New fuel

The magnox fuel element is essentially a cylindrical bar of natural uranium encased in a magnesium al­loy can. Both the uranium and the can material have additional trace elements to provide the required met­allurgical properties. The bar is about 25 mm diameter by 750 mm in length (the Berkeley and Hunterston elements are about half this length) and a weight of 10 11 kg. It is ‘grooved’ and during manufacture the can is pressurised down into the bar. This procedure is employed to provide intimate bar-to-can contact and to present ratcheting between the bar and can, due to thermal changes during operation. The can is dosed by end caps which are screwed and seal welded into the ends after the bar has been inserted. A ther­mal disc of sintered alumina is positioned between the bar and end cap to reduce the transfer of heat to the end fittings. The latter are screwed into the can to provide a cup at the top end of the element and a cone at its lower end, thus providing a suitable means ot stacking the elements in a channel. The top end
fitting is designed to accept the jaws of a grab used for handling procedures. In most designs this fitting is provided with a link which is sprung, allowing it to press against the channel wall thus reducing the tendency of the element to rattle or rotate within the channel. The method of stacking the fuel in the Berkeley reactors differs from the Hunterston A reactors in that instead of the lower dements sup­porting those above, each element is individually sup­ported. The Berkeley element is contained in a struc­ture made up of graphite struts and steel bridges so that the weight of element above is taken by the struts. The Hunterston A element is contained in a graphite sleeve which supports the weight. This design has the additional advantage that the graphite ad­jacent to the fuel in the channel (which experiences greater radiation damage) is replaced when the fuel is exchanged.

There are two types of cans, polyzonal or helical finning and herringbone finning (Fig 3,42). The poly­zonal can is produced by an extrusion process and the helix formed by twisting the can which is stif­fened by a splitter and brace assembly or cage. The herringbone can is also an extrusion followed by ma­chining of the tugs and fins. The polyzonal can has to some extent been superseded by the herringbone design since the latter has greater resistance to fin waving during operation. With increased dwell times in the reactor, the thermal changes cause the finning to assume a wavy characteristic (multiple longitudinal bowing) with a permanent set, some fins touching and

IriG. 3.42 Comparison of the polyzonal (helical) and herringbone fuel cans

resulting in a reduced heat transfer efficiency. Since the herringbone fin length is less than that of the polyzonal element, the fin waving tendency is reduced. It is usual for reactors to be wholly charged with either polyzonal or herringbone fuel. The exception is Oldbury where a mixed channel is employed with three polyzonal elements occupying the bottom chan­nel positions with herringbone elements on top of them. It will be noted that the helix of adjacent quad­rants of the herringbone element are such that rota­tional forces resulting from the flow of the coolant gas cancel out. However, it does mean that radially adjacent lugs are at differing temperatures.