Within a subproject of the RAPHAEL-Program, which is part of the 6 th EURATOM Framework Program supervised by the European Commission it was investigated whether the use of a Hybrid Magnetic Bearing Concept (HMBC) will be beneficial for a blower application. As in the RAPHAEL program the subproject “Component Development” deals with R&D on components of High Temperature Reactor Technology (HTR), a major focus is on safety- and reliability-related issues. That implies special requirements for the support of high speed rotating shafts in HTR-Applications that only can be satisfied by using Active Magnetic Bearings (AMB). Regarding safety and competitiveness, AMBs are considered key components for the support of rotating HTR-components due to their technical features. AMBs are characterized by an electromagnetic actuator that is generating the bearing force depending on the clearance between stator and rotor, in which the rotor is levitated. Therefore an active control of the coil current is necessary. Furthermore, Touch Down Bearings (TDB) are needed to avoid damages in case of an emergency shut down or in case of energy supply losses. This contribution provides an internal insight on the advantages of a Hybrid Magnetic Bearing Concept that is characterized by a completely Active Magnetic Bearing-supported vertical arranged rotor and an additional permanent magnetic Radial Bearing. One benefit of the HMBC is an additional radial guidance of the shaft that may reduce the loads while dropping into the Touch Down Bearings e.g. in case of energy supply losses of the AMBs. Reduced loads on the TDBs will increase their life cycle and the availability of the AMB supported component. The Scope of this R&D-Project, which will be described more detailed in this contribution, includes the analytical modeling and simulation of the dynamic behavior of the Hybrid Magnetic Bearing System, the modification of the completely AMB-supported test facility FLP500 with a radial PMB and the experimental tests and validation of the analytical models to provide recommendations for the investigated blower application as an HTR-component. Furthermore, the effects occurring during the modification of the test facility and the approach that was necessary to solve unexpected problems will be described.

HTR projects have been launched within the European Union Framework Programmes (FP’s) to consolidate and advance HTR and VHTR technology within Europe. This paper reviews the main achievements arising out of the work in the area of materials and component development. The programme to date addresses material and qualification requirements for the reactor pressure vessel, high temperature resistant alloys and technological development aspects of the power circuit components, material property needs and issues for the graphite core and requirements for Codes and Standards. The experimental programme includes irradiation and feature testing, tests on reduced scale mock-ups and bearings, corrosion, modelling and analysis issues. For the 6th Framework activities which are current, the main European research focus on VHTR is through the RAPHAEL-IP. Results and main conclusions from the work are reported, also a summary of the status of the test work and recommendations for future actions. This programme of work provides important results for the International Generation IV VHTR Materials and Components Research and Development programme as part of the EURATOM contribution.

A power-generating unit with the high-temperature helium reactor (GT-MHR) has a turbomachine (TM) that is intended for both conversion of coolant thermal energy into electric power in the direct gas-turbine cycle, and provision of helium circulation in the primary circuit. The vertically oriented TM is placed in the central area of the power conversion unit (PCU). TM consists of a turbocompressor (TC) and a generator. Their rotors are joined with a diaphragm coupling and supported by electro-magnetic bearings (EMB). The complexity and novelty of the task of the full electromagnetic suspension system development requires thorough stepwise experimental work, from small-scale physical models to full-scale specimen. On this purpose, the following is planned within the framework of the GT-MHR Project: investigations of the “flexible” rotor small-scale mockup with electro-magnetic bearings (“Minimockup” test facility); tests of the radial EMB; tests of the position sensors; tests of the TM rotor scale model; tests of the TM catcher bearings (CB) friction pairs; tests of the CB mockups; tests of EMB and CB pilot samples and investigation of the full-scale electromagnetic suspension system as a part of full-scale turbocompressor tests. The rotor scale model (RSM) tests aim at investigation of dynamics of rotor supported by electromagnetic bearings to validate GT-MHR turbomachine serviceability. Like the full-scale turbomachine rotor, the RSM consist of two parts: the generator rotor model and the turbocompressor rotor model that are joined with a coupling. Both flexible and rigid coupling options are tested. Each rotor is supported by one axial and two radial EMBs. The rotor is arranged vertically. The RSM rotor length is 10.54 m, and mass is 1171 kg. The designs of physical model elements, namely of the turbine, compressors, generator and exciter, are simplified and performed with account of rigid characteristics, which are identical to those of the full-scale turbomachine elements.

Cooling helium of HTRs is expected to contain a low level of impurities: oxidizing gasses and carbon-bearing species. Reference structural materials for pipes and heat exchangers are chromia-former nickel base alloys — typically alloys 617 and 230 — and, as is generally the case in any high temperature process, their long term corrosion resistance relies on the growth of a surface chromium-oxide that can act as a barrier against corrosive species. This implies that the HTR environment must allow for oxidation of these alloys to occur, while it remains not too oxidizing against in-core graphite. First, studies on the surface reactivity under various impure helium containing low partial pressures of H2, H2O, CO and CH4 show that alloys 617 and 230 oxidize in many atmospheres from intermediate temperatures up to 890–970°C, depending on the exact gas composition. However when heated above a critical temperature, the surface oxide becomes unstable: it was demonstrated that at the scale/alloy interface the surface oxide interacts with the carbon from the material. These investigations have established an environmental area that promotes oxidation. When expose in oxidizing HTR helium, alloys 617 and 230 actually develop a sustainable surface scale over thousands of hours. On the other hand if the scale is destabilized by reaction with the carbon, the oxide is not protective anymore and the alloy surface interacts with gaseous impurities. In the case of CH4-containg atmospheres, this causes rapid carburization in the form of precipitation of coarse carbides on the surface and in the bulk. Carburization was shown to induce an extensive embrittlement of the alloys. In CH4-free helium mixtures, alloys decarburize with a global loss of carbon and dissolution of the pre-existing carbides. As carbides take part to the alloy strengthening at high temperature, it is expected that decarburization impacts the creep properties. Carburization and decarburization degrade rapidly the alloy properties and thus result in an unacceptably high risk on the material integrity at high temperature. Therefore, the purification system shall control the gas composition in order to make this unique helium atmosphere compatible with the in-core graphite as well as with structural materials. This paper reviews the data on the corrosion behavior of structural material in HTR and draws some conclusion on appropriate helium chemistry regarding the material compatibility at high temperature.

The high-temperature gas-cooled reactor technology is the only nuclear technology capable of achieving coolant temperatures as high as 950 °C and at the same time ensuring safe and efficient production of both electricity and hydrogen. OKBM and GA started independent research in this area in the 1990s. In 1995, OKBM in cooperation with GA started development of the GT-MHR design which combines a safe modular reactor and a power conversion unit based on the high-efficiency Brayton cycle. The power conversion unit in the GT-MHR design has integral configuration, with vertical arrangement of the turbomachine consisting of a synchronous generator and a turbocompressor. Active electromagnetic bearings are used as supports. In order to select optimal technical solutions, the effect of the following factors on the design was considered: vertical or horizontal arrangement, submerged or remote generator with oil bearings, and different turbomachine rotor speeds. Application of electromagnetic bearings and diaphragm coupling between the rotors, integral arrangement of the turbomachine inside the power conversion system vessel, and use of helium as coolant required performance of comprehensive analyses and experiments. For this purpose, the helium turbomachine technology demonstration program was developed and is currently being implemented. This technology demonstration program aims at validating the quantitative and qualitative characteristics of such turbomachine components as electromagnetic and catcher bearings, control system, computer codes, generator, diaphragm coupling, turbocompressor, etc. At the concluding stage of the technology demonstration program, a full-scale turbocompressor model will be tested at a helium test facility. The present paper lists the main parameters of the GT-MHR turbomachine and describes the status of experimental validation of its components.