Description

This PR adds two minor enhancements to support FW-CADIS on large fusion models like JET.

Specifically, it makes the following changes:

1. Addition of ZERO_FLUX_CUTOFF macro (define here as 10^-22) to ensure that global adjoint external sources (defined as 1/forward flux) do not tend towards infinity. Especially in low ray density situations, you might end up with a really small flux estimate in a small cell, and we don't want to end up having this source dominate the overall adjoint source term. We check the flux against this new macro when computing the adjoint external source terms, and just set the external source to zero. While we generally want to make weight windows that bias particles towards areas of low flux, there is a limit to what is detectable. In this case, a 22 order of magnitude flux attenuation would essentially produce particle count rates that would be well below background rates even for the strongest fission/fusion starting sources, meaning fluxes below this level become meaningless in practice.

2. When pulling data from the MGXS interface, check Chi for values of NaN. For some reason, when I generate MGXS data with the model.convert_to_multigroup() function on JET, some materials will end up having extremely low sigma_f values but will return NaN for Chi. This may be caused by a bug in the MGXS interface, but for now I simply fix the random ray interface to check for this case and set Chi to zero.

3. Fixed a variable capitalization to be more consistent.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)