Tarmac grout for Dounreay Decommissioning
When a bespoke blend of high performance grout was needed for round-the-clock works as part of the 2.9 billion pounds Dounreay nuclear decommissioning programme, cementitious blending expert Tarmac CMS Pozament was called upon to deliver a specialised product.
Geotechnical contractor, Ritchies, part of the Nuttall Group, specified a grout with a complex mix of ingredients to meet demanding performance criteria. Due to the exacting formula and large volumes required, Tarmac CMS Pozament was contracted to supply a dry grout blend ready for mixing on-site. Over 1,000 tonnes was produced to complete the job, delivered at a distance of over 500 miles from Tarmac CMS Pozament's Derbyshire base to Dounreay on the north coast of Scotland.
Tarmac's grout was used to infill a Liquid Effluent Discharge Tunnel (LEDT) connected to a radioactive waste shaft, 60 metres underground. The requirement was to permanently seal sections of the LEDT to prevent seawater seeping into an isolation curtain that will eventually be created around the waste shaft. The extreme conditions meant it was essential that the grout had a low permeability, shrinkage resistance, and the ability to be pumped at pressure down a series of boreholes.
The grout was pressure injected over 13 days in one continuous process to avoid any joints, which meant Tarmac had to deliver a total of 416 m3 of dry mix around the clock to keep the process uninterrupted.
"This was an exciting challenge to get our teeth into," said Bryan Smith, commercial manager, Tarmac CMS Pozament. "The sheer volume of dry blend grout and distances involved made the task a big one, but add to that the technical requirements, plus a tight delivery window and the job becomes even more interesting!"
Initially, the dry blend was delivered to the site in one tonne IBCs, close to each borehole. However, it quickly became apparent that the strong coastal winds caused an issue with dust produced by the mixing of the grout on-site.
"With Tarmac CMS Pozament's help, we were able to respond quickly to the dust issue," commented Iain Robertson, site manager of Ritchies. "They switched deliveries of the dry blend to bulk road tankers, direct to a nearby concrete batcher, enabling them to mix the grout off-site and transport it in mixer trucks, directly into on-site hoppers. This removed the dust issue from the equation, without interrupting the critical continuous pour."
Due for completion in 2033, the decommissioning and clean-up of the Dounreay nuclear research complex is one of the largest and most technically challenging projects in the country. In addition to the enormous size of the 130-acre site, the decommissioning of the two experimental 'fast breeder' reactors, plus the isolation of the radioactive waste shaft, in preparation for removal of its waste, places extreme demands on both engineering techniques and materials employed.
