To serve in evaluating the safety of an FBR steam generator, the heat transfer characteristics are studied on a twophase flow comprising sodium and hydrogen gas constituting a reaction jet, and a simple model is derived of the effective heat transfer coeflicient (HTC) between flowing fluid and conduit tube wall. Comparison of the model with experimental data lead to the following conclusions: The HTC presented by the two-phase reaction jet-which proves to have an upper limit-is equivalent in

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... roximation to that presented by single-phase sodium flow. The last-mentioned HTC being significantly greater than that presented by hydrogen gas, the heat flux and HTC of the two-phase flow can in approximation be written in simpler form as functions of the HTC of single-phase sodium flow, sodium temperature and void fraction. The upper limit of sodium temperature-considered to be equal to the sodium boiling point-would be dependent on pressure. On the other hand, hydrogen gas temperature can readily surpass that of sodium, so that the maximum temperature difference between the two could reach several hundreds degree centigrade. Hydrogen gas provides negligible heating effect, so that the apparent HTC would decrease with increasing hydrogen gas temperature. Sodium conversely provides surpassing cooling effect on the conduit tube wall, and this would prevent the wall temperature from rising sugnificantly above that of sodium, even if simultaneously exposed to high-temperature hydrogen gas.