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[en] It is by now fairly widely known that the high temperature reactor (HTR) is a unique nuclear energy source which can supply heat at temperatures up to 10000C for application in chemical processes, for which previously exclusively conbustion heat sources have been used. With the HTR, it is possible to apply nuclear energy not only for electrical power production but also for the synthesis of liquid or gaseous energy carrier. Nuclear coal gasification appears most promising as the first step for the demonstration and industrial application of nuclear process heat technology in the Federal Republic of Germany. Reactor manufacturers and coal mining companies in co-operation with the Nuclear Research Center established a joint project in 1975 for the development of an HTR with a coolant outlet temperature of 950C, for the development and testing of nuclear coal gasification, for the detailed engineering of a prototype plant consisting of an HTR and gasification plant and finally for the construction and operation of this prototype plant for nuclear process heat (PNP). The authors describe the status of the PNP-project and the scope for future development. (Auth.)
[en] The movement of individual pebbles in a simulated recirculating pebble bed nuclear reactor has been investigated. Experimental and statistical results are presented concerning the transit number spectra of groups of pebbles and the transit of individual pebbles in recirculated random packings of identical pebbles. A cylindrical vessel with a variable base angle and a single axial outlet contained the pebbles which varied in diameter, shape, and specific gravity. The pebbles flowed downward under the influence of gravity and through a rotating extractor in the base. Transit number spectra were found by seeding many coloured pebbles at a particular radius and noting the amount of recirculation required before they emerged. While the packing descended, a radioactively tagged pebble inserted at the top of the bed was tracked as it moved through the vessel. Tracks and velocities of pebbles for various combinations of seeding radius, bed and pebble parameters were determined. Using these tracks, patterns and effects of flow zones were deduced. (Auth.)
[en] Assuming a granular material to consist of a random assembly of spheres in contact, a nonlinear but elastic constitutive law is derived. Use is made of the nonlinear force-displacement relationships of the contact loads in the normal and tangential directions. The Lame constants of this isotropic continuum, which is not necessarily homogeneous, depends on the elastic constants of the spheres, the density of the array and either the mean pressure or the volumetric strain. (orig.)
[en] Highlights: ► The experimental installation has been built to investigate the pebble flow. ► The feasibility of two-region pebble bed reactor has been verified. ► The pebble flow is more uniform in a taller vessel than that in a lower vessel. ► Larger base cone angle will decrease the scale of the stagnant zone. - Abstract: The pebble flow is the principal issue for the design of the pebble bed reactor. In order to verify the feasibility of a two-region-designed pebble bed reactor, the experimental installation with a taller vessel has been built, which is proportional to the real pebble bed reactor. With the aid of the experimental installation, the stable establishment and maintenance of the two-region arrangement has been verified, at the same time, the applicability of the DEM program has been also validated. Research results show: (1) The pebble's bouncing on the free surface is an important factor for the mixing of the different colored pebbles. (2) Through the guide plates installed in the top of the pebble packing, the size of the mixing zone can be reduced from 6–7 times to 3–4 times the pebble diameter. (3) The relationship between the width of the central region and the ratio of loading pebbles is approximately linear in the taller vessel. (4) The heighten part of the pebble packing can improve the uniformity of the flowing in the lower. (5) To increase the base cone angle can decrease the scale of the stagnant zone. All of these conclusions are meaningful to the design of the real pebble reactor.
[en] Highlights: → Establishment of a modeling capability to simulate the integral thermal operation of an HTR. → The focus was on the capability rather than improving the assumptions and correlations. → All assumptions and correlations applied were taken from previous credible research. → It was shown that it is possible to establish such a capability using current information. → With this capability, it is now possible to test updated correlations and determine the integral effect. - Abstract: A CFD method was developed to conduct integral thermal reactor analysis for the complete Reactor Unit of the Pebble Bed Modular Reactor (Pty) Ltd (PBMR). The requirement was however also to include very detailed aspects such as leakage and bypass flow paths through the reflector blocks and sleeves. The aim was therefore to investigate the influence of leakage and bypass flow on the thermal performance of the Reactor Unit in an integral fashion. The focus of this paper is to discuss the methodology that was developed. The discussion will firstly highlight all the required inputs, elaborate briefly on the underlying theory and how this was implemented into the CFD modeling capability. Results will be discussed briefly, but the focus is on the methodology.
[en] At very high temperatures loaded metallic structures undergo creep deformations. The generated creep strains are connected with stress relaxations, stress redistributions and/or progressive deformations. In mainly load controlled situations the behaviour of the material can be described by a non-linear viscous flow law (Norton power play). A stress-deformation analysis of complex structures can be carried out by finite element codes in which the mentioned constitutive equation is implemented. The code PERMAS-VISCOUS was used to analyse the stress state of a notched tension bar and the deformation behaviour of a reformer tube of HTR under external pressure undergoing a creep collapse. The relation to experimental findings is also given. (orig./HP)
[en] In a modular high temperature reactor (HTR) the ceramic reflector surrounding the core is completely enclosed by metallic core internals. These comprise the cylindrical core barrel, the bottom structure supporting the ceramic internals, and the top thermal shield which locates the ceramic internals in the axial direction and provides a shield between the top head of the reactor pressure vessel (RPV) and the core. Radial location of the ceramic internals is provided by supports installed over the entire height of the core barrel and arranged according to the column-type design of the ceramic reflector. The nozzle for attachment of the hot gas duct is located below the core to the side of the hot gas plenum. This nozzle is connected to both the core barrel and the ceramic internals. Part of the nozzle is therefore made of graphite. Loads from the metallic as well as the ceramic internals are transmitted to the bottom head of the RPV. It is here that the core barrel is supported, thus allowing it to undergo axial thermal expansion in the upward direction. The barrel is sealed off at the top in order to permit access to the RPV top head for maintenance purposes when the reactor has been shut down. A system of expansion joints located above the top thermal shield provides a seal between the core barrel and the RPV. (orig.)