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Methodology to identify appropriate options to manage tritiated waste
Fusion Engineering and Design xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Methodology to identify appropriate options to manage tritiated waste ⁎
Christelle Decanis , Michal Kresina, Daniel Canas CEA DEN, 13108 Cadarache, France
A R T I C L E I N F O
A B S T R A C T
Keywords: Waste management Tritiated waste Comparative analyses BAT Waste management tool
Fusion facilities will use tritium as a part of fuel. As a consequence, most of waste is expected to be contaminated with tritium and specific waste management strategies need to be developed taking into account physical and chemical properties of tritium. In France, the reference strategy for waste exceeding acceptance limits for disposal due to its tritium inventory is an interim storage period up to 50 years, corresponding approximately to detritiation factor of 16 due to radioactive decay. This article is mainly focused on the most tritiated categories of waste that would require an interim storage period longer than 50 years. Alternative strategies or additional processing options have been sought for these categories and the article defines methodology of identification of the Best Available Technique (BAT) for reduction of tritium inventory in the waste. Furthermore, the methodology is demonstrated on the case of purely tritiated metallic waste made from stainless steel. The methodology has been divided into three main steps: 1) Identification of the most relevant detritiation techniques; 2) Comparative analysis; and 3) Economical assessment. In order to simplify the first step, a software tool has been developed by CEA and its main features are described in this article. The results given by this tool are then analysed in the second step, which takes into account several non-economic criteria. Results of the comparative analysis show that detritiation of purely tritiated metallic waste made from stainless steel appears to be very attractive, allowing a significant reduction of interim storage duration before disposal in a surface repository. The last step, briefly described in this article, is ongoing and will economically assess the results from the previous steps.
1. Introduction
2. Tritium behaviour
Radioactive waste will be generated during operation and decommissioning of fusion facilities. This waste will be either contaminated by tritium, or activated by neutrons resulting from the fusion reaction. Because most of the waste is expected to be tritiated, a specific waste management strategy needed to be developed, taking into account the physical and chemical properties of tritium, such as its high coefficient of diffusivity in metals, outgassing, and its half-life. In France, the reference management strategy for tritiated waste with levels of activity exceeding the acceptance limits for final disposal is interim storage phase allowing for tritium decay before shipment to a final repository. Other options for reducing the tritium content in the waste have been studied as alternatives or complement to the reference strategy. In this article, different options for management of purely tritiated waste are described and compared to each other.
Tritium (T or 3H), which will be used as a thermonuclear fuel for the fusion reaction with deuterium, is a radioactive isotope of hydrogen with similar chemical properties and behaviour as hydrogen. Tritium exists in three chemical forms: gaseous tritium (HT), tritiated water (HTO), and organically bounded tritium (OBT). Tritium occurs naturally due to reactions between cosmic particles and the atoms in the upper atmosphere but the quantities are extremely small. Therefore it is artificially produced in fission reactors, and, in the future, in tritium breeding blankets where tritium is bred by the nuclear reaction 6Li(n, α)3H [1,3]. Regardless of its origin, tritium is extremely mobile in the environment and in all biological systems, which means it can be easily absorbed into most materials. Tritium is subsequently off-gassed after absorption, resulting in the contamination of air. The parameters defining the uptake and transport of tritium through materials include diffusivity, solubility, permeability, trapping coefficients, and recombination-rate coefficients. All of these parameters affect the choice of materials for fusion
⁎
Corresponding author. E-mail address: [email protected] (C. Decanis).
https://doi.org/10.1016/j.fusengdes.2018.02.008 Received 7 August 2017; Received in revised form 21 December 2017; Accepted 4 February 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Decanis, C., Fusion Engineering and Design (2018), https://doi.org/10.1016/j.fusengdes.2018.02.008
Fusion Engineering and Design xxx (xxxx) xxx–xxx
C. Decanis et al.
a temperature provokes the migration of tritium into the depth of the processed part instead of removing it. The form of released tritium is mainly tritiated water vapour (HTO) [3,4,6]. From a technical point of view, detritiation techniques should be adapted to the type of waste, the size of contaminated components, the type of contamination (superficial or in the bulk), and the activity levels of tritium and other radionuclides.
machines, which can be divided into three main categories: plasmafacing materials, structural materials and components of the fuel cycle systems. Typical plasma-facing materials, which compose the first wall of a vacuum vessel and are exposed to high particle fluxes and power of plasma, are martensitic steel, tungsten, and beryllium. In the case of structural materials, the most used types are ferritic and austenitic steels, especially 316L stainless steel, Inconel, copper and its alloys, and aluminium and its alloys [1,4]. Because most of the materials used to build fusion machines are metals with relatively high permeability at high temperature, tritium permeation is identified as one of the critical issues that can cause a large tritium inventory in the most contaminated parts and therefore may significantly complicate maintenance and management of tritiated waste in fusion facilities. Tritium permeability is defined as the steady state diffusion flux (i.e., t → ∞) that is expressed by Fick’s first law for diffusion:
dc J = −D ⎛ ⎞ ⎝ dx ⎠
3. Comparison of detritiation techniques: goals and methodology 3.1. Input data Most of the tritiated waste from experimental fusion devices will not be routed directly to the disposal facility due to the high level of tritium expected. One of the main recommendation that has been endorsed by the French Government is to set up a temporary storage site to allow for tritium decay (for 50 years if necessary, duration based on feedback from existing storage facilities) until the waste can be accepted for disposal. In addition, the French Nuclear Safety Authority (ASN) lists a number of recommendations for managing tritiated waste without any disposal solution in notice No. 2009-AV-0075. The two main following measures are recommended with respect to the interim storage:
(1)
where D is the tritium diffusion coefficient that follows Arrhenius-type dependence on temperature:
D = D0 exp (−ED/ RT )
(2)
– The impact of increasing the storage period beyond 50 years should be analysed with respect to the design of the interim storage facility. – Detritiation processes for highly out-gassing waste and purely tritiated waste should be considered in order to reduce the tritium releases, in particular in interim storage facilities.
where D0 is a constant and ED is the activation energy of diffusion [1,5]. The profile of tritium inventory distribution in metals is composed of two regions, which are a sub-surface layer (several hundreds micrometres) followed by a decrease in the tritium concentration towards the bulk [2,6]. In this article, we will focus on 316L stainless steel showing a very high tritium concentration in the sub-surface layer followed by a sharp decrease towards the bulk. The shape of tritium inventory distribution as well as the depth of the sub-surface layer is affected by many factors, such as temperature, gas pressure, and duration of exposure [1,2,6]. Although the mechanism of tritium release from solid materials is a complex process, it allows reduction of tritium inventory and also tritium off-gassing, which represents a good opportunity for waste processing. Reduction of tritium inventory, detritiation, can be performed by different techniques but only some of them enable in-depth detritiation, which is essential for processing of wastes that are highly loaded with tritium, such as those expected from future fusion facilities. Components of the fuel cycle systems are not located in the fusion machine but have to be decommissioned, detritiated and stored as well concerning their tritium content. Thermal detritiation techniques using elevated temperatures for an extended heating period in order to achieve a high detritiation factor are considered in this article. Thermal detritiation performed at too low
The current management strategy for tritiated waste is based on interim storage. Nevertheless, detritiation of this waste offers opportunities of reducing duration of interim storage and recovering tritium associated with a reduction of the releases for the most tritiated waste. The management options for tritiated waste are shown in Fig. 1. Two main branches can be identified in Fig. 1. The top one corresponds to the interim storage of tritiated waste, which represents the current management strategy, while the bottom one represents a detritiation process. The combination of both detritiation techniques and interim storage is also shown in Fig. 1. A comparative study was carried-out to assess the best available technique (BAT) for the processing of metallic highly tritiated waste. The target to reach is a tritium specific activity of 0.01 MBq/g. This value makes sense for a purely tritiated waste package as it corresponds to: – highest tritium activity level acceptable in the French VLLW disposal facility – average tritium activity level acceptable in the French LLW disposal1 facility – higher tritium activity level acceptable in the UK LLW disposal facility.2 In order to show the impact of the tritium activity and the waste amount on the choice of the BAT, two scenarios are taken into account in the comparison. The first scenario (S1) assumed waste with the following 1 For French LLW, maximal activity limits for 143 selected radionuclides, limits on outgassing of gaseous radionuclides, and limits on total activity of packages have been set. The tritium specific activity is limited to 0.2 MBq/g per package and the outgassing limit is 0.2 Bq/g/day. Based on the outgassing limit and a tritium outgassing rate of 1% per package and per year [9], another limit on tritium specific activity can be calculated. This value is 0.0073 MBq/g and is more stringent than the limit on the tritium specific activity. The total activity limits are 1 GBq per 200-l compacted drum and 50 GBq per any other packages. 2 in the UK, the threshold between low level waste (LLW) and intermediate level waste (ILW) is 0.012 MBq/g for ϒ- and β- emitters [11].
Fig. 1. Waste management options for metallic waste.
2
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• Thermal desorption up to melting • Isotope exchange • Chemical or electrochemical etching (with or without ultrasounds) • Treatment by plasma or laser • Surface washing and cleaning (with or without ultrasounds) • Vacuum desorption • Surface treatment using adhesive film • Polishing • Surface removal (skimming)
specifications: – 50 metric tons of waste containing 316L stainless steel – Tritium specific activity up to 3 MBq/g. The following assumptions on the waste that will be processed were taken into account for the second scenario (S2): – 300 metric tons of waste containing 316L stainless steel – Tritium specific activity up to 30 MBq/g.
Generally speaking, surface treatment methods (e.g. leaching, flushing, vacuum, chemical abrasion) are unsuitable for parts that are highly and massively loaded with tritium, such as those expected from fusion devices. However, these methods may prove useful on parts that are only slightly contaminated or that are not very thick. Surface removal, like skimming, would also allow separating the top layer from the bulk and be treated separately, increasing the efficiency of the global treatment. Nonetheless, tritiated secondary effluents generated by certain chemical etching methods could be more difficult to manage. The methods enabling more in-depth detritiation involve a thermal treatment, which must sometimes be combined with the use of a gas to enable isotope exchange. In addition, after each treatment, experience shows that tritium tends to migrate from the depths of the part to the surface and subsurface once again. Therefore, it appears interesting to re-process the waste if the residual activity is too high.
The first scenario corresponds to the amount of purely tritiated waste expected over a period of several years from an experimental fusion facility during its operational phase like ITER. The second scenario corresponds to the amount of waste during its deactivation phase [8]. This study takes into account the recommendations of the abovementioned of the French regulatory body [7]. 3.2. Detritiation techniques From a technical point of view, detritiation methods should be adapted to the type of waste, the size of contaminated components, the type of contamination (superficial or in the bulk), and the activity levels of tritium and other radionuclides. This means that a preliminary characterisation of the material should be performed. This characterisation includes several steps, such as taking smear-tests, measuring a tritium outgassing rate, and investigation of operational conditions to which the material has been exposed (tritium pressure, total duration of contact with tritium, chemical form of tritium etc.). Operational parameters affecting the efficiency of detritiation processes are expected to be temperature, duration of treatment, and type of atmosphere (different gases or vacuum) [9,10]. The secondary waste generated is mainly in the form of tritiated water (HTO) after gaseous effluent treatment. There are three possibilities for managing the secondary waste. It can be released into the environment, processed for tritium recovery, or stored. The management choice will be directly related to the produced volume of tritiated water, the amount of tritium contained in the water, and the purity of water.
– Focus on thermal treatment In this article, we will focus on thermal treatment using elevated temperatures for an extended heating period in order to achieve a high detritiation factor. These detritiation techniques are more complex than interim storage as they are performed in a suitable furnace under different atmosphere, such as hydrogen, ozone, oxygen, air, steam, or in a vacuum. The process of metal waste by thermal treatment is composed of three main steps.
• Sorting and segregation by material type, cutting up and preparing the metal loads • Heating the waste at an usual temperature of 1000 °C and up to 1700 °C depending on the type of materials to detritiate • Cooling and treating the processed waste (taking out of the furnace,
– Interim storage The duration of interim storage to allow for tritium decay is estimated to last about 50 years, which corresponds to a detritiation factor of 16. This duration is based on feedback from existing storage facilities. The main advantages of interim storage are that it generates little secondary waste and offers a well-tried solution for all types of waste. The main disadvantage of interim storage is its duration does not provide a high detritiation factor. In addition, after the storage period, the waste will have to comply with the acceptance criteria of the final repositories which can differ from the criteria existing at the time of its production and therefore can complicate the shipment of packages to the disposal facility. It is very important to secure with the disposal site, from the beginning of the acceptance of the packages, after the interim storage period.
cleaning and measurement of tritium outgassing). The main part of the gaseous tritium is transformed into tritiated water and further managed.
The principle of tritium migration or permeation in a metal is based on a mechanism of solution and diffusion. Generally speaking, there is a link between the treatment temperature and the efficiency of the treatment: the higher the treatment temperature, the more efficient the treatment. As mentioned before, the size has also an impact on the efficiency of thermal treatment: the smaller and thinner, the more efficient the thermal detritiation process. The thermal treatment of metallic waste produces tritiated water that must be managed. During the treatment, the volume of waste is not reduced unless the metal is melted. The thermal treatment is expected to reduce the tritium activity by a factor 10–1000 [9]. The detritiation factor is highly dependent on the selected thermal treatment technique and the treated material. The choice of the most suitable detritiation technique is not only linked to the highest detritiation factor or the lowest production of secondary waste, but this choice is also affected by many other parameters, such as public acceptance, the releases of tritium into the
– Detritiation techniques for metallic waste Results of the preliminary characterisation may indicate where tritium contamination is mainly present, either on the surface or in the bulk of the material, which is an important factor affecting the choice of a suitable detritiation technique. All known detritiation techniques for metallic waste have been investigated. A list of these techniques is given below: 3
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Table 1 Criteria applied in the comparative analysis − metallic waste. Criterion
Parameter
Scoring criteria
Environment
Global releases per year (waste/gas) Public acceptance
Amount of tritium released
Safety
Technical feasibility
Sustainability Long-term stewardship burden Secondary waste management Public exposure Occupational exposure Tritium incident management Process reliability Process complexity Process efficiency
Possible nuisance felt by the public Time between production of the waste and shipment to disposal Existence of a suitable route Added annual exposure Level of exposure Detritiation equipment available Maturity of techniques Design and operating conditions Volume and activity reduction
environment, level of exposure, and costs. 3.3. Methodology The main purpose of this chapter is to take into account all the parameters affecting the choice of management strategy. Table 1 shows the technical criteria applied and explains the different choices. A set of clearly independent parameters was chosen, with each parameter having been assigned the same weighting. Nonetheless, an overall weighting of 40% was assigned to the environment, 30% to safety and 30% to technical aspects. These values are based on the feedback at CEA at the conceptual design phase. Cost which is a critical parameter is taken into account at the preliminary design stage. Each parameter was graded by a group of 6 experts specialised in the following fields: radwaste management, tritium management, safety, project management, detritiation techniques for metallic waste and fusion energy. A grading was assigned quantitatively whenever possible (for example “amount of tritium released” or “added annual exposure”). Otherwise, grades were attributed from a qualitative perspective (for example “possible nuisance felt by the public”). The scores obtained for each criterion in terms of environment, safety and technical feasibility will be presented in the next chapters.
Fig. 2. General algorithm of the software dedicated to tritiated waste management.
repositories. We have focused on purely tritiated metallic waste and its possible treatment. The current strategy for all types of waste that cannot be accepted by final repositories is an interim storage of 50 years. In order to be able to compare results of different management strategies, it was necessary and important to include also the current strategy. In addition, in some specific cases, it could be worth combining detritiation methods with interim storage, and therefore this strategy was also taken into account. Fig. 3 shows all possible strategies that can be simulated by the software and Table 2 presents possible combination for metallic waste.
4. Results 4.1. Identification of appropriated techniques
– Conclusion about the software
In order to allow waste management experts to establish the different waste management strategies for tritiated waste to compare, a specific software program was developed. The name of the software is TWIT that stands for Tritiated Waste Investigation Tool.
The software can simulate different management strategies for tritiated waste. The main advantage is that users can easily change all the parameters of a waste management strategy and observe the impact of these changes. In particular, it is possible to modify the detritiation factor, depending on the selected technique and the material treated. In the case of study described in chapter 3.1, considering a detritiation factor of around 200 for thermal treatment, the 2 scenarios lead to the comparison of 3 possible strategies:
– General overview of the software The source code is written in Visual Basic for the application language and is composed of 10 modules. Fig. 2 shows a general algorithm of TWIT. As can be seen in this figure, the whole calculation process performed by TWIT comprises several steps, which can be divided into two groups, input data and code calculation.
– 3 consecutives interim storage periods – 2 consecutives thermal treatments – 1 thermal treatment followed by interim storage.
– Processing
4.2. Comparative analysis of waste management opportunities
As mentioned above, the main purpose of the software is to allow waste management experts to compare different management strategies. Therefore, it was necessary to investigate all possibilities for management of tritiated waste that cannot be accepted directly by final
The three possibilities identified through the software for the management of the tritiated waste defined in scenario 1 and in scenario 2 have been compared and scored through the criteria mentioned in Section 3. The results of the analysis and scoring are shown in the 4
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Fig. 4. Comparative analysis – metallic waste.
environment, technical and safety criteria. If we consider the input parameters, the global analysis leads to the following statements: 4.2.1. Tritium activity The interim storage solution is suitable for lowly tritiated waste. Thermal treatment can handle higher levels of tritium in the radwaste as the detritiation factor is higher and waste can be re-processed. 4.2.2. Secondary waste Interim storage does not generate secondary waste. Thermal treatment is more interesting if combined with a water detritiation system to deal with tritiated water produced. 4.2.3. Capability of the facilities Interim storage needs to be designed taking into account the global amount of waste to detritiate. Thermal treatment detritiation rates can be adapted to the waste production flows. 4.2.4. Sustainability Interim storage allows us to take advantage of a new BAT not available at the time of production of the waste. However, it represents a mid-term stewardship burden A thermal treatment makes it possible to close the fusion fuel cycle by tritium recovery. An economic assessment of the methods is ongoing in order to identify the most cost-effective one. The following costs will be taken into account for this assessment:
Fig. 3. Possible management strategies.
Table 2 Management strategies for metallic waste available in TWIT. Duration of interim storage
Type of treatment
Thermal treatment
No treatment
=0
>0
Thermal treatment + Final disposal
Thermal treatment + Interim storage + Final disposal Interim storage + Finaldisposal
Final disposal
– Design and construction cost – Maintenance and refurbishment + modifications due to the 10yearly inspections – Operational cost – Transport to the disposal site – Disposal cost – Tritium recovery (cost and profit)
following figure. The columns represent scores obtained for each criterion. The maximum score was fixed at 100. As seen in Fig. 4, the analysis of the scoring is the following:
This work is not finished, nevertheless the next steps are launched.
– Environmental criterion is higher for the thermal treatment due to the reduction of the long-term stewardship burden compared to the interim storage solutions and the releases reduction, – Safety is better for the thermal treatment as it offers an insite emergency response in case of a tritium concern, – From the technical point of view, there is a balance bertween the complexity of the process which is higher for the thermal treatment and the process efficiency which is lower for the interim storage. As a result, this criterion doesn’t differentiate the two solutions.
4.3. Next steps The work programme is focused on the facility and process design that will be necessary to develop and optimise the detritiation process for each type of material generated by fusion technology comprises the following steps: 4.3.1. Interim storage
• The capacity will be determined on the basis of the tritiated rad-
At this point, an analysis of the sensitivity of the results is carried out considering changes in the overall weighting distribution of
waste inventories (final packages volumes)
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• Preliminary and detailed design of the facility in view of the con-
for irradiating waste such as LL-ILW. The economic relevance of the entire waste management system including the secondary waste is ongoing, but first the chosen processes will need to be optimised via engineering studies including R&D to validate the performances of the detritiation process and the tritium recovery efficiency.
struction.
4.3.2. Thermal treatment
• Preliminary and detailed design of the process with the following
References
objectives: ○ Adaptation of the process to several materials: ○ Be, W ○ Steel ○ Identification of the parameters to reduce the secondary waste. ○ Validation of the performances of the Detritiation Facility: in particular, Detritiation Factor achievable. The detritiation factor has a big impact on the final decision for management strategies. ○ Compliance of the Tritium recovery system with the waste characteristics.
[1] R.A. Causey, R.A. Karnesky, C. San Marchi, Tritium barriers and tritium diffusion in fusion reactors, Compr. Nucl. Mater. 4 (2012) 511–549. [2] A.N. Perevezentsev, A.C. Bell, L.A. Rivkis, V.M. Filin, V.V. Gushin, M.I. Belyakov, V.I. Bulkin, I.M. Kravchenko, I.A. Ionessian, Y. Torikai, M. Matsuyama, K. Watanabe, A.I. Markin, Comparative study of the tritium distribution in metals, J. Nucl. Mater. 372 (2008) 263–276. [3] K.Y. Wong, B. Hircq, R.A. Jalbert, W.T. Shmayda, Tritium decontamination of machine components and walls, Fusion Eng. Des. 16 (1991) 159–172. [4] A.N. Perevezentsev, A.C. Bell, L.A. Rivkis, V.M. Filin, V.V. Gushin, M.I. Belyakov, V.I. Bulkin, I.G. Prykina, I.M. Kravchenko, A.A. Semenov, A.I. Davidov, S.P. Eliseev, D.V. Titov, Experimental trials of methods for metal detritiation for JET, Fusion Sci. Technol. 52 (2007). [5] S.K. Lee, H.S. Kim, S.J. Noh, J.H. Han, Hydrogen Permeability, Diffusivity, and Solubility of SUS 316L Stainless Steel in the Temperature Range 400 to 800 °C for Fusion Reactor Applications, J. Korean Phys. Soc. 59 (5) (2011) 3019–3023. [6] R.-D. Penzhorn, Y. Torikai, K. Watanabe, M. Matsuyama, A.N. Perevezentsev, On the fate of tritium in thermally treated stainless steel type 316L, J. Nucl. Mater. 429 (2012) 346–352. [7] ASN (regulator body) opinion n (2009-AV-0075 dated 25 August 2009 on the studies in view of the elaboration of the French National Plan for the Management of Radioactive Materials. ASN website http://www.asn.fr. [8] L. Rodriguez-Rodrigo, P. Cortes, Optimisation de la gestion du tritium dans le projet ITER, Journées Tritium Société Française de Radioprotection 23–24 September (2009). [9] French Nuclear Safety Authority, Tritium, website http://livre-blanc-tritium.asn.fr. (2010). [10] K. Liger, P. Trabuc, J. Mascarade, M. Troulay, C. Perrais, S. Tosti, F. Borgognoni, Preliminary results from a detritiation facility dedicated to soft housekeeping waste and tritium valorization, Fusion Eng. Des. 89 (9–10) (2014) 2103–2107. [11] Guidance on UK Low Level Waste Management Legislation, NWP-REP-090–Issue 1–Oct 2015.
This work programme is expected to last 3 years. 5. Conclusion A specific analysis is required for each radwaste category so as to reduce the tritium inventory in the waste. This paper focuses on metallic waste expected to be generated during the operational phase of a fusion reactor. The main advantage of the interim storage current strategy is its application to all types of tritiated waste. However, in the case of metallic waste, the most efficient technique is a thermal treatment, which, besides the highest detritiation factor, also offers the opportunity to recover tritium and give a short-term solution which is an advantage. Even though some detritiation processes are efficient, the implementation and operation of these processes can be costly, especially
6
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Methodology to identify appropriate options to manage tritiated waste ⁎
Christelle Decanis , Michal Kresina, Daniel Canas CEA DEN, 13108 Cadarache, France
A R T I C L E I N F O
A B S T R A C T
Keywords: Waste management Tritiated waste Comparative analyses BAT Waste management tool
Fusion facilities will use tritium as a part of fuel. As a consequence, most of waste is expected to be contaminated with tritium and specific waste management strategies need to be developed taking into account physical and chemical properties of tritium. In France, the reference strategy for waste exceeding acceptance limits for disposal due to its tritium inventory is an interim storage period up to 50 years, corresponding approximately to detritiation factor of 16 due to radioactive decay. This article is mainly focused on the most tritiated categories of waste that would require an interim storage period longer than 50 years. Alternative strategies or additional processing options have been sought for these categories and the article defines methodology of identification of the Best Available Technique (BAT) for reduction of tritium inventory in the waste. Furthermore, the methodology is demonstrated on the case of purely tritiated metallic waste made from stainless steel. The methodology has been divided into three main steps: 1) Identification of the most relevant detritiation techniques; 2) Comparative analysis; and 3) Economical assessment. In order to simplify the first step, a software tool has been developed by CEA and its main features are described in this article. The results given by this tool are then analysed in the second step, which takes into account several non-economic criteria. Results of the comparative analysis show that detritiation of purely tritiated metallic waste made from stainless steel appears to be very attractive, allowing a significant reduction of interim storage duration before disposal in a surface repository. The last step, briefly described in this article, is ongoing and will economically assess the results from the previous steps.
1. Introduction
2. Tritium behaviour
Radioactive waste will be generated during operation and decommissioning of fusion facilities. This waste will be either contaminated by tritium, or activated by neutrons resulting from the fusion reaction. Because most of the waste is expected to be tritiated, a specific waste management strategy needed to be developed, taking into account the physical and chemical properties of tritium, such as its high coefficient of diffusivity in metals, outgassing, and its half-life. In France, the reference management strategy for tritiated waste with levels of activity exceeding the acceptance limits for final disposal is interim storage phase allowing for tritium decay before shipment to a final repository. Other options for reducing the tritium content in the waste have been studied as alternatives or complement to the reference strategy. In this article, different options for management of purely tritiated waste are described and compared to each other.
Tritium (T or 3H), which will be used as a thermonuclear fuel for the fusion reaction with deuterium, is a radioactive isotope of hydrogen with similar chemical properties and behaviour as hydrogen. Tritium exists in three chemical forms: gaseous tritium (HT), tritiated water (HTO), and organically bounded tritium (OBT). Tritium occurs naturally due to reactions between cosmic particles and the atoms in the upper atmosphere but the quantities are extremely small. Therefore it is artificially produced in fission reactors, and, in the future, in tritium breeding blankets where tritium is bred by the nuclear reaction 6Li(n, α)3H [1,3]. Regardless of its origin, tritium is extremely mobile in the environment and in all biological systems, which means it can be easily absorbed into most materials. Tritium is subsequently off-gassed after absorption, resulting in the contamination of air. The parameters defining the uptake and transport of tritium through materials include diffusivity, solubility, permeability, trapping coefficients, and recombination-rate coefficients. All of these parameters affect the choice of materials for fusion
⁎
Corresponding author. E-mail address: [email protected] (C. Decanis).
https://doi.org/10.1016/j.fusengdes.2018.02.008 Received 7 August 2017; Received in revised form 21 December 2017; Accepted 4 February 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Decanis, C., Fusion Engineering and Design (2018), https://doi.org/10.1016/j.fusengdes.2018.02.008
Fusion Engineering and Design xxx (xxxx) xxx–xxx
C. Decanis et al.
a temperature provokes the migration of tritium into the depth of the processed part instead of removing it. The form of released tritium is mainly tritiated water vapour (HTO) [3,4,6]. From a technical point of view, detritiation techniques should be adapted to the type of waste, the size of contaminated components, the type of contamination (superficial or in the bulk), and the activity levels of tritium and other radionuclides.
machines, which can be divided into three main categories: plasmafacing materials, structural materials and components of the fuel cycle systems. Typical plasma-facing materials, which compose the first wall of a vacuum vessel and are exposed to high particle fluxes and power of plasma, are martensitic steel, tungsten, and beryllium. In the case of structural materials, the most used types are ferritic and austenitic steels, especially 316L stainless steel, Inconel, copper and its alloys, and aluminium and its alloys [1,4]. Because most of the materials used to build fusion machines are metals with relatively high permeability at high temperature, tritium permeation is identified as one of the critical issues that can cause a large tritium inventory in the most contaminated parts and therefore may significantly complicate maintenance and management of tritiated waste in fusion facilities. Tritium permeability is defined as the steady state diffusion flux (i.e., t → ∞) that is expressed by Fick’s first law for diffusion:
dc J = −D ⎛ ⎞ ⎝ dx ⎠
3. Comparison of detritiation techniques: goals and methodology 3.1. Input data Most of the tritiated waste from experimental fusion devices will not be routed directly to the disposal facility due to the high level of tritium expected. One of the main recommendation that has been endorsed by the French Government is to set up a temporary storage site to allow for tritium decay (for 50 years if necessary, duration based on feedback from existing storage facilities) until the waste can be accepted for disposal. In addition, the French Nuclear Safety Authority (ASN) lists a number of recommendations for managing tritiated waste without any disposal solution in notice No. 2009-AV-0075. The two main following measures are recommended with respect to the interim storage:
(1)
where D is the tritium diffusion coefficient that follows Arrhenius-type dependence on temperature:
D = D0 exp (−ED/ RT )
(2)
– The impact of increasing the storage period beyond 50 years should be analysed with respect to the design of the interim storage facility. – Detritiation processes for highly out-gassing waste and purely tritiated waste should be considered in order to reduce the tritium releases, in particular in interim storage facilities.
where D0 is a constant and ED is the activation energy of diffusion [1,5]. The profile of tritium inventory distribution in metals is composed of two regions, which are a sub-surface layer (several hundreds micrometres) followed by a decrease in the tritium concentration towards the bulk [2,6]. In this article, we will focus on 316L stainless steel showing a very high tritium concentration in the sub-surface layer followed by a sharp decrease towards the bulk. The shape of tritium inventory distribution as well as the depth of the sub-surface layer is affected by many factors, such as temperature, gas pressure, and duration of exposure [1,2,6]. Although the mechanism of tritium release from solid materials is a complex process, it allows reduction of tritium inventory and also tritium off-gassing, which represents a good opportunity for waste processing. Reduction of tritium inventory, detritiation, can be performed by different techniques but only some of them enable in-depth detritiation, which is essential for processing of wastes that are highly loaded with tritium, such as those expected from future fusion facilities. Components of the fuel cycle systems are not located in the fusion machine but have to be decommissioned, detritiated and stored as well concerning their tritium content. Thermal detritiation techniques using elevated temperatures for an extended heating period in order to achieve a high detritiation factor are considered in this article. Thermal detritiation performed at too low
The current management strategy for tritiated waste is based on interim storage. Nevertheless, detritiation of this waste offers opportunities of reducing duration of interim storage and recovering tritium associated with a reduction of the releases for the most tritiated waste. The management options for tritiated waste are shown in Fig. 1. Two main branches can be identified in Fig. 1. The top one corresponds to the interim storage of tritiated waste, which represents the current management strategy, while the bottom one represents a detritiation process. The combination of both detritiation techniques and interim storage is also shown in Fig. 1. A comparative study was carried-out to assess the best available technique (BAT) for the processing of metallic highly tritiated waste. The target to reach is a tritium specific activity of 0.01 MBq/g. This value makes sense for a purely tritiated waste package as it corresponds to: – highest tritium activity level acceptable in the French VLLW disposal facility – average tritium activity level acceptable in the French LLW disposal1 facility – higher tritium activity level acceptable in the UK LLW disposal facility.2 In order to show the impact of the tritium activity and the waste amount on the choice of the BAT, two scenarios are taken into account in the comparison. The first scenario (S1) assumed waste with the following 1 For French LLW, maximal activity limits for 143 selected radionuclides, limits on outgassing of gaseous radionuclides, and limits on total activity of packages have been set. The tritium specific activity is limited to 0.2 MBq/g per package and the outgassing limit is 0.2 Bq/g/day. Based on the outgassing limit and a tritium outgassing rate of 1% per package and per year [9], another limit on tritium specific activity can be calculated. This value is 0.0073 MBq/g and is more stringent than the limit on the tritium specific activity. The total activity limits are 1 GBq per 200-l compacted drum and 50 GBq per any other packages. 2 in the UK, the threshold between low level waste (LLW) and intermediate level waste (ILW) is 0.012 MBq/g for ϒ- and β- emitters [11].
Fig. 1. Waste management options for metallic waste.
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• Thermal desorption up to melting • Isotope exchange • Chemical or electrochemical etching (with or without ultrasounds) • Treatment by plasma or laser • Surface washing and cleaning (with or without ultrasounds) • Vacuum desorption • Surface treatment using adhesive film • Polishing • Surface removal (skimming)
specifications: – 50 metric tons of waste containing 316L stainless steel – Tritium specific activity up to 3 MBq/g. The following assumptions on the waste that will be processed were taken into account for the second scenario (S2): – 300 metric tons of waste containing 316L stainless steel – Tritium specific activity up to 30 MBq/g.
Generally speaking, surface treatment methods (e.g. leaching, flushing, vacuum, chemical abrasion) are unsuitable for parts that are highly and massively loaded with tritium, such as those expected from fusion devices. However, these methods may prove useful on parts that are only slightly contaminated or that are not very thick. Surface removal, like skimming, would also allow separating the top layer from the bulk and be treated separately, increasing the efficiency of the global treatment. Nonetheless, tritiated secondary effluents generated by certain chemical etching methods could be more difficult to manage. The methods enabling more in-depth detritiation involve a thermal treatment, which must sometimes be combined with the use of a gas to enable isotope exchange. In addition, after each treatment, experience shows that tritium tends to migrate from the depths of the part to the surface and subsurface once again. Therefore, it appears interesting to re-process the waste if the residual activity is too high.
The first scenario corresponds to the amount of purely tritiated waste expected over a period of several years from an experimental fusion facility during its operational phase like ITER. The second scenario corresponds to the amount of waste during its deactivation phase [8]. This study takes into account the recommendations of the abovementioned of the French regulatory body [7]. 3.2. Detritiation techniques From a technical point of view, detritiation methods should be adapted to the type of waste, the size of contaminated components, the type of contamination (superficial or in the bulk), and the activity levels of tritium and other radionuclides. This means that a preliminary characterisation of the material should be performed. This characterisation includes several steps, such as taking smear-tests, measuring a tritium outgassing rate, and investigation of operational conditions to which the material has been exposed (tritium pressure, total duration of contact with tritium, chemical form of tritium etc.). Operational parameters affecting the efficiency of detritiation processes are expected to be temperature, duration of treatment, and type of atmosphere (different gases or vacuum) [9,10]. The secondary waste generated is mainly in the form of tritiated water (HTO) after gaseous effluent treatment. There are three possibilities for managing the secondary waste. It can be released into the environment, processed for tritium recovery, or stored. The management choice will be directly related to the produced volume of tritiated water, the amount of tritium contained in the water, and the purity of water.
– Focus on thermal treatment In this article, we will focus on thermal treatment using elevated temperatures for an extended heating period in order to achieve a high detritiation factor. These detritiation techniques are more complex than interim storage as they are performed in a suitable furnace under different atmosphere, such as hydrogen, ozone, oxygen, air, steam, or in a vacuum. The process of metal waste by thermal treatment is composed of three main steps.
• Sorting and segregation by material type, cutting up and preparing the metal loads • Heating the waste at an usual temperature of 1000 °C and up to 1700 °C depending on the type of materials to detritiate • Cooling and treating the processed waste (taking out of the furnace,
– Interim storage The duration of interim storage to allow for tritium decay is estimated to last about 50 years, which corresponds to a detritiation factor of 16. This duration is based on feedback from existing storage facilities. The main advantages of interim storage are that it generates little secondary waste and offers a well-tried solution for all types of waste. The main disadvantage of interim storage is its duration does not provide a high detritiation factor. In addition, after the storage period, the waste will have to comply with the acceptance criteria of the final repositories which can differ from the criteria existing at the time of its production and therefore can complicate the shipment of packages to the disposal facility. It is very important to secure with the disposal site, from the beginning of the acceptance of the packages, after the interim storage period.
cleaning and measurement of tritium outgassing). The main part of the gaseous tritium is transformed into tritiated water and further managed.
The principle of tritium migration or permeation in a metal is based on a mechanism of solution and diffusion. Generally speaking, there is a link between the treatment temperature and the efficiency of the treatment: the higher the treatment temperature, the more efficient the treatment. As mentioned before, the size has also an impact on the efficiency of thermal treatment: the smaller and thinner, the more efficient the thermal detritiation process. The thermal treatment of metallic waste produces tritiated water that must be managed. During the treatment, the volume of waste is not reduced unless the metal is melted. The thermal treatment is expected to reduce the tritium activity by a factor 10–1000 [9]. The detritiation factor is highly dependent on the selected thermal treatment technique and the treated material. The choice of the most suitable detritiation technique is not only linked to the highest detritiation factor or the lowest production of secondary waste, but this choice is also affected by many other parameters, such as public acceptance, the releases of tritium into the
– Detritiation techniques for metallic waste Results of the preliminary characterisation may indicate where tritium contamination is mainly present, either on the surface or in the bulk of the material, which is an important factor affecting the choice of a suitable detritiation technique. All known detritiation techniques for metallic waste have been investigated. A list of these techniques is given below: 3
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Table 1 Criteria applied in the comparative analysis − metallic waste. Criterion
Parameter
Scoring criteria
Environment
Global releases per year (waste/gas) Public acceptance
Amount of tritium released
Safety
Technical feasibility
Sustainability Long-term stewardship burden Secondary waste management Public exposure Occupational exposure Tritium incident management Process reliability Process complexity Process efficiency
Possible nuisance felt by the public Time between production of the waste and shipment to disposal Existence of a suitable route Added annual exposure Level of exposure Detritiation equipment available Maturity of techniques Design and operating conditions Volume and activity reduction
environment, level of exposure, and costs. 3.3. Methodology The main purpose of this chapter is to take into account all the parameters affecting the choice of management strategy. Table 1 shows the technical criteria applied and explains the different choices. A set of clearly independent parameters was chosen, with each parameter having been assigned the same weighting. Nonetheless, an overall weighting of 40% was assigned to the environment, 30% to safety and 30% to technical aspects. These values are based on the feedback at CEA at the conceptual design phase. Cost which is a critical parameter is taken into account at the preliminary design stage. Each parameter was graded by a group of 6 experts specialised in the following fields: radwaste management, tritium management, safety, project management, detritiation techniques for metallic waste and fusion energy. A grading was assigned quantitatively whenever possible (for example “amount of tritium released” or “added annual exposure”). Otherwise, grades were attributed from a qualitative perspective (for example “possible nuisance felt by the public”). The scores obtained for each criterion in terms of environment, safety and technical feasibility will be presented in the next chapters.
Fig. 2. General algorithm of the software dedicated to tritiated waste management.
repositories. We have focused on purely tritiated metallic waste and its possible treatment. The current strategy for all types of waste that cannot be accepted by final repositories is an interim storage of 50 years. In order to be able to compare results of different management strategies, it was necessary and important to include also the current strategy. In addition, in some specific cases, it could be worth combining detritiation methods with interim storage, and therefore this strategy was also taken into account. Fig. 3 shows all possible strategies that can be simulated by the software and Table 2 presents possible combination for metallic waste.
4. Results 4.1. Identification of appropriated techniques
– Conclusion about the software
In order to allow waste management experts to establish the different waste management strategies for tritiated waste to compare, a specific software program was developed. The name of the software is TWIT that stands for Tritiated Waste Investigation Tool.
The software can simulate different management strategies for tritiated waste. The main advantage is that users can easily change all the parameters of a waste management strategy and observe the impact of these changes. In particular, it is possible to modify the detritiation factor, depending on the selected technique and the material treated. In the case of study described in chapter 3.1, considering a detritiation factor of around 200 for thermal treatment, the 2 scenarios lead to the comparison of 3 possible strategies:
– General overview of the software The source code is written in Visual Basic for the application language and is composed of 10 modules. Fig. 2 shows a general algorithm of TWIT. As can be seen in this figure, the whole calculation process performed by TWIT comprises several steps, which can be divided into two groups, input data and code calculation.
– 3 consecutives interim storage periods – 2 consecutives thermal treatments – 1 thermal treatment followed by interim storage.
– Processing
4.2. Comparative analysis of waste management opportunities
As mentioned above, the main purpose of the software is to allow waste management experts to compare different management strategies. Therefore, it was necessary to investigate all possibilities for management of tritiated waste that cannot be accepted directly by final
The three possibilities identified through the software for the management of the tritiated waste defined in scenario 1 and in scenario 2 have been compared and scored through the criteria mentioned in Section 3. The results of the analysis and scoring are shown in the 4
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Fig. 4. Comparative analysis – metallic waste.
environment, technical and safety criteria. If we consider the input parameters, the global analysis leads to the following statements: 4.2.1. Tritium activity The interim storage solution is suitable for lowly tritiated waste. Thermal treatment can handle higher levels of tritium in the radwaste as the detritiation factor is higher and waste can be re-processed. 4.2.2. Secondary waste Interim storage does not generate secondary waste. Thermal treatment is more interesting if combined with a water detritiation system to deal with tritiated water produced. 4.2.3. Capability of the facilities Interim storage needs to be designed taking into account the global amount of waste to detritiate. Thermal treatment detritiation rates can be adapted to the waste production flows. 4.2.4. Sustainability Interim storage allows us to take advantage of a new BAT not available at the time of production of the waste. However, it represents a mid-term stewardship burden A thermal treatment makes it possible to close the fusion fuel cycle by tritium recovery. An economic assessment of the methods is ongoing in order to identify the most cost-effective one. The following costs will be taken into account for this assessment:
Fig. 3. Possible management strategies.
Table 2 Management strategies for metallic waste available in TWIT. Duration of interim storage
Type of treatment
Thermal treatment
No treatment
=0
>0
Thermal treatment + Final disposal
Thermal treatment + Interim storage + Final disposal Interim storage + Finaldisposal
Final disposal
– Design and construction cost – Maintenance and refurbishment + modifications due to the 10yearly inspections – Operational cost – Transport to the disposal site – Disposal cost – Tritium recovery (cost and profit)
following figure. The columns represent scores obtained for each criterion. The maximum score was fixed at 100. As seen in Fig. 4, the analysis of the scoring is the following:
This work is not finished, nevertheless the next steps are launched.
– Environmental criterion is higher for the thermal treatment due to the reduction of the long-term stewardship burden compared to the interim storage solutions and the releases reduction, – Safety is better for the thermal treatment as it offers an insite emergency response in case of a tritium concern, – From the technical point of view, there is a balance bertween the complexity of the process which is higher for the thermal treatment and the process efficiency which is lower for the interim storage. As a result, this criterion doesn’t differentiate the two solutions.
4.3. Next steps The work programme is focused on the facility and process design that will be necessary to develop and optimise the detritiation process for each type of material generated by fusion technology comprises the following steps: 4.3.1. Interim storage
• The capacity will be determined on the basis of the tritiated rad-
At this point, an analysis of the sensitivity of the results is carried out considering changes in the overall weighting distribution of
waste inventories (final packages volumes)
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• Preliminary and detailed design of the facility in view of the con-
for irradiating waste such as LL-ILW. The economic relevance of the entire waste management system including the secondary waste is ongoing, but first the chosen processes will need to be optimised via engineering studies including R&D to validate the performances of the detritiation process and the tritium recovery efficiency.
struction.
4.3.2. Thermal treatment
• Preliminary and detailed design of the process with the following
References
objectives: ○ Adaptation of the process to several materials: ○ Be, W ○ Steel ○ Identification of the parameters to reduce the secondary waste. ○ Validation of the performances of the Detritiation Facility: in particular, Detritiation Factor achievable. The detritiation factor has a big impact on the final decision for management strategies. ○ Compliance of the Tritium recovery system with the waste characteristics.
[1] R.A. Causey, R.A. Karnesky, C. San Marchi, Tritium barriers and tritium diffusion in fusion reactors, Compr. Nucl. Mater. 4 (2012) 511–549. [2] A.N. Perevezentsev, A.C. Bell, L.A. Rivkis, V.M. Filin, V.V. Gushin, M.I. Belyakov, V.I. Bulkin, I.M. Kravchenko, I.A. Ionessian, Y. Torikai, M. Matsuyama, K. Watanabe, A.I. Markin, Comparative study of the tritium distribution in metals, J. Nucl. Mater. 372 (2008) 263–276. [3] K.Y. Wong, B. Hircq, R.A. Jalbert, W.T. Shmayda, Tritium decontamination of machine components and walls, Fusion Eng. Des. 16 (1991) 159–172. [4] A.N. Perevezentsev, A.C. Bell, L.A. Rivkis, V.M. Filin, V.V. Gushin, M.I. Belyakov, V.I. Bulkin, I.G. Prykina, I.M. Kravchenko, A.A. Semenov, A.I. Davidov, S.P. Eliseev, D.V. Titov, Experimental trials of methods for metal detritiation for JET, Fusion Sci. Technol. 52 (2007). [5] S.K. Lee, H.S. Kim, S.J. Noh, J.H. Han, Hydrogen Permeability, Diffusivity, and Solubility of SUS 316L Stainless Steel in the Temperature Range 400 to 800 °C for Fusion Reactor Applications, J. Korean Phys. Soc. 59 (5) (2011) 3019–3023. [6] R.-D. Penzhorn, Y. Torikai, K. Watanabe, M. Matsuyama, A.N. Perevezentsev, On the fate of tritium in thermally treated stainless steel type 316L, J. Nucl. Mater. 429 (2012) 346–352. [7] ASN (regulator body) opinion n (2009-AV-0075 dated 25 August 2009 on the studies in view of the elaboration of the French National Plan for the Management of Radioactive Materials. ASN website http://www.asn.fr. [8] L. Rodriguez-Rodrigo, P. Cortes, Optimisation de la gestion du tritium dans le projet ITER, Journées Tritium Société Française de Radioprotection 23–24 September (2009). [9] French Nuclear Safety Authority, Tritium, website http://livre-blanc-tritium.asn.fr. (2010). [10] K. Liger, P. Trabuc, J. Mascarade, M. Troulay, C. Perrais, S. Tosti, F. Borgognoni, Preliminary results from a detritiation facility dedicated to soft housekeeping waste and tritium valorization, Fusion Eng. Des. 89 (9–10) (2014) 2103–2107. [11] Guidance on UK Low Level Waste Management Legislation, NWP-REP-090–Issue 1–Oct 2015.
This work programme is expected to last 3 years. 5. Conclusion A specific analysis is required for each radwaste category so as to reduce the tritium inventory in the waste. This paper focuses on metallic waste expected to be generated during the operational phase of a fusion reactor. The main advantage of the interim storage current strategy is its application to all types of tritiated waste. However, in the case of metallic waste, the most efficient technique is a thermal treatment, which, besides the highest detritiation factor, also offers the opportunity to recover tritium and give a short-term solution which is an advantage. Even though some detritiation processes are efficient, the implementation and operation of these processes can be costly, especially
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