[en] Siemens KWU has been developing a High Conversion Reactor in cooperation with the Swiss PSI, KfK and the Technical University of Braunschweig for several years. An HCR requires a tight lattice which is easier to accomplish in a triangular pitch as compared to a square one. The triangular pitch leads to a hexagonal fuel assembly. Beyond the potential of tightening the lattice, there are other advantages of hexagonal fuel assemblies which are valid also for moderation ratios in the range of conventional PWRs. The major current disadvantage is that some of the important design codes have to be adapted to the hexagonal geometry. 2 refs, 7 figs

[en] Two features of the HCR define the main design areas: the triangular pitch of the fuel bundle and the higher density of control assemblies. The triangular pitch requires a novel spacer design. Several solutions have been identified. A honey comb grid assembled from bent straps was selected for further detailed development. In order to obtain specimens for thermohydraulic tests and manufacturing experiences about 50 spacers were built to present. The higher control assembly (CA) density necessary to compensate reactivity in a harder neutron spectrum requires to join at seven positions seven spiders to one ''super RCC'' and to one CA drive. Due to the fact that part of reactivity will be compensated by CA a safety mechanism is required which prevents any rod ejection failure. Furthermore, an adequate coolant flow path especially for the central fuel assembly at a ''super RCC position'', must be provided. A safety mechanism which unlatches drive and control element if the upward motion is larger than a regular step has been designed. The function was demonstrated. The required flow path from the central element into the plenum can be provided by a novel control rod guide structure. A template encloses the complete assembly of seven control rod spiders. The flow area and guide path are separated such that the coolant flow cannot induce adverse vibrations. This structure is extended almost directly from the fuel rod bundle into the upper plenum. The remaining flow path is sufficient. The guide structure itself is latched at the assembly in order to prevent mismatch between the upper guide and the assembly internal guide tubes. (author). 2 refs, 8 figs, 1 tab

[en] A hydraulically driven control rod is being developed at Kraftwerk Union for integration in the primary system of a small nuclear district heating reactor. An elaborate test program, under way for --3 yr, was initiated with a plexiglass rig to understand the basic principles. A design specification list was prepared, taking reactor boundary conditions and relevant German rules and regulations into account. Subsequently, an atmospheric loop for testing of components at 20 to 90⁰C was erected. The objectives involved optimization of individual components such as a piston/cylinder drive unit, electromagnetic valves, and an ultrasonic position indication system as well as verification of computer codes. Based on the results obtained, full-scale components were designed and fabricated for a prototype test rig, which is currently in operation. Thus far, all atmospheric tests in this rig have been completed. Investigations under reactor temperature and pressure, followed by endurance tests, are under way. All tests to date have shown a reliable functioning of the hydraulic drive, including a novel ultrasonic position indication system