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More waste, less speed

Top of Dounreay's decommissioning challenges is undoubtedly the removal of 700m3 of intermediate level rubbish dumped in a 65m deep unlined construction shaft sunk originally as a muck removal route for a 1950s water cooling outlet tunnel. The 4.6m diameter shaft was used as a dump until 1971.

For years water has been pumped out of the top creating a negative pressure around the shaft and therefore a net inflow of groundwater. But the surrounding deposits of sandstone, siltstone and mudstone are fractured and engineers are keen to see the shaft emptied as soon as possible.

As in all decommissioning timetables, safety considerations rule and 'soon' is relative, with nothing likely to actually come out of the shaft until at least 2014.

Two months ago, in the midst all the latest flack being levelled at Dounreay, the Government gave engineers the final go ahead to embark on a £350M, 20 year world-first exercise to isolate the shaft, remove and treat its contents and then backfill it.

Tender documents are now being prepared for a major £4M hydrogeological survey to determine the exact rock regime and, early next year, the first of some half dozen deep boreholes will be sunk around the shaft to allow up to five years of detailed monitoring.

At some stage during this monitoring engineers will choose one of two shaft isolation methods. It will be encircled either by a ring of 300mm thick secant piled diaphragm walling excavated at least 80m deep or by an equal depth ground freezing regime.

At this early stage, Dounreay's head of waste storage Dr Doug Graham reckons either method would probably achieve the aim but he is in no hurry to decide. He might even sink a second full size trial shaft to test the techniques.

Both methods have pros and cons and - at 80m plus - would demand leading edge technology.

The required verticality of sinking reinforced walling panels would be a challenge, but the end result would provide a tangible barrier against escape. However the wall itself could eventually become a contaminated structure requiring treatment.

Ground freezing would be easier but less 'visible' and begs the question of how long to leave the freeze switched on.

While shaft protection is being evaluated, remotely operated grabs will be developed to individually remove every item of rubbish from deep inside the hole. Surrounding water might first be removed and treated to aid visibility.

But while Mathews and his team are busy dismantling Dounreay, Graham will actually be building new secure buildings to house handling, treatment and storage facilities for his freshly recovered waste from the shaft.

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