Resonance Escape Probability
See also: Nuclear Resonance
p ~ 0.75
It is obvious the resonance escape probability is strongly influenced by the arrangement and the geometry of the reactor core. In a homogenous reactor core, where fuel nuclei are surrounded by many of moderator nuclei, there is a significantly higher probability, that the resonance neutron will collide with the fuel nucleus. Therefore in homogenous reactor cores, the resonance escape probability is lower than in heterogeneous cores.
On the other hand, in a heterogeneous reactor core, all the fuel nuclei are in fuel pellets that are encapsulated within a fuel rod. This arrangement increases the probability, that the fast neutron will escape the fuel matrix. The neutron slows down in the moderator where there are no atoms of 238U and 240Pu present. Therefore in heterogeneous reactor cores, the resonance escape probability is significantly higher than in homogenous cores.