Search

For calculations of high-temperature gas cooled reactors with low-enrichment fuel it is important to know the plutonium cross sections accurately. Therefore, a calculational method was developed, by which the plutonium cross-section data of the ENDF/B-IV library can be examined. This method uses zero- and one-dimensional neutron transport calculations to collapse the basic data into one-group cross sections, which then can be compared with experimental values obtained from integral tests. For comparison the data from the critical experiment CESAR-II of the CEN Cadarache were utilized. In this experiment samples with different plutonium concentrations and with different mixtures of the plutonium isotopes were oscillated inside the core at different temperatures between 20 °C and 360 °C. Thesimulation of this experiment through the calculational model developed in the present study showed that the ENDF/B-IV data for plutonium are reasonable. Based on this result the behaviour of the plutonium isotopes and of the entire low-enrichment high-temperature-reactor fuel at higher temperatures was studied.

Experiments and checking computations for investigating the initial-core poisoning of the pebble bed high temperature reactor are described. Following the example of the THTR-300, THTR absorber elements poisoned with hafnium/boron were added to the THTR fuel- and graphite elements of the KAHTER core. Three different hafnium-boron poisoned core loadings, corresponding to 2.7, 5.3 and 8 % reactivity compensation, were used in the experiments. For purposes of comparison, two cores exclusively boron poisoned were also studied. The poisoning of these cores correspond to 2.7 and 8 % reactivity compensation. The experiments and checking computations should serve to test the accuracy of the theoretical models and data sets in modeling the reactivity effects of absorber poisoned elements in the THTR. In particular, the applicability of the nuclear data of hafnium and the treatment of resonance calculations should be verified. In addition to determining critical masses and keff, special emphasis was placed in the experiments on the exact determination of all reactivity effects. In some cases, repeated loading of a configuration also provided a measure of the reproducibility of keff. The experiments were checked computationally using the GAMTEREX code package and the program system RSYST. These two computation packages contain different data bases, - although the hafnium data are identical -, and the computing models differ in certain phases of the calculations. Both code systems compute keff values to within the present accuracy requirements, whereas the program system RSYST gives better agreement with experimental measurements.

Theoretical analysis on a loading experiment of a pebble bed reactor particularly in regard to the loading method, has been performed. This report describes the calculating models and gives information for the first loading. An alternative loading scheme has been examined, in which at the beginning of the loading experiment the lower part of the core is filled with graphite spheres only. It is shown that nitrogen as a neutron poison for compensating excess reactivity may be used.

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.