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Stabilising and reducing the volume of nuclear waste through plasma vitrification

Stabilising and reducing the volume of nuclear waste through plasma vitrification

Author: Leigh Wakefield, civil nuclear director

Thermal treatment of nuclear waste offers a range of advantages over more traditional compaction, grouting and/or direct disposal by reducing volume and stabilisation of waste materials.

Traditional processes for the treatment of low-level nuclear waste often involve compaction and grouting. This has a number of shortfalls:

  1. It can lead to a net increase in the volume of waste due to the addition of the grout
  2. It isn’t suitable for all waste, in particular those with high organic or reactive contents as the wasteform will not truly be passively stable
  3. The resulting waste can have limited long-term stability due to the degradation of organic and other material and therefore result in the requirement for reprocessing with associated volume increases.

Costain and Valinger (previously Tetronics) have been working together to develop the plasma vitrification process for the last 10 years. The initial concept was developed with funding from the Nuclear Decommissioning Authority and Innovate UK to respond to the call to develop new technologies to reduce the costs of nuclear waste management in the UK. The technology combines two distinct established technologies to deliver an innovative process for treatment of waste.

 

The plasma furnace – a tried and tested process
Plasma furnaces have been used for decades to deliver high intensity localised heating in a controlled manner suitable for precious metals recovery (for example from spent catalytic converters or mining).

 

Plasma vitrification
Plasma furnace schematic, showing electrodes and product melt pool in

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 are employed to 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 produces a robust waste product
Vitrification of nuclear waste is a well-established process, often used for treatment of high-level waste. The waste produced from vitrification is physically and mechanically robust, dense, leaching resistant and therefore has demonstrable long-term stability. Glass forming and stabilising components are added to the furnace which binds to the non-volatile components of the waste, to leave a vitrified product with far higher radionuclide retention efficiencies than would otherwise be possible.


The plasma vitrification process

 

Plasma vitrification process

Waste can be added to the plasma furnace through two routes, depending on the physical characteristics of the waste:

  1. Feed from above This is best suited to mobile wastes such as sludges and ion-exchange resins, with waste added via a pipe or screw feeder. This creates a semi-continuous process with particularly efficient furnace operation and high volume reduction.
  2. Pre-loading the crucible The crucible can be removed from the base of the plasma furnace for disposal 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 converted into a vitrified product, 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. In this scenario the crucible forms part of the ultimate waste package. This creates a batch process which is particularly 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 no voidage in the final product. The only waste that cannot be accommodated are pressure vessels and explosive materials.

Applicability of plasma vitrification

Plasma vitrification can be used for both high alpha and beta-gamma waste, and is particularly suited for treatment of three categories of waste:

  1. Wastes with immobilisation issues such as sludges
  2. Wastes which are reactive during processing
  3. Wastes which remain reactive during storage under traditional processing methods.

 

Safety considerations
Through the development of plasma vitrification process and design, nuclear safety has been the lead consideration. This has resulted in a number of key features supporting 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 operation and maintenance can be undertaken remotely if required.

 

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 ranged from ion-exchange resins to Plutonium Contaminated Material (PCM).

We have 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.

Thermal treatment has a significant part to play in delivering the Nuclear Sector Deal goal of using innovation to drive a 20% reduction in the lifetime cost of nuclear decommissioning.

We are continuing to develop the design to enable installation in an active environment to deliver real change to the processing of nuclear waste and improve 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 further 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.

Costain is supporting the Government’s £12bn Ten Point Plan and the role nuclear will play in realising the UK’s net zero carbon target by helping to evolve technology that will better treat and manage nuclear waste.

Article first published in Nuclear Engineering International November 2020.