Sellafield Ltd has been fined £380,000 after a production worker was contaminated with plutonium.
Jonathan Greggain, 51, was working within a glovebox (sealed container) at the nuclear fuel reprocessing and decommissioning site in Seascale, Cumbria, when he felt a pricking sensation on his left hand as he handled a conductivity probe.
Unbeknown to Mr Greggain, one of the legs of the probe had corroded in the acidic environment and effectively become a dangerously sharp object, Carlisle Crown Court heard.
Tests later revealed he had sustained an internal dose of plutonium, which has slightly increased his risk of developing cancer.
Mr Greggain, who was present in court for Tuesday's hearing, is back at work but took six months off following the incident.
In his victim personal statement, he said he had been diagnosed with depression and that the legal proceedings had made him "stressed, depressed, frustrated, angry and disappointed".
Sellafield pleaded guilty on Tuesday to failing to discharge its general health, safety and welfare duty to its employees, contrary to Section 2 of the Health and Safety at Work Act, on or before February 5 2017.
Sentencing, Judge James Adkin said: "It is agreed that hand working within gloveboxes is a frequent activity at the Sellafield site.
"By its plea the defendant has accepted that its employees were exposed to risk while carrying out work within the glovebox in question and that they did not take all reasonably practicable steps to ensure they were not.
"The system of work in place before the incident meant that a probe would only be replaced when it was found to have corroded."
However, he added that it could fairly be considered as an "isolated incident" as Sellafield had 670 gloveboxes on site used for delicate handling of radioactive materials.
Nigel Lawrence QC said plutonium entering the bloodstream via a wound was "by far the most dangerous pathway to individuals" with deposits remaining in body tissues for the lifetime of the person affected.
He said Mr Greggain received some eight times the statutory dose limit per year, although the defence pointed out the effective dose would be received over 50 years from intake and not on the day of the incident itself.
Mr Lawrence continued: "The bottom line, bearing in mind the high risk environment we are concerned with, is that we should have been talking about a proactive system rather than a reactive one."
James Ageros QC, for Sellafield, said the conductivity probes are now replaced every six months regardless of their condition.
A full review of all gloveboxes and potential sharp objects within them had been undertaken and all others were in acceptable order, the court heard.
He said everyone at the company, including chief executive officer Paul Foster who sat in the courtroom, "express their deep regret and remorse for this incident".
In reaching sentence, Judge Adkin noted that Sellafield Ltd was a taxpayer funded non-profit making organisation and that much of the funding is devoted to trying to ensure operations are safe.
Sellafield Ltd was also ordered to pay prosecution costs of £96,753.
Following sentencing Dr Mina Golshan, deputy chief inspector and director of the Office for Nuclear Regulation's Sellafield, Decommissioning, Fuel and Waste Division, said: "This case does not point to any broader concern about the control of risks and hazards across the Sellafield site and was instead related to a specific event and the outcome of our subsequent investigation.
"Sellafield Ltd has already learned lessons from this event, implementing improvements to help prevent a recurrence, and we expect it to continue to seek reasonably practicable improvements in all areas of its work affecting safety."
A spokeswoman for Sellafield said: "Safety and security are our highest priority and we are extremely disappointed that our robust and thorough safety arrangements were unable to prevent the incident in February 2017.
"We apologise for the adverse effects this has had on the welfare of our employee.
"This is a matter we take extremely seriously as reflected by our guilty plea.
"This was an isolated incident that did not cause wider risk to workers beyond this task, nor any broader safety concerns.
"It is not indicative of a failure of the business as a whole."