The nuclear installation decommissioning process is characterized by production of large amount of various radioactive and non-radioactive waste that has to be managed, taking into account its physical, chemical, toxic and radiological properties. Waste management is considered to be one of the key issues within the frame of the decommissioning process. During the decommissioning planning period, the scenarios covering possible routes of materials release into the environment and radioactive waste disposal, should be discussed and evaluated. Unconditional and conditional release to the environment, long-term storage at the nuclear site, near surface or deep geological disposal and relevant material management techniques for achieving the final status should be taken into account in the analysed scenarios. At the level of the final decommissioning plan, it is desirable to have the waste management scenario optimized for local specific facility conditions taking into account a national decommissioning background. The analytical methodology for the evaluation of decommissioning waste management scenarios, presented in the paper, is based on the materials and radioactivity flow modelling, which starts from waste generation activities like pre-dismantling decontamination, selected methods of dismantling, waste treatment and conditioning, up to materials release or conditioned radioactive waste disposal. The necessary input data for scenarios, e.g. nuclear installation inventory database (physical and radiological data), waste processing technologies parameters or material release and waste disposal limits, have to be considered. The analytical methodology principles are implemented into the standardised decommissioning parameters calculation code OMEGA, developed in the DECOM company. In the paper the examples of the methodology implementation for the scenarios optimization are presented and discussed.
The computer code OMEGA was developed for evaluation and optimisation of decommissioning project planning. It implements the standardised cost structure PSL (Proposed Standardised List) developed by IAEA, OECD/NEA, EC [1], as the universal structure for calculation and optimisation of decommissioning scenarios. Dismantling of technological systems especially in radiation ambient is the most important among decommissioning activities.
Recently developed computer code OMEGA for evaluation and optimisation of decommissioning options implements the standardised cost structure (IAEA, OECD/NEA, EC, 1999) as the universal structure [PSL] for calculation and optimisation of decommissioning scenarios. One of the groups of decommissioning activities is the dismantling of systems and structures. These activities variously depend on the complexity of dismantled systems, e.g. reactors, equipment of the primary systems or standard components like pipes, valves, motors, tanks etc. Type and extent of decommissioning activities depends also on local conditions for dismantling like dose rate, decommissioning equipment category, local working conditions, etc. These factors determine the selection of techniques for dismantling, depending on material and radiological status of the equipment (type of technique and their manual or remote application). This approach enables proper planning and performing of individual decommissioning phases.
The nuclear facility decommissioning process is characterized by production of large volumes of radioactive materials to be managed. The materials with radioactivity slightly above the defined unconditional clearance levels could be released conditionally for a specified industrial application in accordance with developed scenario ensuring that the limits for radiation exposure of workers and public will not be exceeded. The paper presents the methodology for identifying and assessment of the physical and radiological parameters of material items suitable for using in defined scenarios of conditional reuse of materials resulting from the decommissioning process. The calculation of mentioned parameters is done by using the analytical code OMEGA with implemented calculation stream for evaluation the conditional release process. The necessary input parameters for the OMEGA code are maximum concentrations of each radionuclide in the conditionally released materials which do not cause the doses above the limit values for the possible scenarios of further use of materials outside the nuclear site. The examples of such scenarios are assessed and presented in other related papers proposed for the ICEM conference (e.g. papers ICEM2011-59120 or ICEM2011-59128). Based on the inventory database of physical and radiological characteristics of decommissioned facility, the calculation algorithm separates the relevant material items with activity concentrations between unconditional and conditional clearance levels. The amount, material composition and radioactivity of such material items are declared as output parameters of the calculation and are also discussed in the paper. Such data could be used as one of the bases in the benefit analysis of potential application of conditional release within the frame of decommissioning waste management.
Calculation of personnel exposure is a one of the main parameters being evaluated within the pre-decommissioning plans together with other decommissioning drivers such as costs, manpower, amounts of RAW and conventional waste and amount of discharged gaseous and liquid effluents. Alongside with manpower, the exposure is an indicator of the decommissioning process for need of staff, and quantifies impact of decommissioning on personnel from the radio hygienic point of view. At the same time it indicates suitability of individual work procedures use for decommissioning activities. For this reason it is important to estimate as precise as possible demands on personnel exposure even during preparatory decommissioning phase to quantify impact of decommissioning on personnel and eventually optimize the decommissioning process, if needed. The most appropriate way of staff exposure estimation during decommissioning preparatory phases is its calculation based on radiological and physical characteristics of equipment to be decommissioned and also quantitative and qualitative characterisation of typical decommissioning activities. On one hand, the methodology of exposure calculation should allow as much as possible realistic description and algorithmisation of exposure ways during decommissioning activities. On the other hand the calculation have to be systematic, well-arranged and clearly definable by appropriate mathematic relations. Calculation can be made by various approaches using more or less sophisticated software solutions from classic MS Excel sheets up to the complex calculation codes. In this paper, a methodology used for personnel exposure calculation and optimization implemented within the complex computer code OMEGA [1] developed at DECOM, a.s. is described.
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