The Nuclear Decommissioning Authority

Huge savings have been achieved from a project to decommission several miles of drains that once carried low-level radioactive effluent across the Harwell site.

The timeframe has shrunk by two-thirds, with associated cost reductions, after the team from Research Sites Restoration Ltd (RSRL) devised a new approach that allows the pipework to be cut up underground and the use of recycled equipment.

If the work continues at its present pace, engineers will have cut the decommissioning time by a remarkable two-thirds from the original projection of three years, while considerably reducing costs.

The site's New Main Active Drain System, which began operations 24 years ago, carried low-level liquid effluent from the major facilities to the Liquid Effluent Treatment Plant (LETP). Now no longer needed, work has begun to decommission the 4,000 metre-long system, which extends across the site and includes a series of delay tanks and 55 manhole-access chambers.

The drain line itself consists of two pipes, one inside the other, with the outer pipe made of bitumen-coated cast iron and the inner pipe of polypropylene.

Senior Project Manager Paul Atyeo said:

"The outer pipe provided back-up containment in case of leaks. The inner pipe does not have joints except within the chambers, greatly reducing the chance of leakage or trapped radioactivity.

"When the system was built, the engineers envisaged the decommissioning process would reverse the construction process. That is, access ramps would be dug and the pipe pulled out and cut up on the surface – like giant spaghetti. But, after much thought, consultation and investigations, the team came up with an idea which we thought would work better."

The team's plan was to gain access to the chambers, feeding the inner pipe out in sections, so it can be cut up underground.

"This new method means many thousands of cutting operations underground, but practical methods were developed to do this which were quicker, more efficient and therefore more cost-effective," added Paul. "And, because less excavation is required, there is less potential for disruption to underground services." Work began in May and was due to be completed soon after Christmas. Since the estimated timescale was three years, a huge saving in time and cost has been achieved.

Further cost-cutting was also achieved by recycling equipment from Harwell's sister site at Winfrith.

With the new method, the outer pipe and access chambers are left underground, but have to be proved to pose no liability to people or the environment.

"Proving the pipeline is clean requires a specialist health physics drain probe, which can crawl down the pipe, taking measurements and sending back images of what it finds," said Paul. "Luckily for us, just such a probe was designed and built at Winfrith some time ago."

"So, we dusted off the probe, refurbished and re-commissioned it, and put it to work. It is performing perfectly to date."

The network of contractors has included EDS, the primary contractor, alongside Nuclear Technologies and Aurora.