ither total cross sections or some form of empirical removal cross sections. The module can make use of a library of removal cross sections on magnetic tape or disc containing microscopic cross sections for a selection of elements in a number of removal groups. For example, there is a standard tape containing neutron data for 32 elements in 18 removal groups.
The calculation is performed in two stages: the removal flux through the shield is calculated in a number of removal groups, and the removal flux and source are then obtained in an output group scheme which may be different. The removal source can be calculated by multiplying the removal flux in each group by the corresponding removal cross section and assigning each result to an output group. Alternatively, a full matrix of transfer cross sections may be used. The module produces a functional representation of the flux and source in each group and each region using Chebyshev series. This ensures that the execution time does not depend on the number of printout points requested, and that the module output is in a form from which values at any point may be calculated.
PARAMETERS (WRS system module No. 35836) Calculates multigroup neutron diffusion parameters for use in the age- diffusion method. Parameters are derived for materials of specified compositions from a library of microscopic data. This data is normally held on a magnetic tape and is written by an auxiliary program which processes files from the UKAEA Nuclear Data Library. For each material and each energy group the module calculates values of the diffusion coefficient D, the inverse slowing down length K and the absorption coefficient Alpha.
REGROUP (WRS system module No. 13774) Converts multigroup flux spectra from one energy group scheme to another. Several spectra can be processed during one execution of the module, and there is a facility for applying scaling factors to the converted spectra. There are two alternative modes of operation. In one mode a 1/E spectrum shape is assumed within each input group, while in the other a shape is deduced from the flux values in adjacent groups. In both cases the total flux is preserved in the conversion.
DIFFUSION (WRS module No. 7539) Solves a set of multigroup diffusion equations for a system represented in one-dimensional plane, cylindrical or spherical geometry.
The system is divided into regions each of which is assigned a material throughout which the diffusion length, the diffusion coefficient and the cross sections are uniform. The module may be supplied with group parameters appropriate to the continuous slowing down model namely the diffusion coefficient, the absorption parameter and the inverse slowing down length.
It then calculates the diffusion length and the cross section for transfer to the next lower group from simple relationships involving the above parameters.
There is provision to allow for buckling due to lateral leakage by supplying a buckling value for each group and region. Void regions may be included. The module may also be used to solve the adjoint problem.
In order to solve the differential equation for each group, a spatial mesh is superimposed on the system and the equation at each mesh point is expressed using finite difference approximations for the derivatives.
These, together with extra equations for the internal and external boundary conditions, form a set of linear equations which, being almost tri-diagonal, are easily solved by elimination and back substitution.
DOCTOR (WRS module No. 1827) Calculates any number of reaction rates given a set of multigroup fluxes at a number of spatial mesh points in one spatial dimension.
The flux values may be read from cards by the module, or may be in a data group which is already open when the module is called. For each energy group, values are supplied at a number of specified points in one spatial dimension, these points being divided into regions in which they are not necessarily spaced evenly. Average cross sections in each group can be supplied for each detector in the input data, or