in each group during the decay process. The module may generate a new mesh for the gamma sources.
GRAB (Gamma Ray Build-up) (WRS system module No. 60221) Calculates the gamma ray flux, dose or heating rate in a slab shield using the build-up factor solution.
Gamma ray sources of different energies may be distributed throughout the system. To summarize the method, an integration is performed over the source volume and source energy range of a kernel representing the contribution to the quantity of interest from uncollided photons, to which is applied a factor that takes into account scattered photons.
The module requires values of the absorption coefficient (i.e. total cross section) and the energy absorption coefficient in each energy group for each material. For five standard groups these may be calculated by the module from the material compositions using a stored set of values of the mass absorption coefficient and the mass energy absorption coefficient for a number of commonly used elements. The module uses the Taylor approximation to the build-up factor, and therefore requires values of the parameters which define the factor for each group. It should be noted that since only one set of values can be used, a choice of build-up factor must be made with regard to the quantity of primary interest and the materials present. For each region, results are produced at the points of a uniform mesh which may be specified by the user. A separate integration is performed for the flux at each output point, and the execution time for the module therefore depends on the total number of output points requested. Dose and heating rates are, however, calculated from the flux values, so that little extra time is required for these. The contributions from selected groups may be totalled according to specifications in the input data. For the first point of every region the module can also print out the contributions to the flux from the source in each region.
MIXGAM (WRS system module No. 1300) Generates macroscopic cross section sets and diffusion constants for specified material compositions for use in gamma ray diffusion calculations (incorporating a removal flight correction) from a library of microscopic cross sections. For the purpose of the first flight calculation the source is assumed to be composed of discrete energies; the diffusion calculation is performed in groups. The data supplied by the module for each material is as follows:
Total cross sections at the source energies.
Scattering cross sections from the source energies into each diffusion group.
Diffusion coefficient (D) and diffusion length (L) for each diffusion group.
Group to group transfer cross sections for the diffusion groups.
Energy deposition factors for each source energy and for each diffusion group.
The module extracts microscopic cross sections for the required elements from a library which contains data for a given set of source energies and a given diffusion group scheme. The standard library supplied with the module contains data for 33 elements. It employs 13 source energies and 32 diffusion groups. This data has been prepared using the GAMCON program which obtains cross sections from the data of Hubbel by reference to the subroutine HEITLER. The module can, however, handle any library of data prepared in the appropriate format.
The group diffusion length and consequent diffusion coefficient are obtained from standard formulae.
REMTWO (WRS system module No. 299) Calculates the uncollided flux due to a distributed source in a one-dimensional plane geometry system. A method is used in which the flux is approximated by a sum of diffusion equation solutions. From the calculated flux a first collision source may be obtained (externally) for use in the diffusion equations.
The REMTWO module may be used for a system which can be represented by a series of plane parallel sided slabs of infinite lateral extent, each composed of a uniform material. Some or all of these regions contain sources, which are of varying strength and have a range of energies. The source