CFD Modeling of Debris Melting Phenomena during Late Phase CANDU 6 Severe Accident

摘要

The objective of this paper was to study the phase change of the debris formed on the CANDU 6 calandria bottom in a postulated accident sequence. The molten pool and crust formation were studied employing the Ansys-Fluent code. The 3D model using Large Eddy Simulation (LES) predicts the conjugate, radiative and convective heat transfer inside and from the corium pool. LES simulations require a very fine grid to capture the crust formation and the free convection flow. This aspect (fine mesh requirement) correlated with the long transient has imposed the use of a slice from the 3D calandria geometry in order not to exceed the computing resources. The preliminary results include heat transfer coefficients, temperature profiles and heat fluxes through calandria wall. From the safety point of view it is very important to maintain a heat flux through the wall below the CHF assuring the integrity of the calandria vessel. This can be achieved by proper cooling of the tank water which contains the vessel. Also, transient duration can be estimated being important in developing guidelines for severe accidents management. The debris physical structure and material properties have large uncertainties in the temperature range of interest. Thus, further sensitivity studies should be carried out in order to better understand the influence of these parameters on this complex phenomenon.