An analytical model of two-phase head and torque of centrifugal pumps was developed. Pressure differences in gas and liquid phases through the pump were assumed to be equal to each other and a two-fluid model was used for evaluation of the pump performance in the case that two-phase flow entered the pump.
Interphase friction had a large effect on calculated results of the two-phase pump performance. Andersen's model of interphase friction in the bubbly flow regime gave the most proper prediction in both normal and reverse direction flows. The analytical results were within data scatter obtained from tests of three model pumps. The agreement between experimental data and the prediction suggests that the present mathematical model is able to describe two-phase flow behavior in the centrifugal pumps.

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Many integral and separate effect experiments have been carried out cooperatively by the Japanese BWR utilities and vendors. Through these results, various counter-current-flow limitation phenomena occurring inside the BWR core, steam and mist cooling effect, multibundle effect etc. have been identified effectively to cool down the core during LOCA transients.
Based on these findings and with other aids, SAFER code, which can better estimate the thermal-hydraulics of the BWR transient by the simple one-dimentional method, has been developed by the BWR vendors to replace with the existing SAFE-REFLOOD-CHASTE code system.
The new code predicts the peak-cladding-temperature for the standard jet-pump BWR LOCA transients to be as high as 500600°C, which is well below the current regulatory limitation. LOCA/ECCS constraints will be reduced for future BWR core managements and ECCS equipments surveillance etc. by the use of new code.

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Circulation pump behavior has large effect on coolant discharge flow rate in case of reactor pipe break. Experiment of two-phase pump performance was conducted as a joint study of Japanese BWR user utilities and makers.
Two-phase head and torque of three centrifugal pumps in high temperature and high pressure (around 6 MPa) steam/water were measured. Head was decreased from single-phase characteristics when gas was mixed in liquid flow in condition with normal flow and normal rotation directions. When flow rate was large enough, two-phase head was about the same as single-phase one in reversal flow conditions. Two-phase head was smoothly increased as flowing steam volumetic concentration increased when flow rate was small and flow direction was reversal. Changes of torque with gas concentration were correspondent to those of head. This suggested that changes of interaction between flow and impellers due to phase slip effected on torque which caused head differences between single- and two-phase flows. Dependence of dimensionless head and torque of three test pumps on steam concentration were almost the same as each other.

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