Temperature fields and heat transfer in free-packed fuel pin bundles cooled by heavy liquid metal

The paper considers heat transfer and temperature fields in a free-packed fuel pin bundle cooled by a heavy liquid metal with different types of spacing. Data is analyzed for three fuel assemblies with a pitch of s/d = 1.33: a smooth fuel pin bundle, a bundle of fuel pins spaced by a bilifar-helix wire wrapper of the “wire-to-wire” type, and a bundle with transverse spacer grids. In the free-packed smooth fuel pin bundle, there are no temperature non-uniformities around the fuel pin periphery in contrast to major general temperature non-uniformities in the bundle with wire wrapping which provides for a major decrease in the heat transfer. In the bundle with transverse spacer grids, the heat transfer only increases in the region of the grids, and, between them, is approximately equal to the heat transfer in the smooth pin bundle. Correlations are presented as recommended for the calculation of Nusselt numbers and the temperature non-uniformities around the fuel pin periphery with the above spacing options.