As previously discussed, Gilsocarbon graphite for the AGRs was manufactured by molding (or press­ing) the spherical filler particles and blocks, result­ing in a semi-isotropic graphite with an anisotropy ratio of ~1.01 (based on the ratio of the orthotropic CTE values). Dimensional change MTR data for Gilsocarbon over a wide range of temperatures is given in Figure 35. There are two sets of data at each temperature; one WG and one AG. This illustrates how isotropic the properties of Gilsocar — bon are, even when irradiated. In Figure 35, it is also clearly illustrated that the higher the irradiation

	140
160
Figure 34 High-fluence irradiation dimensional change in pile grade A graphite irradiated at 600 °C. Reproduced from Brocklehurst, J. E.; Kelly, B. T. Carbon 1993, 31(1), 155-178.
♦ 430 °C ■ 600 °C A 900 °C • 940 °C О 1240 °C □ 1430°C
sP cN
2 .c о
200
0
Figure 35 Dimensional change in Gilsocarbon graphite. Modified from Birch, M.; Brocklehurst, J. E. A review of the behaviour of graphite under the conditions appropriate for the protection of the first wall of a fusion reactor; UKAEA, ND-R-1434(S); 1987; Brocklehurst, J. E.; Kelly, B. T. Carbon 1993, 31(1), 155-178.

50 100 150

Fluence (1020 ncm-2 EDND)

temperature, the sooner the turnaround is reached. At very low fluence, semi-isotropic graphite swells. This swelling can be quite significant as demon­strated by the irradiation of semi-isotropic NBG — 10 at 294 and 691 ° C.67 This behavior has been attributed to the annealing out of residual machining stresses or shrinkage strains, but there are no micro­structural or other experimental observations to val­idate this reasoning.

When graphite is irradiated past turnaround and reaches its original volume, sometimes referred to

as ‘critical fluence,’ the structure of the graphite begins to break down, as illustrated in Figure 36.

4.11.14.1 Effect of Radiolytic Oxidation on Dimensional Change

When designing the UK AGRs, irradiation experi­ments in a carbon dioxide atmosphere were carried out in BR-2 at Mol, Belgium. These experiments were designed to obtain high radiolytic weight loss (~35%) in a very short time, and hence, a low fast

Figure 37 Dimensional changes in preoxidized samples. Modified from Brocklehurst, J. E.; Edwards, J. The fast neutron-induced changes in dimensions and physical properties of near-isotropic graphites irradiated in DFR; UKAEA, TRG Report 2200(S); 1971.

neutron fluence. Some of these specimens, referred to as ‘preoxidized,’ were reirradiated in an inert atmosphere in other MTRs, including DFR, with some achieving reasonably high irradiation fluence. Figure 37 gives the dimensional change behavior of some of these experiments.68

The results show a clear correlation between preoxidized weight loss and dimensional change behavior, indicating an increased dimensional change and delay in turnaround with increased preoxidized weight loss. This has clear implications for the AGRs and required further investigation. Unfortunately, the

graphite MTR experiments designed to carry out simultaneous radiolytic oxidation and fast neutron damage under power reactor conditions were aban­doned because of the closure of the UKAEA MTRs at Harwell in 1990. It is therefore unclear how signif­icant this behavior is for the AGRs. However, there are now MTR experiments being undertaken in HFR (High Flux Reactor) at Petten, the Netherlands to try and address this.

4.11.14.2 Dimensional Change Rate

The constitutive models used to predict stresses in polycrystalline components often do not use dimen­sional change directly but use dimensional change rate. The dimensional change rate and dimensional change of Gilsocarbon graphite irradiated at 550 °C are com­pared in the schematic shown in Figure 38. The turn­around in dimensional change rate occurs earlier than turnaround in dimensional change. In channel-type reactors such as an AGR or Magnox reactor, it is the turnaround in rate that is associated with the peak in­bore stress. Thus, when planning a nuclear graphite MTR experiment, it is important to obtain data in the low to medium fluence range, as well as at high fluence.