Comparative experimental and numerical analysis of the hydraulic behaviour of free-surface flow in the water experiment of the XT-ADS windowless spallation target

If the long-living actinides contained in nuclear waste are subjected to an intense neutron flux, nuclear reactions convert them into less toxic short living actinides. This process, called transmutation, may be accomplished by Accelerator Driven Systems (ADS). The required neutron flux is generated by bombarding a spallation target with high energy protons emerging from an accelerator. In the windowless target currently developed in the Integrated Project EUROTRANS of the European Community the target material is HLM (heavy liquid metal). The HLM flows downwards through a concentric nozzle surrounding the beam tube and forms a conical free surface which is subjected to the beam current. The thermal-hydraulics of the target must ensure that the high amount of heat deposited into the HLM can be safely removed. The current target design was developed by a close interaction of numerical and experimental studies resulting in a profound understanding of the hydraulic behavior. The water experiment built at UCL, Belgium has similar geometry and dimensions as the XT-ADS spallation target. The experimental results are used to validate the numerical methods that will be used in the analysis of the real HLM target. In the current paper the experimental results obtained from the water experiment are compared to numerical results obtained by applying Volume of Fluid methods in combination with cavitation models. The comparison covers the nominal flow case corresponding to a flow rate of 10 l/s and sensitivity study of free surface level to various flow rates and outlet conditions. Numerical results where obtained from the Computational Fluid Dynamics code STAR-CD. This code is selected for its capability to predict sharper free surfaces compared to other CFD codes implemented in the target study. Numerical and experimental results show good agreement, so that the numerical model is considered validated. The results demonstrate that the hydraulic behaviour of the target tolerates slight variation of the flow conditions relative to the design condition (nominal flow case). The results show the capability of the used numerical model to simulate the free surface in the water experiment. Consequently, the numerical code will be used in the design of a real liquid metal windowless spallation for the XT-ADS