British Nuclear Fuels has unveiled plans to build an above-ground nuclear waste dump the size of a football stadium in the heart of Snowdonia National Park, in a move described by environmentalists as a "nightmare".

  The giant building, costing £50m, will store parts of the reactor from Trawsfynydd nuclear power station which was shut down in 1993.

  A planning application by the nuclear authority has prompted calls for a public inquiry from environmental campaigners.

  BNFL says it has no alternative but to build the storage facility at Trawsfynydd. There is currently no central dump anywhere in the UK for storing intermediate level nuclear waste, which includes parts of the reactor left over when a nuclear power station is decommissioned.

  In 1997, plans for an underground test laboratory that would have paved the way for a subterranean dump at Sellafield in Cumbria were rejected by John Gummer, then Secretary of State for the Environment, on account of fears that radioactive waste would seep into the ground.

  The Council for National Parks (CNP), a campaign group, says all possible options for storing the waste must be debated at a public inquiry.

  "The prospect of 100 years or more of nuclear storage in a national park is disgraceful," said CNP spokeswoman Sylvia Davies. "It is a nightmare.

  "Trawsfynydd shows the damaging legacy of nuclear power. These plans must now be subject to rigorous examination at a public inquiry, including all alternatives for decommissioning."

  While building a nuclear power station in a national park would never be allowed today, Trawsfynydd was given the go-ahead just prior to the creation of Snowdonia National Park. The power station was built at Trawsfynydd in the 1950s, generating electricity for 28 years until it was closed eight years ago.

  BNFL delivered its planning application - including an environmental assessment report that filled seven boxes - to Snowdonia National Park Authority last week. The park authority is responsible for planning laws.

  A spokesman for BNFL said the new building would be 120 metres long, about 23 metres high and 30 metres wide. But he said it would be hidden from view by the existing decommissioned power plant, which will be reduced in height to 20 metres.

  He said the planning application followed a long-term consultation with people in nearby villages. "One of the things they called for was to have the visual impact of the site reduced because of its unique position in the national park," said the spokesman. "Our proposal is to lower the height of the roof [of the reactor] and then clad it in local slate and coloured stainless steel. We are also planting 40,000 trees to screen it."

  He attacked the CNP for criticising BNFL's plans before it had even seen them. "We have had support from the local community," said the spokesman. "We have worked hard with them to find out what they want."