The absorption rate in the fuel is
ΣauφuVu
where V is the volume, and the absorption rate in the moderator is
ΣamφmVm
The fraction of thermal neutrons absorbed by the Uranium fuel as compared to the total number of thermal neutron absorptions in the assembly is known as the thermal utilization factor f and is given by
Fig. 8.12 shows a unit cell of a heterogeneous assembly in which the uranium rods of radius r, are placed at regular intervals (pitch). The equivalent cell radius r1 is also indicated.