The company is being prosecuted by nuclear regulators the Environment Agency and the Office for Nuclear Regulation after four bags of mixed general waste, such as plastic, paper, tissues, clothing, wood and metal, from normal operations in controlled areas of the site, were sent to Lillyhall landfill site, in Workington. The bags should have been sent to the Low Level Waste Repository, at Drigg, Cumbria - a specialist facility that treats and stores low level radioactive waste consignments.
"Sellafield found the error was caused by a new monitor, which had passed the bags as 'general' waste making them exempt from strict disposal controls," the Environment Agency said in a statement.
All of the original four bags were retrieved from the landfill and returned to Sellafield for correct disposal.
Ian Parker, Nuclear Regulation Manager for the Environment Agency, said: "We have carried out a thorough investigation in partnership with the Office for Nuclear Regulation (ONR) and have already required Sellafield Ltd to take action to ensure this does not happen again."
Sentencing is expected to take place at Carlisle Crown Court on 8 March 2013.
Sellafield Limited told NEI that it is not in a position to comment any further while the legal process is still underway.