METHOD FOR FLATTENING THE CURVE OF EVOLUTION OF HEAT OF FUEL ELEMENTS WITHIN THE CORE OF A FAST REACTOR

摘要

1510432 Reactors ELECTRICITE DE FRANCE SERVICE NATIONAL 22 Jan 1976 [10 Feb 1975] 02540/76 Heading G6C In a method for flattening the curve of evolution of heat of fuel elements within the core of a fast reactor, wherein the core is constituted by vertical fuel elements each having an axial passage of sufficient diameter to permit the flow of molten fissile material in the event of a power excursion, the diameter of the axial passage is reduced from the elements of the control region of the core to the peripheral elements, the reduction in diameter being proportional to the reduction in neutron flux density. The method can be employed as a complement to the conventional method of producing a variation in the degree of enrichment or as a substitute for this method by retaining a constant enrichment. Each fuel element, Fig. 3, comprises a metallic can 12 housing fissile material 14 in which is formed a hollow axial passage 16, and is provided with a solid upper blanket 18 and a lower blanket 20 constituting a first catchpot 22. A second catchpot 26 of refractory material is located at the extreme lower end of the fuel element, forming the base of a fission-gas expansion chamber 24. The fissile material 14 may comprise a stack of pellets of UO 2 or mixed UO 2 -PuO 2 . The diameter of the axial passage progressively decreases between the centre, 28, of the fuel element and the lower end, 30, of the fissile material 14. This decrease in diameter is such as to maintain the maximum temperature of the fuel below a predetermined temperature. In the event of a fast power excursion, the temperature and pressure of the gas within the passage 16 increase much faster than those of the gas within the catchpot 22 and the chamber 24. The gas is thus impelled downwards, with the result that the fall of molten fuel under the action of gravity will be considerably accelerated particularly at the level of the effective nozzle at the lower end of the fissile material 14.