates the plant in Cumbria, said the leak was contained within the building but confirmed that it was continuing to monitor the grass in the surrounding area for any signs of further contamination.
Mirella Von Lindenfels, press officer for Greenpeace, said: "This is proof, if proof were needed, that Sellafield should close.
"It is in line with the decision two weeks ago at the Ospar (Oslo Paris Commission) meeting in Lisbon that all member-states, including Britain, should have zero emissions of waste into the ocean.
"This means reprocessing cannot continue. BNFL have been putting a brave face on it saying they can manage, but leaks are very serious and very worrying and this simply confirms that Sellafield should shut down."
Cumbrians Opposed to a Radioactive Environment, a local pressure group, said this was the third incident involving an evacuation at the Sellafield site this year.
Martin Forwood, spokesman for Core, said: "This is another in a long line of incidents. It happened in the plutonium fuel plant - plutonium is an unnecessary by-product of what Sellafield does which nobody wants. It is highly dangerous and highly toxic.
"Here we have a community subjected to the risk of leaks or, even worse, an accident on the Chernobyl scale. It could happen.
"We shall continue to pressure for a halt to reprocessing at Sellafield but much of the damage is already done."
A BNFL spokesman said staff were allowed back into the laboratory later yesterday evening, eight hours after the accident.
The company issued a statement saying: "One person was found to have been contaminated and was sent to the site surgery. Initial testing of that individual has not shown any internal contamination and all external contamination has been removed.
"Other personnel involved were monitored prior to being allowed to go home and everyone was found to be clear."
A spokesman later said: "When this happened today, the plant was operational, producing fuel. The laboratory is off the main production process, but it is linked.
"But the Mox facility is a very small part of the Sellafield complex. It only produces eight tonnes of fuel a year, as much as anything to prove that we can produce the material."
Mr Forwood said: "BNFL have not been able to show that there is any demand for this fuel and yet were have to suffer the consequences."Reuse content