Irradiation produces important effects on the graph­ite moderator of an AGR which could lead to the moderator being the life-limiting feature of AGRs, The integrity of the structure of an AGR core, which is essential for the maintenance of coolant How, operation of control rods and refuelling, depends on the interlocked features of the moderator bricks. The strength of these graphite keys and keyways must therefore remain adequate to accommodate the im­posed stresses throughout reactor life in normal op­eration and under seismic loads. Neutron irradiation not only affects the strength of the moderation but also produces dimensional changes which result in internal stresses. These effects are compounded with changes due to oxidation of the graphite, the oxida­tion itself being radiolytically induced.

Considering first the changes in graphite strength, the effect of neutron irradiation is in fact to strengthen the graphite by up to 50*Го over reactor life. The de­tailed changes occurring are complex, but essentially the radiation induced defects obstruct the disloca­tions in the material, thus increasing the strength. This gain in strength is however counteracted by the effects of graphite weight loss due to corrosion. Graphite does not oxidise thermally in carbon dioxide at the temperature existing in AGR moderator, about 450°C, but irradiation of the carbon dioxide produces highly oxidising species which, although short-lived, do cause graphite corrosion from within the pores of the mod­erator. The rate of oxidation is suppressed by addi­tion of corrosion inhibitors carbon monoxide and methane to the coolant, but the amount of inhibitor has to be restricted in order to avoid carbon deposi­tion on fuel pins and in the boilers. Corrosion of graphite equivalent to a weight loss of 20To reduces graphite strength by about a half.

The internal stresses in the graphite bricks arise Irom differential dimensional changes. Neutron irra­diation initially causes graphite to shrink. In the iso­tropic graphite used in AGRs this shrinkage is due to displaced atoms taking up places in the graphite pores. While the graphite is shrinking, the shrinkage is greatest near the channel wall where fast neutron dose is highest. This results in a tensile stress near the channel wall and a compressive stress at the brick edges where the kevways are located. This is a non-damaging situation. However, when the shrinkage eventual!) stops and reverses, then the stresses reverse and the critical keywav area comes under tensile stress. It is this localised stress relative to the local strength which determines whether keyways will fail. The reversal of graphite shrinkage is delayed by oxida­tion of the graphite because oxidation takes place preferentially on the walls of the pores, thus keeping pores open.

The ‘turn-around’ time is therefore dependent on coolant composition, i. e.. on the corrosion inhibitors introduced to the coolant.

The net changes which occur in stress and strength at keyways are shown in Fig 3.14 for typical AGR conditions with a modestly inhibiting coolant. The residual strength, which is the strength minus the in­ternal stress, is seen to increase initially, reduce to its initial value at around 15 full-power years and then continue to reduce quite rapidly as strength decreases and stress increases monotonically.