Add 127k OpenMC model
This PR adds an OpenMC model (
openmc_model.py) for the 127k pebble case. I added a graphite inner, outer, and top/bottom reflector made of a homogeneous mixture of graphite and FLiBe - the porosity is 11.2% based on a 5mm gap between the PB-FHR reflector blocks and other machined fluid channels.
This model should automatically scale to "taller" cores (i.e. with more pebbles) because the core dimensions are inferred from the pebble center coordinates provided in