Computational investigation of the movement of a melted mass to the bottom of the vessel during an off-design accident in a fast reactor

Abstract

According to the safety concept adopted for fast reactors, heat removal from fragments of a destroyed core must occur inside the reactor vessel. The BN-800 design includes a special bottom plate under the bottom of the vessel for retaining and cooling radioactive materials from the core. To substantiate proper functioning of the bottom plate, it is necessary to analyze heat removal from the heat-releasing layer lying above it. The analysis requires a study of the movement of a melted mass from the core; this will make it possible to estimate the residual heat release in the fuel, which depends on the time it takes for the melted mass to reach the bottom plate.As shown in the technical substantiation of the safety of nuclear power plants with BN-800 reactors, during a loss-ofpower accident without actuation of all means for affecting the reactivity, the core does not melt, since negative feedbacks on reactivity result in a decrease of reactor power, even under conditions when sodium boils. However, the possibility of fuel meltdown has not been excluded in the case of prolonged development of an accident under conditions when active or passive means of acting on the reactivity cannot be actuated.In the present paper we present the results of calculations of the motion of a melted heat-releasing mass, formed in such a hypothetical situation, to the protective foundation. We used a conservative approach, in which it is assumed that the fuel moves simultaneously from the parts of the fuel assemblies which have the highest power. The time for the heat releasing mass to reach the foundation will be shortest in this case.It is assumed in the model that initially the layer of melted heat-releasing material comes instantaneously into an ideal contact with the materials in the bottom blanket, which remains during the entire subsequent time period during which they melt. The decrease of the residual heat release in the fuel with time is taken into account. The one-dimensional temperature distribution in the vertical direction in the heat-releasing mass and the lower-lying materials and the depth of melting of the materials under the core are found at each time step. As a result of the calculations, the time of movement of the heat-releasing mass from the core to the pressure chamber and then the melting-through time of its bottom plate are determined. It is assumed that the heat in the heat-releasing layer and the lower-lying materials is transported only by heat conduction. The effective thermal conductivity model is used; the bottom blanket and the lower-lying region are considered to be porous media with parallel conductivity of the corresponding components: in the bottom blanket --uranium dioxide, steel of the fuel element cladding and the fuel assembly casings, sodium (liquid or vapor under the heat-releasing mass); in the zone of gaseous cavities in the fuel elements --gas, steel, and sodium; in the zone of the fuel assembly tail pieces --the steel of the tail pieces and collectors, and sodium. Heat transfer by convect...