A new correlation for CCFL at full scale PWR excluding the Hutze effect in UPTF data

Abstract

The effect of the Hutze (a secondary ECC injection pipe within the hot leg pipe of a Konvoi-type PWR) upon CCFL was experimentally investigated at COLLIDER test facility of the Technical University Munich (TUM). COLLIDER represents a PWR hot leg with a large dimeter of 190 mm and ~¼ scale. A Hutze pipe was built into COLLIDER’s hot leg according to a German Konvoi-type PWR reactor design data. The effect of the Hutze upon CCFL are shown through comparisons with previous investigations at COLLIDER without a Hutze. Visual observations using high-speed recordings are presented at different flow conditions: supercritical conditions at low air velocities, transition into subcritical flow, and the onset of CCFL with comparison against previous recording without a Hutze. Limits of: the onset of hydraulic jumps, onset of CCFL (flooding), and most importantly CCFL characteristics are also shown and compared. The Hutze was found to induce a large impact upon these limits: two hydraulic jumps are observed prior to the onset of CCFL instead of only one (in case without a Hutze), the limits of the onset of hydraulic jumps and onset of CCFL occur at much lower air velocities, and the hysteresis effect (difference between limits of flooding and deflooding) disappears. CCFL characteristics curve is also shifted toward lower values of gas velocities and less liquid flows into the reactor vessel at same air velocities in the case with a Hutze which means a less cooling potential during the reflux condensation mode. Ohnuki correction of the Hutze effect upon CCFL characteristics of 1988 is tested and it seems to work well at small diameter but it overestimates CCFL characteristics for COLLIDER data. The previous agreement between CCFL characteristics obtained from downscaled experiments with D > 50 mm without a Hutze (including COLLIDER data and other large scale facilities) with UPTF data (at full scale) is found to happen because of the Hutze effect which shifts the CCFL characteristics downwards in Wallis diagram and not because of the correct representation of occurring CCFL phenomena. The actual CCFL characteristics for the 1:1 scale without a Hutze are expected to be higher than those reported from data with Hutze at UPTF facility. This indicates that there is still a scale effect even at a large diameter channel, and that most previous downscaled experiments have underestimated CCFL characteristics at full scale. Based upon obtained results, a correlation of CCFL characteristics at full scale excluding the Hutze effect is given. This correlation is of a special interest for the design of new PWR reactors as most new designs does not include a Hutze.