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.