The prolonged closure of a major French atomic reactor after an explosion this month probably costs EDF at least £1m a day, according to experts.

The nuclear plant operator, which will spend £18bn building the UK’s first new nuclear power station in a generation, shut unit 1 at its Flamanville plant after a fire broke out in the turbine hall.

The company initially estimated it would switch on the reactor within a week, but later pushed the date to the end of March. Work begins this week on replacing damaged equipment.

The unexpectedly long closure adds to the financial pressure on EDF, which last week reported a 6.7% decline in core earnings to €16.4bn (£14bn) in 2016. Closures of its French nuclear plants last year, partly for safety checks, have already cost the 85% state-owned company an estimated €1.3bn.

Prof Neil C Hyatt, head of nuclear materials chemistry at the University of Sheffield, said the lost revenue from the reactor closure in Normandy could be £1m per day.

“Bringing a nuclear power plant back online after an unscheduled outage is a complex task and EDF will want to ensure that all parts of the system are working safely and effectively. A short delay to complete the necessary checks is to be expected, given that the outage was unplanned,” he said.

Another expert said the cost of closure could be up to £1.8m per day, depending on energy market prices, and questioned why there was a delay.

“It took operator EDF almost a week to progressively correct the original outage estimate from one day to 50 days. EDF has provided no information as to why the outage time went from a few days to seven weeks,” said Mycle Schneider, a nuclear energy consultant based in Paris.

The 1.3GW reactor at Flamanville is one of a dozen of EDF’s French nuclear fleet currently offline, which the company said was usual for this time of the year.

It did not say why the restart date for the reactor had been revised four times, or why it had jumped from a few days to more than six weeks.

John Large, a nuclear consultant who has advised the UK government, said initial reports that the fire was in a ventilator suggested the offline reactor would be back online within a week or two. Replacing such parts should be relatively straightforward, he said.

He added that the plant’s continued closure would also add to headaches at the French grid operator RTE, which warned of power cuts at the start of winter due to nuclear outages. “The continuing impact on the grid is likely to be significant, especially if a cold snap develops,” Large said.

A second reactor at the plant is still supplying electricity to the French grid. EDF said: “Work on recommissioning the affected equipment has started this week and should last several weeks, with reconnection to the grid planned for the end of March.”