Cleaning up nuclear waste

Materials World magazine
,
1 Dec 2017

The UK’s Nuclear Decommissioning Authority is testing new ways of treating the large quantities of radioactive waste produced by its sites. Matt Buckley, Deborah Ward and Rick Short* explain the possibilities of thermal treatment.

It’s challenging work – to clean up 17 nuclear sites and make them fit for reuse. The programme, driven by the Nuclear Decommissioning Authority (NDA), will take more than 100 years to complete. Many sites will remain in a semi-dormant stage for decades, although radioactive decay renders other buildings more straightforward for eventual demolition.  

Largely funded by the UK taxpayer, current forecasts are that the project will cost more than £100 billion. However, future technological advances are likely to make the process faster and cheaper, while offering potential safety benefits.

Research activities are an important part of the process to identify such technologies, with radioactive waste a particular area of interest. Many thousands of cubic metres already exist and more will arise as facilities are dismantled, ranging from mildly radioactive concrete to highly hazardous materials and facilities that must therefore be tackled using remotely operated equipment.

Managing radioactive waste

The NDA calculates that almost five million cubic metres of radioactive waste needs to be managed, including a legacy of historical waste that was generated before the organisation was set up in 2005.  

By far the largest quantities are at Sellafield, Cumbria, where waste has been accumulating since the earliest days of the UK weapons programme (1950s) and the first generation of nuclear power stations. 

Meanwhile, the continued dismantling and demolition of nuclear facilities also produces large quantities of radioactive waste, and will continue to do so for many years to come. It is also produced during continuing operational activities, such as the reprocessing of spent fuel, which takes place at Sellafield.

Quantities are recorded in the latest UK Radioactive Waste Inventory as follows:

The NDA’s strategy involves converting the higher activity wastes (HAW) into a form that can be safely stored, pending eventual transfer to an underground geological disposal facility (GDF) that is being developed for England and Wales, in line with current government policy – Scotland’s policy is for long-term management at near-surface, near-site facilities. 

More than 650 different HAW waste streams are listed in the UK Radioactive Waste Inventory. Those in the intermediate level waste (ILW) category are the most diverse and complex to deal with. The NDA already has well-established plans in place to manage radioactive waste, but is nonetheless pursuing opportunities for new methods.  

Going thermal

One area of focus is to examine whether thermal technologies could be used to treat some types of radioactive waste – which at present tends to be immobilised in a cement grout, therefore increasing volume – and packaged in containers for storage, pending future disposal. Ongoing storage is required, until a suitable location for a disposal facility is identified.

The process immobilises inorganic contaminants (i.e. radionuclides and heavy metals) present in the waste mixture by incorporating them into a durable glass matrix. Organic wastes are destroyed, while gaseous contaminants released during the melting process are passed through an offgas abatement system and filtered separately.

If thermal treatment is suitable for mixed ILW at the higher end of the radioactivity spectrum, lifetime savings across the NDA could be significant, potentially hundreds of millions of pounds. This is based on reduced waste package volumes, which could mean fewer stores are required. Additional, significant savings from thermal processes could stem from stabilising the waste, destroying chemically reactive species that would enable more straightforward storage and disposal options and avoid re-treatment of waste packages.

Waste is mixed with glass-former (e.g. soils, industrial minerals, glass frit or a combination) and is heated by an electrical current through electrodes lowered into the batch. As the waste melts into reduced-volume molten glass, more can be added to the container. Thermal convection mixes the waste as it melts, avoiding the need for stirring. Exhaust gases pass through an off-gas treatment system.

Thermal treatment is already used to vitrify the extremely radioactive liquid, known as highly active liquor (HAL), which is produced by the reprocessing of spent fuel at Sellafield. The process was developed specifically for liquid feeds and the relatively small volume of arisings from reprocessing. 

These quantities, however, are tiny compared to the much larger amounts of ILW and low level waste (LLW), which are varied both in terms of physical characteristics and volume, for example: 

  • The contents of spent fuel ponds, such as skips used for spent fuel storage 
  • Sludges that have accumulated in the ponds
  • Metallic components from fuel elements
  • Contaminated tools, clothes, paper, rags, etc
  • Contaminated concrete
  • Other parts of buildings – scaffolding, pipework, circuitry

The test rig

Using thermal treatment technology on a larger scale for these kinds of wastes would be a new development for the country. 

Under work funded directly by the NDA, Sellafield, Galson Sciences and the National Nuclear Laboratory are working together to carry out in-container vitrification trials at an existing test rig on the Sellafield site in Cumbria. It was commissioned in 2016 through a test melt using samples of radioactively contaminated soils. 

Experiments are demonstrating that thermal treatment has potential for wider deployment in the NDA estate, and if trials continue to prove successful, a wide range of factors would be considered before a final choice of technology could be agreed, including engineering design, achievable volume reductions, ease of operability, package disposability and costs.  

A work programme has since treated some simulants of common waste types that are present at Sellafield and elsewhere. 

The 12 paired melts used six inactive waste streams that will be followed by six active streams. The results so far confirm that the process has potential to offer volume reduction and vitrification of varying waste streams, including mixed compositions.   

Melts compositions were selected to provide information for Sellafield's waste and were chosen to represent generic challenges across UK waste streams.

Each melt required materials sourcing, adjustment of the precise formulation for the glass-forming materials (frit) and the configuration of how and in what quantities they should be loaded into the steel container. All materials subject to thermal treatment were successfully processed, including those with high levels of water content and of mixed content, such as:

  • Corroded magnox sludge simulant (CMgS) with a very high water content
  • CMgS loaded in a skip. The simulated corroded magnesium sludge was placed in a small-scale metal skip, as typically used at Sellafield, and vitrified at the same time in a single run. This surrogate stream was representative of miscellaneous beta/gamma waste (MBGW) and particularly linked to waste in the First Generation Magnox Storage Pond (FGMSP), one of the NDA’s highest hazard facilities
  • The waste simulant zeolite mineral clinoptilolite, used as an ion exchange medium in the Sellafield Ion Exchange plant (SIXEP). The SIXEP plant removes radionuclides from waste effluent
  • General decommissioning materials, including scaffolding pole, hand tools, wood, paper suits and gloves, filter fragments, soil and oil
  • Uranium containing wastes, such as grout and miscellaneous metals
  • Organic plutonium contaminated material, including PVC suits

Making thermal treatment viable

In each case, the volume reduction factor (VRF) – the ratio of treated product to the raw waste – was significant with most melts surpassing expectations and reaching over 90% volume reduction. when compared to the baseline volume encapsulation in cement grout.  

There is scope to increase this via longer feeding-while-melting operations. This would need to be balanced against any treatment required post-vitrification and the eventual choice of package. Treatment of any secondary waste will also need to be considered. 

Sampling of the vitrified glass blocks and comparison with the starting material is currently underway, using a range of sampling and analytical techniques. These full analytical results will feed into a comprehensive assessment of packaging, storage and disposability across the whole UK radioactive waste inventory.

Similar waste streams within NDA and non-NDA sites have been identified where thermal treatment may be suitable. This includes some uncertainties, such as accuracy of inventory data (physical, chemical and radiological characteristics and volumes), schedules for waste retrieval and existing site strategies for treatment and packaging, as well as the ability to transport raw wastes to other sites for treatment.

Nevertheless, subsequent trials with active waste samples are ongoing and will provide additional valuable information. 

Overall, the NDA’s research will develop an understanding of the parameters that could feed into decisions on the technology and the business case, looking at operational factors, product performance and disposability. This will improve the readiness of technology to the point that thermal treatment could be considered to be a viable alternative to cement encapsulation for some higher activity wastes.

*Matt Buckley is the NDA’s Higher Activity Waste Strategy Development Manager, responsible for a number of strategic projects that aim to bring innovation and improvement to how the NDA’s higher activity waste is managed.

Deborah Ward, a former journalist, is Corporate Communications Manager at the NDA.

Rick Short, a Research Manager at the NDA, completed a PhD and Postdoctoral Research position at the University of Sheffield’s Immobilisation Science Laboratory.