Radioactive water is leaking from a nuclear waste storage building as big as 132 double-decker buses at Sellafield.

Sellafield Ltd said there is no risk to staff nor the wider community as the water, which covers the solid radioactive waste in the silo, will remain in the ground "for some time".

The leak is believed to be originating from the six older compartments of the Magnox Swarf Storage Silo, which has 22 compartments in total. However it is not known how much water has been lost so far.

The majority of the radioactive material stored there is fuel cladding, which Sellafield says has an intermediate level of radioactivity.

A company spokesman said: "This was a known risk which is linked to the work to empty the silo and permanently remove the hazard.

"It is something we have planned and prepared for. Our modelling and understanding of a previous leak in the 1970s shows that the liquid will remain held in the ground under the building for some time.

"This ground is already contaminated. We have a range of options available for containing the material if it begins to migrate. This migration process would take years and gives us a considerable amount of time to respond.

"We continuously monitor groundwater around the facility and this monitoring shows there is no increase in radiation levels."

Sellafield said that while the water is radioactive, the vast majority of the radioactivity in the building (99.5 per cent) is held within the solid waste.

The water is used to cover the solid radioactive material to keep it cool and prevent fires.

The company said: "It is this water which we suspect is leaking from the building. Investigations are ongoing to establish the volume of water lost.

"It is difficult to calculate an exact figure because the water levels in the building are always expected to vary, due to evaporation and other processes."

The original six compartments were built in the 1960s. The building was extended three times up until the 1980s to accommodate more waste.

An original leak in the 1970s ended when the route was ‘plugged’ by solid material in the compartment.

The spokesman said: "Since then we have been continually monitoring the water levels in the building. We would always anticipate some variation in a building of this size, but in October this year we increased our level of monitoring and observation after recent figures showed some increased variations."

The levels of water are constantly monitored at the silo, but increased monitoring began in October.

in-Cumbria sister title, the News & Star understands workers repairing the leak were asked to sign a non-disclosure agreement, preventing them from discussing the details of the work, however Sellafield has denied this.

The spokesman said: "We have been open and transparent about this incident. We have kept our regulators and stakeholders informed throughout and published details on our website on 18 November."

Last month there was another leak in the older part of the site and work is due to take place in the new year.

Sellafield said: "The current suspected leak is in an inaccessible part of the building, which is underground. Our method for dealing with the hazard posed by the building is to remove the waste from the compartments, demolish the building and then deal with the contaminated ground underneath.

"This has been agreed with our regulators. Waste retrievals are scheduled to start next year. Our focus will be on ensuring the radioactive material remains in the already contaminated ground under the facility and there is no risk to our workforce or the wider public."