2.1. Multigroup Cross Sections

2.1.1. Background

OpenSn is not provided with cross-section libraries. Users are expected to supply their own multigroup cross-section data. One may use other open-source software to generate this data (e.g., NJOY, Dragon, OpenMC).

The cross sections are read from a plain text file. The OPENSN_XSFILE format of that file is as follows:

# Add descriptive comments as appropriate
NUM_GROUPS ng
NUM_MOMENTS nmom

SIGMA_T_BEGIN
0 value
.
.
ng-1 value
SIGMA_T_END

SIGMA_A_BEGIN
0 value
.
.
ng-1 value
SIGMA_A_END

TRANSFER_MOMENTS_BEGIN
# Add descriptive comments as appropriate
M_GPRIME_G_VAL 0 0 0 value
.
M_GPRIME_G_VAL moment gprime g value
.
M_GPRIME_G_VAL nmom-1 ng-1 ng-1 value
TRANSFER_MOMENTS_END

2.1.2. Materials

We create a material and add a property to it:

• TRANSPORT_XSECTIONS for the transport cross sections

Recall that lua indexing starts at 1.

-- Add materials
materials = {}
materials[1] = mat.AddMaterial("Material_A")

2.1.3. Cross Sections

We assign the cross sections to the material by loading the file containing the cross sections.

mat.SetProperty(materials[1], TRANSPORT_XSECTIONS, OPENSN_XSFILE, "xs_1g_MatA.xs")

---

2.1.4. The complete input is below:

You can copy/paste the text below or look in the file named tutorials/material/mg_xs.lua:

-- Add materials
materials = {}
materials[1] = mat.AddMaterial("Material_A")

mat.SetProperty(materials[1], TRANSPORT_XSECTIONS, OPENSN_XSFILE, "xs_1g_MatA.xs")