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This report contains a compilation of single contributions, which were presented an the occasion of a series of lectures in theInstitute of Reactor Development in March 1977. Results for Cs-diffusion in coated particles and spherical fuel elements are presented in detail and discussed in respect to their consequences. Besides this it is reported about progress in methodical development. By application of two independent methods the quantitative determination of the "rare" silver Isotope Ag-11om in high burnt-up HTR fuel succeeded. The new developed technique of laser drilling is presented in its basis. With the aid of results for Cs-release and distribution in coated particles the efficiency and the inserting potential of this method are proved. By this first statements about the release behaviour of defect particies can be gained.

Coated fuel particles were investigated in post irradiation experiments. Sr-90-fractional releases from single particles were measured during isothermal anneals. The time-dependent fractional release curves were analyzed with the aid of simple diffusion theory and delivered temperature dependent diffusion constants. For oxide kernel material with burn-up values up to 30% fima it resulted: D$_{Sr-90}$ =3,5 $\cdot$ 10$^{-5}$ $\frac{m ^{2}}{sec} $ e - $\frac{409 000(Joule/mol)} {RT}$ Additionally some measurements were done on kernels with additives and TRISO coated particles. Moreover these data and other data from literature were used in a parametric study to determine the data's influence upon Sr-90 Core releases of 3000 MW$_{th}$-plants. The results show, that all calculated forecasts remain sufficiently low as for oxide fuel, but increase to evidently higher release values as for carbide kernel materials.

The intranuclear-cascade evaporation model (INCE) as implemented in the high energy radiation transport code HETC, subsystem of HERMES (1) is used in the calculation of double differential cross sectionsl of proton induced neutron production. The investigations were done on target elements C, Al, Ta, Ni, W, Pb, and U at 318 MeV incident proton energy and on C, Al, Pb, and U at 800 MeV, respectively. The predictions of the INCE model were compared with experimental data for double differential cross sections taken at 7.5 and 30 degrees scattering angles at the Los Aamos WNR facility utilizing the Time of Flight (ToF) technique at LANL. The calculations performed here are part of a experimental-theoretical program within the LANL-KFA collaboration concernign medium energy cross section measurements mainly neutrons and state of the art computer code validations of these measurements. In general, the model predictions reproduce the correct neutron production for evaporation neutrons and are also in good agreement with the experimental data at high neutron energies. In the energy range dominated by preequilibrion processes an underestimation of experimental yields has to be remarked.

The migrational behaviour of silver in the coated particle fuel, proposed for High-Temperature Reactors, is investigated experimentally. Data are described in the framework of the diffusion model. The diffusion coefficients are derived from the experimental data by a nonlinear least squares fit procedure. The experimental procedures and the theoretical calculations to analyse the data are described extensively. Arrhenius lines D = D$_{o}$ e$^{-Q/RT}$ are presented for U(Th)-O$_{2}$ , PyC and SiC. The silver release in advanced High-Temperature Reactors is prognosticated based an the measured data.

A deep penetration experiment conducted at the Los Alamos WNR facility's Spallation Neutron Target is compared with calculations using intra-nuclear-cascade and S$_{N}$-transport codes installed at KFA-IRE. In the experiment medium energy reactions induced by neutrons between 15 MeV and about 150 MeV inside a quasi infinite slab of iron have been measured using copper foil monitors. Details of the experimental prodedure and the theoretical methods are described. A comparison of absolute reaction rates for both experimentally and theoretically derived reactions is given. The present knowledge of the corresponding monitor reaction cross sections is discussed.