ABSTRACT
This chapter generalizes the simplified multigroup formalism and the equations given in the chapter are at the heart of most of the numerical codes used in reactor physics. These codes give the effective multiplication factor, and also the neutron flux in each group. This flux, which defines in greater or lesser detail the neutron spectra at each point, is not interesting in itself, but enables us to calculate all the useful quantities. The chapter presents some information about multigroup transport theory. The increasing power of calculational tools has enabled us to abandon this semi-empirical theory which gave good distributions of thermal neutrons but which became very questionable from the moment it was a matter of knowing rapid neutron fluxes, just those which contribute the most too biological doses. The same number of groups, transport calculations is much more costly than diffusion calculations.
