The impact of qas in sodium flow on the temoerature variation of an LMFBR rod b~ndle was studied in two type~ of exoeriments: • The gas fraction of the subchannels as well as the aas bubble spectra across the outlet of an unheated 61-rod bundle with wire spacers were measured in water/air flow. For uniform and non-uniform gas distribution at the bundle inlet, the determined averaqed qas fractions across the subchannels at the bundle outlet were uniform and non-uniform, respectively.The measured bubble spectra show a. dependency between the bubble frequencies, the contact times and the gas fractions of a subchannel . . A model to compute the transient temperature distributions within a heated rod was supported by experiments in a sodium/ argon slug flow. The measurements show that the fluid and the rod temperature variations increase with increasing heat flux, gas fraction and contact time of a single bubble at the wall.The knowledge of the averaged gas fractions in the subchannels allows to calculate for oractical reasons the mean subchannel coolant and wall temperature ~ises, the knowledge of the contact time of a bubble at the wall allows to calculate the wall temper3ture variations due to the presence of gas in the sodium flow of an LMFBR rod bundle.

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