The SABRE code for fuel rod cluster thermohydraulics

Abstract

This paper describes the capabilities of the SABRE code for the calculation of single phase and two phase fluid flow and temperature in fuel pin bundles, discusses the methods used in the modelling and solution of the problem, and presents some results including comparison with experiments. The SABRE code permits calculation of steady-state or transient, single or two phase, flows and the geometrical options include general representation of grids, wire wraps, multiple blockages, bowed pins, etc. Transient flows may be calculated using either semi-implicit or fully implicit time solution methods and the temperature distributions within the fuel pins are determined as well as the velocity and temperature of the coolant. Two phase flows are calculated using a homogeneous boiling model, with the possibility of a specified slip between the two phases. General inlet boundary conditions are available (including pressure, velocity, total mass flow) and these may vary linearly with time; the outlet boundary condition is taken as constant pressure. The treatment of grids allows for irreversible head losses at entry and exit. The wire wrap model introduces a grid resistance tensor with its principal axes along and perpendicular to the wire, resulting in a very satisfactory modelling of inducement of swirl. The derivation and solution of the difference equations is discussed. Emphasis is given to the derivation of the spatial differences in triangular subchannel geometry, and the use of central, upwind or vector upwind schemes. The method of solution of the difference equations is described for both steady state and transient problems. Together with these topics we consider the problems involved in turbulence modelling and how it is implemented in SABRE. This includes supporting work with a fine scale curvilinear coordinate programme to provide turbulence source data. The problem of modelling boiling flows is discussed, with particular reference to the numerical problems caused by the rapid density change on boiling. The final part of the paper presents applications of the code to the analysis of blockage situations, the study of flow and power transients and analysis of natural circulation within clusters to demonstrate the scope of the code and compare with available experimental results. The comparisons include the calculation of a flow pressure drop characteristic of a boiling channel showing the Ledinegg instability, examples of overpower and flow rundown transients which lead to coolant boiling, and calculation of natural circulation within a rod cluster.