The Nuclear Decommissioning Authority

Finally, the end is in sight for two facilities that deal with the radioactive sludge from Winfrith's most famous experiment, the Steam Generating Heavy Water Reactor, or SGHWR.

Four tall concrete tanks, more than four metres high, stored sludge generated during a 23-year operational life that ended when SGHWR closed in 1990. But before the External Active Sludge Tanks (EAST) could be decommissioned and taken apart, their contents needed to be retrieved and stabilised in grout mix. This led to construction of the £12 million Waste Encapsulation Treatment Plant (WETP) by contractor Nuvia, which opened in 2005 and completed its mission last year after producing 1,068 stainless steel drums which will be stored on-site pending a final disposal solution.

WETP was demolished at the end of last year (2011) and EAST is on schedule to follow suit by March 2012, marking a major milestone in the site's restoration.

Over the years, the project has thrown up some unexpected obstacles for the team from Research Sites Restoration Ltd (RSRL), which were solved with a combination of ingenuity and innovation.

RSRL project manager, Cliff Strange, said:

"All went reasonably smoothly until we reached the final stages, when we discovered a quantity of dense material in the bottom of the tanks. It was mostly silica and generally referred to as 'sandy material'. The recovery of this unexpected material required different methods and a fair bit of innovation."

Nuvia, in conjunction with RSRL, developed a process to recover the sandy material using an industrial vacuum-pumping system – called Big Brute – and encapsulate it in 40-litre containers, within prepared 200-litre waste drums. The sandy material was recovered in 20kg batches and mixed with a premixed grout in a temporary encapsulation rig, also developed by Nuvia.

With the tanks – whose walls were more than half a metre thick - now empty, work could begin on decontamination, dismantling and cutting up the concrete. This work is now nearing completion with three-quarters of the tank structure removed.

In November 2010, an engineer entered one of the tanks to carry out a radiological survey, the first time anyone had been inside for 40 years. Considerable progress has since been made on removing the tank roofs and walls, using innovative diamond wire-cutting techniques.

Most of the concrete arising from dismantling the tanks will be disposed of either as Very Low Level Waste (VLLW) or exempt material.

Dismantling has also thrown up challenges, with investigations required to establish the source of a suspected 'hot spot' in the pipe corridor.

Cliff said:

"Though this 'hot spot' is only the size of a little fingernail, the exacting standards to which we operate mean it must be fully looked into and properly dealt with."

The final piece of the jigsaw was the demolition of WETP, with the entire project due for completion by March 2012.

Unique place in UK's nuclear history

The remote heathland of Winfrith in Dorset was the second site chosen by the Government to carry out research on experimental reactors in the post-war years.

Harwell, in Oxfordshire, had been the first site selected to help pioneer the UK's drive to harness nuclear fission for electricity generation.

Back in 1957, Winfrith had good road and rail connections to London, as well as a large supply of essential fresh cooling water and potential workers from Bournemouth and Weymouth.

By the mid-1960s, the site had nine research and experimental reactors, with colourful names such as Zenith, Zero, Dimple, Dragon and Zebra. An apprentice training school and post-irradiation examination facility were also key parts of the unique world-leading facilities which harnessed some of the finest engineering and scientific expertise of the time.

The most famous landmark was the iconic Steam Generating Heavy Water Reactor (SGHWR), the only Winfrith reactor to provide power to the national grid. In its lifetime, it produced enough power to supply a small town.

The last reactor closed in 1995 and seven have been completely decommissioned and demolished, with only two remaining: DRAGON, a prestigious high-temperature helium-cooled facility which closed in 1976, and the SGHWR which closed in 1990. Both are now in the 'care and maintenance' phase, signalling the completion of early decommissioning work. They will be maintained and monitored until they are dismantled for transfer to the Geological Disposal Facility (GDF).

Meanwhile, in the early 1990s, the eastern part of the site opened its gates to tenant companies and became the Winfrith Technology Centre, with a strong focus on science and innovation. In 2004, the centre was transferred to English Partnerships.

EAST was built to store waste radioactive sludge from the SGHWR, while the role of the WETP which opened in 2005, was to deal with the sludge and other waste.

Now, with more than 30% of the clean-up programme at Winfrith complete, there are no high-hazard nuclear facilities left at the site and around 50% of the original area has been released for commercial use, employing many people.