Visiplan and Alara

Introduction
The decommissioning of BR3 project involved three main operations :
  • the chemical decontamination of the plant primary loop ;
  • the selection and testing of techniques and tools for the remote dismantling of the highly radioactive reactor internals, and then ;
  • the dismantling of all the internals (Stainless Steel AISI 304).

    The BR3 project is a pilot for the decommissioning itself, but has also been extensively used as a pilot project for implementing operational dosimetry (see also and radioprotection optimization throughout the whole project.

    Moreover, as the regulation about Decommissioning and Decontamination (D&D) was not yet complete at the project start up, this project has been seen by the authorities as a test case for different approaches of the problem, and therefore followed in detail by an extensive exchange of information.

  • Practical cases of implementation
    The implementation of the Alara principle on the decommissioning operation was decided at the beginning of the project.
    Therefore, some help in analysing the operation (dose commitment estimate and modelling) was requested from external partners during the design and study phases of the first operations to be carried out : the pre-dismantling decontamination and the dismantling of the reactor thermal shield.
    For the primary loop decontamination, the implementation of the radioprotection optimization exercise was launched when the operation was already started.
    In fact, for this operation, the major dose uptake was due to the old design of the plant where the closure of the reactor head has to be done "hands on", in a radiation field coming from the pressure vessel itself as well as from the contamination of the reactor pool. Most of the optimisation was then concentrated on this task, and resulted in a decrease of the estimated dose commitment of about 30 %, or about 60 man*mSv, by taking appropriate actions.
    The main influencing action decided was the reduction by a factor of two of the number of studs to be fastened, the reactor operating during the decontamination at a much lower pressure than during normal operation (about 20 bar instead of 150 bar).
    The foreseen dose uptake for the whole preparation tasks amounted finally to 139 man*mSv (starting from 200 man*mSv first prevision), and the actual preparation tasks gave rise to about 135 man*mSv. The rest of the decontamination operation was less important for the dose commitment, and the total collective dose uptake for the whole operation, including the evacuation of solid and liquid waste amounted to 159 man*mSv.

    The preparation and the performance of the dismantling of the reactor thermal shield was also analysed on the radioprotection optimization point of view. A complete detailed analysis of the operation was carried out and the whole operation was divided into jobs, tasks and subtasks with attached manpower and ambient dose rate.
    This analysis did not directly influence the work and way of operation but was a very efficient help in analysing afterwards the complete job and determining the important parameters for the dose reduction or for subsequent estimation of dose commitment.
    This analysis showed, for instance, the importance of the "learning effect".

    Impact of the radioprotection optimization
    The application of the "Alara principle" and the optimization of the radioprotection has got different impact on the operations, the strategies and the organization of the works : this will be explained a bit hereafter.

    Impact on the operations

    The main operations involved in the decommissioning of the BR3 reactors were analyzed following different points of view : on the cost minimization, the technological feasibility, the waste minimization and the reduction of the occupational exposure and dose uptake.
    This last one is presently an obliged way to initiate any operation. The dose estimate and the means used to reduce the total dose commitment is part of the operation file to be submitted before the activity starts.

    The impact was first psychological for the responsibles of the individual operations : indeed the obligation to estimate the dose commitment and to reduce it as far as possible induced progressively an "Alara reflex" in the technological approach of any operation. Moreover, the application of the radioprotection optimization led to analyze the different tasks and operations, even the simplest, on a detailed manner leading often to simplification or to dose reduction by application of simple systems. For low complexity operation, the total collective dose estimate is calculated using simple assumptions on the manpower, the dose rate (supposed to be uniform in the different working areas) and the duration.

    Nevertheless, the experience gained has shown that the dose calculated in this way is approximately 1.5 to 2 times overestimated (mostly due to the uniform rate assumption, i.e. the local exposition factor, and to the actual dose exposure time of the operators : they are not 100 % of the time in the radiation field). Therefore, presently, the results of these simple estimates are arbitrarily divided by a factor 2, also to keep the pressure on dose reduction.
    On the other hand, better dose uptake evaluation is needed to better estimate the actual dose commitment, and a new tool is in development (see here under).

    Impact on the strategies

    The optimization of the radioprotection can have important influence on the strategies applied in the D&D operations.
    One typical example was the choice of carrying out a pre-dismantling decontamination of the primary loop, in order to reduce the dose uptake during subsequent dismantling of the loop.
    This was in fact the actual incentive to carry out this operation before beginning any dismantling task. According to rough pre-estimate, the dose savings should at least be of a few man*Sv, regarding the immediate dismantling strategy involved.
    After the operation, which led to a mean decontamination factor of 10, the estimate of dose savings vary between 4.25 and 9 man*Sv depending on the assumptions taken for the dismantling of the primary and auxiliary loops.

    Other applications and impact can be exemplified by selecting different strategies for specific tasks. As example, the dismantling of some upper internals of the reactor (see figure here below) could be done either with a reduced water height in the pool, i.e. about 1.5 m. above the flange, and a dedicated footbridge placed just above the water level, or using the usual water level, i.e. about 8 m. above the reactor flange, with operators working from the usual platform.

    A global cost/benefit analysis, taking into account the dose commitment of both options led finally to select the first option, as it allowed to work easier, in a shorter time and with simpler tools; the dose commitment estimate being similar in both cases.

       

    Impact on the work organization

    Taking into account the lessons learned and the experience gained by the different phases of the project, the organization of the different operations have evolved. The cold testing of the principal operation on actual size mock-ups (see figure here below), applied since the beginning of the project, has proved to be worthwhile as well economically as for training and radioprotection purposes.

    This principle of cold testing and training of operators was applied for all the tasks of dismantling of the reactor internals, but also for other auxiliary tasks, like for instance, the transfer of highly radioactive pieces from the reactor pool to the deactivation pool of the plant using a home designed shielded container.

       
    Cold tests in a test tank for training Correspondant "hot" operations in the reactor pool
    Tools and development
    Different tools were used to improve the optimization of the radioprotection.
    The first one, which had probably the most important influence on the individual dose uptake, was the introduction of electronic dosimeters with alarm level. Indeed, the presence of a sound alarm when the dose rate trespass a certain level indicates directly to the operators that he is coming into a higher level dose rate ambience and even unconsciously, the individual tends to escape from the field.

    For dose prevision and alternative simulation, the computer code "Dosi-ana" from CEPN was used during the implementation of the Radioprotection optimization analysis of the first operations of the decommissioning. This code allowed to analyze into details the different tasks and subtasks and also to simulate different alternatives for carrying out an operation ; and then to select the "best one" regarding the foreseen dose uptake.

    Nevertheless, the Alara-planning is a difficult task especially when multiple radioactive sources are present in the work environment. This task becomes even more difficult when the environment has a complex geometry. Therefore a new calculation tool has been developed at the SCK·CEN : it is called Visiplan.

    This tool makes it possible to plan the work in a 3D virtual environment, based on information about the geometry, the materials and the radiation field.

    The Visiplan methodology is characterized by three different stages :
  • the information gathering and model building stage ;
  • the general analysis stage ;
  • the detailed analysis stage and the work planning ;
  • the follow-up stage.

    In the first stage the computer model of the environment is built based on the known geometry, the materials information and information about the radiation sources of the site. When the sources are known a calculation of the field can be performed immediately.
    When no information about the source intensities are present, a source inference algorithm provides the possibility to determine source strengths from a semi-detailed dose mapping of the working area. The mapping together with the information about the history of the site results in pinpointing the position of the main sources which contribute to the field.
    The end of this stage results in the basic geometry from which other geometries, mostly with supplementary shielding, are derived.

    In the general analysis stage the calculated field is studied and suggestions about shielding techniques are tested and analyzed using calculated dose maps for each of the suggested shielding geometries.
    Once a shielding geometry is chosen, a detailed dose calculation can be performed along a trajectory which is constituted of a series of tasks each characterized by a position, a task description and a work duration. Several trajectories can be calculated in different shielding geometries.
    A set of scenarios can then be build from a selection of trajectories in the different geometries. The intercomparison of these scenarii then leads to an optimization of the work to be performed.

    In the follow-up stage the dose accounts of the workers are compared with the predictions from the model. When large deviations occur a reassessment of the work can be performed by adapting the model to the new information. This makes it possible to adjust, and thus to further optimize the work during its progress.

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    Dose mapping of the BR3 building Reactor (under Operating Deck)
    Conclusions : challenge and limits
    The BR3 reactor dismantling provides a first radioprotection analysis. Its "pilot aspect" made it a challenge for the different aspects of the operation, and in particular for the optimization of the radioprotection.

    Moreover, the implementation of a systematic "Alara approach" within the SCK·CEN is in use and BR3 is regarded as a judicious test case for this implementation.
    The challenge was then double :
  • the decommissioning was a quite "new" operation for the nuclear industry, and
  • the Alara approach was also quite "new" for the operators and contractors involved in the activities.

    Nevertheless, this "innovative" character was also an advantage to implement the radioprotection optimization, as almost no "bad habits" were already existing in this domain.
    This challenge was taken by the SCK·CEN and successfully applied to the different main operations carried out so far. Dose reductions and improvements in the dose estimates were reached.

    Moreover, the operational dosimetry was also implemented successfully, and helped to optimize further the radioprotection approach. But some limits were already touched, and developments appeared necessary to be able to apply this optimization systematically, and in an economical way.
    The first difficulty encountered is the very heavy work involved by the dose estimate evaluation and the very long work needed for introducing the necessary data (up to subtask level) to carry out the detailed estimate of the dose commitments. If this can be easily justified for jobs implying high dose commitment, it is much more difficult to justify for rather simple operations or operations carried out only once.
    The dose distribution estimate in complex installations appeared rapidly also to be a challenge, and the assumptions made to simplify the calculation were often too rough to give really useful results.

    Therefore, developments in radiation field modeling and dose uptake estimation were started to help the "Alara coordinator" to better evaluate the actual dose uptake of operators working in complex environment and in different locations. This resulted in the development of the Visiplan code.
    Dose mapping, which is also one of the input of the dose uptake modeling programme, remains a development area, where different institutions are busy to develop efficient tools and processes.
    And this is particularly true in low radiation field area, where the sources are distributed and where hot spots or main sources are absent or difficult to track.

    Finally, the "spirit of good practice" remains also a very important way to reduce the dose commitment and this must be helped by dedicated tools allowing to assess the assumptions made in the engineering feeling of the designers and conceptors.
    The decommissioning is surely a task where the optimization of the radioprotection has to be applied, and where dose reductions can be gained by appropriate actions and analyzes. Some developments are still to be made, and the approach of the internal contamination should also be considered in such operations. The decommissioning project also has lead to the development of a new safety culture which can be considered as the ultimate goal.
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