Ergebnisse der Untersuchungen zur heterogenen Vergiftung mit bor- bzw. hafniumhaltigen Absorberelementen am Kritischen Experiment zum Hochtemperaturreaktor (KAHTER)

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.