The bill for cleaning up the huge Sellafield nuclear plant in Cumbria will rise even higher than its current estimated level of £70bn as operators struggle to assess the full scale of the task, according to sources close to the project.

The warning comes just days before private sector managers face a grilling from the public accounts committee, which is investigating activities at the facility.

It was hoped that the huge bill – eight times the cost of staging the London Olympics – would be capped at £70bn, but well-placed sources have told the Guardian that the operators are convinced they are still "not at the top" of the cost curve.

Sellafield is regarded as the most dangerous and polluted industrial site in western Europe, not least because it houses 120 tonnes of plutonium, the largest civilian stockpile in the world.

The cost of decommissioning the Calder Hall reactor plus a magnox fuel reprocessing plant at Sellafield has been rising steeply, but the biggest task comes from "ponds" and "silos" filled with old equipment and deteriorating, highly toxic waste.

Nuclear Management Partners (NMP), the private sector consortium that manages the site, declined to comment, but other sources said those engaged in the clean-up were still some way from knowing exactly what was in the storage facilities. "Record-keeping in the past was clearly not what it should have been," said one.

The soaring cost of decommissioning, along with the apparent inefficiency of NMP – and the £230m of dividends it has received – will come under the spotlight on Wednesday at a meeting of the public accounts committee, which is chaired by Margaret Hodge, the straight-talking MP for Barking and Dagenham.

Tom Zarges, the chairman of NMP, will be questioned alongside Tony Price, the managing director of Sellafield, and John Clarke, the chief executive of the state-owned Nuclear Decommissioning Authority (NDA), which is meant to oversee the clean-up process.

The committee has in the past been highly critical of NMP, not least for falling behind on 12 out of 14 key tasks being undertaken in Cumbria.

The senior nuclear executives will also be asked to comment on how £6m of bonuses came to be shared out among NMP bosses over three years and why the consortium paid back £100,000 in expenses that had been wrongly claimed.

The political temperature has been raised by the NDA agreeing to give a further five-year contract to NMP despite its performance being fiercely criticised by accountants in a recent report, which was not initially provided to the committee.

KPMG, working for the decommissioning authority, accused the clean-up group of overspending, failure to reach operational targets and weak leadership at the atomic complex in Cumbria.

Hodge has already said that, in the light of the critical review, it was "inexplicable" that the NDA was prepared to reward the NMP consortium, which she also accused of spending cash "like confetti".

NMP is made up of British firm Amec, American firm URS and Areva, the French engineering group, which is also engaged to help EDF of France build the nuclear plant at Hinkley Point in Somerset.

While the clean-up goes on there has been much speculation about how to deal with the plutonium, which in theory could be used to create dozens of atomic bombs if it fell into the wrong hands. Just storing this material is said to cost £80m a year and tThere are a variety of potential plans for reducing the stockpile, possibly by burning it or turning it into more fuel for reactors.

Talk of building a new mixed-oxide (Mox) fuel reprocessing plant has been undermined by a report out this summer that concluded a previous Mox facility, which closed two years ago, had left taxpayers with a £2.2bn bill rather than the healthy profit that had been promised when it was first constructed.