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V.S.O.P. is a computer code system for the comprehensive numerical simulation of the physics of thermal reactors. It implies the setup of the reactor and of the fuel element, processing of cross sections, neutron spectrum evaluation, neutron diffusion calculation in two or three dimensions, fuel burnup, fuel shuffling, reactor control, thermal hydraulics and fuel cycle costs. The thermal hydraulics part (steady state and time-dependent) is restricted to HTRs and to two spatial dimensions. The code can simulate the reactor operation from the initial core towards the equilibrium core. V.S.O.P.($\underline{99 / 05}$) represents the further development of V.S.O.P. ($\underline{99}$). Compared to its precursor, the code system has been improved in many details . Major improvements and extensions have been included concerning the neutron spectrum calculation, the 3-d neutron diffusion options, and the thermal hydraulic section with respect to `multi-pass'-fuelled pebble-bed cores. This latest code version was developed and tested under the WINDOWS-XP - operating system. The storage requirement for the executables and the basic libraries associated with the code amounts to about 15 MB. Another 5 MB are required - if desired - for storage of the source code ($\backsim$ 65000 Fortran statements).

For a supposed population of nuclear reactors in the FRG during the next 5 decades, the hazard potential of the produced nuclear waste of different fuel cycles using Uranium and Plutonium is calculated by use of the computercode ORIGEN-EXJÜL. The consequences concerning the hazard potential resulting from these strategies are investigated using the radioactivity concentration guides as given in the US-Code of Federal Regulations. In addition, more restrictive values in view of ingestion for some heavy metal isotopes are applied and a comparison of the impacts is made. With regard to the acceptibility of disposing the nuclear waste, its hazard potential is compared with the ingestion hazard, of the natural uranium content in the soil of the FRG.

A 2700 MW$_{th}$ OTTO-pebble bed reactor for a direct cycle helium turbine plant is presented resulting from a technical and economical optimization. The calculational research presents the nuclear properties, thermal hydraulics and tost evaluation. Same more detailed analysis an reactor control fission product releaseand reflector are performed as far as required for assessing this concept. A discussion of alternatives in the design of fuel elements and reflector illustrate the flexibility inherent to this type of reactor. The characteristic data are : Fuel cycle with high enriched uranium and thorium, average power density of 9 MW/m$^{3}$, specific power of 2817 kW$_{th}$ /kg$_{fiss}$, average burn up of 106000 MWd/t, fuel cycle costs of 0.51 Dpf/kWh$_{e}$, maximal nominal fuel temperature of 1156 °C. Technological limits are reached far the fast neutron dosis in the radial reflectar and for the release of fission products. In order to reduce these figures the fuel elements being inserted adjacent to the reflectar are loaded with a reduced fissible and an increased fertile isotopic content. For the fabrication of the coated particles follows the requirement of a lower fissile isotopic contamination in the coatings. The aim of the study is to provide a basis for the pebble bed alternative reference design within the framework of the HHT-project.