Magnox nuclear power stations were introduced in the UK with the design and building of Calder Hall at Sellafield in Cumbria. This plant, claimed to be the first nuclear power station in the world, currently generates 60 MWe in each of its four reactors. It first supplied power to the national grid in October 1956. Its sister station at Chapelcross near Annan in Scotland is also still in operation. Fuel elements used in such stations consist of uranium rods one inch in diameter canned in Magnox, an alloy of magnesium with low neutron absorption and high thermal conductivity.
The initial can design used closely-spaced fins in transverse position to coolant flow direction, an improvement on the longitudinal fins adopted for the Windscale pile. Longitudinal fins suffered from the fact that increasing the number or size of fins in order to increase heat transfer restricts the flow at the base of the fins, which reduces heat transfer [Fortescue and Hall (1957)]. Experimental studies using metal-coated insulating material enabled the determination of heat transfer coefficient distribution over transverse fins [Harris and Wilson (I960)]. Flow visualization studies revealed the formation of vortices between fins thus helping to achieve acceptable heat transfer rates.
When larger commercial versions of Calder Hall were built at Bradwell, Dungeness, Hinkley Point, Sizewell, Hunterston and Trawsfynnid in the UK, at Tokai Murai in Japan and Latina in Italy, the transverse fins were displaced by polyzonal fins of various designs, e.g., spiral fins in quadrants formed by straight splitter fins. Experimental work was carried out on these designs, including optimization of the number and angle of fins. Although average heat transfer rates were higher than for transverse fins, these designs suffered from longitudinal and transverse variations in heat transfer, caused by interruption of spiral flow by splitters forming the quadrants and by blockage produced by braces added to prevent fuel element distortion.
Reactor operation is dictated by maximum can temperature and hence by minimum heat transfer coefficient. The location of this minimum coefficient is dependent on manufacturing variability and could occur at different positions along the can, although usually slightly downstream of a brace. The probabilistic treatment of this problem has been developed by Wilkie (1962).
An improvement over the polyzonal design has been the four-zone "herringbone" design introduced in France. This type is now used in all magnox reactors. But it also suffers from longitudinal and transverse variations in heat transfer caused by fin deformation during reactor operation. (See also Augmentation of Heat Transfer, Single-phase.)
Fortescue, P. and Hall, W. B. (1957) J. Brit. Nucl. Energy Conf. Vol 2. 83.
Harris, M. J. and Wilson, J. T. (1960) Heat Transfer and Fluid Flow Investigations on Large-scale Transverse Fins. Paper 7 at the Joint I. Mech. E. and Brit. Nucl. Energy Soc. Symposium on the use of secondary surfaces for heat transfer with clean gases. London.
Wilkie, D. (1962) The Probability of Obtaining a Low Stanton Number on a Polyzonal Fuel Element. UK Atomic Energy Authority TRG Report 217 (w).