ALARA radiation protection applications at NPP A1 decommissioning, Slovakia, u sing VISIPLAN planning tool - 097

Currently the radio protection optimization and the application of the ALARA principle is becoming an important aspect for all operations carried out in nuclear installations and in particular for decommissioning. Those operations involve changes in the environment and lead in some cases to unrepeatable actions for which the use of very complex and heavy dose calculations (e.g. MCNP, DORTTORT) are not adequate. On the other hand, an oversimplification of the problem is not advisable as this can lead to costly and unnecessary investments or doses that could have been omitted. Another argument against oversimplification is the fact that the total doses in some cases strongly depend on space distribution of dose rate (DR) values and on time duration of works in the locations where DR are strongly elevated. In such a case a more or less complex source model that allows one to calculate the mentioned DR elevations should be employed. It is advantageous if the user has the possibility to assign at these locations also the expected time duration of respective working tasks. In this way, the user would have the possibility to select the most suited work trajectories taking into account work duration, DR and dose account and influence of the radiological and geometrical configuration. In the end a comparison of alternative sets of work trajectories can lead to the selection of the most adequate scenario of the decommissioning works with optimal dose burden to the workers. The SCK.CEN developed during the BR3 reactor decommissioning a graphical interfaced 3D dose assessment tool aimed at the above mentioned dose optimization problems. The tool was improved further and commercialized under the name VISIPLAN 3D ALARA planning tool. The use of VISIPLAN tool at NPP A1 decommissioning was advised by EDF within the IAEA TCP SLR/4008 project missions as a component of a 3D technological chain ensuring acquisition and evaluation of digitized information leading to ALARA optimisation of the developed decommissioning working procedures. VISIPLAN license was purchased by IAEA and granted to VUJE and NPP A1 within the IAEA TC project SLR/4008, covering also the necessary basic and advanced training (at NPP A1 real decommissioning environment), and is currently used by VUJE analysts at NPP A1 decommissioning as an ALARA tool for intervention planning and optimisation. Goal of the paper is to describe and summarize the experiences learned with VISPLAN implementation and ALARA applications at NPP A1 decommissioning. According to the experiences learned during implementation and employment of VISIPLAN planning tool at NPP A1 decommissioning, Slovakia, the following can be stated: Basic and advanced training including real environment exercises and experienced tutorial was very important to speed up the employment of the VISIPLAN tool for ALARA applications at NPPA1. This was possible only thanks to the extensive support of the IAEA within the SLR/4008 TCP project. Exploring and following of the ALARA methodology, built in the VISIPLAN tool, has helped VUJE analysts to implement good practice in ALARA planning for intervention working procedures at dismantling and decommissioning works at NPP A1. Planning of overall time schedule and estimation of time duration for partial working tasks is an important aspect of dose assessment that crucially influences the uncertainty of dose assessments. In practice, this aspect is, usually, conservatively overestimated. Attention has also been paid to uncertainty of dose rate calculations caused by selection of the most suitable build up factor and building of model details