- Email: [email protected]
Muon tomography for the analysis of in-container vitrified products
Applied Radiation and Isotopes 157 (2020) 109033
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: http://www.elsevier.com/locate/apradiso
Muon tomography for the analysis of in-container vitrified products Allan Simpson a, *, Anthony Clarkson b, Simon Gardner b, Ramsey Al Jebali c, Ralf Kaiser b, David Mahon b, Julian Roe a, Matthew Ryan a, Craig Shearer a, Guangliang Yang b a
National Nuclear Laboratory, Central Laboratory, Sellafield, Seascale, Cumbria, CA20 1PG, UK Lynkeos Technology Ltd, University of Glasgow, No. 11 The Square, Glasgow, G12 8QQ, UK c European Spallation Source (ESS), Lund, Sweden b
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermal treatment Muon tomography Analysis Waste management Nuclear waste
Alternate treatment routes for radioactive waste are a key research area for much of the nuclear industry, with potentially significant savings available through volume reduction of waste. Achieving this requires a full and demonstrable understanding of waste product behaviour. For this purpose, the UK’s National Nuclear Laboratory (NNL) has been collaborating with the University of Glasgow and Lynkeos Technology to develop passive techniques for analysis of waste containers over a number of years. In this instance, novel muon tomographic techniques have been applied to the analysis of thermally treated nuclear waste surrogates as part of a project to build and deploy a first of a kind muon imaging system for nuclear waste. The system has been deployed at NNL’s Central Laboratory, Cumbria, UK, to analyse products from a series of thermal treatment technology trials, funded by the Nuclear Decommissioning Authority (NDA) through the Direct Research Portfolio (DRP). Analysis of the waste products using this technique has proven the value of muon analysis in the development of waste management technologies, proving an ability to understand the homogeneity of products and direct further destructive testing. Results from three different thermal treatment trials are presented, with three different surrogate intermediate level waste (ILW) forms in each.
1. Introduction All nuclear operations, be that once through or indeed closed fuel cycles, generate a variety of wastes. The existence of long-lived radio nuclides that have both radiotoxic and chemotoxic effects within these waste streams means that particular care and consideration must be taken when developing appropriate disposal routes. In the UK, this waste is categorised as high-level waste (HLW), intermediate-level waste (ILW), low-level waste (LLW) and very low-level waste (VLLW). Costs attributed to the disposal of radioactive waste are related to volume and classification. For this reason, a research area that is of great interest to the nuclear community is in technologies that minimise the volume of packaged waste, and thus minimise the scale of infrastructure required to safely store and dispose of the waste. Development of any alternative technology, however, requires an ability to prove its capa bility to produce a waste package that will safely contain radionuclides for at least as long as current technologies. All of this requires an array of analytical techniques and an ability to improve understanding of waste package behaviours, to meet appropriate requirements of disposal
facilities, such as Radioactive Waste Management Ltd (RWM) in the UK. Additionally, industrial deployment of thermal treatment would require reliable, non-destructive quality assurance techniques. This paper will discuss a new analytical technique that has been used to support research and development of thermal treatment technologies for the UK’s Nuclear Decommissioning Authority (Nuclear Decom missioning Authority, 2019a). Work by the UK’s National Nuclear Laboratory (NNL), University of Glasgow and recent spin-out Lynkeos Technology Ltd, has used muon scattering tomography to analyse the homogeneity and density distribution of a variety of thermally treated waste forms. 1.1. Veolia GeoMelt in-container vitrification Thermal treatment technologies have been an area of development for the treatment of ILW and HLW over a number of years. Currently, the UK has experience with the vitrification (immobilisation of waste in a glass matrix) of high-activity liquors on the Sellafield site. Deployment of ILW techniques, however, has proven slower. One such process is
* Corresponding author. E-mail address: [email protected] (A. Simpson). https://doi.org/10.1016/j.apradiso.2019.109033 Received 3 June 2019; Received in revised form 13 October 2019; Accepted 27 December 2019 Available online 1 January 2020 0969-8043/© 2020 Elsevier Ltd. All rights reserved.
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Applied Radiation and Isotopes 157 (2020) 109033
GeoMelt®, developed by Battelle for the US Department of Energy in 1980 and applied to in-situ, in-container and in-cell applications since then (World Nuclear News, 2016). To date the GeoMelt® vitrification process has processed over 26,000 MT of waste, predominantly through in-situ vitrification (Veolia Nuclear Solutions, 2019). NNL has been working with Veolia Nuclear Solutions on the devel opment of in-container vitrification (ICV) for ILW over a number of years to test the applicability of thermal treatment to UK waste forms (World Nuclear News, 2016). Using thermal treatment, waste can be immobilised, removed or destroyed. Vitrification results in chemically durable glass wasteform with significant volume reduction. Radionu clides are immobilised in the glass network and are not preferentially leached. Organic material is destroyed by thermal processes, and vola tile radionuclides are removed and captured in the off-gas treatment system. For UK applications, current work is investigating the behaviour of certain ILW waste forms in the process to determine viability of thermally treated waste for disposal in a Geological Disposal Facility. The ICV process is a flexible process and can be designed for a large range of melt sizes (<1 kg to >80 tonnes). It consists of a reusable ICV container, which is filled with a cast refractory box (CRB) that provides the primary containment for melt products (as shown in Fig. 1). Inherent convective mixing during the melt is designed to produce a homoge neous product without the need for pre-treatment. At NNL Central Laboratory, a demonstration system is operated by NNL to develop this process. Three main processes are controlled within the whole system:
to achieve the target temperature and complete melting to the required specification. Total melt time for the system installed at NNL Central Laboratory is usually around 20h, and additional feed is added to the container as the waste volume reduces. The active demonstration system at NNL Central Laboratory has been used to assess the capability of thermal treatment to a range of UK wastes. As the current baseline technology for ILW in the UK is often cementation, thermal treatment is being explored for its potential in volume reduction as an alternative to cemented wasteforms, resulting in reduced costs for long term storage and disposal. 1.2. Lynkeos muon imaging system Cosmic-ray muography for non-destructive analysis has been a field of development since researchers at Los Alamos National Laboratory demonstrated that high-density, high-atomic number materials could be detected from the Coulomb-scattering of muons (Borozdin et al., 2003). Muons are highly penetrating particles which can traverse kilometres of rock, or other dense materials. Recent successes of the technology have included the discovery of a previously unknown additional chamber in the Khufu Pyramid in Egypt (Morishima et al., 2017) and locating fuel within the failed reactors of Fukushima Daichii Nuclear Power Plant in Japan (World Nuclear News, 2017). Another application that has been of interest to the muography community is the characterisation and imaging (as opposed to simply identifying the presence of high-Z materials) of nuclear waste con tainers. These highly engineered structures are intentionally built to shield operators from high energy x-ray and gamma photons, limiting the viability of traditional imaging and characterisation techniques such as high energy x-ray. A long term collaboration between the UK’s Na tional Nuclear Laboratory, University of Glasgow and recent spin-out Lynkeos Technology Ltd has been developing muography techniques for this purpose. Recently, this has resulted in the deployment of a firstof-a-kind Muon Imaging System (MIS) by Lynkeos at NNL’s Central Laboratory on the Sellafield site in Cumbria, UK. The Lynkeos MIS is formed from two detector modules above, and two modules below the imaging volume to enable reconstruction of incoming and Coulomb-scattered muon vectors. Each module is formed of 2048 scintillating fibres, layered orthogonally across four layers and coupled to multi-anode photomultiplier tubes (MAPMTs), resulting in a total of 1,048,576 possible muon interaction points per module across a detection area of approximately 1066 mm � 1066 mm. Resultant signals are fed through discriminators and then logic tests for detection in multiple modules generate a trigger signal. Data is then read out through an FPGA controlled readout system over USB. Further detail of the system construction is provided in (Mahon et al., 2018). Modules are surrounded by black foam PVC to make them lightproof. These are then contained within an aluminium profile frame, including a sample table and heavy-duty gravity roller track to move samples in and out of the active detection volume. To enable straightforward deploy ment on a nuclear licensed site, the Lynkeos MIS was CE marked as an off-the-shelf commercial system during an Innovate UK contract. Extensive optimisation was performed on the system during the con struction phase. The modules where mechanically aligned to a few millimetres precision and final corrections applied in software, mini mising the misalignment to less than half a fibre-width across the detection area. The system was deployed in the active rig hall at NNL’s Central Laboratory, a unique experimental facility within the UK, proving the capability for the technology to be deployed within nuclear regulated facilities. Samples containing kilogram quantities of uranium and other relevant radionuclides can be handled, enabling testing of the system at higher technology readiness levels.
� Power transformation converts utility power into a controllable dual phase output. � An off-gas treatment system is used to treat particulates and volatiles that are driven off in the melt process before discharging and sub sequent treatment via the existing building vent systems. � All of these processes are controlled by SCADA (Supervisory Control and Data Acquisition) process control. During melting, heating due to the electrical resistance of the waste between the graphite electrodes in the CRB is utilised to generate heat within the container. The target temperature is varied by waste type and internal glass temperature is indicated by an IR camera and thermo couples embedded in the melt; manual adjustment of the power is used
Fig. 1. Diagram of the containment structure within the GeoMelt® in-container vitrification process showing a skip like the one used in sample B. 2
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Applied Radiation and Isotopes 157 (2020) 109033
Fig. 2. The Lynkeos MIS installed at the Lynkeos Technology fabrication facility.
2. Methodology
horizontally-drilled cores though the refractory box. X-ray fluorescence spectroscopy (Croudace and Gilligan, 1990), inductively coupled plasma optical emission spectroscopy, or inductively coupled plasma mass spectrometry (Thomas, 2013) are then used to analyse the sample composition. Samples taken from the off-gas system during melting are also subjected to similar techniques and the measured ratios of all products is used to calculate a mass balance based on the input and output fluxes (ie. all input streams and output streams) as shown in Fig. 3. This aids understanding of the inclusion of different materials
Analysis of thermal treatment test melts from the system at NNL Central Laboratory is completed to characterise the performance of the technology with a variety of waste streams. To achieve this, product from the system is sampled and measured in a number of ways (see Fig. 2). Destructive sampling of the glass is undertaken using a core drilling technique developed by NNL. Glass samples are collected from
Fig. 3. Schematic diagram showing inputs and end points/outputs of the ICV process, as a basis for mass balance analysis. 3
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within the final product and thus the viability of thermal treatment for that waste stream. Within this mass balance process, muon tomography is used to un derstand the homogeneity of the thermally treated product and give an understanding of the glass block fluxes as described in Fig. 3. Different materials respond in different ways under thermal treat ment (transfer to the off-gas, immobilisation and thermal destruction) and radionuclides present are incorporated into the glass structure. The goal of waste processing can also vary, particularly for metals. Depen dant on the amount of metal present waste treatment goals will vary from incorporation into the monolith to formation in a separate layer for higher concentrations. Similarly, the degree of melting for metals will vary dependent on the level of activation (surface or full depth) or se curity implications. Some metals are oxidised in the glass (eg., Zn, Cr, Ti, Al, Mg.), and others are not (eg., Fe, Ag) depending on electronegativity and Gibbs free energy and the redox conditions of the melt. Key metallic radionuclides (actinides and Cs-137, Sr-90) preferentially enter the glass, so if they are contained in a steel container, they are “scrubbed” from the steel as it melts to iron. Lynkeos MIS’ usage of highly penetrating muons has been used to understand the internal melt structure and indicate the presence of areas of higher density, such as metal shapes. Non-active melts from a thermal treatment demonstration programme at NNL Central Laboratory were specifically selected based on their likelihood to contain partially degraded material, and thus test the capabilities of the MIS. These selected samples were analysed using the prototype MIS de tector system by Lynkeos at the University of Glasgow. Following this, data was reconstructed and analysed using Lynkeos’ 3D imaging soft ware. Results in agreement with the evidence built up from operational data and post-melt destructive sampling would be a mark of success.
Fig. 5 shows some of the results from this imaging campaign, and an area of higher density within the volume of the thermally treated product can be observed. Lynkeos’ proprietary muon reconstruction software allows a 3D rendering of the muon data, which is shown in Fig. 6 (Mahon et al., 2018). For the thermal treatment demonstration programme, this indicated partial melting of the steel top hat. The 3D rendering of the image shows a region of higher density within the area originally occupied by the steel top hat, indicating that there has not been total melting of the top hat contents within this melt. A Lynkeos developed statistic for the homogeneity of the sample was calculated to be 96% for this sample, indicating consistent melting and integration of the waste streams through the rest of the sample (Mahon et al., 2018). The statistic is defined as the percentage of volume elements that contain a value four-sigma above the average value for sodium boro silicate, in a Gaussian distribution. Muon tomography’s ability to non-destructively assess the degree of melting of large dense items such as steel waste containers is an important step in analysis of the thermally treated items. Results from this process can then be used to define additional analysis on the product which is used to build a complete picture of the product behaviour. This collated information then provides a basis on which to assess the viability of the thermal treatment for that waste stream.
3. Results 3.1. Sample A – steel top-hat Sample A which was imaged using Lynkeos’ MIS was a non-active thermally treated product containing a steel ‘top hat’, partially filled with a mixture of misch metal (a non-active uranium analogue composed of a mix of Ca and La) and dry aged grout cement, as shown in Fig. 4. Steel ‘top hats’ are used for uranic waste storage in the nuclear industry and are formed of a 20 mm-thick stainless steel cylindrical container measuring 300 mm in diameter by 550 mm in height. As a non-active sample, this product was imaged using the Lynkeos prototype imaging system based at the University of Glasgow, of a similar con struction to that described earlier. In total, 21.3 million muon triggers were collected during the imaging campaign over a period of approxi mately 27 days.
Fig. 5. 10 mm horizontal slice taken from Sample A 50 mm above the inner base of the CRB showing a region of higher density. The colour scale represents relative density on a log scale from 1.0 to 1.8 (Mahon et al., 2018).
Fig. 4. Steel top hat containing misch metal and dry aged grout, as filled and then placed in the CRB. 4
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Homogeneity was calculated to be 97% for this sample, indicating consistent melting and integration of the waste streams through most of the sample. It is obvious though from the muon reconstruction that there is an area of higher density near the centre of the block. This takes the shape and form of the original mild steel skip, indicating only partial melting of the steel again. Destructive sampling of the thermally treated product confirmed this when the core drill used was unable to penetrate further than the original location of the skip, supporting the homoge neity value calculated. 3.3. Sample C – miscellaneous decommissioning wasteforms Sample C was constructed of a variety of decommissioning wastes, and the main glass former was clean soil taken from near the Sellafield site in Cumbria, UK (instead of the borosilicate glass used in melts A and B). Items such as concrete, gravel, galvanised steel scaffold poles, hand tools and HEPA (High Efficiency Particulate Air) filter fragments were placed within the crucible volume. Post melt, imaging of this sample on the prototype MIS was carried out with a data collection period of approximately 14 days. Fig. 8 shows two horizontal slices through the block. Whilst there are indicators of small higher density areas and inclusions in the image, overall homogeneity appears good, something that is supported by the calculated homogeneity of 98%. Visual inspection of this sample sup ports this conclusion, with the appearance of mottling and areas of low porosity observed from the top of the crucible. Destructive sampling into the crucible noted samples of consistent texture and porosity at varying heights and depths. The subsequent chemical analysis of these glass samples supports the overall homogeneity of the material, with consis tent composition between samples taken at varying heights and depths within the crucible. The results of the muon tomographic analysis are consistent with the relatively small quantity (5 kg) and greater surface area of the steel in Melt C (5 kg made up of many small galvanised scaffold pieces) than in previous melts (eg. single 20-kg top hat).
Fig. 6. 3D reconstruction of higher density regions of the thermally treated product with CRB removed to show only internal contents. The region of higher density shown in Fig. 5 can be seen to extend vertically within the product here, forming a cylindrical shape (Mahon et al., 2018).
3.2. Sample B – mild steel skip Another non-active melt, sample B is composed of a cubic mild steel skip (an open topped box) as a surrogate for steel skips used in ponds at Sellafield (weighing approximately 11 kg) (Institution of Mechancial Engineers), filled with 28 kg of corroded Magnox sludge (predominantly Mg(OH)2) (Hastings et al., 2007). This is representative of a range of miscellaneous beta/gamma waste streams across the UK nuclear estate, particularly to skips containing sludge generated from the storage of Magnox fuel elements in the First Generation Magnox Storage Pond on Sellafield site (Nuclear Decommissioning Authority, 2019b). Similarly to sample A, this non-active trial was imaged in the Lynkeos prototype detector at the University of Glasgow. Data was collected over a period of approximately 22 days. 2D and 3D reconstructions of the data are shown in Fig. 7.
4. Discussion The objective of the sampling and analysis programme carried out on thermally treated products was to test the capabilities of the MIS against vitrified monolith samples to evaluate whether the findings from this
Fig. 7. 2D and 3D reconstructions of the collected muon data from sample B. The colour scale on the left hand image represents relative density on a log scale from 1.0 to 1.8 (Mahon et al., 2018). 5
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Fig. 8. Horizontal imaging results for sample C. The colour scale represents relative density on a log scale from 1.0 to 1.8 (Mahon et al., 2018).
non-destructive analysis technique correlated with findings from intru sive techniques. Bulk glass homogeneity and the fate of steel waste containers within the glass monolith is a key piece of information needed in order to determine how successful a melt process has been against its processing goals. Whilst homogeneity is not the only indicator of a successful melt, increased homogeneity indicates a greater proportion of the melt ma terial being including within the glass structure. Other indicators of interest include areas of specific high density which may indicate only partial melting or the encapsulation of metals at the base of the crucible. Being able to determine this information with non-destructive analysis offers a potential future melt product evaluation metric and accompanying quality assurance procedure. Whilst the length of data acquisition for muon tomography is extended compared to other tech niques such as high energy X-ray for the purposes described within this paper it is optimal due to the low rate of melts in an R&D setting and the reduced complexity of operating a passive system. The primary sampling methodology used to assess the vitreous monoliths analysed in this study was to core drill through the cast re fractory melter crucible and recover fragments of glass product for subsequent chemical analysis. Glass samples were taken from a diverse range of positions within the monolith with the aim of demonstrating homogeneous distribution of waste materials. Muon imaging was used as the secondary, non-destructive, sampling technique which provided indication of the presence of bulk objects within the monolith. The key benefits of muon tomography were to indicate general homogeneity or heterogeneity of the monolith prior to destructive glass sampling. Two of the case study monoliths contained large bulk items that did not fully melt and remained intact to some degree; one contained small items that were melted and fully incorporated into the melt structure. Where there were partially melted metal shapes they were identified by the MIS successfully and confirmed by sampling. Furthermore, the in formation provided by muon imaging was used to guide core drilling sampling operations to avoid attempting to drill through bulk metallic items, showing the utility of muon tomography to guide destructive sampling. Homogeneity statistics on the melt showed relevant variation in the homogeneity of the product (reduced where bulk materials are present). However, due to the relative volume of the material within the full melt structure (approximately 3% by volume of the melts was metal mono lith) the variation of this statistic is low, as would be expected. Further work to develop a relevant suite of statistics that can more clearly
indicate the presence of bulk objects is now planned to address this. 5. Conclusion Muon tomography has been applied to thermally treated waste simulants to provide analytical information on the ability of it as a nondestructive analysis technique to identify partially melted items in the products. Muon tomography can play an important role in the quality assurance of thermally treated waste forms, demonstrating processing criteria are achieved without compromising the physical integrity of the waste form. Here it was demonstrated that the technique could identify bulk objects within the vitreous monolith product that had not been fully melted and also confirming where items had been fully processed, demonstrating the potential value of the technique to support the development of waste treatment routes. It should be noted of course that the thermal treatment process does not need to achieve full incorpora tion of the waste form to ensure appropriate immobilisation of the waste and incorporation of radionuclides into the glass. Here the results of the muon tomography were both used to direct core drilling to understand the melt performance and for direct comparison with the findings of destructive testing. Funding Funding and support for this work was provided by the UK Nuclear Decommissioning Authority through the Direct Research Portfolio, Sellafield Ltd (UK) and Innovate UK. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to thank Matthew Buckley, Sean Clarke, Mark Dowson, Kevin Finucane, Robert Mills and Mike Moulin-Ramsden for their support in producing this paper.
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Appendix A. Supplementary data
characterization. Phil. Trans. R. Soc. A 377 (2137). https://doi.org/10.1098/ rsta.2018.0048, pp. 20180048. Morishima, K., Kuno, M., Nishio, A., Kitagawa, N., Manabe, Y., Moto, M., Takasaki, F., Fujii, H., Satoh, K., Kodama, H., Hayashi, K., Odaka, S., Procureur, S., Atti� e, D., Bouteille, S., Calvet, D., Filosa, C., Magnier, P., Mandjavidze, I., Riallot, M., Marini, B., Gable, P., Date, Y., Sugiura, M., Elshayeb, Y., Elnady, T., Ezzy, M., Guerriero, E., Steiger, V., Serikoff, N., Mouret, J.-B., Charl�es, B., Helal, H., Tayoubi, M., 2017. “Discovery of a big void in Khufu’s Pyramid by observation of cosmic-ray muons. Nature 552 (7685), 386. https://doi.org/10.1038/nature24647. Nuclear Decommissioning Authority, “Nuclear Decommissioning Authority,” Government Digital Service, [Online]. Available: https://www.gov.uk/government/ organisations/nuclear-decommissioning-authority. [Accessed 12 March 2019]. Nuclear Decommissioning Authority, “UK Radioactive Waste Inventory,” [Online]. Available: https://ukinventory.nda.gov.uk/. [Accessed 05 March 2019]. Thomas, R., 2013. Practical Guide to ICP-MS: a Tutorial for Beginners. CRC Press. Veolia Nuclear Solutions, 23 January 2019. New GeoMelt® Vitrification System Successfully Completes Hot Commissioning and its First Demonstration Melt. Veolia Nuclear Solutions [Online]. Available: https://www.nuclearsolutions.veolia.com /en/news/new-geomelt-vitrification-system-successfully-completes-hot-commissio ning-and-its-first. (Accessed 5 March 2019). World Nuclear News, 25 July 2016. Inauguration of GeoMelt System at Sellafield. World Nuclear News [Online]. Available: http://www.world-nuclear-news.org/Articles/In auguration-of-GeoMelt-system-at-Sellafield. (Accessed 5 March 2019). World Nuclear News, 2017. Muons Suggest Location of Fuel in Unit 3. World Nuclear News [Online]. Available: http://www.world-nuclear-news.org/RS-Muonssuggest-location-of-fuel-in-unit-3-0210174.html. (Accessed 5 March 2019).
Supplementary data to this article can be found online at https://doi. org/10.1016/j.apradiso.2019.109033. References Borozdin, K.N., Hogan, G.E., Morris, C., Priedhorsky, W.C., Saunders, A., Schultz, L.J., Teasdale, M.E., 2003. Radiographic imaging with cosmic-ray muons. Nature 422 (6929), 277. https://doi.org/10.1038/422277a. Croudace, I.W., Gilligan, J.M., 1990. “Versatile and accurate trace element determinations in iron-rich and other geological samples using x-ray fluorescence analysis. X Ray Spectrom. 19 (3), 117–123. https://doi.org/10.1002/ xrs.1300190307. Hastings, J., Rhodes, D., Fellerman, A., McKendrick, D., Dixon, C., 2007. New approaches for sludge management in the nuclear industry. Powder Technol. 174 (1–2), 18–24. https://doi.org/10.1016/j.powtec.2006.10.015. Institution of Mechancial Engineers, 10 October 2011. Work Starts to Clear Sellafield Nuclear Fuel Pond. Institution of Mechancial Engineers [Online]. Available: htt p://www.imeche.org/news/news-article/work-starts-to-clear-sellafield-nuclearfuel-pond. (Accessed 5 March 2019). Mahon, D., Clarkson, A., Gardner, S., Ireland, D., Jebali, R., Kaiser, R., Ryan, M., Shearer, C., Yang, G., 2018. First-of-a-kind muography for nuclear waste
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Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: http://www.elsevier.com/locate/apradiso
Muon tomography for the analysis of in-container vitrified products Allan Simpson a, *, Anthony Clarkson b, Simon Gardner b, Ramsey Al Jebali c, Ralf Kaiser b, David Mahon b, Julian Roe a, Matthew Ryan a, Craig Shearer a, Guangliang Yang b a
National Nuclear Laboratory, Central Laboratory, Sellafield, Seascale, Cumbria, CA20 1PG, UK Lynkeos Technology Ltd, University of Glasgow, No. 11 The Square, Glasgow, G12 8QQ, UK c European Spallation Source (ESS), Lund, Sweden b
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermal treatment Muon tomography Analysis Waste management Nuclear waste
Alternate treatment routes for radioactive waste are a key research area for much of the nuclear industry, with potentially significant savings available through volume reduction of waste. Achieving this requires a full and demonstrable understanding of waste product behaviour. For this purpose, the UK’s National Nuclear Laboratory (NNL) has been collaborating with the University of Glasgow and Lynkeos Technology to develop passive techniques for analysis of waste containers over a number of years. In this instance, novel muon tomographic techniques have been applied to the analysis of thermally treated nuclear waste surrogates as part of a project to build and deploy a first of a kind muon imaging system for nuclear waste. The system has been deployed at NNL’s Central Laboratory, Cumbria, UK, to analyse products from a series of thermal treatment technology trials, funded by the Nuclear Decommissioning Authority (NDA) through the Direct Research Portfolio (DRP). Analysis of the waste products using this technique has proven the value of muon analysis in the development of waste management technologies, proving an ability to understand the homogeneity of products and direct further destructive testing. Results from three different thermal treatment trials are presented, with three different surrogate intermediate level waste (ILW) forms in each.
1. Introduction All nuclear operations, be that once through or indeed closed fuel cycles, generate a variety of wastes. The existence of long-lived radio nuclides that have both radiotoxic and chemotoxic effects within these waste streams means that particular care and consideration must be taken when developing appropriate disposal routes. In the UK, this waste is categorised as high-level waste (HLW), intermediate-level waste (ILW), low-level waste (LLW) and very low-level waste (VLLW). Costs attributed to the disposal of radioactive waste are related to volume and classification. For this reason, a research area that is of great interest to the nuclear community is in technologies that minimise the volume of packaged waste, and thus minimise the scale of infrastructure required to safely store and dispose of the waste. Development of any alternative technology, however, requires an ability to prove its capa bility to produce a waste package that will safely contain radionuclides for at least as long as current technologies. All of this requires an array of analytical techniques and an ability to improve understanding of waste package behaviours, to meet appropriate requirements of disposal
facilities, such as Radioactive Waste Management Ltd (RWM) in the UK. Additionally, industrial deployment of thermal treatment would require reliable, non-destructive quality assurance techniques. This paper will discuss a new analytical technique that has been used to support research and development of thermal treatment technologies for the UK’s Nuclear Decommissioning Authority (Nuclear Decom missioning Authority, 2019a). Work by the UK’s National Nuclear Laboratory (NNL), University of Glasgow and recent spin-out Lynkeos Technology Ltd, has used muon scattering tomography to analyse the homogeneity and density distribution of a variety of thermally treated waste forms. 1.1. Veolia GeoMelt in-container vitrification Thermal treatment technologies have been an area of development for the treatment of ILW and HLW over a number of years. Currently, the UK has experience with the vitrification (immobilisation of waste in a glass matrix) of high-activity liquors on the Sellafield site. Deployment of ILW techniques, however, has proven slower. One such process is
* Corresponding author. E-mail address: [email protected] (A. Simpson). https://doi.org/10.1016/j.apradiso.2019.109033 Received 3 June 2019; Received in revised form 13 October 2019; Accepted 27 December 2019 Available online 1 January 2020 0969-8043/© 2020 Elsevier Ltd. All rights reserved.
A. Simpson et al.
Applied Radiation and Isotopes 157 (2020) 109033
GeoMelt®, developed by Battelle for the US Department of Energy in 1980 and applied to in-situ, in-container and in-cell applications since then (World Nuclear News, 2016). To date the GeoMelt® vitrification process has processed over 26,000 MT of waste, predominantly through in-situ vitrification (Veolia Nuclear Solutions, 2019). NNL has been working with Veolia Nuclear Solutions on the devel opment of in-container vitrification (ICV) for ILW over a number of years to test the applicability of thermal treatment to UK waste forms (World Nuclear News, 2016). Using thermal treatment, waste can be immobilised, removed or destroyed. Vitrification results in chemically durable glass wasteform with significant volume reduction. Radionu clides are immobilised in the glass network and are not preferentially leached. Organic material is destroyed by thermal processes, and vola tile radionuclides are removed and captured in the off-gas treatment system. For UK applications, current work is investigating the behaviour of certain ILW waste forms in the process to determine viability of thermally treated waste for disposal in a Geological Disposal Facility. The ICV process is a flexible process and can be designed for a large range of melt sizes (<1 kg to >80 tonnes). It consists of a reusable ICV container, which is filled with a cast refractory box (CRB) that provides the primary containment for melt products (as shown in Fig. 1). Inherent convective mixing during the melt is designed to produce a homoge neous product without the need for pre-treatment. At NNL Central Laboratory, a demonstration system is operated by NNL to develop this process. Three main processes are controlled within the whole system:
to achieve the target temperature and complete melting to the required specification. Total melt time for the system installed at NNL Central Laboratory is usually around 20h, and additional feed is added to the container as the waste volume reduces. The active demonstration system at NNL Central Laboratory has been used to assess the capability of thermal treatment to a range of UK wastes. As the current baseline technology for ILW in the UK is often cementation, thermal treatment is being explored for its potential in volume reduction as an alternative to cemented wasteforms, resulting in reduced costs for long term storage and disposal. 1.2. Lynkeos muon imaging system Cosmic-ray muography for non-destructive analysis has been a field of development since researchers at Los Alamos National Laboratory demonstrated that high-density, high-atomic number materials could be detected from the Coulomb-scattering of muons (Borozdin et al., 2003). Muons are highly penetrating particles which can traverse kilometres of rock, or other dense materials. Recent successes of the technology have included the discovery of a previously unknown additional chamber in the Khufu Pyramid in Egypt (Morishima et al., 2017) and locating fuel within the failed reactors of Fukushima Daichii Nuclear Power Plant in Japan (World Nuclear News, 2017). Another application that has been of interest to the muography community is the characterisation and imaging (as opposed to simply identifying the presence of high-Z materials) of nuclear waste con tainers. These highly engineered structures are intentionally built to shield operators from high energy x-ray and gamma photons, limiting the viability of traditional imaging and characterisation techniques such as high energy x-ray. A long term collaboration between the UK’s Na tional Nuclear Laboratory, University of Glasgow and recent spin-out Lynkeos Technology Ltd has been developing muography techniques for this purpose. Recently, this has resulted in the deployment of a firstof-a-kind Muon Imaging System (MIS) by Lynkeos at NNL’s Central Laboratory on the Sellafield site in Cumbria, UK. The Lynkeos MIS is formed from two detector modules above, and two modules below the imaging volume to enable reconstruction of incoming and Coulomb-scattered muon vectors. Each module is formed of 2048 scintillating fibres, layered orthogonally across four layers and coupled to multi-anode photomultiplier tubes (MAPMTs), resulting in a total of 1,048,576 possible muon interaction points per module across a detection area of approximately 1066 mm � 1066 mm. Resultant signals are fed through discriminators and then logic tests for detection in multiple modules generate a trigger signal. Data is then read out through an FPGA controlled readout system over USB. Further detail of the system construction is provided in (Mahon et al., 2018). Modules are surrounded by black foam PVC to make them lightproof. These are then contained within an aluminium profile frame, including a sample table and heavy-duty gravity roller track to move samples in and out of the active detection volume. To enable straightforward deploy ment on a nuclear licensed site, the Lynkeos MIS was CE marked as an off-the-shelf commercial system during an Innovate UK contract. Extensive optimisation was performed on the system during the con struction phase. The modules where mechanically aligned to a few millimetres precision and final corrections applied in software, mini mising the misalignment to less than half a fibre-width across the detection area. The system was deployed in the active rig hall at NNL’s Central Laboratory, a unique experimental facility within the UK, proving the capability for the technology to be deployed within nuclear regulated facilities. Samples containing kilogram quantities of uranium and other relevant radionuclides can be handled, enabling testing of the system at higher technology readiness levels.
� Power transformation converts utility power into a controllable dual phase output. � An off-gas treatment system is used to treat particulates and volatiles that are driven off in the melt process before discharging and sub sequent treatment via the existing building vent systems. � All of these processes are controlled by SCADA (Supervisory Control and Data Acquisition) process control. During melting, heating due to the electrical resistance of the waste between the graphite electrodes in the CRB is utilised to generate heat within the container. The target temperature is varied by waste type and internal glass temperature is indicated by an IR camera and thermo couples embedded in the melt; manual adjustment of the power is used
Fig. 1. Diagram of the containment structure within the GeoMelt® in-container vitrification process showing a skip like the one used in sample B. 2
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Fig. 2. The Lynkeos MIS installed at the Lynkeos Technology fabrication facility.
2. Methodology
horizontally-drilled cores though the refractory box. X-ray fluorescence spectroscopy (Croudace and Gilligan, 1990), inductively coupled plasma optical emission spectroscopy, or inductively coupled plasma mass spectrometry (Thomas, 2013) are then used to analyse the sample composition. Samples taken from the off-gas system during melting are also subjected to similar techniques and the measured ratios of all products is used to calculate a mass balance based on the input and output fluxes (ie. all input streams and output streams) as shown in Fig. 3. This aids understanding of the inclusion of different materials
Analysis of thermal treatment test melts from the system at NNL Central Laboratory is completed to characterise the performance of the technology with a variety of waste streams. To achieve this, product from the system is sampled and measured in a number of ways (see Fig. 2). Destructive sampling of the glass is undertaken using a core drilling technique developed by NNL. Glass samples are collected from
Fig. 3. Schematic diagram showing inputs and end points/outputs of the ICV process, as a basis for mass balance analysis. 3
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within the final product and thus the viability of thermal treatment for that waste stream. Within this mass balance process, muon tomography is used to un derstand the homogeneity of the thermally treated product and give an understanding of the glass block fluxes as described in Fig. 3. Different materials respond in different ways under thermal treat ment (transfer to the off-gas, immobilisation and thermal destruction) and radionuclides present are incorporated into the glass structure. The goal of waste processing can also vary, particularly for metals. Depen dant on the amount of metal present waste treatment goals will vary from incorporation into the monolith to formation in a separate layer for higher concentrations. Similarly, the degree of melting for metals will vary dependent on the level of activation (surface or full depth) or se curity implications. Some metals are oxidised in the glass (eg., Zn, Cr, Ti, Al, Mg.), and others are not (eg., Fe, Ag) depending on electronegativity and Gibbs free energy and the redox conditions of the melt. Key metallic radionuclides (actinides and Cs-137, Sr-90) preferentially enter the glass, so if they are contained in a steel container, they are “scrubbed” from the steel as it melts to iron. Lynkeos MIS’ usage of highly penetrating muons has been used to understand the internal melt structure and indicate the presence of areas of higher density, such as metal shapes. Non-active melts from a thermal treatment demonstration programme at NNL Central Laboratory were specifically selected based on their likelihood to contain partially degraded material, and thus test the capabilities of the MIS. These selected samples were analysed using the prototype MIS de tector system by Lynkeos at the University of Glasgow. Following this, data was reconstructed and analysed using Lynkeos’ 3D imaging soft ware. Results in agreement with the evidence built up from operational data and post-melt destructive sampling would be a mark of success.
Fig. 5 shows some of the results from this imaging campaign, and an area of higher density within the volume of the thermally treated product can be observed. Lynkeos’ proprietary muon reconstruction software allows a 3D rendering of the muon data, which is shown in Fig. 6 (Mahon et al., 2018). For the thermal treatment demonstration programme, this indicated partial melting of the steel top hat. The 3D rendering of the image shows a region of higher density within the area originally occupied by the steel top hat, indicating that there has not been total melting of the top hat contents within this melt. A Lynkeos developed statistic for the homogeneity of the sample was calculated to be 96% for this sample, indicating consistent melting and integration of the waste streams through the rest of the sample (Mahon et al., 2018). The statistic is defined as the percentage of volume elements that contain a value four-sigma above the average value for sodium boro silicate, in a Gaussian distribution. Muon tomography’s ability to non-destructively assess the degree of melting of large dense items such as steel waste containers is an important step in analysis of the thermally treated items. Results from this process can then be used to define additional analysis on the product which is used to build a complete picture of the product behaviour. This collated information then provides a basis on which to assess the viability of the thermal treatment for that waste stream.
3. Results 3.1. Sample A – steel top-hat Sample A which was imaged using Lynkeos’ MIS was a non-active thermally treated product containing a steel ‘top hat’, partially filled with a mixture of misch metal (a non-active uranium analogue composed of a mix of Ca and La) and dry aged grout cement, as shown in Fig. 4. Steel ‘top hats’ are used for uranic waste storage in the nuclear industry and are formed of a 20 mm-thick stainless steel cylindrical container measuring 300 mm in diameter by 550 mm in height. As a non-active sample, this product was imaged using the Lynkeos prototype imaging system based at the University of Glasgow, of a similar con struction to that described earlier. In total, 21.3 million muon triggers were collected during the imaging campaign over a period of approxi mately 27 days.
Fig. 5. 10 mm horizontal slice taken from Sample A 50 mm above the inner base of the CRB showing a region of higher density. The colour scale represents relative density on a log scale from 1.0 to 1.8 (Mahon et al., 2018).
Fig. 4. Steel top hat containing misch metal and dry aged grout, as filled and then placed in the CRB. 4
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Homogeneity was calculated to be 97% for this sample, indicating consistent melting and integration of the waste streams through most of the sample. It is obvious though from the muon reconstruction that there is an area of higher density near the centre of the block. This takes the shape and form of the original mild steel skip, indicating only partial melting of the steel again. Destructive sampling of the thermally treated product confirmed this when the core drill used was unable to penetrate further than the original location of the skip, supporting the homoge neity value calculated. 3.3. Sample C – miscellaneous decommissioning wasteforms Sample C was constructed of a variety of decommissioning wastes, and the main glass former was clean soil taken from near the Sellafield site in Cumbria, UK (instead of the borosilicate glass used in melts A and B). Items such as concrete, gravel, galvanised steel scaffold poles, hand tools and HEPA (High Efficiency Particulate Air) filter fragments were placed within the crucible volume. Post melt, imaging of this sample on the prototype MIS was carried out with a data collection period of approximately 14 days. Fig. 8 shows two horizontal slices through the block. Whilst there are indicators of small higher density areas and inclusions in the image, overall homogeneity appears good, something that is supported by the calculated homogeneity of 98%. Visual inspection of this sample sup ports this conclusion, with the appearance of mottling and areas of low porosity observed from the top of the crucible. Destructive sampling into the crucible noted samples of consistent texture and porosity at varying heights and depths. The subsequent chemical analysis of these glass samples supports the overall homogeneity of the material, with consis tent composition between samples taken at varying heights and depths within the crucible. The results of the muon tomographic analysis are consistent with the relatively small quantity (5 kg) and greater surface area of the steel in Melt C (5 kg made up of many small galvanised scaffold pieces) than in previous melts (eg. single 20-kg top hat).
Fig. 6. 3D reconstruction of higher density regions of the thermally treated product with CRB removed to show only internal contents. The region of higher density shown in Fig. 5 can be seen to extend vertically within the product here, forming a cylindrical shape (Mahon et al., 2018).
3.2. Sample B – mild steel skip Another non-active melt, sample B is composed of a cubic mild steel skip (an open topped box) as a surrogate for steel skips used in ponds at Sellafield (weighing approximately 11 kg) (Institution of Mechancial Engineers), filled with 28 kg of corroded Magnox sludge (predominantly Mg(OH)2) (Hastings et al., 2007). This is representative of a range of miscellaneous beta/gamma waste streams across the UK nuclear estate, particularly to skips containing sludge generated from the storage of Magnox fuel elements in the First Generation Magnox Storage Pond on Sellafield site (Nuclear Decommissioning Authority, 2019b). Similarly to sample A, this non-active trial was imaged in the Lynkeos prototype detector at the University of Glasgow. Data was collected over a period of approximately 22 days. 2D and 3D reconstructions of the data are shown in Fig. 7.
4. Discussion The objective of the sampling and analysis programme carried out on thermally treated products was to test the capabilities of the MIS against vitrified monolith samples to evaluate whether the findings from this
Fig. 7. 2D and 3D reconstructions of the collected muon data from sample B. The colour scale on the left hand image represents relative density on a log scale from 1.0 to 1.8 (Mahon et al., 2018). 5
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Fig. 8. Horizontal imaging results for sample C. The colour scale represents relative density on a log scale from 1.0 to 1.8 (Mahon et al., 2018).
non-destructive analysis technique correlated with findings from intru sive techniques. Bulk glass homogeneity and the fate of steel waste containers within the glass monolith is a key piece of information needed in order to determine how successful a melt process has been against its processing goals. Whilst homogeneity is not the only indicator of a successful melt, increased homogeneity indicates a greater proportion of the melt ma terial being including within the glass structure. Other indicators of interest include areas of specific high density which may indicate only partial melting or the encapsulation of metals at the base of the crucible. Being able to determine this information with non-destructive analysis offers a potential future melt product evaluation metric and accompanying quality assurance procedure. Whilst the length of data acquisition for muon tomography is extended compared to other tech niques such as high energy X-ray for the purposes described within this paper it is optimal due to the low rate of melts in an R&D setting and the reduced complexity of operating a passive system. The primary sampling methodology used to assess the vitreous monoliths analysed in this study was to core drill through the cast re fractory melter crucible and recover fragments of glass product for subsequent chemical analysis. Glass samples were taken from a diverse range of positions within the monolith with the aim of demonstrating homogeneous distribution of waste materials. Muon imaging was used as the secondary, non-destructive, sampling technique which provided indication of the presence of bulk objects within the monolith. The key benefits of muon tomography were to indicate general homogeneity or heterogeneity of the monolith prior to destructive glass sampling. Two of the case study monoliths contained large bulk items that did not fully melt and remained intact to some degree; one contained small items that were melted and fully incorporated into the melt structure. Where there were partially melted metal shapes they were identified by the MIS successfully and confirmed by sampling. Furthermore, the in formation provided by muon imaging was used to guide core drilling sampling operations to avoid attempting to drill through bulk metallic items, showing the utility of muon tomography to guide destructive sampling. Homogeneity statistics on the melt showed relevant variation in the homogeneity of the product (reduced where bulk materials are present). However, due to the relative volume of the material within the full melt structure (approximately 3% by volume of the melts was metal mono lith) the variation of this statistic is low, as would be expected. Further work to develop a relevant suite of statistics that can more clearly
indicate the presence of bulk objects is now planned to address this. 5. Conclusion Muon tomography has been applied to thermally treated waste simulants to provide analytical information on the ability of it as a nondestructive analysis technique to identify partially melted items in the products. Muon tomography can play an important role in the quality assurance of thermally treated waste forms, demonstrating processing criteria are achieved without compromising the physical integrity of the waste form. Here it was demonstrated that the technique could identify bulk objects within the vitreous monolith product that had not been fully melted and also confirming where items had been fully processed, demonstrating the potential value of the technique to support the development of waste treatment routes. It should be noted of course that the thermal treatment process does not need to achieve full incorpora tion of the waste form to ensure appropriate immobilisation of the waste and incorporation of radionuclides into the glass. Here the results of the muon tomography were both used to direct core drilling to understand the melt performance and for direct comparison with the findings of destructive testing. Funding Funding and support for this work was provided by the UK Nuclear Decommissioning Authority through the Direct Research Portfolio, Sellafield Ltd (UK) and Innovate UK. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to thank Matthew Buckley, Sean Clarke, Mark Dowson, Kevin Finucane, Robert Mills and Mike Moulin-Ramsden for their support in producing this paper.
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Appendix A. Supplementary data
characterization. Phil. Trans. R. Soc. A 377 (2137). https://doi.org/10.1098/ rsta.2018.0048, pp. 20180048. Morishima, K., Kuno, M., Nishio, A., Kitagawa, N., Manabe, Y., Moto, M., Takasaki, F., Fujii, H., Satoh, K., Kodama, H., Hayashi, K., Odaka, S., Procureur, S., Atti� e, D., Bouteille, S., Calvet, D., Filosa, C., Magnier, P., Mandjavidze, I., Riallot, M., Marini, B., Gable, P., Date, Y., Sugiura, M., Elshayeb, Y., Elnady, T., Ezzy, M., Guerriero, E., Steiger, V., Serikoff, N., Mouret, J.-B., Charl�es, B., Helal, H., Tayoubi, M., 2017. “Discovery of a big void in Khufu’s Pyramid by observation of cosmic-ray muons. Nature 552 (7685), 386. https://doi.org/10.1038/nature24647. Nuclear Decommissioning Authority, “Nuclear Decommissioning Authority,” Government Digital Service, [Online]. Available: https://www.gov.uk/government/ organisations/nuclear-decommissioning-authority. [Accessed 12 March 2019]. Nuclear Decommissioning Authority, “UK Radioactive Waste Inventory,” [Online]. Available: https://ukinventory.nda.gov.uk/. [Accessed 05 March 2019]. Thomas, R., 2013. Practical Guide to ICP-MS: a Tutorial for Beginners. CRC Press. Veolia Nuclear Solutions, 23 January 2019. New GeoMelt® Vitrification System Successfully Completes Hot Commissioning and its First Demonstration Melt. Veolia Nuclear Solutions [Online]. Available: https://www.nuclearsolutions.veolia.com /en/news/new-geomelt-vitrification-system-successfully-completes-hot-commissio ning-and-its-first. (Accessed 5 March 2019). World Nuclear News, 25 July 2016. Inauguration of GeoMelt System at Sellafield. World Nuclear News [Online]. Available: http://www.world-nuclear-news.org/Articles/In auguration-of-GeoMelt-system-at-Sellafield. (Accessed 5 March 2019). World Nuclear News, 2017. Muons Suggest Location of Fuel in Unit 3. World Nuclear News [Online]. Available: http://www.world-nuclear-news.org/RS-Muonssuggest-location-of-fuel-in-unit-3-0210174.html. (Accessed 5 March 2019).
Supplementary data to this article can be found online at https://doi. org/10.1016/j.apradiso.2019.109033. References Borozdin, K.N., Hogan, G.E., Morris, C., Priedhorsky, W.C., Saunders, A., Schultz, L.J., Teasdale, M.E., 2003. Radiographic imaging with cosmic-ray muons. Nature 422 (6929), 277. https://doi.org/10.1038/422277a. Croudace, I.W., Gilligan, J.M., 1990. “Versatile and accurate trace element determinations in iron-rich and other geological samples using x-ray fluorescence analysis. X Ray Spectrom. 19 (3), 117–123. https://doi.org/10.1002/ xrs.1300190307. Hastings, J., Rhodes, D., Fellerman, A., McKendrick, D., Dixon, C., 2007. New approaches for sludge management in the nuclear industry. Powder Technol. 174 (1–2), 18–24. https://doi.org/10.1016/j.powtec.2006.10.015. Institution of Mechancial Engineers, 10 October 2011. Work Starts to Clear Sellafield Nuclear Fuel Pond. Institution of Mechancial Engineers [Online]. Available: htt p://www.imeche.org/news/news-article/work-starts-to-clear-sellafield-nuclearfuel-pond. (Accessed 5 March 2019). Mahon, D., Clarkson, A., Gardner, S., Ireland, D., Jebali, R., Kaiser, R., Ryan, M., Shearer, C., Yang, G., 2018. First-of-a-kind muography for nuclear waste
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