Thermohydraulic characteristics of reactor fuel assemblies with partial blockage of the flow channel

Abstract

Blockage of the flow channel in a fuel assembly of a nuclear reactor changes the coolant pressure, temperature, and temperature fields. A redistribution of the flow rate and pressure in the flow occurs at the location of the blockage, recirculation flows arise behind the blockage, and at the same time the hydraulic resistance increases and, correspondingly, the coolant flow rate through the fuel assembly decreases. Stagnation of the coolant flow increases the overall heating of the coolant. The degradation of heat removal increases the temperature in the region of the blockage. All of this makes the blockage of a fuel assembly a potential source of accidents associated with overheating of fuel elements, possible local boiling of the coolant, and slow destruction of the fuel element cladding; which can increase the scale of the blockage and give rise to accidents that can spread. This makes it important to investigate blocked fuel assemblies.Blocked bundles of rods have been investigated since the end of the 1960s in Germany, USA, and Japan. Such investigations have also been performed in Russia, France, Belgium, the Netherlands, and Great Britain. A list of works with an indication of some characteristics is given in Table 1 [ . Additional information can be found in [42,[46][47][48][49][50][51].The experimental works were performed on models of fuel assemblies cooled by water, thorium--potassium alloy, sodium, and air. Systematic investigations were conducted for LMFBR type reactors (the reactor "Mondzyu," Japan, SNR-300, Germany, and so on). Currently available computational codes for analyzing the thermal hydraulics of fuel assemblies with blockages are also illustrated in Table 1.As one can see, substantial experimental and computational data from investigations of the thermal hydraulics of blocked fuel assemblies have now been accumulated. Nonetheless, not enough investigations (especially experimental) have been performed for parameters characteristic for Russian fast reactors (fuel element lattice spacing 1, 1 < s/d < 1.17, spacing of fuel elements by means of wire wrap with pitch hid = 15, coolant --sodium).In the last few years, thermal hydraulic investigations of blocked model fuel assemblies have been performed at the Main Science Center of the Russian Federation --Physics and Power Engineering Institute for a wide range of values of the parameters, including the parameters of Russian fast reactors. The results of these investigations are reported in this paper together with a generalization of the data. HYDRODYNAMIC INVESTIGATIONSThe model assemblies (Fig. 1) consist of 19 tubes (elements), arranged in a triangular lattice and enclosed in a hexagonal jacket. The diameter of the tubes is 19 mm, and the tubes are separated by top and bottom lattices, as well as a wire wrap, in the case of the ribbed variant. Partial blockage of the flow channel is accomplished with a plate placed in the central or nearwall zones of the flow channel of a tube bundle 400 mm from the bottom lattice. The degree of blockage ...