It follows ONR’s investigation into the incident which took place on 5 February, 2017.
A spokesman for the ONR said: “For legal reasons we are unable to comment further on the details of the case which is now the subject of active court proceedings.”
Proceedings are due to start at Workington Magistrates Court, where Sellafield will face charges for offences under Section 2 (1) of the Health and Safety at Work Act (1974).
Sellafield was last fined £700,000 in February 2013 for wrongly dumping radioactive waste. Four bags of low-level waste were sent to a landfill site in Workington.
The site covers 6 square kilometres and is home to more than 200 nuclear facilities and the largest inventory of untreated nuclear waste in the world.
Sellafield Ltd is a subsidiary of Nuclear Decommissioning Authority, a non-departmental public body and so not subject to Crown Immunity. It employs more than 11,000 people.
The decision announced on 11 May is the first prosecution from the nuclear safety regulator since it was made independent from the HSE in 2014.