Handling radioactive waste

The UK could halve the storage space needed for low- or intermediate-level radioactive waste retrieved from decommissioned nuclear power stations using a new processing technique that can handle multiple kinds of hazardous material.

The patented technology from Babcock Nuclear Services (BNS), the nuclear arm of Babcock International, encapsulates solid and sludge radioactive waste in a cement mixture within cube-shaped containers.

The containers, which measure 1.6m3 and hold tons of material, can then be transported to nuclear-waste storage facilities.

The technique, which is the key feature of BNS’ Versatile Encapsulation Plant (VEP), differs from current technology that must handle solid and sludge waste in two separate plants.

Encapsulation technology traditionally involves mixing low- and intermediate-level radioactive waste with a cement material to effectively reduce its radionuclie mobility. The waste materials generally behave as additives and become chemically or physically incorporated into the cement material, which is commonly blast-furnace slag.

With current technologies, cement powders are added to slurried waste in a drum and mixed with a paddle. The BNS technique, however, encapsulates the waste and cement in the cube-shaped container without a paddle.

Doug Kirk, technical consultant at BNS and inventor of VEP, said the system does this by first concentrating the sludge, which is generally diluted so that it can be retrieved from the reactor.

The sludge is settled and concentrated in a tank for 12 hours before transferring to one side of an inline mixer. Kirk described the inline mixer as a 50mm-diameter tube containing a set of static veins designed to agitate and mix the sludge.

‘It’s a perfectly standard piece of technology used a lot in the oil and gas industry,’ he said. ‘We have used it previously for blending different types of resins but it has not been used in this application before.’

Kirk said the other side of the inline mixer is fed with a wet blast-furnace slag grout mixture. ‘Those two streams are mixed together as they pass through the inline mixer so what comes out at the end is a grout, sludge mix,’ he said. ‘The advantage of that is when it leaves the inline mixer, the job is done so it doesn’t matter what shape of container you are actually allowing the fluid to collect in.’

Kirk noted that the cubed container increases storage volume by 25 per cent compared to a spherical drum with paddle.

‘The other advantage is because there are no moving parts inside the container you could also put solid waste in,’ he said, adding other encapsulation plants cannot handle both solid and sludge waste together and must run two separate operations. ‘We can store up to 25 per cent more solids compared to other methods.’

Similarly, a problem with current systems is that a reusable internal paddle requires cleaning, a process that is time-consuming and generates radioactive secondary waste.

All of this, BNS claims, could help cut the storage area needed for nuclear waste in half. The company believes this could reduce the nationwide nuclear reactor decommissioning budget by millions of pounds. Over the next century the decommissioning programme will likely produce thousands of waste packages that will be retrieved, conditioned and stored for no less than £40bn.

It is estimated that the bulk of the UK’s expenditure will be spent decommissioning the Sellafield site in north-west England, the country’s largest nuclear site.

The site was officially shut down in 2005 and taken over in 2004 by the Nuclear Decommissioning Authority (NDA), a public organisation under the UK’s Energy Act 2004 responsible for the decommissioning and cleanup of nuclear reactors.

Approximately 18 months ago, Kirk said the NDA contracted BNS along with other nuclear service companies to trial encapsulation technologies at the Sellafield site. BNS is now proof-of-scale trialling VEP against competing technologies as part of a Sellafield Project risk-reduction initiative.

While Kirk said he did not know the details of the other competing encapsulation technologies, he could confirm BNS’s technique will be the only one that does not use a drum and paddle mixer.

The low-level waste from Sellafield is currently sent to a repository site in Drigg, Cumbria. Sellafield, like other nuclear reactor sites across the UK, also has significant quantities of intermediate level radioactive wastes.

The NDA believes mixing such waste in cement grout within containers is a good short-term solution for storage, but long-term management will also need to be considered.

The NDA has stated that the best option for long-term storage is deep geological disposal. The organisation envisions storing the waste in a repository excavated in a stable rock formation, deep underground.

There has still been no site chosen for such a repository and the NDA claims it could take up to 11 years to find one. Even then, construction would be likely to take another 20 years and it would not begin to be filled with waste until 2040. It could then take up to 60 years to fill the sites.

Meanwhile, there will be plenty of work for those in the encapsulation industry to do. According to the NDA, the UK has 23 reactors generating one-fifth of its electricity and all but one of these reactors will be decommissioned by 2023.

Kirk claims the BNS method for encapsulation would be the right solution for future projects. ‘We want customers to know that this technology exists and there are alternatives to the traditional ways of doing things,’ he said.

Siobhan Wagner