Radiation protection during decontamination and decommissioning operations at Marcoule

Transcription

1 Radiation protection during decontamination and decommissioning operations at Marcoule Philipp Blaise (1), Jean-Louis Garcia (1), Joël Chardin (2) (1) Commissariat à l Énergie Atomique CEA/Marcoule BP 17171, Bagnols sur Cèze cedex, France (2) AREVA NC Marcoule, BP 76170, Bagnols sur Cèze Cedex, France Abstract. Natural uranium-gas-graphite reactor fuel was reprocessed at Marcoule, in southern France, from 1958 until the reprocessing plant was shut down at the end of Decontamination and dismantling operations began immediately thereafter, with the CEA as the site nuclear operator and AREVA as the prime contractor. The decommissioning program now in progress will be completed by 2040, and concerns more than 1000 rooms, metric tons of waste, and 4.3 million Man-hours. This paper summarizes the status at the end of 2006 of one of the largest decommissioning projects in the world. The dismantling program is based on prior decontamination to diminish the source term using the expertise of the plant operating personnel still present on the site. The ALARA principle is applied during the three successive phases of each project: definition of a reference scenario, optimization of the selected scenario, optimization of the operating procedures. The decommissioning techniques implemented are discussed in detail and involve nuclear measurements, personal protection, and the actual decontamination and dismantling work. All these operations require extensive preparation. These principles are illustrated through an example of a dismantling operation involving a collective dose of 450 ManmSv for more than Man-hours. The results for the period from 1998 to 2006 demonstrate the effectiveness of radiation protection during decommissioning. The occupational dose is directly related to the total number of Man-hours. Optimization of the work as the project progresses allows a reduction of about 25% in the collective doses compared with the initial projections, and the annual individual doses are less than 12 msv. It has become apparent that the organization of radiation protection departments must be adapted to the context of dismantling projects, notably in terms of the proficiency level of the radiation protection staff, involvement of the radiation protection department from the initial engineering studies, and ensuring the initial radiological characterization is as accurate as possible Keywords: radiation protection, decontamination, dismantling, project 1. General Natural uranium-gas-graphite (UNGG) reactor fuel was reprocessed at Marcoule (figure 1), in southern France, beginning in The reprocessing consisted of separating the plutonium from the spent fuel for French defense purposes. Beginning in the 1970s, UP1 also reprocessed fuel from other electricity-producing UNGG reactors. After 40 years of reprocessing activity, production ceased in the plant at the end of 1997 and the decontamination and dismantling operations began immediately thereafter. Today the site hosts 3500 employees together with 1500 persons employed by outside contractors. figure 1: view of Marcoule Philipp Blaise, 1

2 The CEA has concentrated its research program on sustainable management of radioactive materials and waste at Marcoule. As the site nuclear operator the CEA is also the contracting authority for dismantling operations at Marcoule. AREVA NC is the industrial operator of the support facilities for effluent and solid waste treatment, and ensures project management for the decontamination and decommissioning operations. A joint CEA-AREVA coordinating committee ensures a coherent radiation protection policy. The radiation protection department (SPR) of the CEA, the nuclear operator, defines the general procedures applicable on the Marcoule site. The AREVA SPR defines specific radiation protection procedures for individual D&D projects. When projects are carried out by subcontracting firms, these procedures are established jointly with each firm s radiation protection unit. Dismantling concerns the reprocessing plant, the decladding units, the vitrification facility and the fission product storage unit, and the supporting facilities for liquid effluent and solid waste treatment. These operations are scheduled for completion around 2040, in accordance with the program schedule. The support facilities are currently in operation, and will be dismantled between 2035 and The scope of the program can be summarized in a few figures. Decommissioning the reprocessing plant, the decladding units and the vitrification facility covers 1000 rooms to be dismantled, representing m 3 of contaminated and irradiating zones. The operations will produce metric tons of low- and very lowlevel waste and will require Man.hours of preparatory studies and Man.hours in situ. By the end of 2006, 6100 tons of equipment had been dismantled and Man.hours of work completed; 95% of the initial radioactivity had been eliminated with a collective dose of about 4000 Man.mSv. 2. General principles The D&D operations are carried out in accordance with the principles detailed below. 2.1 Prior decontamination Decontamination prior to the actual dismantling facilitates the subsequent tasks by allowing hands-on operation or remote operation using simple equipment. Even if subsequent teleoperated dismantling proves necessary, the reduction in radiological activity also reduces the doses incurred during equipment maintenance and waste management. Prior decontamination also permits more effective waste sorting and categorization. The liquid effluents resulting from decontamination operations are sent to the liquid waste treatment station and to a vitrification facility to concentrate the radioactivity and reduce the ultimate waste volume. 2.2 Dismantling The CEA opts for rapid dismantling when the facility entails high operating or investment cost for maintenance and surveillance, or when it contains long-lived radioelements such as plutonium or other actinides for which there is little advantage in waiting for radioactive decay. 2.3 Operator expertise At Marcoule the CEA chose to undertake the radioactive cleanup work immediately to reduce the risk levels as soon as possible and to benefit from the broad experience of the operating personnel still present on the site. 2.4 ALARA The fundamental issues of decontamination and dismantling projects in a nuclear facility concern environmental and operator protection. The CEA applies the ALARA optimization principle and selects the best technical solution based on prior examination of all the options. The methodology follows the procedure indicated in the figure 2. Philipp Blaise, 2

3 After expressing and substantiating the need to carry out the task, and after radiological characterization of the work environment, the possible scenarios capable of meeting the objective are examined. Each scenario is quantitatively assessed according to predefined criteria: cost, deadline, dosimetry, liquid and solid waste volume, technical feasibility, operator safety, security. The optimum solution then becomes the benchmark scenario, which is broken down into phases that are individually optimized by considering and analyzing variants according to the same criteria as above. Each phase continues to be improved during the third step, in which the detailed operating procedures are examined, as well as the dose limits and possible hold points. Expression and assessment of requirements Radiological characterization Inventory of scenarios Selection of optimum scenario Optimization of phases in selected scenario Optimization of operating procedures Definition of dose constraints and hold points Project execution and monitoring Results and lessons learned from experience Figure 2: Application of the ALARA principle The project is followed radiologically by continuously monitoring the occupational doses and by checking at least daily that the radiological conditions correspond to the expected values. Periodic meetings are scheduled to implement additional improvements as the project advances. The employees with the highest doses are subject to individual dosimetry monitoring. The validity of the overall project dosimetry predictions is verified after the each phase of the procedure. On completion of the project, the results and lessons learned are applied to other similar operations. 2.5 Environmental protection Environmental protection involves continual efforts to minimize the use of consumables such as gloves, protective clothing, etc., to limit power and utility fluid consumption, and to reduce the volume and activity of radioactive and chemical liquid and solid waste. 2.6 Setup and preparation Decontamination and dismantling operations require setup and preparation work that depends on the projects. The full procedure includes compiling initial data, specifying methods and equipment, developing suitable tools if necessary, validating the equipment and operating procedures on mockups and by simulation, and training the operators on mockups. 3. Techniques For dismantling programs we implement and develop techniques in the following areas: nuclear measurements, personal protection, and decontamination and dismantling operations. Each of these categories is discussed below. Philipp Blaise, 3

4 3.1 Nuclear measurements Nuclear measurements are used at every stage of decontamination and dismantling: to determine the initial radiological state, to define the provisions necessary to protect the workforce, for radiological monitoring purposes, for final inspection of the work zone, and to determine the material and waste categories. Radiological characterization can be performed: by direct measurements: dose rate measurement and surface contamination measurement directly or by smear tests. These measurements are easy to carry out and allow exhaustive verification of the premises and equipment. by measurement using more complex equipment in situ, for example gamma imaging and in situ gamma spectrometry to localize hot spots in relatively inaccessible areas. by taking samples, core sampling, or smear tests for laboratory analysis and measurement. This method allows fine measurements but requires representative samples of the premises and equipment to be characterized. 3.2 Personal protection Biological shielding materials (for example lead mats) are used for protection against external exposure. For decontamination and dismantling projects the shielding is often removable and can be repositioned as the work progresses. Cleanup underwater substantially reduces the occupational doses by using the radiological shielding properties of water. This requires real-time dosimetry monitoring of the divers combined with an intercom system. Protection against internal exposure is ensured mainly by collective shielding such as airlocks and temporary cells equipped with mobile ventilation and filtration systems. These provisions are often supplemented by individual protective equipment such as special clothing, ventilated suits, and respirators. 3.3 Decontamination techniques The purpose of decontamination is to concentrate the radioactivity into a minimal volume of waste and facilitate the task of the operators by diminishing the radiological constraints. Decontamination techniques are generally based on chemical and mechanical processes: Chemical: repeated rinsing with conventional and then specific reagents. This approach generally involves low occupational doses for rinsing process equipment. Applying this strategy eliminated 95% of the initial radiological activity. The CEA has developed solidifiable gels and decontaminating foams that reduce the effluent quantities generated when decontaminating polluted surfaces. This type of decontaminant will be used for the fission product storage tanks in the vitrification facility. Rinsing will considerably lower the dose rates and minimize the volume of high-level waste. Mechanical: additional means can be used depending on the extent of fixed contamination: vacuum removal of fine particles or solutions; swabbing; mechanical cleaning (scraping, chipping, shotblasting or sandblasting, high- and very high-pressure water jetting). These techniques are liable to generate aerosols that must be immobilized. 3.4 Dismantling techniques Cutting operations Cutting operations are carried out mainly during the dismantling phase, using industrial tools modified as necessary for working conditions in a nuclear environment. Tools are selected according to the material being cut, the accessibility and confinement of the cutting site. Steel is cut using saws (band saws, circular saws, etc.), cutting discs, flame cutting or plasma cutting processes, etc. For concrete we use jackhammers, diamond cable saws, high-pressure grit blasting, etc. When aerosols or gases are likely to be dispersed, specific confinement and filtration systems are installed. Teleoperation Teleoperation allows dismantling in highly irradiating zones and limits the difficulty of the work. Lifting and handling equipment is also operated by remote control, including telemanipulators and semiautomatic Philipp Blaise, 4

5 equipment usable remotely. For this type of work the objective of radiation protection is mainly to reduce the doses incurred during manual equipment maintenance operations. 4. Example: dismantling a filter room figure 3: view of filterroom The filter room (figure 3) was part of the off-gas treatment system in a spent fuel dissolution facility. The first step was to remove the filter elements and package them as waste, working from a new enclosure built on the roof of the filter room. The filter elements were removed and packaged under excellent conventional and nuclear safety conditions while minimizing the risks of dispersion of radioactive materials. Removal of the filter elements reduced the ambient dose rates from the initial levels of 20 to 300 mgy/h to between 15 and 50 mgy/h. Three scenarios were examined for dismantling the filter casings: Remove the casings and cut them up outside the cell in an airlock to avoid the severe radiological environment and cramped working conditions in the cell: this scenario was excluded because of the risk of breaching the containment. Decontaminate the materials prior to cutting in situ: this scenario was excluded in view of the uncertainty on the technical feasibility of prior decontamination. Cut up the casings in situ by teleoperation (figure 4) and clean the sheet metal after cutting: this option was selected as the benchmark scenario. From an occupational dose standpoint the three scenarios were equivalent. Once the solution of teleoperation was selected, and considering the ambient irradiation level, the methodological options followed logically: cut up the filter casings with a disc cutter, eliminate the internal contamination from the casings by vacuuming, bind the residual contamination in a coat of varnish, perform final cutting of the sheet metal sections, remove dust and debris from the floor by vacuuming, wash the scrap metal sheets in a cleaning tunnel, and package the waste. The initial dose estimate was about 800 Man.mSv; the ALARA approach reduced this value to 650 Man..mSv. The optimization performed as the work progressed lowered the final accumulated dose to 450 Man.mSv. The annual individual doses were less than 12 msv. The complete project lasted 4 years with Man.hours of work, and generated 50 metric tons of waste. Philipp Blaise, 5

6 figure 4: Teleoperated device figure 5: Maintenance station Optimization concerned the following points in particular: Remote-controlled devices and cutting tools were designed and developed to address all the situations likely to occur during the project. Tests were conducted on unirradiated metal structures representative of the equipment in the filter room to detect and correct equipment malfunctions, as well as to train the operators in the remote operation of each type of cutting tool. Onboard diagnostic systems limited the frequency and duration of equipment maintenance operations. To limit the doses incurred during equipment maintenance or tool-changing operations an opening (figure 5) was created to allow the work to be performed outside the irradiating zone. With increased remote surveillance and remote dosimetry, the work was carried out from a control room and waste conditioning procedures were adapted according to the measured irradiation level. The first four filter casings were cut up during a preliminary test phase; the resulting experience allowed us to modify the cutting, vacuuming, and waste conditioning operating procedures. 5. Operating results for Organization The organization was gradually modified to meet the dismantling requirements: The nonrepetitive nature of the operations led us to modify the proficiency level of the radiation protection staff, and to shift the focus from measurements to prior analysis. The radiation protection department was involved in the initial engineering studies and was thus able to evaluate the aspects related to radiological constraints. Dosimetry issues led the radiation protection department of the prime contractor and its subcontractors working in close collaboration throughout the dismantling program. A specific organization was set up for the characterization work prior to actual dismantling. A health physicist who had worked in the facilities during production operation was included in this organization. 5.2 Dosimetry Collective dosimetry The total collective dose was about 4000 Man.mSv. The collective dose incurred was about three-quaters the predicted level on average. The difference can be attributed to the optimization carried out during the project. After the initial startup period, the dose sustained per Man-hour diminished (figure6) and stabilized over the duration of the D&D program. These results can be attributed to the effectiveness of prior decontamination, the use of teleoperation, the removal of radioactive materials from the facility, and the lessons learned from experience with the first D&D projects. Philipp Blaise, 6

7 Figure 6: dose evolution msv /h YEAR Personal dosimetry The primary goal of radiation protection is to reduce the doses incurred by the most highly exposed employees. Over the entire dismantling period the maximum annual dose was less than 12 msv, and no more than two employees sustained an annual dose between 10 and 12 msv (figure 7). These results were obtained by systematic attention to reducing individual doses by all the persons involved in the dismantling program. Individual dosimetry in msv per year workers figure 7: Individual dosimetry ED = 0 0 < ED 2 2 < ED 6 6 < ED < ED < ED 20 >20 6 Conclusion This overview shows that dismantling activities are carried out with full control over radiation protection of the personnel. Radiation protection is subject to continual change as dismantling progresses, with regard to the application of radiation protection principles, D&D techniques, and organizational structure. This is the result of a determined effort to improve and to adapt to the fundamental transition from production operations to dismantling activities. The cooperation established between the new staff working on the analysis and design of D&D projects and the personnel who had previously operated the facilities ensured optimum implementation of new concepts in the field. For this project the CEA, as contracting authority, and AREVA, the prime contractor, demonstrated full control over radiation protection applied to the decontamination and dismantling of fuel cycle facilities. Philipp Blaise, 7