Innovative way to plug tubes at BEPO

A novel technique using expanding polyurethane foam has been used to prevent radioactive materials escaping from storage tubes at the British Experimental Pile Zero (BEPO) at Harwell, which is being decommissioned.

A novel technique using expanding polyurethane (PU) foam has been used to prevent radioactive materials escaping from storage tubes at the British Experimental Pile Zero (BEPO) at Harwell, which is being decommissioned.

Aurora workers prepare the storage tubes for filling with foam (Image: SVI)

The 6 MW BEPO experimental reactor was commissioned in 1948 and used primarily for the production of radioisotopes, general irradiations, chemical engineering experiments and as a source of neutrons for nuclear measurements. It was used to demonstrate that commercial power reactors could be viable and was a forerunner of the Windscale Piles. BEPO operated until 1968 and defuelling was completed the following year.

Whilst BEPO was in operation, a large concrete block containing some 250 tubes was used to store fuel elements and rigs from BEPO. Around 175 of these tubes are over 8 meters long and only 3.5cm in diameter.

These storage tubes were exposed in March 2013 for the first time since 1969 after a type of sprayed on concrete known as 'gunite' had been removed. The concrete block surrounding the storage tubes is to be demolished so the risk of fracturing the tubes within the structure was deemed too high without first encapsulating any loose contamination.

Contractors carried out tests to determine the levels of residual radioactivity in the tubes. These levels were found to be higher than previously anticipated.

Research Sites Restoration Limited, which manages the Harwell site on behalf of the Nuclear Decommissioning Authority, said: "In order to allow the decommissioning of the block to be carried out effectively, it was therefore necessary to fix the radioactivity in place." It noted that traditional methods to fix contamination in place were not appropriate due to the size, shape and positioning of the tubes."

Following successful trials using expanding PU foam, a contract to conduct the foam sealing work was awarded to Aurora Health Physics Services Ltd and Steve Vick International (SVI).

Aurora had carried out the characterization work on the storage block and was able to work in partnership with SVI to ensure that the best method to seal the storage tubes was identified. SVI was able to design and manufacture different PU foams and foam installation kits to suit the 11 different configurations of storage tubes.

The work was completed on time and on budget in March 2014.

The BEPO reactor itself will remain in care and maintenance until around 2040, when the core and remaining facilities will be completely decommissioned.

Researched and written
by World Nuclear News