Temperature fields in fast reactor fuel assemblies with shape changes

Abstract

The lifetime of fast reactor fuel elements is determined by a number of factors (stress level, initial defects in the fuel Cladding, the corrosive effect of fission fragments on the clad, etc.); among these, an important role is playedby the temperature level, as well as by the value and nature of the azimuthal nonuniformity of the temperature field, particularly for the fuel assemblies with the highest heat-release rate.Experience in operating high-flux fast reactors (BN-350, BN-600, Ph~nix) has shown [1][2][3] significant changes in the shape of fuel assemblies over the operating cycle, changes associated with swelling and radiation creep of materials (Fig. I). Changes occur in the shape of the fuel assembly jacket and in the geometric characteristics of the bundle and the fuel elements along the fuel assembly cross section and length.In recent years, there has been great interest in investigating fuel assembly temperature regimes for deformed fuel lattices. Experimental and calculational investigations of fuel assembly temperature fields have been performed for misalignment of fuel elements in a bundle and for their buckling and deformation of the fuel bundle resulting from swelling and temperature field nonuniformities [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. The results of these investigations show an increase in the temperature level and in the temperature nonuniformity in fuel assemblies with shape changes.Experimental Investigation of the Maximum Azimuthal Temperature Nonuniformity in a Deformed Fuel Lattice. The systematic investigations of temperature fields in deformed fuel lattices which were performed at the Soviet Physics and Energy Institute allow us to analyze the laws of temperature field formation and to obtain concrete relationships for the characteristic variants of lattice deformation [4,5,8,13,14]. Experiments were performed on a model fuel assembly of the BN-600 type, which consisted of 37 simulated fuel elements situated in a triangular lattice with a relative pitch s/d = 1.185 and enclosed in a hexagonal casing. The relative gap between the casing and the simulated rods was A/(s --d) = 0.5 [5].Although bundle deformation resulting from a single (group) fuel element misalignment which is parallel along the whole length is an idealized model, its analysis gives important practical information on the limiting temperature nonuniformities.The temperature field for an azimuthal (along the perimeter of afuel assembly bundle) misalignment, for example, of a fuel element adjacent to the wall is nonsymmetric: the maximum temperature occurs in the area of a unit cell with a decreased flow section (Fig. 2a), the temperature nonuniformity is not the same over the whole region of energy output (Fig. 2b) and significantly exceeds the nonuniformity for nominal geometric conditions (Fig. 2c). The nature of the temperature field is qualitatively identical for fuel assemblies with ribbed and smooth fuel elements and with and without spacers in the unit cells adjacent to the wall.Representing ...