Substantial penalty expected to be handed down to the operators of Cumbrian plant

The safety record of Britain's nuclear industry will be tarnished tomorrow when managers at the Sellafield complex in Cumbria are fined for exposing staff to radioactive contamination.

A substantial penalty is expected to be imposed by Carlisle crown court following a successful criminal prosecution brought by the Health and Safety Executive.

Concerns about conditions at the plant come just a week after an eminent group of scientists and military experts described as "ludicrous" the manner in which 100 tonnes of plutonium was stored at Sellafield – and at a time when the wider nuclear industry is trying to build public support for a new generation of reactors.

Sellafield, now owned by Amec, Areva of France and URS Washington of the US, pleaded guilty this summer to failing to discharge its duties under section 3 of the Health and Safety at Work Act 1974. Two contractors were exposed to danger while they were refurbishing a concrete floor at the plutonium storage plant.

A spokesman for Sellafield declined to comment but industry executives said the company was braced for a financial hit. The Cumbrian facility was fined £500,000 plus costs of more than £50,000 three years ago following the discovery of a large leak of highly radioactive materials at its Thermal Oxide Reprocessing Plant (Thorp).

The incident dated back to April 2005, and although no one was injured there was concern that the leak had continued for eight months before it was detected.

Sellafield was then under the control of the British Nuclear Group – an arm of the state-owned BNFL – which has since ceded control to the private sector consortium Nuclear Management Partners, which is made up of Amec and its two partners.

BNFL was itself fined £30,000 and ordered to pay £21,000 costs in January 2004 after a diving accident when the company was checking underwater outfall pipes.

Tomorrow's court appearance follows safety concerns raised by anti-nuclear campaigners the British Pugwash Group in a report on Britain's plutonium stockpile, which is centred on Sellafield. Retired general Sir Hugh Beach, one of the report's authors, told the BBC: "It's a total absurdity that we should have 100 tonnes of separated plutonium sitting up at Sellafield in tin cans ... that is manifestly ludicrous."

The BPG, named after the venue of a 1957 nuclear safety conference in Nova Scotia, fear the nuclear stockpile could become a target for terrorists.These difficulties are highlighted as the government and industry try to move ahead with plans to build more than half a dozen new nuclear plants to generate low carbon electricity.

The Health and Safety Executive has already taken a tough line on the designs for new reactors by telling Areva and Westinghouse of the US that it needs much more work to be done before it could give the green light to the plants they have proposed for construction here.