Graphite corrosion can be expressed either as G(-C) (the number of carbon atoms removed per 100 eV of
energy adsorbed) or as oxidation rate (the fractional weight of graphite oxidised per mWh/g of energy adsorbed). Graphite corrosion is measured by either a weight loss or a C-14 technique. Weight loss mea­surements involve weighing a sample, irradiating to a known dose in a known coolant composition and then reweighing the sample. During irradiation in-pore carbon deposits are formed, increasing with increasing G(-CH. t), and thus allowance must be made for these. The quantity of deposit is measured using a differ­ential thermal oxidation measurement in air. Using the graphite weight loss the G(-C) value is calculated using:

D x £ x P

where T is the absolute temperature, К

W is the graphite weight change, ppm D is the cumulative dose, mWh/g £ is the graphite open pore volume, cm3/g P is the gas pressure, bar

C-14 measurements are made by initially labelling a small graphite sample (typically 10-50 g) with C-14 by either manufacturing the graphite from C-14 la­belled coke and pitch or by heating a sample of C-12 graphite to 3000°C in the presence of C-14 labelled carbon monoxide when exchange between the gas and solid phases occur. The sample is then enclosed in a steel capsule and irradiated in a known radiation field with a gas of known composition passing through the capsule. The graphite oxidises and releases CuO into the gas stream, this being oxidised to CI4C>2 which

oitected and measured using either a gas or solid phase scintillation counter. G(-C) is calculated:

6.69 x 1010 x C Q T

here c is the effluent activity, Bq/cm3 q is the gas flow rate, cm3TP/s S is the graphite specific activity, Bq/g D is the dose rate, mW/g £ is the graphite open pore volume, cm3 p is the gas pressure, bar M is the molecular weight of CO2

The relationship between G(-C) and oxidation rate (OR) is given by: OR = G(-C) x £ x 8.1 x ІСГ10 at 41 bar and 673°C, where £ is the graphite open pore volume cm3/(100 cm3).

The mechanism of radiolytic graphite corrosion is a complex process proceeding via the formation of an ionised carbon dioxide molecule, to a carbon monoxide molecule and an oxidising species. In the absence of other materials these will recombine to carbon dioxide giving it its apparent radiation sta­bility. The lifetime of the oxidising species is typi­cally 10“7 s, equivalent to a range of a few microns at reactor pressures. Radiolytic corrosion therefore occurs throughout the graphite brick and not solely at the geometric surface. Early experiments demonstrated that the reaction rate was controlled by the rate of energy deposition within the coolant in the pores, i. e., the rate increases with increasing pore volume and increasing gas density (increasing pressure and decreasing temperature).

In magnox reactors the primary inhibitor of the reaction is carbon monoxide, which is permitted to build up to a maximum of 1.5v/o. The mechanism of inhibition is that as the carbon monoxide in­creases, the probability that the oxidising species will recombine before they react with the graphite surface also increases. In addition the small quantities of hydrogen and water (total of 25-100 vpm) form a slightly protective complex on the surface.

In AGR reactors, where methane concentrations up to 400 vpm are used together with up to 1.5 v/o carbon monoxide, the formation of a protective sur­face complex due to the radiolytic destruction of methane is the main mechanism of graphite inhibi­tion. In large diameter pores only a fraction of the oxidising species will reach the graphite surface be­fore being deactivated resulting in a net formation of protective species and a build up of carbon deposits. In the smaller diameter pores all the oxidising species feach the graphite surface leading to a net removal of carbon. Hence in a AGR a most important para­meter is the volume of small diameter pores (0.05 f*m -* 5 цт).

The difference of the pore distribution in which corrosion occurs between magnox and AGR coolant compositions has a very significant effect on the re­lationship between weight loss and cumulative dose. The basic equation relating the two parameters at constant reactor power is

(А2/100тге) log (1 + Ct A) — ACt 100 — g0t

where go is the initial oxidation rate given by go = 2.10 x 103 £G(-C) D ~ /<r0у

D is the dose rate, W/g

£ is the graphite open pore volume, cm-Vg

P is the gas pressure, bar

T is the temperature, °A

A = 100xe/(l — 7Te)

xe is the effective open pore volume, cm3/cm3 Ct is the % weight loss after time, t years.

For magnox coolant compositions the effective open pore volume to be used is the total initial open pore volume, but for AGR coolant compositions the vol­ume is only a fraction (20-30%) of the initial open pore volume. The equation indicates that, for any given initial oxidation rate, the cumulative weight loss increases with decreasing initial pore volume. This has been experimentally verified in a realistic AGR coolant composition and the results are shown in Fig 1.42 as a comparison of xe = open pore volume and irt ш 0.2 open pore volume. This clearly demon­strates the effect; but at the highest cumulative doses the weight loss becomes increasingly lower than pre­dicted by the above simple model, being due to the increasing diameter of the small diameter pores. More complicated corrosion models have been developed which allow for this effect as well as the influence of closed pore volume opening due to oxidation of the graphite.

Effect of coolant composition on graphite oxidation rates

In magnox reactors the two primary impurities in the coolant are carbon monoxide and hydrogen/water. For the former, increases up to 1.5 v/o decrease the corrosion rate by a factor of up to 3 compared to pure carbon dioxide, but further increases in carbon monoxide concentration do not significantly reduce the rate further. Increases in hydrogen/water concen­tration also reduce the oxidation rate up to 150 vpm but at higher concentrations little further decrease occurs. The operating levels of carbon monoxide and hydrogen/water which should be maximised to reduce

graphite corrosion have to be optimised with respect to carbon deposition on fuel pins and corrosion of circuit materials.

In AGR reactors the water concentration is in the range 200-500 vpm, due to its formation from methane destruction, and over this range has virtual­ly no effect on graphite oxidation rate. Therefore the main impurities that exert the major effect on graphite oxidation rate are the carbon monoxide and methane concentrations. A wide range of experiments using moderator graphite were carried out in the DIDO Materials Testing Reactor at AERE Harwell, the Siloe Reactor at CEA Grenoble and in the gamma irra­diation facility at Berkeley Nuclear Laboratory using both weight loss and C-14 techniques. Analysis of the results, ignoring thin specimens (< 2—4 mm) when geometric surface effects dominate, has shown good agreement between all the facilities, and the ‘mean’ oxidation rate against coolant composition is shown in Fig 1.43 and is given by the equation:

OR = 2.7 [0.26 — 1.53 log jo [(CO) + 8(CH4)]j

Г 1 1

————————- 0,5

0.55 + 1.29(CO)

I

1 + 2is(™±)

CO /

Where OR is the oxidation rate at 41 bar and 673 K, and (CO) and (CH4) are the carbon monoxide and methane concentrations, Vo.

METHANE ICMO, »om Fig. 1.43 Effect of coolant composition on AGR moderator oxidation rates

Variations of ±30% about the mean data are found between different samples but no difference has been observed between the materials of two manufacturers. Anglo Great Lakes Corporation and British Acheson Electrode Ltd.

The effect of increasing methane concentration is always to decrease the oxidation rate although the effect tends to level out above 400-500 vpm. The effect of carbon monoxide concentration is more com­plex and demonstrates the relative effects of the two mechanisms of graphite inhibition. At low carbon monoxide concentrations the gas phase recombina­tion of active species is low and hence at low meth­ane concentrations the oxidation rate is high. As the methane concentration is increased the rate of meth­ane destruction increases and the increased surface inhibition leads to very low oxidation rates. At high carbon monoxide concentrations the gas phase recom­
bination of active species is high and hence at low methane concentration the oxidation rate is low. As the methane concentration is increased the rate of destruction of methane increases but, because this reaction is inversely proportional to carbon monoxide concentration, the increase in methane destruction and consequent decrease in oxidation rate is not as great as for low carbon monoxide concentrations. In the technological range of interest of methane concen­tration (200-400 vpm) the carbon monoxide concen­tration has little effect on oxidation rate and the exact choice has to involve other constraints.

A similar but more limited range of irradiation tests has been completed for AGR sleeve graphite and generally the mean oxidation rate has been shown to be twice the mean oxidation rate of AGR moderator graphite namely:

OR = 5.4 [0.26 — 1.53 logio [(CO) + 8(CH4>]]

[———- !———

[ 0.55 + 1.29(CO)

of predicted low methane concentration. A greater benefit can be achieved if the coolant is forced through the graphite by applying a pressure drop across the brick. In this situation the major graphite parameter controlling the methane depletion is the ‘permeability’, particularly at reactor pressure, the ‘viscous flow co­efficient’, Bo. Again this parameter should be maxi­mised within other constraints. However, ‘permeability’ is a difficult parameter to control and can vary by up to two orders of magnitude from one brick to another even within one graphite type. This is overcome in reactor design by testing every brick and preferentially laying the core so that bricks of high permeability are positioned in areas of high load or high cumulative dose. This has minimised the depletion of methane within the brick and maximised core integrity.

A series of computer programs generically known as ‘DIFFUSE’ have been written to solve the com­plex interactions. An example of the predicted corro­sion profile resulting from the programs is shown in Fig 1.44.

10.4.1 Graphite corrosion (thermal)