Some methods used in lattice calculations

The basic problem of reactor physics is to obtain the neutron and gamma distributions throughout a reactor. The neutron and gamma distributions are described by solving the transport or Boltzmann equation, with today's it is not feasible to solve this equation for entire reactor, core plus reflector, during its entire lifetime for both short-lived transients and long-term depletion effects. Therefore the computational process is divided into four stages, detailed in this paper. The remainder of this paper describe some of the methods that are used in lattice codes that can handle complex geometries. Section 2 summarizes the generation of a nuclear-data library without involving any methods. Section 3 sketches how in Helios arbitrary two-dimensional systems are described. Section 4 outlines a method to evaluating resonance shielding in such systems. Section 5 deals with the spatial solution of the transport equation in those systems. Section 6, finally, shows how the linearization of complex depletion chains is still feasible if so-called break isotopes are introduced. (author)