Experimental investigation of the critical heat flux in 8 mm circular tubes and in 7-rod bundles

In a broad framed research program to develop a high conversion pressurized water reactor (HCPWR), experiments of critical heat flux (CHF) in 8 mm circular tubes and in closely spaced 7-rod bundles are performed. Freon 12 is used as working fluid. The influence of different parameters on CHF, e.g. pressure, mass flux, steam quality, tube diameter, spacers and power distributions is investigated. CHF fluid-to-fluid modelling is used to transfer the test results to water conditions. The CHF experiments with 8 mm tubes yield an increase of critical heat flux with increasing mass flux under HCPWR conditions. A HCPWR therefore shows an attractive CHF-behaviour due to its high mass fluxes compared to a conventional PWR. The test results in 8 mm tubes, combined with the test results of other authors, underline that correlations in the literature can not reproduce the diameter influence on CHF with sufficient accuracy. The experiments with 7-rod bundles show on the basis of the same hydraulic diameters much lower CHF values compared to the tube experiments. The influence of spacers (grid spacer and wire wrap) on CHF is found to be substantially governed by the local steam quality. For low steam quality wire wraps lead to higher CHF. An opposite influence of spacers on CHF is found for high steam quality. CHF increases during the transition from uniform to non-uniform power distributions. The test results with different radial power distributions can qualitatively be explained by using a subchannel analysis. These experiments deliver a data base for the verification of subchannel analysis codes. (orig./GL)