ICONE15-10140 MOLECULAR DYNAMIC SIMULATION OF THERMAL CREEP IN 7-PIN LMFBR BUNDLE UNDER HYPOTHETICAL ACCIDENT CONDITIONS

Abstract

Rarefied gases confined by material walls with tangential temperature gradient flow in the direction of the gradient, even in absence of gravity. This "thermal creep" phenomenon, which often occurs associated with thermophoretic forces, due to temperature gradients across the fluid, is currently being investigated in a variety of cases, ranging from micro-machine engineering, as pumping devices without components motion, to nuclear applications, intended to remove radioactive micro-particles, thus preventing atmospheric contamination. In the frame of safety analysis of fast breeder reactors, the sequence of events following a Transient Over-Power (TOP) or Transient Under Cooling (TUC) accident, which may lead to severe code damage and eventually to core disruption, is generally referred to as Hypothetical Core Disruptive Accident (HCDA). Numerical analysis with the Direct Simulation Monte Carlo (DSMC) method were made with the computer program THEMIS/3D.2 for a 60-degree sector of a 7-pin bundle, with hexagonal lattice, simulating sodium coolant vaporization, film condensation and rewetting sequences. Beside, thermophoretic forces acting upon spherical fuel particles of variable sizes, released by hypothetical disintegration of fuel pellets, were simulated. Under accident conditions, the coolant channel in a fuel bundle can get chocked by stagnating sodium vapour, displaying a zero net mass flux along the channel, but characterized by creep flow along the surfaces of the fuel claddings, thus contributing to the dispersal of fuel debris along the walls of the fuel elements. The present work investigates numerically the importance of keeping into account the impact of creep flows in the coolant upon the dispersal of fuel debris, hence upon the formation of fuel crusts when debris are dispersed and deposited on still intact fuel elements.