Thermal-hydraulics of hollow geometry reactor fuel element in transient conditions

The paper presents a proposed mathematical model for hollow geometry fuel element transient analysis. In derivation of the unsteady conduction discretisation equation, definition is made of some mean temperature over the time step with a weighted factor that takes the inertia of the temperature field into account. The optimal relaxation for this type of geometry is analyzed and included in the numerical scheme. The solution of the temperature field over the fuel element and related coolant channels is analyzed on the basis of boundary fuel element temperatures and the maximum fuel element temperature radius iteration procedure. The paper gives fuel element thermal analyses of a hypothetical large break LOCA on the hot leg of a primary system, using the developed model. Also, an analysis of the effects of individual parameters on the thermal calculations of a reactor fuel channel with a hollow fuel element in transient conditions, is presented. Emphasis is put on the influence of the fuel's thermophysical properties and on the effects of a delay in the emergency core shutdown system on changes in the fuel and coolant temperature during a loss of coolant accident.