British Nuclear Fuels (BNFL) today reported an operating loss of £210 million sterling and said performance in some of its areas was "unacceptable".
The Cumbria-based nuclear fuel operator said problems at his Wylfa Power Station in north Wales had contributed to a poor performance in generation by its Magnox stations.
Loss after exceptional items in the last financial year was £66 million, compared with £337 million the year before.
The Magnox business had a pre-tax, pre-exceptional loss of £199 million, which significantly affected the company's financial results.
BNFL chairman Mr Hugh Collum said the company was rebuilding customer confidence and said "significant progress" had been made in addressing issues raised by Health and Safety reports into the company last year.
He said: "It has been a year of substantial progress during which we have accelerated change throughout the company.
"Although there has been significant improvement in operating performance in some parts of the business, there are areas where performance was unacceptable."
But he said the outlook for the business was encouraging: "We are operating at a time when the prospects for a renaissance in nuclear power are brighter than for many years."
PA