Thermal treatment of ILW

The current concept for disposal of higher activity radioactive wastes in England and Wales is a single geological disposal facility for all intermediate level wastes (ILW) and high level wastes (HLW) arising from current and historic nuclear operations. Here, we apply the UK definition of HLW and ILW, for which the activity exceeds the threshold of 4GBq a-activity or 12GBq ß,?-activity per tonne for low level waste (LLW) designation, and for which heat generation is either a definitive consideration for the design of storage or disposal facilities (in the case of HLW), or may be neglected (in the case of ILW). Vitrification of HLW arising from the reprocessing of nuclear fuels, in an alkali borosilicate glass matrix, has been adopted as the most appropriate waste treatment technology by USA, UK, Russia, France and Germany and has been deployed on an industrial scale since at least 1977 [1]. In contrast, the application of vitrification or other high-temperature technologies to ILW is not widespread, although plasma treatment of wastes within the UK ILW envelope has been routinely practiced at the Zwilag facility in Switzerland, which entered operations in 2004 [2]. The Kurion GeoMelt® in situ and in container vitrification system has also been used to treat LLW, ILW and mixed wastes at various locations in the US and Australia since the mid 1990s [3-5].

Reference to the UK Radioactive Waste Inventory 2010 shows that the projected volume of ILW generated by current and historic nuclear fuel cycle operations to be 287,000 m3 (that is, excluding wastes from nuclear new build) [6]. The strategy for treatment of ILW in current Lifecycle Baseline Plans is encapsulation by an ordinary portland cement matrix, combined with super-compaction where appropriate. Since the waste incorporation rates of encapsulation processes are generally low, the estimated final volumes of conditioned and packaged ILW swell to 378,000 m3 and 488,000 m3, respectively; contained within 287,000 individual packages. In contrast to encapsulation processes, thermal treatment technologies offer the potential to reduce conditioned and packaged waste volumes due to the destruction of combustible materials, removal of entrained water, and minimisation of void space. Depending on the waste to be treated and technology platform selected, the conditioned waste volumes may be reduced by a factor of between 2 to 100, compared to the volume of unconditioned waste [2]. Clearly, therefore, thermal treatment of ILW would be expected to translate into considerable financial, environmental, safety, and security benefits, arising from the lower volume and number of waste packages requiring interim storage, transport, and emplacement in a geological disposal facility.

Thermal treatment technologies also offer the opportunity of treating ILW waste streams which are incompatible with cement encapsulation, due to unfavourable reactions between the waste material and the hyperalkaline cement matrix. Such wastes include some reactive metals, organic ion exchange resins, and plutonium-contaminated materials (PCM). The materials produced by thermal treatment processes generally show enhanced passive safety, arising from oxidation of the metallic waste fraction and destruction of organic components, compared to their cement-encapsulated counterparts. Thermal technologies could, in principle, be applied to the treatment of a wide spectrum of generic waste types in the ILW inventory, including (but not limited to): metals (mainly steels, but also zircaloy, aluminium, copper), organics (cellulosics, plastics, and rubber), concrete, cement, sand, sludges, flocs and contaminated soil.

Wasteforms

In general, the objective of thermal processes is to immobilise radionuclides and chemotoxic elements within a host matrix. Vitrification processes involve melting of waste materials and additives to achieve a monolithic glass, glass-ceramic or slag product. A separate metal phase may also be produced in some vitrification processes. Ceramic processing involves reaction between waste and additives (if required) and sintering of the product grains, primarily via solid-state diffusion, to achieve a monolithic polycrystalline product. The waste itself typically forms an integral and functional part of the host matrix produced by thermal treatment.

Potential technology platforms

In general, the key performance requirements for thermal treatment technologies may be stated as:

• Achieve maximum passive safety in the conditioned product by removing the reactivity of problematic waste components
• Achieve maximum volume reduction, with respect to unconditioned waste or other baseline technology, with minimal additives
• Achieve maximum retention of radionuclides and chemotoxic elements in the conditioned product
• Generate minimal secondary wastes
• Meet the product acceptance criteria for geological disposal.

Joule heated ceramic melters

comprise a refractory-lined reaction vessel within which glass forming additives, or frit, are melted with waste material, to produce an essentially homogeneous glass. The melt is sustained by resistive heating applied by passing an electric current between submerged electrodes. Volatilisation of some radionuclides may be reduced by maintaining a layer of cold material above the melt surface, referred to as a "cold cap". In general, the process requires a homogeneous waste feed, in the form of a slurry or calcine; glass-forming additives or frit may be added to the slurry or separately to the melter vessel.

In container or in situ vitrification

is essentially a distinct variant of the Joule heated melter concept, using resistive heating applied by sacrificial electrodes to produce a melt pool from the waste to be treated and suitable glass or ceramic forming additives, if required. The concept utilises a sacrificial refractory (lined) metal container which also forms the packaging for the wasteform and is designed for storage, transport and disposal. A key advantage of the ICV or ISV systems is the ability to operate at high melt temperatures, up to around 1800°C, which allows for greater glass chemistry flexibility. The constraints on waste feed are much less proscriptive, requiring only sufficient size reduction and handling mechanisms to transfer waste to the container. The product wasteform may be a homogeneous or heterogeneous assemblage of glass, ceramic or metallic phases.

Induction melters comprise

a metallic reaction vessel, within which the waste and additives to be processed are melted by induction currents arising from the application of an external radio frequency field. In the cold crucible melter concept, the exterior of the melter vessel may be forcibly cooled to maintain a protective layer of solid material at the interface the melt pool and inner surface of the melter vessel. The use of a protective 'skull', combined with exterior heating applied in the cold crucible concept provides, in principle, greater flexibility over the Joule heated ceramic melter concept, both in allowing more corrosive or refractory wastes to be processed, and the manufacture of glass, glass-ceramic, or ceramic wasteforms.

Plasma melting systems

may be broadly classified as non-transferred or transferred designs, both use the electric discharge between two electrodes (or torches), of opposite bias, to produce a melt pool from waste and additive materials. Non-transferred plasma systems utilise the electric arc between electrodes to ionise and heat a working gas, which, in turn, transfers heat to the melt pool. Transferred plasma systems utilise the melt pool itself as the cathode, and energy is transferred directly to the melt via electric discharge from the anode. A key advantage of plasma melting systems is the potential to employ very high processing temperatures, since the plasma temperature may be sustained in excess of 5000°C. The off gas treatment requirements for plasma systems are typically more demanding, compared to the other technologies discussed here, due to the burden of volatiles and entrained particulates.

Hot isostatic pressing (HIP)

utilises a combination of high temperature (generally, up to 1300°C) and applied pressure (argon gas, 10-200 MPa) to achieve incorporation of radionuclides into a glass or ceramic host phase, by melting or solid-state reaction, with simultaneous exclusion of entrained porosity to achieve near theoretical density of the final packaged product. The reaction vessel is a metal container, designed to undergo controlled deformation under applied pressure, and which forms the primary packaging for the wasteform, subject to any required overpack post-processing. In general, the HIP process requires a homogeneous feed of fine powder and additives; volatiles constituents must be removed prior to the processing cycle.

Application to UK ILW

In recent years, thermal treatment of several generic inactive UK intermediate level waste streams was demonstrated using both laboratory-scale capabilities and pilot-scale facilities [7-18]. An example of successful development from hypothesis, through laboratory proof of concept to pilot scale inactive demonstration was the vitrification of wet ILW arising from a UK reactor site comprising spent ion exchange resins, active effluent treatment plant (AETP) sludge, pond water treatment plant (PWTP) sludge and sand pressure filter (SPF) waste [7-12]. The process treatment requirements specified by the waste producer included:

• A target upper melt temperature of 1200°C; use of minimal glass forming additives; viscosity < 100,000 cP at the melting system operating temperature; and maximised Cs-137 and Ru-106 retention in the waste product (Cs >80%)
• The production of a wasteform with a homogeneous distribution of phases and leach resistance of better than 1 g/m2/d for Cs, Sr and Na at pH = 10.5, T = 50°C over a 28-day test period (relevant to the conditions anticipated in a cementitious GDF, see below).

Compatible glass compositions were identified based on an appraisal of the chemical, physical and radiological inventory of the wastes, combined with a knowledge of vitrification techniques, phase diagrams, and development of a bespoke database of measured and modelled properties of 80 glass compositions. A programme of proof-of-concept research at the laboratory scale demonstrated that the eight candidate glass compositions meet or surpass the specified process and waste product requirements. For dry ion exchange resins (which limited overall waste loading) an incorporation rate exceeding 35 wt% was achieved and, relative to the volume of unconditioned waste, the conditioned waste volume was reduced by 30%; for comparison, cement encapsulation of an equivalent quantity of ion exchange resin was shown to increase the conditioned waste volume by 300%. The results and predictions of laboratory-scale research were used to inform and parameterise pilot-scale inactive demonstrations using Joule heated and plasma vitrification technologies, and the wasteforms produced successfully validated the laboratory proof of concept studies.

Vitrification of a magnox sludge (derived from degraded magnox fuel cladding) and a clinoptilolite/sand ILW was demonstrated using Joule heated ceramic melter technology [13]. Laboratory-scale crucible experiments demonstrated 30-40 wt% incorporation of Magnox waste sludge in glass formulations, with additions of Al2O3, B2O3, Na2O, and SiO2; and 75-80 wt% incorporation of clinoptilolite/sand wastes in glass formulations, with the addition of B2O3, Li2O, and Na2O. Successful validation was achieved using a Joule heated ceramic melter, operating at 1130-1175°C, with a melt surface area of 0.021 m2 and a glass inventory of 8 kg. The waste feed was introduced to the melter as an aqueous slurry using a peristaltic pump. Both off gas and particulate emissions from this demonstration were within the envelope of normal melter operations. The retention of the Cs inventory was 80-90%, whereas only 20-40% of the Re inventory (as a surrogate for Tc) was retained. The vitrified products were shown to pass the toxicity characteristics leaching procedure (TCLP) and product consistency test B (PCT-B) thresholds for reference US waste glasses. The estimated volume reduction factors, relative to the unconditioned waste, were 1.6 for the Magnox sludge and 2.5 for the clinoptilolite/sand waste. It was estimated by the authors that a single Joule heated ceramic melter with a melt surface area of 1.0 m2 could process both waste streams (1200m3 of each waste type) within six years, with reasonable assumptions regarding plant availability and performance.

The thermal treatment of four generic UK ILW waste types was demonstrated using in container vitrification (ICV) technology in two separate pilot-scale trials [14]. The first trial considered co-treatment of a PCM and magnox sludge simulant, using a two-stage process involving the initial processing of the PCM within the container, followed by a predetermined addition of magnox sludge. The product comprised separate slag and metal layers, which was expected given the substantial metal fraction in the waste feed. Mass balance calculations showed 68% of the Cs and 93% of the Re inventories to be immobilised (primarily within the slag and metal fractions, respectively), with the remainder captured in the HEPA filters. Independent study showed the slag material to be highly durable [12], as discussed below. The second trial treated a mixture of simulant Sellafield pile fuel cladding silo and clinoptilolite/sand waste in a single batch treatment process (HEPA filters from the first trial were also included to demonstrate treatment of such secondary wastes). The product again comprised separate glass and metal layers, with 99% of the Cs inventory recovered in the glass (Re could not be analysed). Investigation of product durability was not undertaken and the estimated volume reduction factors were not reported.

More recent testing demonstrated ICV treatment of reactive metals and organics, typical of materials found in a Sellafield ponds solids skip, using a single-stage top-down melt [15]. A waste simulant mixture containing 41 kg of steel, magnesium, aluminium, cerium, lanthanum and various organics was treated, producing a two-phase glass and iron product. The reactive metals were fully oxidized/passivated and post-melt product testing showed 91-99% of tracer metals uniformly distributed within the glass/metal product. Durability testing using the PCT-B method showed that the product performance exceeded US DOE regulatory requirements

The application of plasma melting technology to the treatment of simulant PCM, magnox sludge, and clinoptilolite/sand ILW has been demonstrated with a twin-electrode plasma reactor, utilising sacrificial graphite electrodes and an enclosed cold crucible of copper construction. Plasma vitrification of a simulant PCM waste, using CeO2 as a PuO2 surrogate, was demonstrated at waste loadings of up to 54.1 wt% of PCM simulant with additives of CaO, Al2O3 and SiO2 [16, 17]. The metallic fraction of the waste was almost completely oxidised to yield a slag-type waste form. The precise volume reduction factor was not reported but was suggested to be significant. It was estimated that 98-100% of the CeO2 inventory was retained within the slag product, with Ce partitioned exclusively into the glass phase. The slag products were found to be highly durable, with Si release rates of ~0.03 g/m2/d over a period of 28 days in deionised water at 90°C, using the PCT-B method; no Ce release was detected under these conditions. As expected, the measured Si and Ce release rates of 0.1 and 0.001 g/m2/d were significantly higher in experiments conducted in buffered pH 11 solution (to simulate the hyperalkaline environment of a cementitious GDF) at 90°C, but, nevertheless, point to a highly durable product.

Successful plasma vitrification of clinoptilolite/sand (with unspecified additives) and magnox sludge simulant (with additives of SiO2 and Al2O3) was also reported [18]. This achieved substantial volume reductions of 64% and 59%, respectively, with respect to unconditioned wastes. For comparison, cement encapsulation of Magnox sludge and clinoptilolite/sand ILW was estimated to result in an increase in conditioned waste volume of 318% and 91%, respectively. The product resulting from plasma treatment of Magnox sludge was reported to be a homogeneous glass and subsequent independent study showed this material to be highly durable, as discussed below [12]. An analysis of the comparative costs of treating the broad category of wet intermediate level wastes, which constitutes approximately 20% of the total UK ILW inventory, estimated the total cost of treatment, packaging, storage and transport to a GDF to be GBP 1.3 billion for cement encapsulation and GBP 287 million for plasma treatment. The analysis assumed an average reduction across the wet ILW inventory of 57% (which is considered reasonable, on the basis of the trials referred to above). The capital costs of cement encapsulation and plasma treatment plant construction were determined to be similar and were excluded from the analysis.

Geological disposal of vitrified ILW

One option for disposal of thermally-treated UK ILW would be emplacement with cement-encapsulated ILW within an engineered GDF, using a basic backfill comprised of ordinary Portland cement and portlandite. The purpose of this backfill is to chemically condition the near field to hyperalkaline pH, such that the solubilities of some radionuclides are limited and their sorption enhanced (particularly actinides). Until recently, the behaviour of vitrified waste products in hyperalkaline environments was a key knowledge gap. However, a seminal study of the dissolution behaviour of vitrified ILW glasses in saturated Ca(OH)2 solution (pH ~ 12) for up to 42 days, at temperatures between 30 and 90°C, demonstrated considerably lower glass dissolution rates than expected from extrapolation of data for silicate glasses at lower pH [10,11,16]. The materials studied included a laboratory simulant ILW glass and products from in container vitrification of PCM and magnox sludge wastes and plasma vitrification of magnox sludge and clinoptilolite/sand, as described above. The low dissolution rates of vitrified ILW products under hyperalkaline conditions is considered to be a consequence of the formation of a protective calcium silicate hydrate layer on the altered glass surface. Although evidence to date suggests that the hyperalkaline conditions of a cementitious GDF may not be detrimental to the durability of vitrified wasteforms, the potential impact of Ca depletion from the backfill on its chemical conditioning capacity may be a concern in the event of co-disposal of vitrified and cement ILW. The radiation stability of vitrified UK ILW wasteforms was also recently demonstrated [12].

Outlook

From an international view point, deployment of thermal treatment technologies for the treatment of lower and higher activity wastes is an established, although not widespread, practice. Commonly-cited barriers to implementation of thermal treatment technologies for UK higher activity wastes are listed below.

Complexity of process operation.

Thermal processes may require more complex engineering than a cement encapsulation plant operating at ambient temperature, but this is within current design and operating experience. It could also be argued that the in container and in situ vitrification are less complex compared to cement encapsulation.

Radionuclide volatility requiring expensive and complex off gas treatment.

The retention of H-3, C-14, and I-129 in thermal treatment processes is typically low or negligible, with the possible exception of hot isostatic pressing. The potential volatility of Cs and Tc in vitrification technologies may be mitigated by process conditions, use of a cold cap, and selected additives, as shown by the demonstration scale trials, described above. It is clear that an off gas system will be required but is within current design and operating experience.

Low level of technical maturity.

The technology platforms described here have been successfully deployed to treat radioactive and/or hazardous wastes, in the international context, and have been proven at laboratory, demonstration and pilot scale for the treatment of UK ILW. Thermal treatment facilities for higher-activity wastes are operating or under construction in Switzerland, USA, Russia, Korea and Australia. Thermal treatment facilities for low activity wastes are operated in most countries utilising nuclear power generation [2,19].

Higher overall lifetime waste management costs.

A comprehensive appraisal of overall life time costs of thermal treatment, compared to cement encapsulation, is not yet available, but must include the costs of plant construction, operations, packaging, interim storage, monitoring, transport, and disposal. Due consideration must also be given to the overall environmental impact, including emissions, secondary waste generation, and carbon footprint. However, rudimentary analysis, highlighted above, suggests that substantial overall waste management cost savings may be achieved through deployment of thermal treatment technologies.

It may therefore be appreciated that the available evidence does not support the contention that the aforementioned issues constitute significant barriers to the thermal treatment of UK ILW. It has also been shown that thermal treatment of UK ILW offers the potential to deliver a conditioned wasteform with enhanced passive safety and durability, and an overall reduction in packaged waste volume, compared to the current baseline technology of cement encapsulation. Indeed, thermal processes may also be required to treat a significant fraction of the ILW inventory that is incompatible with the current baseline technology. Furthermore, minimising the volume of packaged ILW through the application of thermal treatment technologies may offer enhanced flexibility in the siting, design and construction of a GDF, if the capacity of suitable host rock were limited by local geological features.

In summary, there is a clear need to develop an estate-wide strategy for the deployment of thermal technologies for the treatment of UK ILW, including definition of acceptance of criteria for geological disposal of waste forms from thermal treatment. This strategy should include a detailed appraisal of the role of thermal treatment technologies in accelerating the delivery of life cycle baseline plans for decommissioning and geological disposal. A transparent and evidence-based appraisal of the whole life cycle costs of thermal treatment technologies, compared to current baseline processes, is also required. Consideration should be given to the feasibility of a national thermal treatment facility, utilising multiple technology platforms to process a wide spectrum of higher activity wastes, which could be expected to minimise costs, as discussed by IAEA [2]. Although the demonstration and commissioning of high-maturity treatment facilities is likely to be a key issue for timely technology insertion, a fundamental scientific understanding of the process and waste product are essential. Consequently, there is a need to nurture the declining and fragmented UK capability in basic science of glass and ceramic science that is required to support technology selection, development and deployment, and deliver the scientific safety case for geological disposal. Since the required knowledge, skills, and capability required to successfully adopt thermal treatment processes is held within site licence companies, government agencies, regulators, the supply chain, and research laboratories, an alliance of these organisations in a special interest group, supported by a 'collaboratory' of research excellence, is proposed to achieve timely estate-wide deployment of thermal treatment technologies for ILW in the UK.

Neil C. Hyatt, Department of Materials Science and Engineering, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK and Mike James, Sellafield Ltd., Sellafield Site, Seascale, Cumbria CA20 1PG, UK. The opinions expressed are those of the authors and do not necessarily represent positions of the host organisations or research sponsors.

References

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