Radioactive waste management and clearance of accelerator waste at CERN

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1 Radioactive waste management and clearance of accelerator waste at CERN Luisa Ulrici *, Pierre Bonnal, Doris Forkel-Wirth, Matteo Magistris, Hans-Georg Menzel CERN, European Laboratory for Particle Physics, 1211 Geneva 23, Switzerland. Abstract. The European Organization for Nuclear Research (CERN, Geneva, CH) has been operating accelerators for high-energy physics for more than 50 years. The interactions of the accelerated particles (for example protons up to 450 GeV and soon up to 7 TeV) and their secondaries with matter in various nuclear processes lead to the activation of accelerator components and other material. The resulting range of radionuclides depends on the irradiation history and the composition of the material. If accelerator components come to the end of their operational lifetime they will be disposed of as waste. This waste requires radiological characterization in order to be either declared as radioactive waste or, if appropriate, to be cleared and released. Different methods for the evaluation of the radionuclide inventory of activated components are currently under investigation at CERN. Due to its international status, CERN defines and applies its own set of regulations for operational radiation protection, which are comparable, but not necessarily identical, to those of the two CERN Host States: France and Switzerland. In the context of radioactive waste management in general - and of clearance in particular - however, CERN has to take account of host regulations. The differences between the French and Swiss regulations have a practical impact on the procedures to be applied at CERN. This paper provides a description of operational radioactive waste management at CERN, with focus on the methods for the radiological characterisation of the waste. Examples of the application of clearance and a comparison between the Swiss and the French regulations in this field are provided. KEYWORDS: clearance; low-level radioactive waste; high-energy accelerators, radionuclide inventory. 1. Introduction The European Organization for Nuclear Research (CERN) has been operating a number of accelerators for fundamental and applied research for more than 50 years. The machines, located near Geneva on both French and Swiss territory, are designed to accelerate particles to energies of 33 TeV (lead ions), 7 TeV (protons) and 100 GeV (electrons, positrons facility dismantled in 2000). The operation of CERN particle accelerators entails the production of radioactive waste, which results from maintenance activities as well as upgrading and dismantling of the accelerator components and nearby infrastructures. CERN has continuously applied specific procedures and methods for the management and temporary storage of radioactive waste. These are based on a system of technical procedures and regulations that CERN has derived from the national regulation of the two Host States (France and Switzerland) and adapted to its specific status of international Organization. CERN is equipped with a temporary storage for radioactive waste, where all waste originated in CERN installations (e.g. accelerators, laboratories, experimental areas) is stored prior to its elimination outside the Organization. No elimination pathway can be defined without the knowledge of the radionuclide inventory, which is the result of the radiological characterisation of the waste. CERN still stores historical radioactive waste, for which the determination of radionuclide inventory is a complex procedure. 2. CERN operational radioactive waste management Radioactive waste are received, treated and radiologically characterised in the CERN Waste Treatment Centre, where pre-conditioning techniques are applied in order to reach a safe and stable configuration of the waste prior to their storage. * Presenting author, luisa.ulrici@cern.ch 1

2 CERN facilities straddle across the Swiss-French border. CERN has established a set of Safety rules which are applicable on its site in order to guarantee protection against ionizing radiation to people and to the environment. In the contest of radioactive waste in general - and of clearance in particular - CERN has to take into account the regulations of the two Host-States. The two Host-States have stipulated agreements with CERN in form of bilateral conventions which define, among other issues, the commitment to accept radioactive waste generated at CERN in proportions related to the geographical position of the installation where the radioactive waste is created. The management of radioactive waste at CERN is characterized by the following actions reception, preliminary radiological characterisation (dose-rate based), registration in the database; temporary storage in safe conditions. This step could involve volume reduction and separation of components according to their material composition. Separate storage is provided for waste originated on Swiss and French territory, according with the agreements taken with the Host- States; minimisation of induced radioactivity by appropriate choice of material at the design stage of the facility; periodical elimination of the radioactive waste for which an elimination pathways exists (e.g. burnable waste). The steadily increasing amount of radioactive waste in the last years, together with the limited disposal campaigns, is leading to the saturation of CERN storage areas. This situation resulted in the definition of a project for the implementation of a new policy in the radioactive waste management, which started in 2003 with the following main goals: enlargement of the current temporary storage facility within CERN; establishment of a new waste treatment centre establishment of new waste elimination pathways. The basic step for the implementation of this project is the complete knowledge of the types of waste, radioactivity level, amount of materials and their history, the legal requirements for their handling and the expected disposal pathways. This is part of a comprehensive management system for the radioactive waste that considers the life of a waste from the source to its final elimination. 3. Production of waste 3.1 Type of waste The majority of the radioactive waste from a high energy accelerator is of solid, metallic nature and consists of magnets, vacuum pipes and other components, as well as material from the infrastructure (e.g. electrical and signal cables, electronic cards, iron and concrete shielding and support structures). The typical materials which are used for the construction of particle accelerators are: iron and zinc, especially for magnets and cable trays; copper, used for the coils of the magnets and for electric cables; normal and stainless steel, used for supports, pipes for water cooling systems and machine components; aluminium for power cables and pipes; plastics and resins, used as insulator of electric cables; graphite, for collimator jaws, beam absorbers and beam dumps; concrete, used for walls and as biological shielding from radiation; earth, which is exposed to radiation in the case of underground facilities. 2

3 The heaviest element which can be encountered is lead, used as shielding material against photons and high-energy electrons. In order to minimize the activation of lead, its use should be avoided during operation of hadron accelerators. The list of materials given above covers the majority of CERN solid waste. However, within the same family (e.g., steel) there are actually many different types of material which differ in density and presence of trace elements. The trace elements are particularly important for neutron capture where high cross-sections can compensate for the small content. 3.2 Activation mechanisms From a physics point of view, the events responsible for material activation are the nuclear interactions. In particle accelerators the range of possible reactions is very large and includes neutrons, protons, photons and pions with energy range from a fraction of ev (netrons) to a few TeV (protons). Charged particles trigger electromagnetic reactions like Compton, photoelectric effect and pair production. The resulting electrons and photons can trigger further reactions and lead to an electromagnetic shower (EM shower). As soon as the energy of the projectile particle exceeds 10 MeV, secondary particles have enough energy to trigger further nuclear interactions, giving rise to a hadronic shower. The hadron accelerators (protons, ions) constitute the main part of the actual configuration of the accelerator chain at CERN. With the commissioning of LHC foreseen in the last quarter of 2008, about 50 km of accelerator tunnels will be dedicated to hadrons. The activation in hadron accelerators is primarily due to spallation reactions, which are nuclear reactions between high-energy particles (energy > 100 MeV) and stable nuclei of the target material. The resulting nuclides are normally poor in neutrons, instable - and therefore radioactive - and with atomic number Z lower than the original target element. Another dominant reaction channel is the production of radionuclides via absorption of thermal neutrons, like for example in the reaction 59 Co(n, γ) 60 Co. Characteristic of accelerator waste is the presence of a large number of different radionuclides, mainly neutron-poor and normally with atomic number below 70. It is expected that the residual activity produced in the components of electron accelerators is lower than in hadron accelerators. In the case of electron accelerators, the majority of the radionuclides is produced by (γ, n) reactions, or in high energy electron accelerators (like the Large Electron Positron Collider, dismantled in 2000), via (γ, xpyn) reactions. Secondary particles like protons and neutrons can at their turn produce activation. Table 1 gives a summary of the main radionuclides, measurable by gamma spectrometry, which can be found in electron accelerator waste [1]. The importance of a reaction channel is determined by the frequency of occurrence and the half-life of the product nuclide. The radioactive nuclides which are of relevance for the radiological characterization have a half-life longer than one year. The frequency of occurrence depends on the production rate, which is the product of the reaction cross-section, the particle spectrum and the material composition. It is only by considering these three factors together that one can judge the importance of the different nuclear processes on a case-by-case basis. Table 2 shows examples of the production channels of selected radionuclides produced in several materials at hadron accelerators [2, 3]. In contrast with radioactive waste originated in nuclear power plants, in accelerator waste there is low probability to produce long lived alpha emitters. Furthermore, as long as destructive machining or corrosion is avoided, the contamination risk is negligible. 4. Characterisation of waste A fundamental prerequisite for the establishment of elimination pathways is the knowledge of the residual activity in the waste and the complete radionuclide inventory. Therefore, the establishment of methods for the radiological characterization is integral part of the radioactive waste management project. 3

4 Table 1: Main radionuclides, measurable by gamma spectrometry, generated in different materials irradiated in high-energy electron accelerators. [1] Material Nuclide Most important reactions t 1/2 Aluminum Na Al(γ,2p3n), 24 Mg(γ,pn) ; 2.6 y 54 Mn Mn(γ,n), 56 Fe(γ,pn), 54 Cr(p,n) ; 312 d Copper 51 Cr Mn Mn Co Co 50 Cr(n,γ), 52 Cr(γ,n) Cr(p,n) Mn(γ,n); Fe(p,n); Cu(γ,2p4n); 27.7 d 5.6 d d 77.7 d d 58 Co Cu(γ,2p3n); 70.9 d 60 Co Cu(γ,2pn), 65 Cu(γ,2p3n); 5.27 y Lead Stainless Steel Iron-Concrete 105 Ag 122 Sb 124 Sb 203 Hg 202 Tl 46 Sc 48 V 51 Cr 52 Mn 54 Mn 59 Fe 56 Co 57 Co 58 Co 60 Co 95 Nb 48 V 51 Cr 52 Mn 54 Mn 59 Fe 56 Co 107 Ag(γ,2n) 121 Sb(n,γ) 123 Sb(n,γ) 202 Hg(n,γ), 206 Pb(γ,2pn), 207 Pb(γ,2p,2n) ; 204 Pb(γ,pn) 47 Ti(γ,n), 48 Ti(γ,pn) ; 50 Cr(γ,pn); 50 Cr(n,γ), 52 Cr(γ,n); 53 Cr(γ,2n), 56 Fe(γ,2p3n) 52 Cr(p,n), 53 Cr(p,2n), 54 Fe(γ,pn); 55 Mn(γ,n), 56 Fe(γ,pn), 54 Cr(p,n); 58 Fe(n,γ), 62 Ni(γ,2pn); 56 Fe(p,n), 57 Fe(p,2n), 58 Ni(γ,pn), 59 Co(γ,3n); 58 Ni(γ,p), 57 Fe(p,n), 59 Co(γ,2n); 60 Ni(γ,p), 59 Co(γ,n), 58 Fe(p,n); 63 Cu(γ,2pn), 61 Ni(γ,p); 96 Mo(γ,p), 97 Mo(γ,pn) 50 Cr(γ,pn); 50 Cr(n,γ), 52 Cr(γ,n); 53 Cr(γ,2n), 56 Fe(γ,2p3n) 52 Cr(p,n), 53 Cr(p,2n), 54 Fe(γ,pn); 55 Mn(γ,n), 56 Fe(γ,pn), 54 Cr(p,n); 58 Fe(n,γ), 62 Ni(γ,2pn); 57 Fe(p,2n), 58 Ni(γ,pn), 56 Fe(p,n) 41.3 d 2.7 d 60.2 d 46.6 d 12.2 d 84 d 16 d 27.7 d 5.6 d d 44.5 d 77.3 d d 70.9 d 5.27 y 34.9 d d 27.7 d 5.6 d d 44.5 d 77.7 d Table 2: Main production channels of selected radionuclides in different materials irradiated at hadron accelerators. In case of spallation reactions (Spall), the main target elements responsible for the production of the radionuclide are given in brackets [2, 3]. isotope T 1/2 Copper Stainless steel Aluminum Concrete 7 Be 53.3 d Spall (Al, Cu) Spall (C, N) Spall (Al) Spall (O, C) 22 Na 2.6 y Spall (Al, Cu) Spall (Fe, Ni) Spall (Al) Spall (Ca, Si, Al) 46 Sc 83.8 d Spall (Cu) Spall (Fe, Cr, Mn) Spall (Mn)* Spall (Fe) 54 Mn d Spall (Cu) Spall (Fe, Mn) Spall (Mn, Fe)* Spall (Fe) 57 Co d Spall (Cu) 58 Ni(n, pn); / / 58 Co 70.8 d Spall (Cu) 58 Ni(n, p) / 59 Co(n,2n)* 59 Co(n,2n)* 60 Co 5.27 y Spall (Cu) 60 Ni(n, p); / 59 Co(n,γ) * 59 Co(n,γ) * 59 Co(n,γ)* 65 Zn d 65 Cu(p,n) 65 Cu(p, n)* Ni(α, n) / / * reactions on impurities of the material. 4

5 At present there is no single method for the radiological characterization which can be applied to all items of waste from any particle accelerator, because the material activation strongly depends on the irradiation conditions, which are specific to each machine. The approaches adopted by other laboratories in Europe span from analytical calculations which are the result of a 15-year study like at the Paul Scherrer Institute (PSI, Switzerland), to high-technology measurement systems like at EU- JRC (Joint Research Centre of the European Union in Ispra, Italy) which require a consistent budget investment. The radiological characterisation of historical waste poses several technical challenges because of the large number of factors that influence the radionuclide inventory (irradiation spectra, duration of the irradiation, material composition, etc.) and the partial loss of information about some of these factors over the years (e.g. incomplete data logging of the irradiation conditions, lack of information on material composition). Specific characterisation methods are needed in order to cope with the different cases. CERN is presently developing different characterization methods, each with a different field of validity with a view to cover all the different categories of radioactive waste. The goal set in the development of a method to meet the needs of CERN and its large complex of radioactive waste is an optimization of techniques and resources in order to meet the requirements of the final repositories in France and in Switzerland on a few-year time scale and with a limited budget. These methods are briefly described in the next paragraphs. 4.1 The matrix method The matrix method is similar to the one developed and used at other research centers (e.g. Paul Scherrer Institute in Switzerland) to characterize items irradiated in secondary particle fields [4]. The method is based on the calculation of nuclide production yields for selected target materials and particle spectra, which are representative of the accelerator. The calculations require tabulated crosssections for all particles, energies and reactions of interest. For the specific case of CERN, the activating particles are neutrons, protons, pions and photons and the energy ranges from fractions of ev to a few TeV. Only part of the required cross sections exists in literature. When experimental data are not available, the cross sections can be calculated with the Monte Carlo code FLUKA [5, 6]. In addition, representative spectra should be either calculated via Monte Carlo or measured [7]. The matrix method has the great advantage of estimating the activity of virtually any nuclide, even in those special cases when the activity is below the detection limit of the available detectors or when the nuclide is a pure beta emitter and the phenomenon of self-absorption hinders the measurements. In the case of clearance, this method can be used to define constraints on the activity of gamma emitters in such a way that when these constraints are met, the activity of hard-to-measure nuclides is automatically below the clearance levels. The activity of gamma emitters can then be evaluated by means of dose rate measurements. The application of the matrix method to an item of waste requires the initial effort of calculating the representative spectrum. Inside massive objects and near the beam line the spectrum changes dramatically with position. This method can therefore be applied to thin objects (e.g., cables and electronic cards) which are fairly distant from the beam line and which do not perturb the radiation environment with self-absorption. 4.2 Direct Monte Carlo Calculation If the radiological history and the material composition are well known, the induced radioactivity can be calculated directly with the Monte Carlo code FLUKA. In particular, the latest version of FLUKA (2007) has an improved evaporation and multi-fragmentation model which allows for a more accurate prediction of induced radioactivity. Recent experiments in the CERN-EU High-Energy Reference Field (CERF) facility have shown that most nuclides produced in materials commonly used in 5

6 accelerators are predicted within 20% accuracy [3] (see Table 3). These experiments consisted in the irradiation of material samples by a hadronic beam and in the comparison of the experimental results with the corresponding Monte-Carlo calculations. The results have also shown the importance of the knowledge of the exact composition of materials: an impurity content of a material of high cross section could result in the production of important radionuclides in the final material. Example is given in Table 3 for aluminum, whose impurities are listed in column 3 and are responsible for the production of radionuclides with Z above the one of aluminum. Table 3: Ratio of the calculated (with Monte-Carlo code FLUKA) and measured specific activity for aluminum samples irradiated in a hadronic beam. Most of the radionuclides are predicted within 20% uncertainty. The presence of impurities (listed in column 3) explains of the production of radionuclides with Z above the one of aluminum [3]. Radionuclide t 1/2 FLUKA/Exp Impurities 7 Be 53.29d 0.71 ± Na 2.60y 0.76 ± Na 14.96h 0.81 ± 0.03 Mg 27 Mg 9.46m 1.52 ± 0.25 Mg 44m Sc 58.60h 1.08 ± 0.17 Fe, Mn 46 Sc 83.79d 0.79 ± 0.18 Mn, (Ti, Fe) 47 Sc 80.28h 1.04 ± 0.15 Mn, (Ti, Fe) 48 V 15.97d 1.07 ± 0.13 Fe, Mn 51 Cr 27.70d 0.86 ± 0.16 Fe, Mn 52 Mn 5.59d 0.88 ± 0.07 Fe, Mn 54 Mn d 0.96 ± 0.12 Mn, Fe 56 Mn 2.58h 1.53 ± 0.25 Mn 57 Co d 0.66 ± 0.24 Cu, Zn, Ni 58 Co 70.82d 0.82 ± 0.19 Cu, Zn, Ni The direct FLUKA calculation is particularly appropriate for large and complex items. Monte Carlo allows transporting particles through complex geometries and predicting induced radioactivity even in those cases where the radiation field changes dramatically with position. The drawback for these capabilities is that each specific study is very time consuming in terms of geometry implementation, computational time and data analysis. Moreover, the results are valid for the specific case studied but, as a general rule, cannot be extended to other cases without important investments in time. FLUKA predictions of induced radioactivity have been benchmarked in a number of occasions. Nevertheless, in order to be accepted for free-release the characterization must be completed by gamma spectroscopy measurements and cannot be based on Monte Carlo methods alone. Experimental validation of the calculations is recommendable also because of uncertainties associated to material composition and simplifications in the FLUKA geometry. 4.3 The Fingerprint method The fingerprint method is currently used in nuclear power plants, where a pre-defined radionuclide inventory (the so-called fingerprints) is scaled to the measured dose rate to obtain the final radionuclide inventory. The method has been adapted to the requirements of particle accelerators. In particular, the fingerprints are calculated with Monte Carlo simulations and the normalization is based on gamma spectroscopy measurement of samples [8]. 6

7 This method was applied for the first time on a set of irradiated targets from the ISOLDE facility at CERN - an on-line isotope mass separator facility at CERN, where a 1.4 GeV proton beam is sent to different targets to generate radioactive ions from spallation reactions. Extensive Monte Carlo simulations allowed selecting categories of targets with similar mixtures of radioactive nuclides. The fingerprint of each category was calculated both with Monte Carlo and with gamma spectroscopy. The application of the fingerprint method to the characterization of the ISOLDE targets has been validated by performing gamma-spectroscopy measurements on several targets and comparing the results with the predicted radionuclide inventory. A detailed description of the benchmark is found in references [8, 9]. Measurements and simulations were in good agreement and together could provide information on trace elements and difficult-to-detect nuclides. This method requires extensive Monte Carlo simulations to calculate the fingerprints and gamma spectroscopy measurements on the confirmation samples. The fingerprint method can be applied to a large fraction of radioactive waste from particle accelerators, namely the items which have been exposed to a space dependent radiation field (e.g., on the beam line) with a complex irradiation cycle and which are made of several subcomponents: it is therefore complementary to the matrix method. Among the disadvantages of the fingerprint method, it is worth mentioning that it is neither applicable to future machines (because no conformation samples are available) nor to items with unknown radiological history. 4.4 The statistical method All models used in the characterization, as well as the input data, have associated uncertainties. These uncertainties arise through assumptions made about the irradiation conditions and through the lack of knowledge about material composition. In those cases where the uncertainties are relatively high (e.g., mixed waste), the characterization is based on probability distributions of values rather than exact values, following the laws of the Bayesian theory. The application of the Bayesian theory to measurements of induced radioactivity has found its way in Germany - at least in the form of guidelines [10]. In the final repository in France, proof of fulfillment of the acceptance criteria can be based on statistical considerations. However, no precise guideline is given concerning the recommended statistical techniques nor the Bayesian method is mentioned. The application of the statistical method to Monte Carlo predictions of induced radioactivity is still an unexplored domain. It is evident that if there are large uncertainties in the predictions, the decision threshold is relatively small and it becomes difficult to prove that the item of waste is below the free-release values. The statistical method which is being developed at CERN can be summarized as follows: collection of the available information and choice of the probability distributions for the input values; calculations with Monte Carlo, measurements, application of the Matrix or the Fingerprint method to predict the induced radioactivity; calculation of the propagation of uncertainty and estimate of the error associated to the output values; comparison with the decision threshold, i.e. the guideline values for free-release minus the absolute error in the predictions. If the result is close to the decision threshold and if the financial impact of the decision is worth investing additional resources, the procedure can be repeated to reduce the uncertainty and thus set the decision threshold at a higher level. This method can be virtually applied to any item of waste, but it is particularly appropriate for objects with unknown radiological history and for mixed waste. 7

8 5 Elimination of radioactive waste 5.1 Elimination pathways The level of induced radioactivity in CERN radioactive waste varies considerably depending on the type of accelerator, on the location of the item with respect to beam losses and on the waiting time following activation. A majority of the material presents very low level residual activity at the time of its elimination and there is neither contamination nor alpha emitters. Recycling, after proper clearance measurement, is certainly a reasonable elimination pathway for at least a fraction of this material. There is nevertheless a fraction of waste which has detectable and - in a limited number of cases - medium levels of radioactivity. This waste must be eliminated towards final repositories respecting the laws imposed by the national authorities. The technical specifications set by the final repository for low-level waste in France and the ones foreseen for the final repository of similar category in Switzerland (in planning) require the definition of the radionuclide inventory at the best achievable precision. In general the list of radionuclide shall be as exhaustive as possible (including alpha and pure beta emitters, difficult to be assessed by measurement) and the relative activities shall be declared with an uncertainty lower than a factor of Clearance in France The French legislation does not provide unconditional clearance levels for specific activity in waste to be released on the public domain. Release of waste may only be allowed on a case by case basis if supported by a detailed theoretical study and confirmed by experimental measurements. The aim of the theoretical study, called zoning, is the classification of the installation in different zones: those that might contain radioactive waste and those where activation and contamination are impossible under normal machine operation. This distinction is made on the basis of the knowledge of the installation operation and on calculations or simulations. It must be confirmed by measurements, though it cannot be established on the basis of measurements only [11, 12]. A major constraint connected to the zoning philosophy is that components classified as radioactive in the zoning study cannot be declassified as conventional after simple measurement, no matter how accurate the measurement is in showing no traces of induced radioactivity. Waste declassification is possible only after revision of the whole zoning study, which means after reasoning on the causes and physical phenomena that prevented the waste from becoming radioactive, and a study of the possible impact of the dose to the public. Experience at CERN suggests that the zoning study shall be as realistic as possible. If the zoning study is too conservative (pessimistic), the installation s owner will eventually have to store as radioactive a large amount of waste, which would actually pose no radiological risk if cleared. On the other hand, the zoning cannot be too optimistic either, as the detection of anomalies (i.e. material classified as conventional in the waste study and measured as radioactive) could bring the dismantlement operations to the stop by the French authorities. The procedure currently applied across CERN in the absence of the zoning of a specific installation is that all components evacuated from a supervised and controlled radiation-area are considered as potentially radioactive and are systematically checked for induced radioactivity. The advantage of having the zoning of a machine during its operation is linked to the possibility to manage normal flux of equipment on a routine basis. Furthermore the determination of the waste zoning allows an estimate of the volume of waste generated by an installation during its entire lifetime, which is of outmost importance for the prevision and planning of budgetary expenses connected to waste elimination. The knowledge of the irradiation field and activation reactions, completed by the estimate of exchange rate of equipment in maintenance interventions or planned 8

9 modifications, helps in establishing a good estimate of the volume of an interim storage for radioactive waste and the related resources needed for the elimination of the radioactive waste. On the other hand, experience has shown that the zoning as defined in the French Doctrine turns out to be very costly to implement for large installations such as CERN accelerators, where only a fraction of the material is certain to be directly irradiated (e.g. dumps, collimators, etc.), while the remaining material which actually represents the largest fraction in terms of volume and weight - will be exposed to beam losses distributed over the machine (e.g., beam-gas interactions and accidental irradiation). The zoning in high-energy accelerators can be very cumbersome because of the big dimensions of the installations, the lack of information on the exact location of beam losses, the incomplete inventory of the installed components and their composition, as well as the uncertain history of irradiation in the accelerator. Furthermore, the zoning philosophy largely relies on the traceability for the identification and localization of future waste at any time of their operational lifetime. The implementation of a complete traceability system in an installation like the former LEP (Large Electron Positron collider - 27 km of circumference and several tenths of thousands of components) is very time consuming and requires important infrastructures (databases, barcode readers etc.) [13]. 5.3 Clearance in Switzerland CERN adopted in its internal radiation protection regulation the approach to free release of material as stated in the Swiss Ordonnance [14]. This regulation applies only on radioactive waste originated in CERN installations on Swiss territory. It consists in the assumption that all material or component evacuated from a radiation regulated area (for example a supervised or controlled radiation area) is potentially radioactive. Only after a complete free-release measurement aimed at the control of the respect of the limitation as set in the regulation, the material or component can be free released from regulatory control. The measurement procedure includes the respect of nuclide-specific contamination values, a dose rate measured at a distance of 10 cm below 0.1 μsv/h (after background subtraction) and a nuclide specific limitation on specific activity and total activity. Studies have shown that for massive components the limiting criterion for the free release of material is the dose rate at 10 cm [15]. These results are confirmed by the practice. In the latest years the radiation protection practice at CERN evolved towards the measurement of the dose rate at contact. A small, though measurable, deviation from the background requires a very thorough analysis to assess the possibility of free release of the material, even if the specific and total activity are well below the nuclide-specific limits. This precaution takes into account the increasing number and sensitivity of gate monitors as well as the attention paid by the public to matters related to radioactive waste. 6. Conclusions The high-energy accelerator facilities at CERN produce a wide variety and large volumes of radioactive waste. In 2003 a specific project was launched for the implementation of new practices in the management of radioactive waste: in particular the project aims at the establishment of ad-hoc methods for the characterisation of waste in view of their elimination, the implementation of new elimination pathways, the improvement of the temporary storage at CERN and the planning of a new treatment facility for the conditioning of radioactive waste. Many of the components evacuated from the accelerator facilities during maintenance work and the majority of the infrastructure material (support structures, shielding material, walls) originated during dismantling and decommissioning contain only small amounts of induced radioactivity, if any at all, and could be regarded as candidates for clearance. However, at present the approaches to clearance and the related criteria are different in the two Host States: France and Switzerland. CERN is an international organization and therefore it is self-regulated in matter of safety by a policy that is compatible with the legislation of the Host-States and that takes into account the Organization s 9

10 mission and interest. The current differences in the Swiss and French radioactive-waste regulation complicate the management of radioactive waste and the operational practices at CERN. Discussion with the relevant French and Swiss Authorities has started in order to find a pragmatic solution to be applied at CERN. REFERENCES [1] FORKEL-WIRTH, D., HOFERT, M., On the Release of Radioactive Material Produced at High-Energy Accelerators, Proc. 2 nd International Symposium Release of Radioactive Material from Regulatory Control. Hamburg, Germany, 8-10 November 1999, (1999). Ed. TUV NORD Akademie. [2] BRUGGER, M., et al., Benchmark study of radionuclides production with FLUKA, Proc. of the Conference The Monte-Carlo Method: Versatility Unbounded in a Dynamic Computing World, Chattanooga, Tennessee, USA, April (2005). [3] BRUGGER, M., et al., Validation of the FLUKA Monte Carlo for predicting induced radioactivity in high-energy accelerators, Nucl. Instr. Meth. in Phys. Res. A, NIM A, 562 (2006), issue 2, [4] TEICHMANN, S., et al., Charakterisierung und Klassifizierung radioaktiver Abfälle aus den Beschleunigeranlagen des PSI, Publication Fortschritte im Strahlenschutz: Strahlenschutzaspekte bei der Entsorgung radioaktiver Stoffe, 37. Jahrestagung des Fachverbandes für Strahlenschutz e.v., Basel, Switzerland, ISSN , 192, (2005). [5] FASSO, A., et al. FLUKA: a multi-particle transport code, CERN , INFN/TC_05/11, SLAC-R-773 (2005). [6] FASSO, A., et al. The physics models of FLUKA: status and recent developments Conf Computing in High Energy and Nuclear Physics (CHEP 2003), La Jolla, CA, USA, March 2003, (2003). [7] MAGISTRIS, M., The matrix method for radiological characterization of radioactive waste, Nucl. Instr. Meth. in Phys. Res. B, NIM B, 626 (2007), 182. [8] MAGISTRIS, M., ULRICI, L., The fingerprint method for characterization of radioactive waste in hadron accelerators, Nucl. Instr. Meth. in Phys. Res. A, NIM A, 591 (2008), [9] ULRICI, L., et al., Radionuclide characterization studies of radioactive waste produced at high-energy accelerators, Nucl. Instr. Meth. in Phys. Res. A, NIM A, 562 (2006), [10] WEISE, K., Bayesian-statistical decision threshold, detection limit and confidence interval in nuclear radiation measurement. Kerntechnik, 63: (1998). [11] DIRECTION GENERALE DE LA SURETE NUCLEAIRE ET DE LA RADIOPROTECTION, Metodologies d assainissement des installations nucleaires. Note DGSNR/SD3 n. 0349/2004, 11 Mai [12] BRIGAUD, O., Management of very low level waste from nuclear installations. Proceedings of the 2 nd International Symposium Release of Radioactive Material from Regulatory Control. Hamburg, Germany, 8-10 November 1999, (1999). Ed. TUV NORD Akademie. [13] SILARI, M., ULRICI, L., Investigation of induced radioactivity in the Large Electron Positron collider for its decommissioning, Nucl. Instr. Meth. in Phys. Res. A, NIM A, 526 (2004), [14] FEDERATION SUISSE, Radiation Protection Ordinance 22 Juni 1994 (StSV), , [15] DONJOUX, Y., Etallonage et utilisation d un détecteur de radiation microspec-2. Technical report, Ecole d Ingénieurs de Genève (1995), Diplôme de Physique Nucléaire. 10