Plasma torch incineration

Established encapsulation techniques for radioactive waste, such as cementation, can often increase the overall waste volume by a factor of three, but there are alternative treatments available that can actually reduce waste volumes significantly, while producing a passive and stable waste-form suitable for long term geological storage. One such process offering the potential to satisfy all these traits is waste vitrification.

The accurate characterisation of nuclear waste is often extremely difficult and, in many cases, simply not possible. The intrinsic versatility of plasma-based technology to process a wide range of heterogeneous waste-forms under a single standard configuration, coupled with significant waste reduction factors, makes it very attractive to the nuclear sector.

High temperature plasma torches are heat sources that exhibit unique functionality, high power intensity and wide-ranging versatility. They combine the clean heat of an electric arc with the stability, directionality and control of a gas flame. They can be applied within an oxygen-free environment, heating to very high temperatures in the absence of combustion, which offers excellent process control over both chemistry and thermal conditions. It is this unique ability to fine tune the thermal environment that gives plasma technology the versatility to process a wide range of heterogeneous and potentially uncharacterised nuclear waste streams.

The capacity for volume reduction exhibited by this dense waste-form are truly impressive. Waste volume is reduced by at least 50 times that of previously untreated radioactive waste, over ten times that of pre-compacted waste, and by a factor of at least two for previously super-compacted wastes.

Plasma-based thermal processes are already being used readily within the municipal and medical waste treatment industries. However, despite the potential benefits, and the fact that plasma-based processes are founded on a relatively reliable and mature technology, application within the nuclear sector has been limited. The primary hurdle for the nuclear industry has been the development of downstream systems for the environmentally effective and safe clean-up of the process off-gas.

The Plasma Melting Facility process design draws on both the technology and operating experience from other waste treatment plants such as the Centralised Treatment Facility (CILVA) in Belgium and ZWILAG plant in Switzerland. This has been scaled-up, adapted and re-engineered by a consortium led by the Spanish engineer-constructor Iberdrola to meet the higher waste category and throughput requirements of the KNPP. The plant can process 250 tonnes of waste per year, based on a maximum solids input of 65kg/hr for its maximum 4000 hours of operation per year.

The PMF has been primarily designed to receive and process three types of heterogeneous radioactive waste from the Kozloduy site, including untreated waste in bags, along with compacted and super-compacted waste contained within 200 litre steel drums. Additionally, the facility incorporates the ability to reprocess its own secondary waste, as well as any raw liquid waste via direct injection into the plasma melting chamber. Prior to the main plasma melting stage, the incoming drum and bag feed is passed through a two-stage shredder to break both waste and container down into relatively small and uniform media of approximately 8cm.

Figure 1: Plasma melting facility diagrammatic flowsheet

At the heart of the PMF is the Primary Treatment Chamber (PTC), which is a semi-continuous plasma torch furnace with a molten slag capacity of 200 litres. A DC-powered 500kW plasma torch (non-transferred arc), centred at the top of the furnace, is used to rapidly heat the incoming semi-continuous waste stream to 1500°C. Within this harsh thermal environment, a large majority of the organic material within the waste is gasified into smaller volatile hydrocarbons, while the inorganic constituents are vitrified and transformed into a molten glassy slag. Upon cooling, this slag forms a dense glassy monolith that is passive and highly stable on a geological time frame, meeting Bulgarian regulatory conditions of acceptance for storage within a geological nuclear waste repository. It is this transformation of incoming waste into a dense vitrified glassy media that forms the basis of the volume reduction and waste passivation process.

Figure 2: Proposed plant layout

The plasma melting chamber generates two principal product streams: the primary molten glassy slag product and a secondary contaminated hydrocarbon off-gas. The molten slag collects within the base of the PTC and is subsequently processed on a batch-wise basis. As the PTC capacity is reached, a furnace tipping mechanism pours each molten batch into a 200 litre double-layer cooling mould. Once cooled and solidified, the inner mould containing the highly dense and vitrified waste-form is removed and placed into a steel drum suitable for long-term storage. This mould-in-drum encapsulation system exhibits excellent self-shielding properties and ensures that there is no possibility of external contamination on the outer surfaces of the final product drum. The clean drum can then be lidded, sealed and removed from the PMF via an airlock system, ready for direct export to a waste repository. The remaining outer iron cooling mould is circulated around on an internal carousel and recycled back into the process via the automated addition of a fresh inner mould.

Meeting environmental requirements

While the highly controllable thermal environment within a plasma furnace can be manipulated to keep off-gas emissions to a minimum, the clean-up process still remains particularly challenging when treating nuclear media, due to the additional inherent constraints for the removal of radioactive components from the off-gas stream. These treatment systems, which are often larger and more complex than the source process, must be highly reliable and effective. They must capture and remove, as far as is practically possible, the radioactive material and other hazardous contaminants within the off-gas stream, so that all gases released to the atmosphere are within safe and environmentally acceptable levels.

The proposed bespoke off-gas system for the PMF (which applies similar principles to those applied at the CILVA incinerator in Belgium, which has demonstrated high performance and an excellent safety record) consists of six sequential stages. Each stage has a specific role within the treatment process. The system involves a mixture of gas cooling and heating preparatory processes, combined with secondary oxidation, two-phase filtration, gas scrubbing and catalytic reduction stages.

Most of the secondary waste captured within this off-gas system is fed back into the plasma melting stage for reprocessing. An emissions monitoring system, including a cumulative sampling system for assessing radiological output, is used to continuously assess the composition of the off-gas, prior to the final emission of the cleansed off-gasses to the plant stack.

Plasma technology’s great versatility, coupled with the significant financial and environmental benefits offered by the substantial waste reduction factors that can be achieved, amounts to a valuable opportunity for the nuclear sector. This plasma-based melting system clearly offers numerous advantages over conventional waste encapsulation techniques. The capacity to process a wide variety of heterogeneous waste while producing a very stable and passive product, is highly desirable within the decommissioning industry.

Paul Blood, process engineer, Babcock Nuclear Business Unit, 2100 Daresbury Park, Daresbury, Warrington WA4 4HS