Heterogeneous Reactors

A reactor core has a lattice structure with fuel, moderator and structural material arranged periodically. The structure of the fuel pellets is heterogeneous; they are covered by claddings, and moderator surrounds these. Figure 7.1 shows a horizontal cross-sectional view of a BWR fuel assembly. When the mean free path of neutrons is equal to or shorter than the diameter of a fuel rod (or the thickness of a fuel plate), the reactor is called heterogeneous. In LWRs, a mean free path of a few centimeters is comparable to the fuel rod diameter. Additionally, since the behavior of neutrons differs greatly between the fuel and moderator, it is necessary to treat them as a heterogeneous system. Fast reactors may be approximately treated as a homogeneous system since the mean free path is a few tens of centimeters.