Sellafield Ltd is to be sentenced for a health and safety breach after an employee was exposed to plutonium.
In February the Office for Nuclear Regulation (ONR), which is prosecuting the case, said bosses at the Cumbrian nuclear fuel reprocessing and decommissioning site confirmed it admitted the offence.
Sellafield is said to have failed to discharge its general health, safety and welfare duty to its employees, contrary to section 2 of the Health and Safety at Work Act.
The maximum penalty is an unlimited fine.
Full details of the allegations have not been made public but at a previous court hearing it was disclosed it involved an exposure of plutonium to a worker at Sellafield's Seascale plant on or before February 2 2017.
Lawyers for Sellafield Ltd previously said the company was not ready to enter a plea until its own appointed expert had reported back their findings.
Sentencing at Carlisle Crown Court by Judge James Adkin takes place from 10.30am on Tuesday.