[1] Purpose of lattice calculation

Even using a high performance computer, a direct core calculation with several tens of thousands of fuel pins is difficult to perform in its heterogeneous geometry model form, using fine-groups (e. g., 107 groups in SRAC) of a prepared reactor constant library. The Monte Carlo method can handle such a core calculation, but it is not easy to obtain enough accuracy for a local calculation or small reactivity because of accompanying statistical errors. Hence, the Monte Carlo method is not employed for nuclear design calculations requiring a fast calculation time. Instead, the nuclear design calculation is performed in two steps: lattice calculation in a two-dimensional (2D) infinite arrangement of fuel rods or assemblies and core calculation in a three­dimensional (3D) whole core. The lattice calculation prepares few-group homogenized cross sections which maintain the important energy dependence (neutron spectrum) of nuclear reactions, as shown in Fig. 2.8, and this reduces the core calculation cost in terms of time and memory. Since final design parameters in the core calculation are not concerned with the energy depen­dence, the spatial dependence such as for the power distribution is important.