Release of fuel and fission products from hot sodium

In case of a hypothetical core disruptive accident with tank failure, major quantities of sodium, fuel and fission products will be released from the SNR core catcher into the containment atmosphere due to mechanical, chemical and evaporation processes. Since the core catcher is a long term aerosol source, it is important to study the retention capability of sodium for fuel and fission products, the release, rates and the release mechanisms. In the KfK-NALA-program, experiments on this subject were performed with UO2, Cs, J and Sr in sodium on a small laboratory scale (LS) and on a larger technical scale (TS). Some experimental details and the most important results are described. In case of the LS experiments, about 100 g of sodium were contaminated with amounts of the order of 1 g of the substance to be investigated and heated up in a small stainless steel vessel (7 cm diameter, 10 cm height) to temperatures between 400 deg. C and 900 deg. C. The contamination substances were UO2 powder, UO2 pellets, Cs metallic, NaJ powder and SrO powder. The released aerosols were transported by a variable inert gas stream into cold traps and a filter. After washing and titration, the sodium release rate and the fuel and fission product traces were determined. The TS experiments were performed in the 4 m3 vessel NABRAUS with mixtures of 2 kg of sodium and 20 g of UO2 powder in a heating pan (surface area 0.72 m2) at temperatures between 500 deg C and 700 deg C. The oxygen content of the containment atmosphere could be varied from 0 to 21%. During the experiment, aerosol probes were taken at various sampling ports of the vessel. The uranium content of the aerosol was measured by a fluorometric method. Cs was determined by neutron activation , Sr by atomic absorption spectroscopy, and J by iodometric titration. Experiments with a preheating phase to support uranate production did not lead to a significant increase of the uranium release rate. LS and TS experiments are in good agreement. The height distribution of U, J and Sr in the vessel pool was analyzed on a layer-by-layer basis after some LS experiments. Very inhomogeneous distributions were found. Surface concentration and released concentration are approximately the same or at least of the same order of magnitude. This is a strong support for the assumption that mechanical processes dominate in the release mechanism. Three examples are given. The sodium release rate was found to be strongly dependent on temperature and gas convection at the surface. Between 400 deg. C and 900 deg. C, it increased by four orders of magnitude, roughly proportional to the sodium vapor pressure