AGRs in the UK use enriched uranium dioxide (UO2) fuel contained within a stainless steel clad or ‘can’. Each fuel element, w’hich is approximately 1 m in length, consists of a bundle of 36 fuel pins, each 14.5 mm diameter and of the same enrichment, supported within a 190 mm bore double-graphite sleeve assem­bly. This serves both to insulate the moderator bricks within the reactor fuel channels from the hot coolant passing over the fuel, and also to permit the flow of cooler gas around the outer sleeve of the fuel element to further cool the moderator. As shown in Fig 3.53, the fuel pins, arranged in three concentric rings of 6, 12 and 18 are attached to machined grids at their lower ends and traverse two additional support braces at the centre and upper end of the fuel element. A central guide tube accommodates the threading of a tie-bar which takes up the full weight of the fuel stack {usually eight elements) during subsequent handling of the completed stringer by the charge machine.

The cans are made from niobium-stabilised stain­less steel, the overall volume of which needs to be mmimi>ed on grounds of neutron economy, leading a fundamental!} ‘thin can’ design of pin. Heat transter is enhanced via surface-roughening of the can m the term of machined transverse annular ribs, although some of the earlier AGR fuel elements con­tained helically-ribbed cans. At the pin ends, cup-

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shaped end caps are welded to the main can wall in two places to provide a double-seal between the fuel and the reactor environment and the pins are locked into the fuel element grid at their lower ends. Each pin contains typically 64 hollow UCb fuel pellets, some of which are specially grooved to receive the can during its pressurisation onto the fuel in a helium gas environment, during the final stages of pin manu­

facture. These ‘anti-stacking’ grooves (ASGs) prevent gaps opening up in the pellet stack during service as a result of differential movements between the fuel and the can. If present, such gaps would allow the can to deform into them under the influence of the external reactor gas pressure, which would eventually lead to localised weaknesses in the can wall and hence provide a possible mechanism for failure. Fuel cur­rently in use at Hinkley Point and Hunterston con­tains either 14 or 22 ASGs per pin. The top two positions within the fuel stack (elements 7 and 8) are assembled only with elements containing 22 ASG pins, since these positions have shown themselves to be more prone to inter-pellet gap formation than those lower down the stack. An increase in the number of ASGs reduces gapping to a minimum. The 14 ASG fuel is assembled into positions 1 to 6. At each end of the pellet stack there is an insulating pellet made from sintered alumina powder, the function of which is to protect the can at the pin ends, a region of poor heat transfer, from excessive temperatures. The UCb fuel pellets contain a hollow central bore (typi­cally 6.35 mm dia. in current Hinkley Point feed fuel) which acts as a void for the accumulation of fission gases produced during reactor operation. Since the rate of release of fission gas depends strongly upon the fuel temperature, the presence of the bore also restricts the quantity of gas actually released from within the fuel by effectively lowering the centre UO2 temperature for a given heat rating. The degree of enrichment of the fuel in U-235 varies across the reactor core and also between stations, but generally lies between l№o and 3% by weight.