An experimental study of CHF for return-to-power condition at main steam line breaks using a non-uniformly heated vertical rod bundle

An experimental study on critical heat flux (CHF) has been performed for water flow in a non-uniformly heated vertical 3x3 rod bundle under low flow and a wide range of pressure conditions. Especially, this experiment was focused on the analysis of the CHF trend and the applicability of conventional CHF correlations in return-to-power condition of main steam line break accident whose conditions might be low mass flux, intermediate pressure, and high inlet subcooling. The effects of mass flux, inlet subcooling and pressure on the CHF are consistent with previous understandings. The effects of mass flux on the CHF are relatively large in low pressure conditions. The effects of inlet subcooling and pressure on the CHF decrease as the mass flux decreases. The effect of pressure on the CHF shows complex behavior in low pressure conditions, and becomes relatively small as the mass flux decreases. As the local critical quality increases, the local heat flux at CHF location sharply decreases at high mass flux or low critical quality. However, at low mass flux or high critical quality, the local heat flux at CHF location shows nearly constant value regardless of the critical quality. Conventional CHF correlations predict the present return-to-power CHF data with reasonable accuracies. However, the prediction capabilities become worse in low mass flux of about 100 kg/m2s