It is quite some time since the CERN Radiation Protection Group last submitted a contribution to the Accelerator Radiation Safety Newsletter. This update on our work and other aspects is intended to be a new start for offering contributions more regularly, as in the past. Because of the gap since our last information, we have prepared a summary of activities and results of our work in the years 2000 and 2001. Future contributions are likely to concentrate more on one or a few specific topics.
In early 2000, the leadership of the Radiation Protection Group was passed from Manfred Höfert to Hans Menzel. At the end of June 2000, Manfred retired from CERN, and he lives now in Freiburg, Germany. His retirement was marked by a scientific workshop organized in his honor and attended by many of his colleagues and friends from CERN and from numerous European institutes. (By the way, Manfred has his own homepage).
In May 2001, the organizational structure of the Radiation Protection Group was modified in order to adjust the allocation of human resources to changed needs in operational radiation protection, radioactive waste management, and radiation safety studies for new installations and projects, in particular for the Large Hadron Collider (LHC). This restructuring was also in response to recommendations given by a panel of experts that reviewed in 2000 the CERN Radiation Protection Group and its work. The new structure is also expected to contribute to an improved transfer of knowledge from senior and experienced staff to younger radiation protection physicists and technicians.
The changes included the creation of a section dedicated to radioactive waste management and of a new section responsible for the operational radiation protection for the On-Line Isotope Mass Separator (ISOLDE) and the Neutron Time-of-Flight (n-TOF) Facility (as well as for individual monitoring at CERN). The section concerned with instrumentation and logistics, which, among others, is responsible for the maintenance and installation of fixed radiation monitors at CERN, has been transferred from another Technical Inspection and Safety (TIS) group (TE) and integrated into the Radiation Protection Group. Finally, steps have been taken to complement the vertical structure by a horizontal approach in terms of projects in which staff from different sections within the group, from different groups within the TIS Division, and from other divisions are involved. An example of this is the project group on radiation monitoring for the future LHC (RAMSES project, see below).
If you are interested in learning more about the Radiation Protection Group and its current activities you can visit our new web site.
Among the wide range of responsibilities and duties of the Radiation Protection Group, the involvement in the dismantling of the Large Electron Positron (LEP) accelerator and experiments stands out in 2000 and 2001. Therefore a more detailed report is given on this work.
The future installation of the LHC in the LEP tunnel required the dismantling of LEP after 11 years of operation. Before dismantling could start, a complete assessment had to be undertaken of the expected amount of low-level radioactive material to be removed during the accelerator decommissioning. The induced radioactivity in electron accelerators is far less important than in proton accelerators. However, LEP was classified as a nuclear basic installation (installation nucléaire de base, INB) in France, and the decommissioning procedure had to follow the corresponding restrictive INB regulations. The French law does not provide unconditional clearance levels (i.e., threshold values below which the material can be regarded as nonradioactive) in terms of specific activity in materials to be released into the public domain. Material may only be released if a comprehensive theoretical study -- supported by measurements -- proves which parts of the machine have or have not been subjected to activation.
To demonstrate that most of the LEP components were "nonradioactive," proof had to be provided that beam losses around the ring, or any other activation mechanism, could have only produced "insignificant" amounts of radioactivity. For the zoning study, 1/10 of the exemption limits as given by the European Directive (EU) in any material were taken as an operational limit. For most radionuclides found in accelerator components, the EU exemption limit is 10 Bq/g (e.g., exceptions are tritium and beryllium-7, for which the values are 106 Bq/g and 103 Bq/g, respectively).
A total of around 30,000 tons of equipment had to be removed from the LEP machine areas and a further 10,000 tons from the experiments. This material was divided into four categories: equipment to be stored for future re-use at CERN or somewhere else, either "conventional" (i.e., free from activation) or radioactive, and material to be eliminated as a waste, again either conventional (sold as scrap) or slightly radioactive (stored on the CERN site).
Four sources of activation were identified in LEP: distributed beam losses, localized beam losses, synchrotron radiation, and high-energy x-rays emitted by the superconducting radiofrequency (RF) cavities. These sources were investigated by Monte Carlo calculations and gamma spectrometry measurements. The experimental data and the Monte Carlo results were used to establish conversion coefficients from unit lost beam power to induced specific activity in the various LEP materials for all radionuclides with half-lives longer than 60 days. This method can be regarded as an extension of the approach used by Swanson in his textbook (IAEA Technical Reports Series No. 188) to evaluate induced activity in linear electron accelerators. Swanson's data can, in fact, only be used to calculate the radioactivity induced in materials directly irradiated by a high-energy electron beam totally absorbed in the material. They could not be employed in the case of LEP, where the problem was rather to assess the weak radioactivity induced in materials by the stray radiation generated by beam losses in nearby components. These conversion coefficients were used to predict the induced activity in the LEP materials for typical beam loss scenarios -- a maximum of a few watts of beam power lost locally.
A similar study was conducted for the four large experiments L3, ALEPH, OPAL, and DELPHI, by evaluating the four potential sources of induced radioactivity: e+/e- annihilation events, two-photon events, e+/e- Bhabha scattering events, and beam-related radiation (i.e., synchrotron radiation and off-momentum beam particles).
Decommissioning started early in 2001 and was essentially completed by February 2002. Every single piece removed from the machine tunnel and from the experiments was checked for induced radioactivity in the underground areas. A second check was performed via a highly sensitive gate monitor, equipped with six large-area NaI scintillator detectors, on all transports leaving the CERN site. Complementary gamma-spectrometry measurements were performed on samples of the various materials. The measurements have, to a large extent, confirmed the predictions.
The individual doses for the entire dismantling operation were very low, with a collective dose for the almost 250 people involved of less than 7 mSv. The maximum individual dose was less than 0.6 mSv.
From a radiation protection point-of-view, the radiation checks performed in the underground areas were very accurate. In fact, only a few trucks set off alarms at the gate monitor, indicating that weakly activated items had escaped the first measurement. The gate monitor was very reliable and allowed detection of very weakly radioactive materials. The final measure of the success achieved is that none of the about 1600 lorries which left CERN with metallic scrap came back because of detection of radioactivity when received by the scrap dealer.
In 2000, the outsourcing of the provision and evaluation of individual neutron dosimeters had been prepared. In 2001, a contract was concluded and the Individual Dosimetry Service received the first batches of neutron dosimeters from a commercial supplier in April. Initial steps have been taken with a view to outsource the supply and evaluation of dosimeters for all types of radiation -- gamma, beta, and neutron -- by the middle of 2003. The total collective dose for all persons supplied with a CERN personal dosimeter has decreased. This is due to several factors, including the reduced work in experimental areas. Another reason is the reduced number of individuals with individual doses exceeding 1 mSv due to improved optimization of work in high-dose-rate areas.
Operational radiation protection is a continuous task for the group at any time of the year; however, the workload is particularly high in the shutdown periods. In 2001, this increase was even more pronounced than usual due to the termination of the running of LEP and associated dismantling work at LEP and the Super Proton Synchrotron (SPS).
During the replacement of cooling water pipes in the PS, asbestos was found in the pipe insulation. This called for the application of specific working procedures because of the mixed radiological and chemical hazard.
Operational radiation protection at ISOLDE differs from that at the rest of CERN in several ways. In particular, there is a risk for contamination with and incorporation of radioactive substances. As a consequence (and in order to improve the efficiency of the use of the ISOLDE facility), the Proton Synchrotron (PS) Division has initiated a "consolidation program" for ISOLDE, and the Radiation Protection Group has increased its surveillance efforts by increasing the allocated manpower. The consolidation program is aimed at improving the operational radiation safety and is well under way. In 2001, various improvements of the experimental hall have been planned in close collaboration with the TIS Division and the Radiation Protection Group (TIS-RP). The hall is soon expected to meet the regulatory requirements for a radioactive laboratory of type "C." This will formally permit the handling of the significant activity amounts usual for ISOLDE.
The quantities of radioactivity released into the atmosphere during the normal operation of the CERN accelerator installations in 2000 and in 2001 were well below legal limits, and the radiological impact remained negligible. The quantities of long-lived radionuclides drained from CERN into watercourses were small as well. The effective doses to the critical groups of the population due to these radioactive releases were far below the limit of 200 microSv. Therefore, concentrations of all possible radionuclides that may have been released from CERN into the environment stayed below the emission limits. This was confirmed by analyzing a number of environmental samples.
In 2000 and 2001, the measurements of doses from stray radiation indicate full compliance with CERN's policy of limiting the dose at the site boundary to less than 1500 microSv. No member of the general public should have received an annual effective dose in excess of 300 microSv due to the operation of CERN facilities. In fact, the maximum dose from CERN activities received by the most affected critical group of the population amounted to less than 17 microSv, which is about 6% of the dose limit and approximately 2% of the ambient dose from natural sources in the region. This is roughly 10 times less than variations of ambient natural doses in the Geneva region and the Pays de Gex.
Several members of the Radiation Protection Group are involved in studying and providing guidance and recommendations concerning radiation safety and protection issues for the future LHC and CERN Neutrino Gran Sasso (CNGS) facilities. They participate actively in numerous LHC and CNGS committees, in order for them to be informed about the progress in design details. The activation of different materials including air and water is being calculated, as well as the remnant dose rate together with shielding requirements, all being calculated using the FLUKA program.
Detailed Monte Carlo studies of radiation shielding in the ECA4/ECX4 area of the SPS were performed. This SPS area is very critical during LHC and CNGS operation because the SPS beam will be extracted from here for injection into LHC and CNGS. The studies included also the verification of the radiation shielding for the present SPS beam situation and resulted in an improvement of the existing shielding. The calculations for the future SPS beam were completed and recommendations for the necessary shielding were made. Radiation studies are also being made in order to determine access conditions for the injection tunnel into LHC and CNGS.
The radiation levels involved in the operation of the future LHC RF cavities, in particular during conditioning, were determined by an extensive measuring campaign. The results of the measurements were used as input for Monte Carlo simulations necessary to determine the radiation shielding and the access conditions for the RF region in LHC.
The n-TOF collaboration completed the commissioning of their facility composed of a lead target (which is bombarded by 4-GeV protons), beam line, and experimental area. Measurements by the Radiation Protection Group revealed significant activation of concrete and air close to the target. Additional shielding has been installed in order to improve the situation.
In continuation of work carried out by an earlier working group, in 2001 a project group with members of the Radiation Protection Group and two other groups was set up in order to prepare the future Radiation Monitoring System for the Environment and Safety (RAMSES) for the LHC. The types of monitors range from area monitors in zones accessible during LHC operation and during shutdown periods to instruments monitoring stray radiation and air and water releases. An internal CERN workshop was organized to inform all divisions and groups involved on the plans of RAMSES and to obtain feedback. Tests of industrial products have been carried out. The experience gained will be used to define the technical specifications for the tendering procedure.
The Radiation Protection Group provides a number of services to other divisions in CERN. This forms a significant fraction of the total workload of the group.
A total volume of about 400 m3 of radioactive waste was received in the radioactive waste center in 2000, and in 2001, a similar amount was received and partially preconditioned according to internal radiation protection rules. The delivery of radioactive material coming from the dismantling of LEP as well as from the SPS increased progressively in 2001. The organization of the reception, interim storage, and traceability of all components required a considerable effort by the new waste section. The material has been stored in areas within the tunnel of the former Intersecting Storage Ring (ISR). During 2001, a new building was established dedicated to the interim storage of depleted uranium (DU), defined as matière nucléaire de base (basic nuclear material) in French legislation, including DU coming from the dismantling of the L3 experiment in LEP.
A new CERN policy for the management and treatment of radioactive waste was developed in 2001. This policy is needed to improve the storage conditions further, to meet current technical and regulatory standards, and to rationalize the approach to the financial implications of radioactive waste management. Core elements of this policy are the establishment of a CERN interim storage facility including a special waste-conditioning center, the establishment of waste elimination pathways to national repositories, and the minimization of radioactive waste in future installations.
The group is also responsible for monitoring the safety of lasers used at CERN and for the storage and handling of DU. Together with the Experimental Physics (EP) Division, a total of 281 tons of DU from the UA1 experiment stored in Building 265 were removed, packed, and placed in containers ready to be shipped to the USA in early 2002. 212 tons of DU from the L3 experiment were transferred from LEP Point 2 to an interim storage area on the Meyrin site.
As in previous years, the CERN-EU High-Energy Reference Field (CERF) facility (which simulates the cosmic-ray radiation field in the atmosphere at 10-12 km altitude) was operated by the Radiation Protection Group for two periods and used by participants involved in radiation protection studies for aircrew from several European and non-European countries.
Environmental Releases and Exposure of the Public
New Installations and Projects
Services of the Radiation Protection Group and Supporting Activities