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DECOMMISSIONING THE DOUNREAY WASTE SHAFT

Update 9 October 2004

In 1959, the Scottish Office authorised a disused vertical shaft at Dounreay as a disposal facility for solid radioactive waste. Approximately 750m3 of radioactive waste accumulated in the 65-metre deep shaft until 1977, when disposals ceased following an explosion.

It was apparent that the water-filled shaft did not meet modern standards for the disposal of intermediate -level waste. Advances in technology meant it was feasible by 1998 to conclude the waste could be retrieved and the facility decommissioned. The waste shaft is one of the great decommissioning challenges in the UK today. The engineering and scientific skills needed to carry out this work to the highest standards of radiological and environmental protection can enhance the international status of the UK in general, and UKAEA Dounreay in particular, in the field of nuclear clean-up expertise.

UKAEA has developed a major programme of work to prepare for the retrieval of waste from the shaft. All of this work is subject to stringent regulatory agreement. Key tasks to prepare for the decommissioning the shaft are identified below:

This project update focusses on accelerated progress in two areas – hydraulic isolation of the shaft from the geosphere and reinforcement of the plug.

Hydraulic isolation
For most of its depth, the shaft is unlined. Throughout its history, groundwater entering the shaft has become contaminated with radioactivity. Most of this is pumped out, filtered, sampled and discharged to sea in accordance with the site’s current waste disposal authorisation. Over time, a small proportion of this water – equivalent in radiological terms to three per cent of that discharged to sea – has migrated through the shaft and contaminated rock on the seaward side of the shaft.

When the waste is retrieved, the operation will disturb the column and spread radioactivity in the water. If the volume of water entering the shaft is not controlled, a large volume of liquid intermediate-level waste will need to be managed along with the solid intermediate-level waste. To prevent this, the shaft needs to be isolated from the geosphere by installing a “curtain” around it, diverting the natural flow of groundwater away from the shaft.

This will also safeguard against the remote possibility of a major leak before waste retrieval begins. At present, the risk of leakage is the main environmental hazard associated with the shaft.

Before deciding how to isolate the shaft, UKAEA sought the views of stakeholders on the clean-up of the surrounding rock. The affected area of rock is believed to extend for 300 metres on the seaward side and options range from natural decay of the radioactivity to sealing it in-situ, flushing and excavation.

Even though such remedial action would not be implemented until after the shaft is emptied, it is important that UKAEA understands the views of stakeholders today because the chosen method of isolation will penetrate areas of rock that are contaminated.

UKAEA discussed the “end state” for the rock with panels of stakeholders and received 32 responses to a consultation document. Some stakeholders felt natural decay of the radioactivity should be allowed, others believed some form of in-situ immobilisation would provide additional reassurance and a small number wanted the rock to be excavated. Some stakeholders expressed the view that a final decision should be taken once the waste had been retrieved from the shaft and more detailed analysis could be carried out. This would mean the “end state” for the shaft would be consistent with future decisions about the “end state” for cleaning up the Dounreay site as a whole.

UKAEA welcomes these views. While it accepts that a final decision cannot be taken until the shaft is emptied, the consultation process has enabled UKAEA to conclude that grouting should be its preferred method of hydraulic isolation of the shaft. This would immobilise any activity in a 10m-wide ring of rock around the shaft but does not preclude its excavation at a later date if this was deemed appropriate.

The next steps
Subject to the appropriate regulatory approvals, isolating the shaft with a curtain of grout will require 350-400 boreholes to be drilled to depths of 80 metres in an oval ring around the shaft. The grout would be injected in the boreholes and forced into the fissures in the rock to create a solid 360 degrees curtain. The wall would begin at a distance of five metres from the shaft, and form a solid barrier 10 metres wide.

UKAEA obtained planning consent in 2004 for further exploratory boreholes to understand better the radioactive contamination in this region of the rock and this work is scheduled for Autumn 2004. Grouting trials are scheduled for Spring 2005 and a start to civil works at the shaft is planned for Autumn 2005. The grouting operation is expected to take between two and four years to complete.

Plug reinforcement
A stub tunnel that connected the base of the shaft to the main effluent tunnel was plugged before the first waste disposal took place. This concrete plug was designed to withstand the normal variations in water pressure in the ground. During waste retrieval operations, the plug may experience pressure loadings in excess of the design criteria, and therefore needs to be reinforced before such work begins. Immediately prior to installing the grout curtain, and subject to regulatory approvals, high-strength concrete will be injected into the length of the stub tunnel between the plug and the Y-junction where it meets the effluent tunnel.

This will ensure the integrity of the plug in the worst-case scenario of changes in pressure during waste retrieval. The enlarged plug will form part of the grout wall around the shaft.

Essential steps to waste retrieval
The work now proposed by UKAEA means the shaft will be isolated from the geosphere on an earlier timescale than previously forecast. It will create a barrier to block any further migration of radioactivity from the shaft and provides stakeholders with additional reassurance that particles sent to the shaft for disposal in the 1960s and early 1970s cannot escape into the marine environment. In essence, it means the major environmental hazard associated with the shaft in its present condition will be mitigated. Shaft isolation and plug reinforcement are essential in order to meet the highest standards of safety and environmental protection during the subsequent operation to retrieve the waste. The completion of this work, ahead of schedule underlines, UKAEA Dounreay’s commitment to reducing and eliminating the hazardous legacies of fast reactor research and development at the earliest opportunity. And by delivering the highest standards of safety and environmental protection in the implementation of this programme, UKAEA and its contractors can enhance Dounreay’s reputation as an international centre of expertise in nuclear clean-up.