SAFETY AND RADIOACTIVE WASTE MANAGEMENT ASPECTS OF THE IGNITOR FUSION EXPERIMENT. M. Zucchetti, A. Ciampichetti

Transcription

1 SAFETY AND RADIOACTIVE WASTE MANAGEMENT ASPECTS OF THE IGNITOR FUSION EXPERIMENT M. Zucchetti, A. Ciampichetti DENER, Politecnico di Torino Corso Duca degli Abruzzi, Torino (Italy) Ignitor is a nuclear fusion experiment aimed at studying Deuterium-Tritium plasmas. If European proposed waste management strategies were applied, all Ignitor radioactive materials could be recycled or declassified to non-radioactive material. We have applied the Italian waste management regulations to the IGNITOR experiment radioactive materials: none of them should be classified in the High Level Waste category but the vessel, and most materials are classified as LLW (Low Level Waste). The machine has very low radiological risks and environmental impact. I. INTRODUCTION Within the frame of the International Energy Agency (IEA) Co-operative Program on the Environmental, Safety and Economic Aspects of Fusion Power, an international collaborative study on radioactive waste has been initiated to examine the backend of the reactor materials cycle as an important stage in maximising the environmental benefits of fusion as energy provider. The study defines the management procedures for active materials following the change out of replaceable components and decommissioning of fusion facilities. 1 The main goal of this activity is the definition of waste management strategies for fusion experiments and reactors. In this paper, we have applied a waste management strategy proposed by the European Union in the frame of the above-quoted collaborative study and compared the results to those obtained applying instead the Italian waste management regulations to the IGNITOR experiment radioactive materials. II. FUSION RADIOACTIVE WASTE MANAGEMENT Most radioactive waste generated from fusion reactors will be activated solid metallic material from the main machine components. Some component will also have surface contamination including tritium. The dominating waste stream is generated in the decommissioning stage, while - for fission spent fuel is the main issue. A great deal of the decommissioning waste has a very low activity concentration, especially when a long period of intermediate decay is anticipated. Radioactive nuclides in fusion waste are mainly metallic activation products and tritium. Therefore, fusion waste is quite different from fission waste, both in type of material and isotopic composition: fusion waste does not include plutonium, fission products, transuranics and normally no alpha-emitting nuclides, and it is generally shorter-lived than fission waste. In most countries with a nuclear program, the waste management strategies are based on deep geological disposal of High-Level Waste (HLW), while a less sophisticated disposal method, mostly a nearsurface type repository, is used for Low-Level or Intermediate-Level Waste (LLW/ILW) 2. In order to maximise the environmental benefits of fusion power generation, it is important to clearly define the parameters governing the back-end of the materials cycle. A fusion-specific, approach is necessary and needs to be developed. Recycling of materials and clearance (i.e. declassification to non-radioactive material) are the two recommended options for reducing the amount of fusion waste, while disposal as low-level waste (LLW) could be an alternative route for specific materials and components. An example of application of an international waste management strategy and a national fissionoriented regulation to fusion is given in the following: it concerns the siting of the IGNITOR fusion experimental reactor in Italy. III. IGNITOR,ITS SAFETY AND ITS SITING Ignitor is a proposed compact high-magnetic field tokamak aimed at studying plasma burning conditions in Deuterium-Tritium plasmas 3. Ignitor has a major radius of 1.3 m, minor radii of 0.47 m and 0.87 m, a peak plasma temperature of 12 kev, a peak plasma density of ions/m 3, at a maximum fusion power of 90 MW. Pulses at different power levels are planned, with either DD or DT operation. The tokamak main components are: a molybdenum first wall (volume: 2 m 3 ), an INCONEL625 vacuum vessel (4.4 m 3 ), the Cu-based toroidal magnets (12.2 m 3 ), and the AISI316 machine structure (named "C-Clamp, 24 m 3 ). The IGNITOR experimental reactor operation lifetime will be divided into two phases: in the second one, tritium and neutron activated materials will be present, however quite moderately. Ignitor has a 814 FUSION SCIENCE AND TECHNOLOGY VOL. 56 AUG. 2009

2 scheduled activity running for ten years. After a first period with aneutronic plasmas (H and He), useful for the setup for the machine, a second period with DD plasmas will follow. After this, starting from the third year, DT discharges will begin: tritium concentration will grow from an initial 5% to 50% (at the beginning of the fourth year) and plasma power will increase. Maximum, i.e., machine end-of-life, irradiation (ten years) is considered. Total operation time of Ignitor is quite small. In fact, after a discharge of about 4 s, pauses last for hours. In total, during the ten-years life of the machine, about 9000 s of DD plasmas and 6000 s of DT plasmas are scheduled, with also about 9000 s of aneutronic plasmas. The total radioactive inventory at end-of-life irradiation has been estimated in about Bq of activation products at shutdown, about Bq after 1 week, Bq after 1 year, Bq after 50 years of decay. The tritium inventory in the whole machine, including the fuel system, amounts to a few grams. The machine has very low radiological risks and environmental impact. However, most of the accident source terms that impact the environment come from the tritium inventory. Tritium is an important radioactive source term for IGNITOR, where it can be found as free gas, captured gas or in an oxidised form (HTO). The total tritium inventory does not exceed Bq ( Ci, 13 g), before the design modification of the first wall material. The substitution of the graphite tiles with molybdenum ones some relevant effect on tritium inventory. It turns out that the global tritium inventory in the machine is reduced by this design modification, with reduction of 1.7 g of T in the first wall, summing up in total to a few grams (Ref. 4). We have always supposed that adequate detritiation of material is carried out before disposal. Concerning Tritium inventory and tritiated waste, Tritium Handling System has been recently redesigned and positively safety assessed. Localisation of this experiment in Italy has seen growing attention during the last years. The determination to carry out this project in Italy has driven to taking into account several candidate sites. In the nineties, a proposed site concerned the EUREX plant in Saluggia (Italy), a place that fully fitted all requirements for a nuclear installation. Further assessments and evaluations were performed for the ENEL TERNA plant in Rondissone (Italy). The two sites are only a few kilometres distant each other. Safety assessments and siting evaluations were performed for those sites, which excellent results in terms of extremely low environmental impact 5-7. Accidental sequences have been identified: Machine and Systems analyses have been performed according to a Probabilistic Risk Assessment (PRA). It consists in identifying the initiating events using a qualitative analysis (Failure Mode and Effect Analysis), followed by the analysis of accidental scenarios by means of the Event Tree analysis, related to plant system functions. Each sequence born from the Initiating Events has been studied in terms of frequency and consequences. Consequences to the plant itself, to the personnel and to the population have been estimated by applying ad hoc phenomenological models. The result of these analyses consists of a list of main sequences, each one characterised by its own frequency and radioactive release. Figure 1 Core of the IGNITOR Tokamak Since the environmental releases of Ignitor are only atmospheric ones, using as an input for the population dose code the meteorological data-and population distribution around the site, the critical group of population (MEI = Most Exposed Individual) has been determined. Results of the accidental sequences study are reported in Ref. 7: in particular, accidents with a frequency higher than 10-7 /y, categorized as beyond design accidents bring to a maximum dose corresponding to the worst cases accidents in term of impact on the population. Design basis accidents have lower consequences to the public. For these worst accidents, collective doses have been also calculated, and bring to negligible health effects. Environmental impact during normal operation has also been assessed, and it shows negligible individual and collective doses. Recently, due to several technical reasons, it was identified a new site, the Caorso Site, in northern Italy, a nuclear site were a nuclear power reactor was sited. The power reactor is presently under the decommissioning phase, and the site could be easily utilised for Ignitor siting and operation. The actual cost of building a new experiment can be considerably contained if infrastructures are already available on its envisioned site. The facilities of the Caorso site (near Piacenza, Italy), have been analyzed in view of their utilization for the operation of the Ignitor machine. The main feature of the site is its FUSION SCIENCE AND TECHNOLOGY VOL. 56 AUG

3 robust connection to the electrical national power grid that can take the disturbance caused by IGNITOR discharges with the highest magnetic fields and plasma currents, avoiding the need for rotating fly-wheels generators. Other assets include a vast building that can house the machine core and the associated diagnostic systems with modest modifications. A lay-out of the Ignitor plant, including the tritium laboratory and other service areas, the distribution of the components of the electrical power supply system and of the He gas cooling system are presented in Ref. 8. Preliminary Safety analyses and activities for the siting of Ignitor in Caorso are currently under way 8. Ignitor main safety requirements are the following: The IGNITOR experiment must protect the health and safety of the facility personnel and of the public, by maintaining an effective defence against hazards, IGNITOR must maintain an operation that is environmentally acceptable to present and future generations, and to satisfy the two basic requirements for environmental feasibility of fusion: 1. No need of public evacuation in case of the worst accident 2. No production of waste that could be a burden for future generations, i.e., minimisation of the production of long-lived radwaste Finally the experiment must be such to be easily sited in Italy, according both to international and to that country s regulations. The question concerning radioactive waste production and classification will be addressed in this paper for IGNITOR, according to international and Italian regulations. IV. INTERNATIONAL WASTE MANAGEMENT FOR IGNITOR A new integrated waste management strategy for fusion has been proposed within the frame of an IEA study This study addresses an integrated approach to the management procedures for active materials following the changeout of replaceable components and decommissioning of fusion facilities. We define this as the back-end of the fusion materials cycle. Just recently, both clearance and recycling concepts and limits have been revised by national and international organizations. These revisions and their consequences have been examined in the study. More importantly, a new radioactive materials management strategy has been proposed for the clearance, recycling, and disposal approaches. An integrated activated materials management strategy has been proposed: it divides the active materials according to the Regulatory Route (unconditional clearance, conditional clearance, noclearance) and the Management Route (recycling/reuse, disposal) with a matrix linking the two routes. Moreover, an approach to the technical difficulty of recycling or waste conditioning, based on a scoring system, depending on the handling and cooling requirements of the components and materials, completed the approach to make it a really integrated system. In conclusion, the parameters that govern the back-end of the fusion materials cycle are clearly defined, with a new fusionspecific approach for the entire back-end cycle of fusion materials. The proposal is a comprehensive one: it considers the evacuation routes for the waste and materials, the handling difficulties, as well as the critical issues and challenges facing all three approaches: recycling, clearance, and disposal. Concerning recycling, it is a question dealing not only with radiation protection, but also with metallurgy, materials science, shielding and remote handling techniques. A wide experience in these fields is available from fission research: a study of the application of existing techniques to fusion radioactive materials is quite useful, to assess whether and when recycling of such materials is feasible or convenient; radiological, technical, economic and strategic questions have to be considered. An example of this may be found in (Ref. 12). For the moment, no recycling of radioactive material is admitted or foreseen in Italy, mainly since this country does not have any Nuclear Program. However, since most of fusion waste comes from relatively low activated material, in shielded position from the plasma, it is appropriate to explore the possibility of finding alternative pathways for the management of such waste, in order to minimise the use of final repositories, based upon two main concepts: Recycling of moderately radioactive materials within the nuclear industry. Declassification of the lowest activated materials to non-active material (Clearance), based upon an extension to fusion of documents issued by NRC, IAEA and European Commission (Ref. 13). An international common strategy for fusion radioactive materials should be proposed, focusing especially on materials recycling: this might integrate the national regulations for the fusion case. TABLE I. International Classification of Ignitor Radioactive Materials and Components Component Material Classification Decay Time Volume (m 3 ) Vacuum Vessel INCONEL625 alloy Shielded recycling or LLW 60 years 4.4 First Wall Molybdenum Hands-on recycling 10 years 2 Magnet Copper Hands-on recycling 60 years 12.2 C-Clamp Structure AISI 316 steel Hands-on recycling (40%) 40 years 24 Clearance (60%) Cryostat Composite material Clearance 20 years FUSION SCIENCE AND TECHNOLOGY VOL. 56 AUG. 2009

4 The question concerning Ignitor radioactive waste production and classification has been addressed, applying the management procedures defined in the IEA study. The main results are the following: all the Ignitor materials can be declassified to non-radioactive materials (clearable materials), or recycled within the nuclear industry, after a relatively short interim decay time. Concerning recycling, the so-called hands-on handling (HOH) limit is fulfilled by all the materials (first wall, magnets and part of the structure), except for the vessel material. Part of the structure and the cryostat can be classified as clearable material. Results are available in Table I. V. ITALIAN WASTE MANAGEMENT FOR IGNITOR We will briefly report here the result of the application of Italian waste management regulations to Ignitor. Italian regulations deal with National Laws on radioactive materials 14, and with Technical Guides from the Italian nuclear regulatory committee ( Guida Tecnica 26 and others 15 ). Wastes are classified into three categories ( I Categoria = First category = Very Low Level Waste, II Categoria = Second Category = Low Level Waste, III Categoria = Third Category = High Level Waste) according to concentration limits for radionuclides. Without going into detail, the boundary between second and third category, for activated metallic materials, is a concentration of 3700 Bq/g for long-lived nuclides (T ½ > 100 y), Bq/g for medium-lived nuclides (5 y < T ½ < 100 y) and Bq/g for shortlived nuclides. This limit deals with waste that has been conditioned and treated for disposal. Second category may be defined Low Level Waste since it may be disposed of in surface or near-surface disposal sites, in a similar way to Low Level Waste eligible for Shallow Land Burial according to the United States Waste Regulation 10CFR61 (Ref. 16). First category waste may be defined Very Low Level Waste since it decays down to radioactive concentration levels comparable to natural substances in a maximum decay period of some years. (e.g. < 10 years). Concerning clearance (immediate declassification to non-radioactive material ), a recent regulation has been issued in Italy 17, concerning the Allontanamento (Italian word for clearance ) of solid radioactive spent materials. This regulation is necessary for the ongoing decommissioning activities of the four shut down Italian fission reactors. Concentration limits are issued for each relevant nuclide, however they may be partially summarised as follows: a non-alpha-emitter metallic material may be cleared, if its specific activity is less than 1 Bq/g. For other materials than metallic ones and concrete, the limit is 0.1 Bq/g, while for concrete the limit is almost half-way, depending on the type of nuclides 17. Recycling in Italy is permitted for cleared material only. We have applied this set of regulations to IGNITOR. Activation data were taken from (Ref. 6) and from new ad-hoc calculations. The main results of the study are the following: if Italian regulations were applied as-they-are to this experiment, no radioactive material should be classified in the Italian High Level Waste category ( III Categoria ), but the Vessel material (INCONEL625 alloy). The low volume of this component (about 4 m 3 ) shows the small safety relevance of this problem. Most of the material can be classified as LLW ( II Categoria ), with a total volume of about 35 m 3. Another remarkable point is the decay time necessary for the First Wall (Molybdenum) material to decay within the LLW limit, i.e., 100 years: furthermore, a 25% dilution is necessary to this material in order to fulfil the limit for LLW. Results are shown in Table II. Italian regulation on declassification to nonradioactive waste ( Allontanamento ) is so restrictive that, in principle, none of the considered material is eligible for this classification, however the cryostat is VLLW, i.e., it takes a relatively short time before they decay to radioactive concentrations similar to those of some natural materials. VI. CONCLUSIONS We have identified in recycling within the nuclear industry for LLW activated materials and clearance (declassification to non-radioactive material) as possible solutions to avoid their disposal into the environment. If international waste management strategies proposed within the frame of an IEA study were applied, all Ignitor radioactive materials could be recycled or declassified to non-radioactive material. In particular, Vacuum Vessel (INCONEL625), First Wall (Molybdenum), Magnets (Copper) and part of the C- Clamp (AISI316L) could be easily recycled within the nuclear industry, while all the other materials (rest of the C-Clamp and Cryostat) could be cleared, that is, declassified to non-active material. No waste disposal of any kind should be necessary. We have applied the Italian waste management regulations to the IGNITOR experiment radioactive materials. The main results of the study are the following: if Italian regulations were applied as-they-are to this experiment, no radioactive material should be classified in the Italian High Level Waste category ( III Categoria ), but the Vessel (results have been refined compared to those of Rfe. 6 with the inclusion of impurities). Most of the material can be classified as LLW ( II Categoria ), with a total volume of about 35 m 3. The rest is VLLW, that is, decays to radioactive concentrations similar to those of some natural materials after a short while. The presence of Mo instead of graphite lead to a very modest increase of the waste quantity. FUSION SCIENCE AND TECHNOLOGY VOL. 56 AUG

5 This shows how conservative Italian waste management regulations are, and also that they could need some revision in order to take into account the peculiar characteristics of fusion activated materials (lower radiotoxicity than fission waste). All these results confirm that IGNITOR fulfils the requirement of no production of waste that could be a burden for future generations, i.e., minimisation of the production of long-lived radioactive waste. TABLE II. Application of Italian Radwaste Regulations to IGNITOR Radioactive materials Component Material Classification Necessary Decay Time Vessel INCONEL625 alloy HLW (III Categoria) -- First Wall Molybdenum LLW (II Categoria)* 100 years Magnet Copper LLW (II Categoria) 25 years C-Clamp Structure AISI 316 steel LLW (II Categoria) 18 years Cryostat Composite material VLLW (I Categoria)** Less than 5 years. * A 25% dilution is necessary to fulfil the limit for LLW. ** For Cryostat, Declassification to non-radioactive waste ( Allontanamento ) could be immediately possible if 14 C was chemically extracted and separated, however this appears not to be a justified procedure, since - after a few years - Cryostat decays to radioactivity concentrations similar to those of some natural substances. REFERENCES 1. International Energy Agency (IEA) Co-operative Program on the Environmental, Safety and Economic (ESE) Aspects of Fusion Power, available at: ia=17 2. K. BRODÉN AND G. OLSSON, Review of earlier studies of waste management options for fusion in Europe, USA and Japan, Report RW-00/26, Studsvik, Sweden, April B. COPPI, A. AIROLDI, F. BOMBARDA, et al., The Ignitor Experiment, Nuclear Fusion, 41, (2001). 4. A. CIAMPICHETTI, M. ZUCCHETTI, Valutazione degli inventari di trizio nelle tegole di molibdeno all interno della camera da vuoto, Politecnico di Torino Report (in Italian) (2006) 5. I. ANTONACI, M. ZUCCHETTI, A. CIAMPICHETTI Radioactive Waste Management And Safety For The Ignitor Fusion Experiment, Fus. Eng. Des , (2005). 6. M. ZUCCHETTI, C. ZUNINO, E. GRASSA, Materials Selection and Shield Design to Improve Ignitor Operational Safety, Fus. Eng. Des., 63-64, (2002). 7. G. MURA, S. ROLLET, M. ZUCCHETTI, R. FORREST, Positive safety impact of Ignitor design modifications, Fus. Eng. Des., 51-52, (2000). 8. S. MIGLIORI, F. BOMBARDA, S. PIERATTINI, G. FAELLI, M. ZUCCHETTI, B. COPPI, Installation of the Ignitor Machine at the Caorso Site, paper submitted at the DPP08 Meeting of The American Physical Society, November 17-21, 2008, Dallas, Texas (USA). 9. M. ZUCCHETTI, L. EL-GUEBALY, R.A. FORREST, T.D. MARSHALL, N.P. TAYLOR, and K. TOBITA, The Feasibility of Recycling and Clearance of Active Materials from a Fusion Power Plant, Jour. Nucl. Mater., , 1355 (2007). 10. L. EL-GUEBALY, R. PAMPIN, and M. ZUCCHETTI, Clearance Considerations for Slightly-Irradiated Components of Fusion Power Plants, Nuclear Fusion, 47, 480 (2007). 11. M. ZUCCHETTI, L. DI PACE, L. EL-GUEBALY, B.N. KOLBASOV, V. MASSAUT, R. PAMPIN, P. WILSON, An integrated approach to the back-end of the fusion materials cycle, Fus. Eng. Des., 83, 1706 (2008) 12. V. MASSAUT, L. OOMS, Decommissioning, Decontamination and Waste Management Strategies fur future Fusion Reactors (based on current fission reactor experience), D&D SCK-CEN, Mol, Belgium, R-3768, 276/03-02, M. ZUCCHETTI, L. DI PACE, L. EL-GUEBALY, B.N. KOLBASOV, V. MASSAUT, R. PAMPIN, P. WILSON, The back-end of the fusion materials cycle, Fusion Technology, (2009). 14. Italian Legislation: Decreto Legislativo n.230, Suppl. Ord. GU n Serie generale (in italian), Decreto Legislativo n.241, Suppl. Ord. GU n Serie Generale (in Italian). 15. ENEA, Guida Tecnica n.26: Gestione dei rifiuti radioattivi, ENEA (Roma), 1990 (in Italian) 16. US Nuclear Regulatory Commission, Licensing Requirements for Land Disposal of Radioactive Waste, 10CFR part 61, US Federal Register, 47, (1982). 17. Italian Legislation: Ordinanza 11 aprile 2003 n. 5, GU n Serie Generale (in Italian). 818 FUSION SCIENCE AND TECHNOLOGY VOL. 56 AUG. 2009