GB1268186A - Improvements in or relating to nuclear reactor fuel elements - Google Patents

Abstract

1,268,186. Fuel elements. UNITED KING- DOM ATOMIC ENERGY AUTHORITY. 21 April, 1969 [26 April, 1968; 16 July, 1968], Nos. 20041/68 and 33902/68. Addition to 1,174,575. Heading G6C. A fuel element of the type specified in the main parent Specification is, in one feature of the invention, in the form of an elongate fuel member and its metallic sheathing includes both internal and external sheathing between which the fuel is disposed, the fuel member thus being of the tubular type and there being an interlayer disposed between and directly contacting the fuel and each of the internal and external sheathing. The interlayers are preferably of a ceramic compound of U of either natural or depleted fissile isotopic abundance, and the fuel is a ceramic compound of Pu, a mixture of ceramic compounds of U and Pu, or a fissile-enriched ceramic compound of U. The interlayers may be in pellet form with the fuel in the form of vibro-compacted ceramic particles, or the interlayers may be of vibrocompacted ceramic particles and the fuel in pellet form, or the interlayers and the fuel may all be in pellet form. Where the fuel is of pellet form, it may be in two annular portions with an annular gap therebetween to provide a fission product reservoir and enable gaseous fission products the better to pass to a gaseous fission product plenum at one end of the fuel element or to gaseous fission product venting means, and also to assist in providing voidage for the accommodation of irradiation-induced swelling. The fuel density is preferably less than 85% of maximum theoretical density. In one embodiment, each fuel pellet has at least three radial ribs extending from its inner and outer surfaces to contact the sheaths, the part-annular spaces between the ribs being occupied by vibrocompacted UO 2 to form the interlayers. Where the fuel is of carbide compounds, a thin, non- fissile, non-fertile compatibility layer is provided between each interlayer and the respective sheathing over at least that axial region of the fuel zone of the fuel member which is subjected to maximum burn-up in operation. Suitable materials for the compatibility layer include MgO, TiO 2 and ZrO 2 . This layer may be applied by spraying or dipping, those parts outside the axial region of maximum burn-up not requiring the layer being masked. Where the fuel member is not of the tubular type, as according to another feature of the invention, it has its interlayer of UC of natural or depleted fissile isotopic abundance, its fuel of PuC, mixed PuC and UC, or enriched UC, and there is additionally provided between the interlayer and the sheathing, a thin compatibility layer of non-fissile, non-fertile material, such as is described with reference to the firstmentioned feature.