As supplied, graphite components contain a significant amount of both open and closed porosity in a variety of shapes and sizes, from the nm scale to the mm scale, as illustrated in Section 4.11.3. The open pore volume (OPV) is defined as the volume of pores accessible to helium, closed pore volume (CPV) is the volume of pores not accessible to helium, and the total pore volume (TPV) is the volume of open and closed pore.
The effect of pore size on the radiolytic oxidation rate was investigated by Labaton et al11 who found
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(b) CH4(vpm)
Figure 10 Inhibition in carbon dioxide due to the addition of carbon monoxide, moisture, and methane.
(a) G_C as function of CO and H2O concentration and
(b)G_C as function of methane (CH4) concentration. Reproduced from Best, J. V.; Stephen, W.; Wickham, A. Prog. Nucl. Energy 1985, 16(2), 127-178.
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the maximum range to be 1.5-5 pm. Taking this into account and referring to eqns [I]—[III] above, the oxidation process will be expected to be more efficient in the smaller pores than in the larger pores.
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This is because in the smaller pores the distance to the wall is less, making it less likely, compared to the case for larger pores, that the active species would be deactivated by collision in the gas phase.
To account for this difference in oxidation rate with pore size for modeling purposes, in the case of the Magnox reactors which did not have CH4 routinely added to the coolant, a pragmatic approach of defining ‘pore efficiency’ was adopted, whereas in the case of the AGRs where CH4 is routinely added, a reactive pore volume (RPV) was defined as being the volume of pores oxidizing in CH4-inhibited coolant gas.
It is also clear that as the oxidation process proceeds, closed porosity will be opened and the pore size distribution will change, thereby changing the oxidation rate.
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Standring further developed this reasoning into a relationship for the cumulative weight loss, Ct, at a constant dose rate:
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where A = (1°°^ and Пє is the effective initial OPV in cm3 cm 3.
However, a reactor is operated at constant power. Replacing the dose rate in eqn [21] by kPt/Wm, where Pt is the reactor thermal power, k is the fraction of the reactor power absorbed in the graphite (~5%), and Wm is the weight of the moderator, gives
kPt P
g0 = 145eeG—C — % per year [23]
Wm T
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From eqn [23], it is clear that the rate of oxidation will increase with loss of moderator mass.
It was shown by Standring that the cumulative weight loss, Ct, for a reactor operated at constant power is given by
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