CFD Analysis of a Possible Leakage of Coolant in the MEGAPIE Spallation Source Target

The paper reports first steps in an analysis, using Computational Fluid Dynamics (CFD), of a leak of hot (>250°C) lead bismuth eutectic (LBE) from the MEGAPIE spallation source target into the helium-filled gap between the target hull and a close-fitting, double-walled safety container. Issues addressed are: (1) determining the initial impact pressure of the LBE on the inner shell of the safety container, which could result in the gap between the shells being closed, and its cooling capacity compromised; (2) LBE freezing on the cold shell surface could delay action of the leak detection system, and hence the beam trip control; and (3) heat transfer from the hot LBE to the D₂O circulating between the safety container walls could induce boiling and subsequent pressurization of the circuit. Steps in the model development needed for the simulation are described, which include, in particular, the implementation of a liquid/gas interface-tracking model based on the Level Set approach. Results confirm that the safety container will be adequately cooled by the D₂O during normal target operation, but first indications are that boiling might occur within the D₂O circuit if substantial amounts of LBE leak from the target vessel unless its outer surface is coated with an insulating layer. Pressures generated by the impact of a slug of LBE on the safety window do not threaten its structural integrity, and no delay of the leak detection equipment as a consequence of LBE freezing is anticipated.