diff --git a/docs/source/usersguide/random_ray.rst b/docs/source/usersguide/random_ray.rst index ff319d6692..032dc85892 100644 --- a/docs/source/usersguide/random_ray.rst +++ b/docs/source/usersguide/random_ray.rst @@ -678,10 +678,14 @@ one of "material_wise", "cell_wise", "stochastic_slab", or * Like ``material_wise``, but clones the material in each cell so every cell gets its own cross sections (each material-filled cell is assigned a distinct macroscopic). - - * Resolves intra-material spatial variation that ``material_wise`` averages - away, e.g. a thick shield or a steep flux gradient within a single material + - * Resolves spatial variation between distinct cells that share a material, + which ``material_wise`` averages away (e.g. a shield wall modelled as + several cells of one material, each covering a different depth) * Captures spatial self shielding between cells filled with the same material - * Most expensive (one cross section set per cell) and a larger library + * Fidelity is limited by the cell definitions, not the source region mesh: + a single large cell uses one cross section set throughout, so a steep + gradient is only resolved if the geometry is split into several cells * Same far-from-source limitation as ``material_wise``: a cell that is not tallied to yields zero cross sections for that cell * - ``stochastic_slab`` @@ -702,6 +706,13 @@ one of "material_wise", "cell_wise", "stochastic_slab", or between materials) * May hang if a material has a k-infinity greater than 1.0 +.. note:: + The ``cell_wise`` method generates one cross section set per cell definition, + not per cell instance. A cell that appears in several locations of a lattice + therefore shares a single cross section set across all of those locations. If + you need distinct cross sections per instance, subdivide the geometry into + separate cells. + When selecting a non-default energy group structure, you can manually define group boundaries or specify the name of a known group structure (a list of which can be found at :data:`openmc.mgxs.GROUP_STRUCTURES`). The ``nparticles``