The HSE said the contractors are unlikely to suffer long-term effects
Sellafield has been fined £75,000 after two contractors inhaled radioactive dust at the nuclear plant.
The workers were drilling an area of floor at the Cumbrian site's plutonium finishing and storage plant in July 2007 when they were contaminated.
The firm, which was also ordered to pay £26,100 legal costs at Carlisle Crown Court, admitted health and safety breaches.
The Health and Safety Executive said the level of dose received was low.
The incident happened while a patch of concrete floor, believed to be contaminated with radiation from spillage some years ago, was being drilled prior to being removed.
The two contractors were contaminated with plutonium by the dust produced from the drilling, some of which they inhaled.
Mark Bassett, the HSE's Superintending Nuclear Inspector, speaking after the hearing, said: "Although the radiation doses in this case were below the statutory dose limits, they could potentially have been higher.
"They should have been zero. The incident highlights the importance of Sellafield following its own arrangements for protecting workers, when undertaking potentially hazardous work with the risk of exposure to radiation.
"Sellafield should have properly assessed those risks, and then appropriately planned, organised and carried out the work."
The HSE has previously said the contractors had not shown any illness arising out of the incident.
A Sellafield spokesman said: "Clearly we regret this incident and the entire management team are dedicated to reducing the risks of incidents like this happening in the future."