Patent GB-1114566-A (GB)
Title: Improvements in or relating to a fuel element assembly for a nuclear reactor
IPC: G21C3/04
Filed: 1965-06-10
Abstract: 1,114,566. Fuel elements. ATOMIC ENERGY BOARD. 10 June, 1965 [11 June, 1964], No. 24666/65. Heading G6C. A fuel element assembly consists of a tubular fuel container having at least two coolant ducts disposed longitudinally inside the fuel container and adjacent a fuel space in the fuel container for accommodating nuclear fuel around the coolant ducts, the coolant ducts having a plurality of longitudinally spaced ports communicating with the fuel space, the one coolant duct being adapted for connection to coolant supply means and the other coolant duct being adapted for connection to coolant withdrawal means. The coolant ducts may comprise separate tubes disposed adjacent and parallel to one another or may be formed by inserting longitudinal internal partitions in a single tube. The fuel may consist of granules, preferably in the form of spheres. The fuel element assembly operating with a liquid metal coolant is preferably also provided with a gas heat shield around the outside of the fuel container. For this purpose, a tube of a material of low neutron cross-section is disposed concentrically around the outside of the fuel container and the annulus so formed is filled with a gas such as nitrogen or helium. The fuelling of the fuel element assembly can be carried out at full load by feeding the fuel granules directly into the fuel space through a feed tube via an isolating valve. The isolating valve prevents the escape of fission gases. Spent fuel is removed from the bottom of the fuel element by means of a rod disposed along the longitudinal axis of the fuel container and adapted to close and open the fuel discharge opening at the bottom of the fuel container by appropriate up and down movements. Alternatively, the bottom end of the fuel container may be provided with a zone or zones adapted to be cooled sufficiently to freeze the liquid metal coolant in such zone or zones so as to block the bottom end of the fuel space by a plug of frozen coolant in which fuel granules are embedded. To remove spent fuel, the plug is melted as by heat from the hot parts of the fuel space. Preferably, two vertically spaced cooling zones are provided so that by alternately melting the plug of one zone while the other is still frozen the batchwise removal of spent fuel can be effected while continuously supporting the fuel column on a plug of solid coolant. By varying the amount of fuel within the fuel space, a means of reactivity control is provided. In an emergency, the reactor could be shut down by discharging all the fuel elements into the spent fuel storage space. The fuel element assemblies of this invention are preferably used with a heavy water moderator at approximately atmospheric pressure. The heavy water is continuously pumped through a heat exchanger in order to prevent boiling. For emergency shut-down, a dump tank with gas-controlled heavy water level regulators can be incorporated so that, in an emergency, the gas pressure can be made to decrease and the heavy water made to flow into the dump tank shutting off the reactor. For fine control of the reactor the heavy water temperature could be varied, although this should normally be as low as possible. Control can conveniently and preferably be effected by the formation of voids in the heavy water by the introduction of a gas such as helium into a void control tube immersed in the heavy water. The only high temperature components in the reactor core are the fuel element assemblies which are separated from the moderator, in the case of a liquid metal being used as coolant, by the gas heat shield referred to above. If the gas is at a pressure somewhat above atmospheric while the pres. sure in the fuel element assembly is maintained at the lowest possible value by removing all permanent gases, and should any leak occur in the fuel element assembly, there would actually be a gas leak inwards making the whole arrangement inherently safe. One or more of the fuel element assemblies may be combined with a heat exchanger to form a replaceable unit, the preferred liquid metal coolant flowing directly from and to the heat exchanger through suitable built-in ducts. A number of such units are immersed in heavy water moderator to form the reactor core.