THE SORPTION OF CESIUM BY STRONTIUM-IMPREGNATED NUCLEAR GRADE GRAPHITE AT HIGH TEMPERATURE

Of the fission product nuclides generated in a typical high-temperature gas-cooled reactor (HTGR), cesium exhibits the most complex transport behavior and is of primary interest, along with strontium, because of the health hazards associated with the potential release of ('137)Cs and ('90)Sr. Both of these elements are known to interact strongly with HTGR fuel element matrix and core structural graphites, exhibiting a Freundlich-type sorption behavior that is tantamount to a chemical bond.;The kinetics of cesium sorption and desorption required three to 15 days to establish near-equilibrium concentrations on the graphite, the behavior suggesting a rapid sorption (or desorption) component followed by a slow diffusion-controlled component. Although both the single- and binary-component desorption isotherms of cesium were of the Freundlich type, a form of hysteresis was observed during sorption at low cesium pressures (< 0.2 Pa). Above the hysteresis branch, and once on the desorption isotherm, sorption and desorption were reversible.;Results of three models, formulated previously by Haire and Zumwalt (1973) to predict mixed-component sorption for sorbates that individually conform to Freundlich behavior, were compared with the observed equilibrium sorption behavior of cesium. The modified Exponential Model was applied to the single-component results, agreeing within 10% at vapor pressures above 0.01 Pa. In the binary case, the Thermodynamic Model with empirically determined activity coefficients represented the experimental desorption data quite well. The FREVAP Model over-predicted cesium vapor pressures at higher strontium concentrations and under-predicted them at lower concentrations. On the basis of these results, the Thermodynamic Model with known activity coefficients is recommended for use in engineering calculations. However, if activity coefficients are not known, the FREVAP model is recommended.;In view of the fundamental interest and practical importance of assessing the behavior of cesium in nuclear grade graphite, the vapor pressure of cesium was determined as a function of cesium and strontium concentration in the reference nuclear graphite H-451. Cesium sorption isotherms were obtained at less than monolayer coverage using the pseudoisopiestic method over the temperature range of 1073 to 1373 K and cesium vapor pressure range of 1 x 10('-4) to 10 Pa. Cesium sorption isotherms were also obtained at 1273 K over a similar vapor pressure range for H-451 graphite diffusion-impregnated with strontium to concentrations ranging from 0.59 to 7.81 mmol Sr/kg graphite. The elements were tagged with ('137)Cs and ('85)Sr, and their concentrations in the graphite samples were monitored in situ.