atw 2018-05v6

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atw Vol. 63 (2018) | Issue 5 ı May OPERATION AND NEW BUILD 304 to be insufficient as their position varies after each operation with those boxes due to replacement of probes. The conclusion is that in case of the IVR-Ex strategy implementation, the replacement of some boxes is needed with modified ones, which will include the burst membrane and sufficient flow channel. The analytical studies on RPV coolability showed very low margin to critical heat flux (CHF) if the cooling is in a water pool. So the idea of the intensification of heat removal appeared and feasibility study on the application of the deflector was performed. The conclusion is that the deflector is technically feasible, but too many new risks are related to its implementation that the deflector cannot be acceptable as the measure for a safety increase. Just a few examples of those risks. The deflector would need to be removed during every outage so the process of its disassembling and later re-assembling using the manipulators would be very risk for the hiting RPV. Also storing of the deflector components, which are irradiated, would be high risk for additional dose rates to personal. So the conclusion is that the deflector is not reasonably applicable. Verification of IVR-EX efficiency for VVER-1000. This topic covers practically all plant analyses for the IVR-EX and also the analytical support for the designing of the experimental facility (called THS-15) on the RPV coolability. This topic also covers the designing of the facility itself. As examples of the activities, it can be mentioned – a study on the heat flux distribution from corium to RPV wall using the FLUENT code, RELAP analyses of RPV cooling with various parameters of deflector, pre-test analyses of the THS-15 facility, supporting scaling analyses, but also a preparation of the methodology for an evaluation of experimental results, a preparation of the experimental matrix and so on. Although it is not part of the project, it is logical to put into this subchapter the information on the building of the THS-15 facility. The facility purpose is to perform tests on the cooling of RPV of the VVER-1000 reactor under IVR-EX conditions. The tests will be focused on the verification of a coolability of heat fluxes, an identification of CHF, and to produce data for the code validation. The experiments will be performed as the part of the WP4 of the EC H2020 Project IVMR (grant agreement number 662157). The recent situation in the facility building is that the production of the last big component is finished and the facility assembling is ongoing. The scheduled com missioning of the facility has to be at the end of November 2017. Then the experimental program will be launched. 3.4.4 Strategy ExVC The alternative strategy to the IVR-EX is the ex-vessel corium cooling one. The idea of the strategy is to spread the corium into the cavity neighboring room (named GA302) and cover the corium with the water to cool it down. Several analytical studies as well as the experimental data from the OECD MCCI and MCCI2 project showed, that the MCCI with the siliceous concrete is not possible to terminate. Based on this observation the idea of the refractory material, which would survive at least several hours till significant corium temperature reduction appeared. The project also defined some topics of a solution for this strategy, starting with additional analysis, but also with the feasibility study on modifications of doors between cavity and room GA302 to make spreading much faster and easier. The second activity was a feasibility study of an instalation of the refractory material. It is very complicated mainly in the cavity due to need of the concrete cooling (it must not be influenced) and also from the perspective of those modification performace as there is really high radiation in the cavity, which practically precludes any work in the cavity. The updated measurement of the radiation in the cavity (in 2017) not only confirmed the level of radiation, but it was more detailed and identified sources, which is not only the RPV itself, but also the thermal and biological shielding near the cavity walls. So it is practically impossible to work there and this shielding was expected to be replaced with new one together with refractory layer installation. So the situation in the cavity is recently not yet solved and will need an alternative solution, which is not yet prepared. Concerning the refractory material, the preliminary study on potential candidates was performed, but before the final decision the experimental testing has to be performed. 3.4.5 Stabilization of containment conditions The title of this chapter differs in the wording with the bullet above, but its meaning is same, because the main task of the activities is to maintain the containment pressure and temperature on an appropriate level for long time. During this year, the project opened this task and the proposal for technical solutions of the heat removal from the containment atmosphere are under development. About nine technical proposals were prepared as basic scheme and two of them are be selected for the feasibility study. Those nine solutions use various approaches – spraying, steam condensation and heat removal, using sophisticated systems with supercritical CO 2 and so on. 4 Conclusions A continuous process of safety enhancement of VVER units in the Czech Republic has been briefly described including a presentation of important milestones and examples of particular safety measures already implemented. Despite the high level of safety reached mainly by the preventive means in the last decades at both VVER sites in the context of Fukushima accident, a new period of enhancement process has been initiated following the stress tests exercise. Use of advanced deterministic analytical approaches and implementation of new preventive safety measures has then taken place together with special attention to mitigative part of potential accidents and relevant strategies and measures. In the paper a special attention was given to the evaluation and implementation of safety measures following stress tests conclusions and R&D activities supporting this process. As examples an implementation of the „design extension condition without core melt“ concept and various activities related to severe accident mitigation strategies are presented in more detailed way. Within the systematic approach to enhance safety also the PSA has been recognized as a very useful tool. The PSA has been mainly used for identification of weak points in design and operation and for an evaluation and prioritization of potential safety measures. The example of new cooling tower measure for the Dukovany site has been used to demonstrate this approach. The work on DEC-A (previously BDBA) safety analyses for Czech NPPs was described as a consequence of the Periodical Safety Review (PSR) after 20 year of the operation of the Dukovany NPP. This effort has been influenced also by initiatives and Operation and New Build Continuous Process of Safety Enhancement in Operation of Czech VVER Units ı J. Duspiva, E. Hofmann, J. Holy, P. Kral and M. Patrik
atw Vol. 63 (2018) | Issue 5 ı May suggestions from European Utility Requirements (EUR), WENRA safety reference levels, and IAEA introduction of the DEC term and concept into the safety standards series. Examples of a use of this approach for Dukovany and Temelín safety analyses have been presented. The programme for severe accidents for Czech NPPs is very extensive, this contribution included mainly the activities related to the Temelín NPP, but also the Dukovany NPP has own program and activities related to the topic of severe accident. The common for both NPPs and not yet mentioned is the training of the staff. The UJV supports both NPPs for longer time with special lessons on the progress of the severe accident and impact of an application of measures to the SA course. Recently the new tool for the training is close to the completion, it is named VINSAP (Visualization of NPP Severe Accident Progress for Training on SAM), the project is sponsored by the Technology Agency of Czech Republic (project no. TH01011086). References [A] National Report on “Stress Tests” of NPP Dukovany and NPP Temelín, Czech Republic, December 2011. [B] [C] Authors National Action Plan (NAcP) on Strenghtening Nuclear Safety of Nuclear Facilities in the Czech Republic, State Office for Nuclear Safety, rev2, Jan 6, 2015. SUJB directive BN-JB-1.7, Selection and Assessment of Design and Beyond Design Events and Risks for Nuclear Power Plants, 2010. J. Duspiva J. Holy P. Kral M. Patrik UJV Rez, a.s. Hlavni 130 25068 Rez, Czech Republic E. Hofmann CEZ, a.s. Duhova 2 14000 Prague, Czech Republic Applications of Underwater-Robotics in Nuclear Power Plants OPERATION AND NEW BUILD 305 Gunnar Fenzel, Dr. Dietmar Nieder and Alexandra Sykora 1 Research project AZURo Cutting and packing of the reactor pressure vessel (RPV) is one important step during decommissioning of nuclear power plants. The RPV and its internals are radiological activated caused by the long standing neutron flux. In particular the internals which – amongst others – retained the fuel assemblies have to be cut and packed under water due to their high radiological activity. In the past this was largely done manually using remote handled tools (such as rods, grippers and cranes). The operation of the remote handled tools is time consuming and enables access to the respective parts by one direction only. Thus the accessibility is strongly restricted slowing down the progress of the work. Hence the costs of the decommissioning are highly increased. In addition, the risk of failures is enlarged by the inflexibility of the tools. Moreover and due to the complex proceeding, the workers are exposed to a certain radiation level leading finally to an averaged increased radiation exposure. Therefore, it was the objective of the research project Automated Cutting of Reactor Pressure Vessels Internals Using Underwater-Robotics (AZURo) to (semi-) automate frequently repeated activities by an underwater robot. This joint research project was sponsored by the German Federal Ministry of Education and Research (BMBF). It was executed together with Fraunhofer-Einrichtung für Gießerei-, Composite- und Verarbeitungstechnik IGCV. The project AZURo started in 2012 and was finished in 2016. The highest degree of innovation is given in the research of the application of industrial robot systems under water and in radiation fields. Thereby there is no direct contact possible, neither with the robot itself nor with the workstation. All works at the system have to be performed remotely monitored respectively remotecontrolled. Key aspects of the development were remote control of the system, optical monitoring and the development of an intuitively designed simulation ambience with automated path planning. Arm of robot Total mass max. load The system supports the operator in planning and execution as well as in handling steps but ensures the control of a human being at all times. Just as well the nuclear requirements such as intervention capability, reproduc ibility and health physics aspects during developing have to be con sidered. 2 Advantages of AZURo The (semi-)automation of cutting and packing activities by means of robots will lead to • a reduction of the local radiation exposure of the involved staff, • a shortening of the performance times of cutting and packing of highly activated components, • a lowering of costs for such projects, and Approx. 1,100 kg 150 kg Protection class IP 68 Working space (spherically) without tools, max. Working space (spherically) with tools, max. | | Tab. 1. Technical Data. Ø 2,194 mm Ø 2,764 mm Operation and New Build Applications of Underwater-Robotics in Nuclear Power Plants ı Gunnar Fenzel, Dr. Dietmar Nieder and Alexandra Sykora
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