4.11.7.1 Introduction

In carbon dioxide (CO2)-cooled reactors, two types of oxidation can occur. The first is thermal oxidation which is purely a chemical reaction between graph­ite and CO2. This reaction is endothermic and is negligible below about 625 °C and is not impor­tant up to 675 °C. The second is radiolytic oxidation that occurs when CO2 is decomposed by ionizing radiation (radiolysis) to form CO and an active oxi­dizing species, which attacks the graphite. Radiolytic oxidation occurs predominantly within the graphite open porosity.

4.11.7.2 Ionizing Radiation

Ionizing irradiation can be defined as that part of a radiation field capable ofionization (charge separation) in CO2 either directly or indirectly. This leads to the creation of reactive species, which may react with the carbon atoms at the surfaces (external and more importantly internal) of the graphite components.

4.11.7.2.1 Energy deposition

Historically, ‘energy deposition’ has been used for a surrogate for ionizing irradiation, most probably because it is easy to measure using calorimetry and can be estimated from the reactor power. Energy deposition, sometimes referred as ‘dose rate,’ in the units of W/g of graphite, is a measure of the total energy absorbed in the gas in unit time from the scattering of g-radiation and fast neutrons.

For a typical Magnox reactor, energy deposition is composed of approximately the following components:

• 36% from the neutrons

• 58% from the gamma

• 6% from the interaction of graphite atoms within the moderator

Of these, it is only the last two that directly con­tribute to ionization of the carbon dioxide gas, mainly through Compton scattering. These ratios will be slightly different in an AGR.

An assumption is made that the dose rate received by the graphite is the same as that absorbed by carbon dioxide within the pores of the graphite and that a fraction k of the fission energy from the fuel causes heating in the moderator. For a typical Magnox reac­tor, k is ^5.6% of the thermal power. The unit G_c is defined as the number of carbon atoms gasified by the oxidizing species produced by the absorption of 100 eV of energy in the CO2 contained within the graphite pores; G_c for pure CO2 = 3.