Measuring the adsorption isotherms of cesium on wall materials of high temperature reactors

The method of measuring the work function in the investigation of the fission product adsorption has important advantages: It enables the measurement of adsorbate concentrations independetly on volume concentrations due to its extremely small information depth of an atomic layer, and enables measurements in equilibrium with the gas phase of the adsorbate. Especially the oscillating capacitor method permits the measurement of the work function and thus of adsorbate concentrations non-destructively and with high sensitivity (typical dissolution of cesium: 0,001 monolayer) in the interesting region below one monolayer. The development and realization of an apparatus based on the oscillating capacitor method is described which is designed for the measurement of adsorption isotherms of cesium on metals (double modulation technique). Adsorption isotherms of cesium were measured on the high-temperature alloys, TZM and nimocast discussed as turbine material. The date obtained embrace the parameter region of Cs vapour pressures between 5 x 10-6 and 5 x 10-9 torr covering degrees between 0.1 and 1 monolayer and temperatures to 1,000 K. The applicability of the data obtained with pure cesium in residual gas-free environment to real circuits with their reactive gas impurities firstly concern the region of temperature above 6450C, where none of the Cs compounds to be expected are stable as resulted from thermochemical calculations. To what extent the data may however be applied to lower temperatures, has to be clarified by experiments for the co-adsorption of various species which should show the influence of the wall on the chemistry of the adsorbates. (orig./HK)