The heat generated in the graphite (or energy depo­sition) is required for the calculation of the graphite temperatures, and in the case of CO2-cooled systems, it is required for the calculations of radiolytic weight loss. Both of these requirements are important in graphite stress analysis calculations.

In the case ofCO2-cooled systems it is assumed that the graphite radiolytic oxidation rate is proportional to the heat generated in the graphite. However, it is ionizing irradiation that causes the dissociation of the CO2. The energy deposition is produced by the inter­action of graphite atoms with three types of particles:

• Neutron interactions with graphite atoms (~40%).

• Fission g-rays (~60%).

• Secondary g-rays caused by absorption by materi­als outside the moderator (e. g., steel fuel pins in AGRs) and by inelastic scattering of carbon atoms (~1% in a Magnox reactor and ~10% in an AGR).

The main source of gammas and neutrons arises from the fuel, mainly from prompt fission, but there are some from delayed fission.

The ratios given above are for a central position in the core and for initial fuel loading. The ratio may change with position in the core and with graphite
weight loss. Furthermore, in graphite material test programs, the ratio between neutron and g-heating is likely to be significantly different, because of the dif­ferent materials used to construct the various reactor cores. It is therefore important that this ratio is known and the implication of a change in this ratio on material property changes, that is, the implication of the ratio between fast neutron damage versus radiolytic weight loss on graphite property changes, is understood.

The gamma and neutron spectrum varies with distance from the fuel and will vary with graphite density (i. e., will change with weight loss) and fuel design. A reactor is run at constant power, and there­fore, as weight loss increases, the spectrum (gamma and neutron) will change and become harsher (higher neutron and g-flux).

In the graphite, charged electrons are produced because of the following:

1. Compton scattering interaction of gamma with electrons within the carbon atoms.

2. Pair product in electrostatic field associated with carbon atoms.

3. Photoelectric absorption.

Compton scattering predominates, but electrons and charged carbon ions are also produced because of the displacement of carbon atoms in the moderator, and in principle this could be calculated.

Energy deposition is the energy released from the first collisions of primary gamma and neutrons.25 Energy deposition is calculated in watts per gram (W/g) and the spatial distribution can be calcu­lated using reactor physics codes such as McBend
(http://www. sercoassurance. com/answers/), WIMS, and WGAM. However, a crude estimation of energy deposition can be made by assuming that ^5% of the reactor power is generated in the graphite. This heat can then be proportioned to the rest of the core using interpolation and form factors, and estimates of the distribution within a moderator brick.

In conclusion, energy deposition is required to calculate graphite temperatures and radiolytic oxida­tion rates. Energy deposition can be estimated but is most accurately calculated using reactor physics codes. However, care must be taken because the ratio between neutron heating and g-heating, or more appropriately a direct measure of the ionizing irradi­ation, is important.

4.11.6.1 The Use of Titanium for Installed Sample Holders

During the construction of the Magnox and AGR reactors, graphite specimens were placed into ‘installed sample holders,’ the intention being that these samples could be removed at a later date to give information on the condition of the graphite core. To enhance the radiolytic weight loss of the graphite in the installed sample holders, titanium was used. Although this only slightly increased the g-heating, it did increase the number of electrons produced, because of an increase in pair production and Compton scattering caused by the higher atomic number or ‘Z-value’ of titanium compared to graphite (22 and 6, respectively).