 strength shows no lateral variation, and is therefore a function of a single spatial coordinate x. The source is defined as a function of x by values given at the points of a mesh which is uniform in each region, while a group treatment is used for the energy dependence. Two cases are distinguished by whether the source energy spectrum is the same throughout the system or varies with position.
In the latter case the spatial variation of source strength is defined separately for each group using a mesh which may be different for each group.
The cross section for each group is defined throughout the system by assigning a material to each region and a set of cross sections to each material. The uncollided flux in group g at position x is given by the integral over all volume of the source strength multiplied by exp(-t) (the number of mean free paths) divided by 4 Pi r**2.
When the integration over the lateral coordinates is performed in this expression, there remains an integral with respect to x in which the integrand involves the E1 function. By substituting an approximation of this function it is possible to express the flux throughout the system as the sum of three solutions of the diffusion equation. By this method, the module calculates flux values at the points of a mesh, and it can print out values at a specified spacing in each region. The mesh can be different from the source mesh and will be set automatically by the module if it is not supplied. The module does not solve the diffusion equations itself but sets the parameters which define them completely and then returns control to the calling program so that the solutions can be obtained externally.