4. METHOD OF SOLUTION
ANISN solves the one-dimensional Boltzmann transport equation for neutrons or gamma-rays in slab, sphere, or cylinder geometry. The source may be fixed, fission or a subcritical combination of the two. Criticality search may be performed on any one of several parameters. Cross sections may be weighted using the space and energy dependent flux generated in solving the transport equation.
ANISN-E : Besides diamond and weighted difference supplementary equations, exponential supplementary equations are available.
The new model:
(1) always gives positive solutions, without using any 'fixup' technique provided that the source is non-negative;
(2) allows, especially in deep penetration problems, the use of larger spatial meshes, hence requires shorter computer times than the ones requested by the diamond model combined with various types of fixup techniques or by weighted difference schemes to get the same accuracy;
(3) supplies solutions that are always reasonable overestimates of the exact solution.
In ANISN-JR, some optional functions are added to increase the utility of the code:
(1) print the total fluxes at boundary points of all mesh intervals. (The original ANISN prints the total fluxes at midpoint only.) (2) calculate, print and plot the lethargy width spectra. (3) print the angular fluxes at only required mesh boundaries or midpoints (maximum 10 points). The original ANISN prints at mid- point of all meshes, and therefore the number of print pages becomes vast according to the number of spatial and angular meshes.
(4) use the asymmetric quadrature set.
(5) calculate and plot the reaction rates for neutron and gamma-ray detectors, and collapse the response functions of detectors.
(6) generate volume-flux weighted cross sections for arbitrary zone or region. In the original ANISN, the cross sections can be collapsed only for a homogeneous zone or region.
(7) collapse into few group cross sections in ANISN, DOT, or TWOTRAN format. (In TWOTRAN format, the l-th Legendre coefficient of the scattering cross section is divided by (2l + 1) and the cross section of (n,2n) reactions is added for use of the coarse-mesh rebalancing technique.)
(8) multiply the average cross section by the density factor, when an option of density factors is used (IDFM=1).
