1. Neutron equations

Depending on the time scales considered when describing the neutronic behavior of a reactor, a distinction is usually made between :

• time-independent "static" behavior (steady-state Boltzmann equation);

• slow" temporal evolution dealing with changes in nuclear fuel composition (Bateman equations);

• fast" transient phases, during which delayed neutrons play a more or less important role, depending on the kinetics of the transient (kinetic equations). The kinetic equations are derived from the time-dependent Boltzmann equation.

It's the large "size" of the Boltzmann equation that makes it so difficult to solve. In the general case, the problem has to be solved in 7 dimensions (three in space, two for the direction of propagation, one for the...