Cooling of heat generating core debris using multiple passive jets in a liquid metal pool of LMFBR

Abstract

A 3D numerical study on coolability of heat generating core debris by multiple passive jets in a liquid metal pool has been carried out. The computational domain considered is a cylindrical enclosure, which resembles the lower plenum of a typical 500 MW LMFBR. The plenum has provisions to ensure the coolability of the core debris resulting from the hypothetical core disruptive accident. The arrangement avoids settlement of core debris on the main vessel and ensures safe retention of destroyed core on core catcher within the main vessel. The enclosure is filled with liquid sodium and is considered as an incompressible fluid. The transient form of governing equations for flow and heat transfer are solved numerically. The k−ε model is used for closing RANS equations while the pressure velocity coupling is done by PISO algorithm to obtain smooth convergence. The developed model is validated against benchmark results on natural convection in low Prandtl fluid available in the literature and also using in-house experimental data. The Rayleigh number based on heat generation rates is varied from 1012 to 1015. Different orientations and angles of passive cooling pipes with time dependent heat generation rate are analyzed. Three different bend angles viz 90°, 135° and 180° with bend orientations of inward, outward and opposite are considered. Also, the influence of central chimney on heat removal is brought out clearly. The fluid flow and heat transfer characteristics are described using isotherms, velocity vectors and in terms of Nusselt number. It is found that cooling pipes with 1800 bend having inward orientation provides favorable cooling capability over all other configurations. The central chimney is found to have profound influence in the heat removal from heat source. A useful correlation for Nusselt number is also proposed for thermal design purposes.