Although it would be inappropriate to devote too much time at this juncture to a detailed description of the AGR, a brief review of some of the funda­mental design concepts are crucial to a clear under­standing of the function of the fuel cycle. Much of the following relates specifically to the AGR design at Hinkley Point В, on the basis that any minor de­partures from it for other AGR stations will be un­important — the principles will be the same.

A typical AGR core consists of a graphite cylinder of approximately 10 m diameter perforated by some 300 or so fuel channels, each about 300 mm in dia­meter, the interstitial positions being occupied by the control rods. The fuel elements themselves are each approximately 1 m long containing 36 stainless steel clad fuel pins located inside a double graphite sleeve. In order to ease the refuelling process the AGR has been provided with single channel access, enabling each fuel assembly to be charged to the reactor via its own standpipe. Each fuel assembly consists of a plug unit, comprising closure unit, gamma shield plug and gag unit, connected to eight fuel elements by a stainless steel tie bar. At its upper end the plug unit is used to seal the standpipe and lower down it con­tains an adjustable gag for controlling the gas flow in the channel. The composite, which can be handled as one entity by the fuelling machine, is some 25 nt in overall height and is called a ‘stringer’, each reactor contains some 300 fuel stringers.

As was common practice with earlier reactor de­signs, the AGR control rod system has been subdivided into groups of coarse (black) and fine or regulating (grey) rods, each with their own separate tasks. A total of 81 rods provide the 44 coarse rods used during start-up and power raising and normally fully withdrawn thereafter, and the 37 regulating rods which offer fine control and are normally partly inserted during operation. It is the regulating rods, whose task is to help maintain control over the radial power distribution during all stages of reactor operation (i. e., start-up, steady power running and refuelling) and so importantlv linked to the fuel cycle, which are of prime interest here. Each is located on a ‘1 in 8’ channel array in interstitial positions (Fig 3.48) and

Fic. 3.48 Regulating rod array
Arrangement of the regulating control rods in
interstitial positions on a 4 in 8’ fuel channel array
provides for a good degree of control over the local
power distribution. The automatic control system
regularly scans the gas temperature conditions in each
of the eight channels in a control ‘box’ and adjusts the
axial position of the rod accordingly.

responds automatically to eight channel gas outlet temperatures in surrounding fuel channels. With this arrangement four fuel channels are situated adjacent to the rod itself, the other four being equidistant from two rods, thus providing a good degree of local control. During operation the axial positions of the individual regulating rods vary from between 15% and 85% penetration, with a mean position, under ideal conditions, of close to 50%. There is a tendency for rods to be well withdrawn when local refuelling is due.