Patent Number: 039309401
Section: summary

This invention relates to fuel sub-assemblies (sometimes also called fuel assemblies) for nuclear reactors having a high power density such as liquid metal cooled breeder reactors which operate predominently on fast neutrons. Fuel subassemblies for this purpose comprise at least one cluster of elongated parallel pins containing fuel (fissile or fertile material), said fuel pins being disposed on a uniform lattice (usually hexagonal) and contained within a shroud or sleeve along which the coolant flows. The relative spacing of the pins in each cluster is determined by means of spacer members (wires, small-diameter tubes or fins) which are helically wound on each fuel pin with a large pitch. In the fuel sub-assemblies which were in use up to the present time, the radial projection of the spacer members was the same in the case of all fuel pins and the distance by which each spacer projected from the pin was identical at all points of the spacer. This arrangement has a drawback : even if the cluster is tightly packed and the shroud is applied against the fuel cluster in perfectly intimate relation, subchannels defined by the outer fuel pins of the cluster and by the shroud exhibit a head loss coefficient which is lower than that of the central sub-channels and the coolant in the outer fuel channels comes into contact with a smaller pin can area. In consequence, the coolant which passes through the outer sub-channels is heated to a lesser extent than the coolant which passes through the inner sub-channels (between inner fuel pins) and therefore has a lower temperature when leaving the cluster. This phenomenon is attenuated to only a partial extent by mixing due to turbulences and heat transfer processes between adjacent sub-channels and remains objectionable. In fact, in respect of a pre-established value of coolant temperature at the outlets of the sub-channels and therefore of the temperature of the fuelpin cans at this level (the temperature being limited by the high-temperature strength of the canning materials), the mean temperature of the coolant at the outlet of the cluster is lower, thereby resulting in a reduction in reactor efficiency. The aim of the invention is to provide a nuclear fuel assembly which meets practical requirements more effectively than assemblies of the prior art, especially insofar as the assembly is no longer attended by the above-noted disadvantage or at least only to a very limited degree. To this end, the invention proposes a nuclear fuel assembly comprising at least one cluster of canned-fuel pins disposed on a uniform lattice within a sleeve through which the coolant is circulated, each fuel pin being provided over at least the greater part of its length with a helical spacer member which determines the spacing between each fuel pin and the adjacent pins or the sleeve. Essentially, the distance to which each spacer member of a lateral fuel pin of the cluster projects radially from said pin in the zones in which said spacer member is applied against the sleeve is smaller than the radial distance of projection of the spacer members which are carried by the central fuel pins of the cluster. In one embodiment of the invention in which the spacer members are constituted by helically wound wires, the wire is flattened in those portions in which it is applied against the sleeve and the flattening is preferably such as to produce a reduction in the rate of flow within the peripheral sub-channels in order to obtain the same temperature at the outlets of all the channels whether said channels are delimited solely by fuel pins or both by fuel pins and by the sleeve. In another embodiment, the spacer members are constituted by wires and the wires of each lateral fuel pin have a smaller diameter than those of the other fuel pins and are threaded through tube sections having an external diameter which is the same as that of the wires carried by the central fuel pins, said tube sections being placed in the zones in which the lateral fuel pins bear on the cental fuel pins or on the adjacent lateral pins. Again according to a particular mode of application of the invention, the sleeve is provided with internal longitudinal ribs which reduce the cross-sectional area of the sub-channels for the flow of coolant between the lateral fuel pins.