Demonstrating plasma vitrification

TRADITIONAL PROCESSES FOR TREATMENT OF intermediate and low-level nuclear waste often involve compaction and grouting. This has a number of disadvantages. It is not suitable for waste with high organic or reactive content, as the wasteform will not truly be passively stable. The resulting waste can have limited long-term stability as organic or other material degrades, so reprocessing may be required, and adding grout can lead to a net increase in waste volume.

Costain and Valinger (previously Tetronics) have been working together to develop a plasma vitrification process for the last 10 years. The initial concept was developed with funding from the UK’s Nuclear Decommissioning Authority and Innovate UK in response to a call for new technologies to reduce the costs of nuclear waste management in the UK.

The technology combines two distinct established technologies.

Plasma furnaces have been used for decades to deliver controlled, high-intensity localised heating suitable for precious metals recovery (for example from spent catalytic converters or mining). The plasma furnace delivers a high temperature and high power density working environment at the centre of the furnace with a high degree of control. An arc is generated between consumable graphite electrodes to avoid intrusive maintenance, and radiated heat creates the melt pool. The outside of the plasma furnace is cooled to ensure containment and thermal management mechanisms minimise degradation of the refractory lining. This provides a highly controllable heat source with an optimised melt pool temperature to limit the loss of semi-volatile radionuclides whilst ensuring full destruction or conversion of the organic, metallic and inorganic material.

Vitrification is a well-established process, often used to treat high-level waste. The vitrified waste is physically and mechanically robust, dense and leach-resistant so it has demonstrable long-term stability. Glass forming and stabilising components are added to the furnace, which bind to the non-volatile components of the waste, at the desired working temperatures, to result in a vitrified product with far higher radionuclide retention efficiencies than would otherwise be possible.

The plasma vitrification process

Waste can be fed to the plasma furnace from above, or pre-loaded into the crucible, depending on the physical characteristics of the waste.

Feeding from above, via a pipe or screw feeder, is best suited to mobile wastes such as sludges and ion-exchange resins. This semi-continuous process offers efficient furnace operation and high volume reduction.

If pre-loaded, the crucible can be removed from the base of the plasma furnace to dispose of the waste product.

When operated in this mode, large containers or items of waste are loaded into the empty crucible and the crucible sealed against the plasma furnace. Once the waste has been vitrified, it is allowed to cool and then removed. At this point, additional waste can be added to the part-filled crucible for additional processing to improve storage volume utilisation efficiency. The crucible forms part of the waste package. This is a batch process, which is useful for processing drums of waste, such as 200 litre drums of plutonium contaminated material, and large miscellaneous items. Thin-walled vessels are readily melted to ensure there are no voids in the final product. The only waste that cannot be accommodated is pressure vessels and explosive materials.

Other process additives

Glass formers can be added via screw feeder into the top of the plasma furnace to ensure the product has the optimum balance between a robust vitreous wasteform and maximum volume reduction and radionuclide retention.

Oxidant (water or air depending on the nature of the waste) may also be required for some waste forms to optimise reaction conditions within the furnace.

Applicability of plasma vitrification

Plasma vitrification can be used for high alpha and beta-gamma waste. It is particularly suited for treatment of wastes that have immobilisation issues (sludges, containers, particles, solvents, oils and greases), are reactive during processing or remain reactive during storage under traditional processing methods.

Safety considerations

Safety has been the lead consideration through the development of plasma vitrification process and design. To support nuclear safety:

• The volume of the reactor is limited, to protect against criticality and exothermic conditions
• There are minimal furnace vessel penetrations, to limit the risk of leakage from seals
• There are no moving parts required near to the furnace
• All aspects of O&M can be undertaken remotely.

Demonstration project results so far

In collaboration with major UK nuclear operators, we have undertaken trials on a range of simulated waste materials, often using surrogates to mimic the behaviours of radionuclides. Wastes included ion-exchange resins and plutonium contaminated material. We found that volume reduction of the waste is heavily dependent on the nature of the waste stream.

In trials undertaken over the last seven years, a stable product was formed with high radionuclide retention rates (typically 95% Ce and Cs, see also Table 1).

Thermal treatment has a significant part to play in delivering one of the goals of the UK’s so-called Nuclear Sector Deal (a government-industry collaboration) — a 20% reduction in the lifetime cost of nuclear decommissioning.

Plasma vitrification is a cost-efficient process route for many problematic waste streams as well as a potential alternative to the current established baseline waste treatment processes. We are continuing to develop the design, to enable installation in an active environment, deliver real change to the processing of nuclear waste and improve the safety and stability of the end product to minimise risk for future generations.

Development of the crucible and disposal package design is also being explored to optimise volume reductions.

The plasma vitrification process has been shown to be flexible to enabling processing of a range of variable liquid and solid waste streams. Reliable, low complexity operation is achieved, producing a low volume, stable, disposable product with low leachability.

Discover more about the process and how to dispose of nuclear waste with plasma technology https://youtu.be/ qDhtEnTZghk

Author information: Leigh Wakefield, Nuclear director at Costain