Free surface modeling and simulation of the water experiment for the XT ADS spallation target

High level nuclear waste may be treated in accelerator driven systems (ADS) so that long-term radiotoxicity can be reduced substantially An ADS combines a particle (proton) accelerator with a sub-critical nuclear reactor core. The protons are injected into a spallation target to produce source neutrons for driving the subcritical core. The cooling of the target is of major concern since the protons depose a very large amount of heat into the target. In the XT-ADS target the protons enter through free surface into a stream of liquid metal (LBE), so that cooling can be ensured by a suitable stable free sum face configuration. The water experiment built at UCL, Belgian aims assisting the design of the XT-ADS spallation target as well as validation of the numerical models to be used in the final design of the planned target. The shape and level of the free surface within the target are essential for its thermal behavior and safety. Many earlier related studies have investigated the effect of various geometrical design parameters on the free surface level and stability. Even though a stable free surface could be established previous designs failed due to cavitation in the feeder. Based on these studies a conclusive design was proposed and numerically investigated. The Star-CD computational fluid dynamics (CFD) code is employed. The volume of fluid model (VOF) and the cavitation models available in the code are applied. Within the present study the behavior of the free surface for d outlet pressure and inlet velocity is investigated. In addition, a design for drag limiters in the feeder is proposed, which consists of parallel plates inserted into the annular feeder tube. These plats increase the pressure losses in the feeder nozzle so that in contrast to previous designs the pressure is sufficiently high avoiding unacceptable cavitation. Our simulations show that the proposed target including the feeder is stable for the tested hydraulic situations. (authors)