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Status on R and D Planning for Supercritical Water Cooled Reactor Systems in the 6. European Framework Programme
Abstract
On July 30, 2003, EURATOM signed the charter to join the Generation IV International Forum and thus to contribute to innovative reactor design and development. Among other concepts, supercritical water cooled reactor systems shall be foreseen as their contribution. In order to support this international forum, a dedicated budget for R and D of innovative concepts is planned for the 6. European Framework Programme of the European Commission. Currently, a detailed work plan for supercritical water cooled reactor systems is being worked out, in order to be presented and decided by the European Commission thereafter. It shall include: - Design studies of a thermal reactor core, its reactor pressure vessel internals and of the balance of plant. - Study of a fast reactor option for sustainable use of fuel and for Plutonium management. - Study of the corrosion behavior and other performances of candidate materials at supercritical pressures. - Detailed investigations of heat transfer and pressure drop at supercritical pressures and at part load operation conditions of the reactor. - Design code improvements and verifications. - Conceptual design and analyses of a suitable safety system. As a result, the program shall enable a thorough assessment of the supercritical water cooled reactor system with a view to determine its future potential. This summary report shall give an overview of the contributions, which are planned to be provided by the EURATOM partners in France, Finland, Germany, Hungary, Switzerland, The Netherlands and The Czech Republic. (authors)
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Within the 5th Framework Programme of the European Commission a project for studying the potential of water reactors using supercritical water/steam conditions was performed. Among the different activities performed within this project emphasis was also set on the major characteristics of a plant configuration. Since the HPLWR has to be considered as a long term development project, the requirements applied for the design are expected to be a combination of existing ones (like EUR) and such discussed for future designs (like Generation IV reactors). This means among others that in such a design, passive means and means for mitigation of severe accidents have to be incorporated. Concerning the primary system the principles of the core configuration and arrangement were outlined as a first conceptual approach. Concerning the fuel assemblies both hexagonal and quadratic types are analyzed. Regarding the containment design, the proposal is to consider a modern BWR containment and in order to introduce passive features into the design, the containment should be provided with a core flooding pool and emergency condensers. The safety systems configuration is intended to provide a passive core flooding pool to replace the amount of water existing in the primary circuits of LWR's. This provision since in case of the HPLWR the mass of water within the RPV is about 1/10 of the primary system water of a BWR or PWR only. Therefore, in case of transients like "loss of offsite power", the safety philosophy of such reactors to maintain the primary pressure and consequently use the heat capacity of the existing water is not appropriate in case of an HPLWR. Instead of using a high pressure feedwater injection it is proposed to initiate the automatic depressurization system (ADS) and use the existing low pressure injection system and the passive flooding system. With this proposed containment and safety concept it is considered that an extensive use of passive systems is reached. A preliminary economic evaluation of the HPLWR plant concept which was performed, indicates that such nuclear plants may have the potential to reach electricity generation costs which can significantly increase the economic advantages of nuclear plants compared with fossil fueled plants. This statement especially holds true if one considers in the future a further increase in fossil fuel prices and the possibility of getting deeper insight into the design of HPLWR's in order to explore the potential of cost reduction more precisely and in more detail.
CATHARE is the French best estimate code used for safety analysis of Light Water Reactors. Two-phase flow is modelled with a two-fluid six-equation model in 0-D, 1-D and 3-D modules. A few months ago, the range of applications was limited to the sub-critical two-phase region, but thanks to the extension of the water and steam properties up to 26 MPa and recent modifications of the standard code version, it is now possible to simulate transients where both supercritical and sub-critical regimes are encountered. This paper describes these recent developments in the modelling of supercritical water flows using the two-fluid model implemented into the CATHARE code and shows the results of the preliminary assessment of this model successfully achieved by running several calculations for 1-D applications: - Steady state in supercritical conditions followed by blow-down - Stationary isenthalpic expansion - Stationary heat-up of supercritical fluid Then, using the modified CATHARE version, first steady-state and transient calculations of a Supercritical Light Water Reactor operating above the critical point were carried out, among them, Loss of Coolant Accidents (LOCAs) considering simple sequences of events. The predicted transient behaviours seem to be correct from supercritical to sub-critical regimes, which confirms the CATHARE capabilities.
Light water reactors using super-critical water as coolant differ from conventional PWR or BWR design by higher plant efficiencies, but also by smaller plant size at comparable electric output. These advantages are partly compensated by new aspects such as the larger coolant temperatures in the core, or the larger coolant density differences which require several new and unconventional design features that need to be addressed in a correct manner and require substantial R&D work. The European community decided to launch the High Performance Light Water Reactor (HPLWR) project, which is now completed, to check the feasibility of the super-critical water cooled reactor concept. The starting point for such studies uses an advanced design by Dobashi et al, [1] for its comprehensive definition of all the relevant aspects of a HPLWR design. During the course of the HPLWR project, an extensive evaluation of core characteristics has been made and alternative options studied. This "reference design" has interesting and innovative features such as water rods which help to flatten the axial power shape as this is one of the main disadvantages of a HPLWR core and also helps to compensate the reactivity swing. However, the "reference design" has features such as a relative small moderation, a moderation very much dependent of the magnitude (and amplitude) of the variation of water flows and of their thermal insulation, a non-uniform water flows in the different sub-channel of the sub assembly and the use of Ni-based alloy cladding materials with a relative strong neutron absorption. With various and appropriate design changes which have been studied during the course of this HPLWR project, the HPLWR "reference concept" could be improved but this, however, would require a substantial design effort with all constraints taken into account. Rather complete new revolutionary designs could also be envisaged but this would even increase the overall design effort. At the conclusions of the HPLWR project, there is a much clearer understanding of what constraints apply to the design, and what is required to upgrade the necessary design tools to pursue the analyses, supported by experiments, within the 6th FP.
A high-pressure test facility which is installed in an accredited laboratory at Siemens' Power Generation Group (KWU) in Erlangen, Germany, and is unique throughout the world due to its wide range of operating conditions, called the BENSON test rig, is now celebrating its 25th anniversary. As the name of the test rig implies, its main purpose is to investigate topics associated with the operation and further development of BENSON boilers. In addition to this, the test facility is also used for issues related to power generation using nuclear and renewable energy sources.
Within the 5. Framework Program of the European Community a High Performance LWR (HPLWR) is investigated, considering a reference design based on work of Oka et. al. [1] The utilization of supercritical water pressure leads to a strong axial variation of the water den-sity in the coolant channels of the fuel assemblies. The reference design contains in each fuel as-sembly a number of moderator rods to enhance the over-all mean water density and three radial and three axial zone enrichments for power flattening. This leads to a very complicated neutron physics behavior of the reactor. The validation of applied calculation procedures is mandatory for such new core concepts. In a first exploratory step, before having new relevant experimental in-formation, validation with Monte Carlo simulations seems to be adequate. A specific feature of the actual HPLWR design is the strong feedback between the neutron physics and the thermal-hydraulics calculations. For the coupling with thermal-hydraulic codes deterministic multi-group neutron physics calculations are recommended. In the following in a first step two-and three-dimensional models for Monte Carlo calculations with MCNP are developed. Comparisons of dif-ferent MCNP models give confidence in the results and show some sensitivities, e.g. with respect to the treatment of the isolation wrappers around the moderator rods. The second step involves the development of a super-cell model of the fuel assembly for deterministic multi-group calcula-tions. Here the comparison with the results of Monte Carlo calculations with MCNP shows the applicability of the super-cell model. Finally, on the basis of this super-cell model a coupling with the thermal-hydraulic code RELAP5 is realized. First results from the coupled calculations are given.
A state-of-the-art study was performed to investigate the operational conditions for in-core and out-of-core materials in a high performance light water reactor (HPLWR) and to evaluate the potential of existing structural materials for application in fuel elements, core structures and out-of-core components. In the conventional parts of a HPLWR-plant the approved materials of supercritical fossil power plants (SCFPP) can be used for given temperatures (⩽600 °C) and pressures (≈250 bar). These are either commercial ferritic/martensitic or austenitic stainless steels. Taking the conditions of existing light water reactors (LWR) into account an assessment of potential cladding materials was made, based on existing creep-rupture data, an extensive analysis of the corrosion in conventional steam power plants and available information on material behaviour under irradiation. As a major result it is shown that for an assumed maximum temperature of 650 °C not only Ni-alloys, but also austenitic stainless steels can be used as cladding materials.
The increase of steam parameters to supercritical conditions could reduce the power generating costs of light water reactors significantly [Proceedings of SCR-2000 (2000) 1]. Core assemblies, however, will differ from current BWR or PWR design. In this context, this paper summarizes the main results related to a thermal-hydraulic design analysis of applicable fuel assemblies. Starting from a thorough literature survey on heat transfer of supercritical fluids, the current status indicates a large deficiency in the prediction of the heat transfer coefficient under reactor prototypical conditions. For the thermal-hydraulic design of such fuel assemblies the sub-channel analysis code Sub-channel Thermal-hydraulic Analysis in Fuel Assemblies under Supercritical conditions (STAFAS) has been developed, which will have a higher numerical efficiency compared to the conventional sub-channel analysis codes. The effect of several design parameters on the thermal-hydraulic behaviour in sub-channels has been investigated. Based on the results achieved so far, two fuel assembly configurations are recommended for further design analysis, i.e. a tight square lattice and a semi-tight hexagonal lattice.
A high temperature reactor cooled and moderated by supercritical light water (SCLWR-H) is designed for assessing its technical feasibility and potential economic improvement. The plant system is the once-through direct cycle. Whole core coolant flows to the turbine. When adopting the conventional ascending flow type water rods, the cold coolant flowing up in the water rod is mixed with that of the fuel channel coolant above the core. Although the maximum surface temperature of the Ni-base alloy cladding is 620°C, the average core outlet temperature is low, 455°C. It, however, reaches 508°C in the core with the descending flow type water rods since no mixing of the coolant occurs above the core. The fuel enrichment is also decreased. The flow rate of each fuel assembly is controlled by an orifice to maximize the outlet coolant temperature. The reactor achieves high thermal efficiency of 44.0% and electric power of 1610 MW. The coolant flow rate per generated electricity is 24% lower than that of an Advanced Boiling Water Reactor (ABWR). This will reduce the size of the balance of plant (BOP) system. The burnup reactivity can be compensated by changing coolant flow rate ratio in the descending flow type water rods to the whole core through the fuel cycle. This reduces the number of control rods. The inlet and outlet coolant temperature and total flow rate need not be changed for keeping the power constant.
The objective of the high performance light water reactor (HPLWR) project is to assess the merit and economic feasibility of a high efficiency LWR operating at thermodynamically supercritical regime. An efficiency of approximately 44% is expected. To accomplish this objective, a highly qualified team of European research institutes and industrial partners together with the University of Tokyo is assessing the major issues pertaining to a new reactor concept, under the co-sponsorship of the European Commission. The assessment has emphasized the recent advancement achieved in this area by Japan. Additionally, it accounts for advanced European reactor design requirements, recent improvements, practical design aspects, availability of plant components and the availability of high temperature materials. The final objective of this project is to reach a conclusion on the potential of the HPLWR to help sustain the nuclear option, by supplying competitively priced electricity, as well as to continue the nuclear competence in LWR technology. The following is a brief summary of the main project achievements:
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