The flux, temperature, and weight loss will vary within each individual graphite component, for example, moderator brick. In addition, the mean component flux, temperature, and weight loss will vary through­out the core. When designing a graphite core, in order to extrapolate data from one component, which has been analyzed in detail, to the other core components, ‘form factors’ are often used, as illu­strated in Figure 13. In typical graphite-moderated reactors, the axial (vertical) flux varies approximately as a cosine with the maximum at center, whereas the radial flux is usually a flattened cosine as illustrated in Figure 13. The exact form of these profiles can be calculated using reactor physics codes. The mean core rating can be calculated from eqn [27]:

 

weight of fuel in reactor(MWd t_1)

 

reactor power

 

[27]

 

Rating

 

and at the time of interest the mean core burnup can be calculated by eqn [28]:

 

Core burnup

 

reactor power

 

[28]

 

Reflector

 

1

 

Individual channel About 320 channels in an AGR (b)

Thus, a mean moderator brick burnup can be calcu­lated by multiplying the mean core burnup by the axial and radial form factor for the particular brick of interest.

4.11.9.1 Fuel End Effects

The relatively small gap between fuel elements has a pronounced effect on the damage to the graphite moderator bricks. This is particularly noticeable in the brick dimensional changes, in both AGRs and RBMK reactors. In assessments, this detail needs to be accounted for and may require a three-dimensional reactor physics calculation.