EDF Energy has been criticised by the chair of a Sizewell watchdog group for failing to carry out a full site survey before it submitted plans for a new radioactive waste store.

The company’s late discovery of thousands of tonnes of concrete waste buried on the site has led to a doubling of both the projected construction period and heavy lorry movements.

Marianne Fellowes, chair of the Sizewell Stakeholder Group (SSG), set up to improve the flow the information between the local community and the nuclear site and its regulators, said the mistake had led to a loss of public confidence.

“It was a huge shock to the public that you didn’t know where the concrete was,” she said.

Joan Girling, a former local county councillor and now a co-opted member of the SSG, said long-term local residents and employees of Sizewell A were well aware of the existence of the buried concrete and EDF should have known.

However, SSG members voted nine to five against a move to seek an apology from the company for not conducting a full site survey prior to submission of the planning application.

The development, approved before the discovery of the concrete, is for a store for the highly radioactive spent fuel rods which are routinely removed from the Sizewell B reactor.

Spent uranium fuel is currently immersed in water in a chamber known as a “pond” to help it cool.

Because it has not proved possible to accommodate the number of rods for which the Sizewell B pond was originally built – owing to new safety advice over density – it will reach capacity in 2015.

EDF has approval from Suffolk Coastal District Council to build a new multi-million pound dry store for the rods which will be encased in steel casks.

John Benn, Sizewell B plant manager, told the latest SSG meeting: “The oversight was not ideal, but we have been totally open and transparent about it since the buried waste was identified.”