Patent Number: 050664530
Section: summary

This invention relates to the control of coolant flow through a nuclear reactor fuel assembly, and is particularly but not exclusively concerned with controlling the rate of flow of liquid metal coolant through a fuel assembly of a fast neutron nuclear reactor, (hereinafter referred to as "a Fast Reactor"). In a nuclear reactor, comprising numerous individual fuel elements, regions of different coolant temperature may develop giving rise to thermal stresses in the core structure of the reactor. To avoid these stresses it is desirable that the nuclear fuel within the reactor operates as far as possible at a uniform temperature, and to achieve this the rate of flow of coolant through different fuel elements of the reactor is usually controlled. Variations in temperature within the reactor are a particular problem in Fast Reactors which contain both fissile fuel and fertile or breeder fuel, since the heat output from the breeder fuel starts at a low level but rises over a period of time to a substantially greater level. Consequently, the temperature of the liquid metal coolant emerging from a Fast Reactor fuel assembly containing breeder fuel rises during the operation of the Reactor towards the temperature of the fissile fuel. It is desirable to stabilise the temperature of the coolant, for example at about 550.degree. C. in a Fast Reactor, so as to maintain efficient heat transfer between the coolant and the fuel, and optimum fuel life. In general, the invention provides an apparatus for controlling the flow of coolant through a nuclear fuel assembly, the apparatus comprising a variable flow restrictor locatable in the fuel assembly, means responsive to neutron radiation at a location in the fuel assembly in a manner to cause neutron induced growth of the responsive means, and a connecting means for connecting the neutron radiation responsive means to the variable flow restrictor for controlling the flow of coolant through the assembly. Preferably, the variable flow restrictor comprises a plurality of longitudinally aligned ducts, and a plugging means having an array of plugging members locatable in some of the ducts, the plugging members being of different lengths so that longitudinal displacement of the plugging means by the connecting means progressively opens or closes some of the ducts. Conveniently, the neutron radiation responsive means comprises at least one elongate member supportable by the fuel assembly and arranged to support an elongate portion of the connecting means, the elongate member comprising at least one material having a greater neutron-induced growth property than that of the elongate portion so as to cause longitudinal displacement of the connecting means as the elongate member grows under neutron irradiation. The elongate member might comprise a rod member extendable within a said elongate portion of tubular form. The rod member might comprise a plurality of rod portions disposed one upon the other, at least some of the rod portions having different neutron-induced growth properties than other said rod portions so as to produce a required longitudinal growth of the rod member under the neutron radiation. Desirably, the rod member is arranged so that an initial longitudinal neutron-induced growth is necessary before the rod member supports the elongate portion. Alternatively, some of the rod portions might be mechanically treated, for example cold worked, so as to introduce a delay in the onset of growth in the rod portions under the neutron irradiation. The elongate member might be of tubular form and the elongate portion might comprise a rod element extending within said tubular elongate member.