The Decommissioning of Accelerator Installations, a Challenge for Radiation Protection in the 21 st Century

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1 The Decommissioning of Accelerator Installations, a Challenge for Radiation Protection in the st Century M. Höfert and D. Forkel-Wirth CERN, European Organization for Nuclear Research, CH Geneva 3, Switzerland INTRODUCTION At the beginning of the th century radioactive material like many other subjects has become a victim of globalisation, however not as a trading good but rather in its quality as an evil, looked upon with disgust and angst. The non-acceptability of radioactivity has led lawmakers to strengthen the rules and lower the limits but these measures have not changed the general attitude of the public. This is why the decommissioning of sometimes big accelerator installations that have come of age and are or will be dismantled has become a major challenge that radiation protection faces. So far, the question of how to deal with the quantities of radioactive material that are generated by accelerators had been pushed aside as, admittedly, also in radiation protection it is more interesting to plan and build new accelerators than to decommission old installations. RADIOACTIVE INVENTORY IN ACCELERATORS When accelerator beams exceed the energy of the Coulomb barrier of an atomic nucleus reactions become possible that induce radioactivity either due to direct interactions of the primary beam or indirect interactions of secondary particles in the surrounding structural materials leading to a multitude of radionuclides. This activation causes remnant ambient dose rates inside accelerator tunnels and target areas but means also that accelerator components when being replaced at the end of their operational lifetime must be treated as radioactive waste. Big quantities of activated material usually arise when a whole accelerator complex is decommissioned although in the case of the presently world s largest facility LEP, an electron-positron collider, relatively few components are activated in comparison with fixed target proton machines. LEP is housed in an underground tunnel of 7 km circumference and its dismantling will make room for the Large Hadron Collider (LHC), a machine with proton beams of 7 TeV intended for a new generation of physics experiments that will hunt for the Higgs boson. Most of the radioactive material produced in and around accelerators is of metallic nature, ranging from vacuum chambers and pumps to magnets and other beam elements. In addition, shielding materials become activated. It must be stressed that the radioactive inventory as well as the specific activity of irradiated material originating from accelerators differ considerably from the radionuclides found in nuclear power plants. In particular, no long-lived alpha activity is produced and rather relatively short-lived beta and gamma emitting isotopes are dominant. In addition, because the material around accelerators is activated in the bulk, a contamination risk only arises when machining of components is performed or material corrodes. A large experience concerning activation processes around high-energy accelerators has been acquired over the last forty years (,,3). Around proton machines the spallation reaction is the most important production path for radionuclides (). High-energy proton beams (E > 00 MeV) and secondary particles when interacting with material bring the initially stable target nuclei into a highly excited state. In a first step, protons and neutrons of these excited nuclei are ejected whilst in a second step an evaporation of neutrons takes place. Thus mainly neutron deficient radionuclides are produced having lower sometimes much lower atomic numbers than their target nuclei. These products decay mainly through positron emission and electron capture - accompanied by the emission of gamma rays - until the bottom of the valley of stability is attained. Secondary particles (p,n) produced in the spallation reaction contribute to the activation of the material too, e. g. by the process of neutron capture. One of the most well-known reaction of this type is the transformation of 59 Co(n,γ) 60 Co. A more detailed description of processes and resulting reaction products is given elsewhere (5). The specific activities of radioactive material produced in accelerator installations vary considerably. They depend on the type of accelerator, on the location of the material with respect to beam losses and the cooling time following activation but they are, in their great majority, rather low. Being activated in the bulk and neither containing high atomic number alpha emitters nor being surface contaminated it is obvious that recycling of the mostly metallic radioactive material from the accelerator environment is not only reasonable but also the most economic approach. EXEMPTION VALUES AND CLEARANCE LEVELS In the field of radioactive materials and waste there are no internationally agreed recommendations like in radiation protection. Both the International and European Basic Safety Standards only contain tables with radionuclide specific exemption limits and do not make recommendation with respect to the clearance of radioactive material. There is a fundamental difference between these two sets of values as a practice is

2 exempted from regulatory control whilst a material is cleared i. e. is no longer be considered as radioactive. Table shows in a synopsis the exemption limits of the Basic Safety standards and various sets of clearance levels for some important radionuclides typically found in accelerator materials. The obvious fact is that clearance levels are numerically smaller than exemption limits, but why can exempted radioactivity not simply be cleared. This is explained by the existence of large amounts of material with low specific activity originating from the dismantling of nuclear installations that must be treated differently with respect to their potential radiation risk than small quantities of radioactivity like low level radioactive sources. Table : Comparison of exemption limits (EL) in the Basic Safety standards (6,7) and clearance levels (CL) in IAEA TECDAO-55 () and in the Recommendation of the German Radiation Protection Commission (9). All values are in Bq/g. Radionuclide 3 H C Na 36 Cl 6 Sc 5 Mn 55 Fe 56 Co 57 Co 5 Co 60 Co 63 Ni 65 Zn 0m Ag 0m Ag 3 Cs 37 Cs 5 Eu 5 Eu 0 Tl EL in the EU and IAEA Basic Safety Standards CL for solid materials in IAEA TECDAO-55 0 * CL in Germany for the reuse without restriction , CL in Germany for the elimination as waste CL in Germany for the melting of metals However, there seems to exist an exception in Switzerland as the Radiation Protection Ordinance contains exemption limits that at the same time serve as clearance levels if an additional condition is fulfilled: when disposing off low level radioactive material the dose rate at 0 cm distance from the surface of a radioactive item must not exceed 00 nsv/h. In fact, for many radionuclides the dose from external radiation determines their radiation risk. The consequence of this condition is illustrated in figure for a massive block of copper or iron activated in the bulk containing 5 Mn and 60 Co two typical long lived radionuclides induced in an accelerator environment. It is seen that due to the dose rate limitation bigger items can only be disposed off as inactive if they have a considerable lower specific activity than the exemption limit. Clearance levels are derived when modelling radioactivity pathways leading to exposure scenarios. The basic and rather universally accepted condition is that an annual dose of 0 µsv should not be exceeded for the most exposed person of the critical group considering any practice involving the recycling or elimination of radioactive material. With a multitude of possible exposure scenarios it is no surprise that many sets of clearance levels - showing for some radionuclides differences of more than an order of magnitude - have been forwarded in recent years with no international consensus so far emerging (9). Those in an IAEA Technical Document are proposed as unconditional clearance levels i. e. there is no longer any restriction for the cleared material. In a different approach the forthcoming German legislation will contain three sets of conditional clearance levels as shown in table (,9). Exposure scenarios are in fact different whether material is eliminated as waste or whether it will be reused. Clearance levels will be higher when, like in the first case, any future contact with the low level material is considered to be improbable than levels for reuse that will lead most likely to exposures of the public. The third intermediate set in table takes the cleaning of metals in the melting process into account where some of the radioactivity initially contained in the metal is known to be transferred to the slag.

3 ACCEPTABILITY OF RADIOACTIVE CLEARANCE With the lack of internationally accepted clearance levels the situation becomes rather difficult with respect to the elimination of low level materials as for all material and waste that enters into the public domain one has to assure that no radioactivity is detectable. This means in practice that dose rates at the surface of inactive items must not exceed a couple of nsv/h, i. e. remain within the variation of natural background radiation and furthermore that the specific activity for 60 Co in the items is below 0. Bq/g. In fact, metallic scrap still giving rise to small dose rates but cleared in Switzerland, which had been exported to Italy to be melted, was stopped at the border when the residual radioactivity was detected using sensitive detection equipment. Presently neither Italy nor France have introduced clearance levels. In the case of Italy decisions whether scrap metal containing traces of radioactivity can be melted are taken on a case to case basis considering the natural dilution in the process. In France the unconditional clearance is not allowed and very low level radioactive material can only be disposed off via predefined and approved elimination pathways (filières) where the traceability of the material must be fully assured. Figure : Exemption limits (LE) of the Swiss Radiation Protection Ordinance (0) and specific activities in Bq/g that lead to dose rates of 00 nsv/h at 0 cm distance from the surface as function of the dimension of the metallic item (iron or copper) WASTE MANAGEMENT AND QUALITY ASSURANCE The management of radioactive material in a high-energy accelerator environment is a continuous process. Obsolete activated vacuum chambers or radioactive cables are delivered for radioactive storage at any time whilst bigger parts like whole activated magnets usually come in during longer shutdown periods. Generally the material is left in pre-storage requiring some radioactive cooling prior to any further handling. Based on the most stringent legislation in the Host States France and Switzerland CERN has its own and unique rules in operational radiation protection accepted by the Authorities and covering the whole domain of the Organisation. The situation is more complicated with respect to radioactive waste. Here not only national laws apply when considering the final disposal of radioactive waste into France or Switzerland but the Authorities also have imposed measures for its treatment while the material is still at CERN as the sites of the Organisation remain national territory. The precept of pre-conditioning radioactive waste is one of these requests where French waste must be pre-conditioned on site in containers homologated in France whereas Switzerland requires the use of their containers as only those are acceptable in their final depository. At present accelerator waste is neither accepted in France nor in Switzerland along the argument that, contrary to waste coming from nuclear industry, its radionuclide inventory is not well known. In particular, the national depositories claim that unknown radionuclides as the result of high-energy spallation reactions will add to the normal radioisotope inventory due to classical (n,γ) and (γ,n) reactions. CERN and the Paul-Scherrer- 3

4 Institute (PSI) have started a pioneering collaboration on the radionuclide composition in activated accelerator material. Both Monte-Carlo techniques and measurements are used to quantify the radionuclides formed where simple gamma spectroscopy in many cases is not sufficient. Chemical methods must be used to determine radionuclides like 55 Fe of 3 H in solid materials. Lately the French Authorities required a quality assurance system for all waste management on the French site which CERN in applying unified rules on its territory must extend to the whole domain of the Organisation. In the particular, in the case of radioactive waste the assurance of traceability is most important. The three basic principles of ICRP must be implemented. This means that a practice producing radioactive waste is only justified if the proper elimination is an integral part of the practice. Any handling of radioactive material and waste must be optimised with respect to the personal doses involved and finally the production of radioactivity must be limited. ELIMINATION STRATEGIES In case the complete radionuclide inventory of accelerator waste material is known its elimination towards the national agencies in charge of the final depository in CERN s host countries becomes possible but is very expensive. The price for the final storage in the Swiss national depository calculated per cubic metre has been increased recently and now amounts to 000 U$ for the usual metallic drum of 00 litres. This, however, is not the full cost as packaging and shipping - although small compared with the elimination fees - is not included. In view of the high costs it is important that all possibilities of reducing the amount of radioactive waste to be disposed off are used as much as possible. In a first step all the radioactive waste at CERN is placed into four convenient and practical categories: Material is put in a Category 0 when in a relatively short time (up to a year) the specific radioactivity will have dropped well below agreed clearance levels. A typical example is 7 Be as a spallation product produced in air and retained in ventilation filters. Category I comprises all material where it can be assumed that the specific activities of the radionuclides contained will drop below clearance levels after 30 years of cooling. For this Category an approximate upper limit on specific activity for material that underwent an initial radioactive cooling time of at least one year is 00 Bq/g. Finally, in Categories II and III all material of higher activity are placed subject to an eventual elimination as radioactive waste where Category III comprises the few items of very high specific activity like production targets, beam collimators and dumps. Material delivered as waste is pre-conditioned. That means in particular material separation, cutting and volume reduction important also for radioactive material of Category I that must be stored for longer periods before it can be released as non-active. Items like whole magnets or other complex accelerator elements are sometimes dismounted to separate materials but also in view of taking off any Category II material of higher activity. Big pieces are sometimes cut to ease future handling. Aluminium vacuum chambers are pressed into cubes. The insulation is either cut off cables or becomes separated from the metal when cables are shredded. Experience shows that insulation material generally falls into Category 0. Although the chosen scheme was agreed upon by the Swiss Authorities and falls in line with a natural characterisation of radioactive waste in an accelerator installations it has to be seen whether such a scheme is acceptable to the French Authorities. CONCLUSIONS In view of the large amount of mostly metallic and only slightly radioactive items originating from the operation of accelerators and considering the high costs for their elimination as radioactive waste it is important that other pathways like the reuse of these materials are investigated. The often mentioned pathway of recycling iron scrap as shielding blocks is rather narrow and economically not feasible as casting blocks from virgin material turns out to be less expensive. Even for cleared material pathways do not exist, as steel mills do not accept such material that would however benefit from further dilution through mixing with inactive material. Note that such an approach is perfectly legal contrary to the dilution of radioactive material with the aim to arrive with the mixture at specific activities below clearance levels. As long as the public does not accept materials that have the flavour of radioactivity any efforts for their recycling and subsequent reuse are doomed. REFERENCES. Marcel Barbier, Induced Radioactivity, North Holland Publishing Company, Amsterdam (969).. A.H. Sullivan, A Guide to Radiation and Radioactivity Levels Near High-Energy Particle Accelerators, Nuclear Technology Publishing, Ashford (99). 3. R.H. Thomas, G. R. Stevenson, Radiological Safety Aspects of the Operation of Proton Accelerators, IAEA Technical Reports Series No. 3, Vienna (9).. R. Serber, Phys. Rev. 7,, (97). 5. D. Forkel-Wirth and M. Höfert, On the Release of Radioactive Material Produced at High-Energy

5 Accelerators, Presented at the nd International Symposium on Release of Radioactive Material from Regulatory Control, Hamburg, November 999, Report CERN-TIS-99-0-RP-CF, (999). 6 International Atomic Energy Agency, International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources, Safety Series No 5-I (99). 7. Directive 96/9/Euratom du Conseil du 3 mai 996 fixant les normes de base relatives à la protection sanitaire de la population et des travailleurs contre les dangers résultant des rayonnements ionisants, Journal officiel des communautés européennes, No L. 59 du 9 juin 996, page, (996).. Empfehlungen der Strahlenschutzkommission : Freigabe von Materialien, Gebäuden und Bodenflächen mit geringfügiger Aktivität aus anzeige- oder genehmigungspflichtigem Umgang, (99). 9. Proceedings nd International Symposium Rlease of Radioactive Material from Regulatory Control, TÜV Nord Akademie, Hamburg (999). 0. Ordonnance suisse sur la radioprotection du juin 99 (99). 5