Hi Shawn,
The problem is that the model itself is very close to critical (keff=0.99551 as you saw). When run in fixed source mode, it becomes a subcritical multiplication problem. However, at the moment, OpenMC can only handle subcritical multiplication problems up to about keff=0.99. Beyond that, too many fission neutrons are created and will eventually exceed the size of the secondary particle bank. When I try to run your problem with a 14 MeV point source in the center of the sphere, I eventually get the following error:
ERROR: Secondary particle bank size limit reached. If you are running a
subcritical multiplication problem, k-effective may be too close to one.
Perhaps MCNPX has a method for handling subcritical multiplication problems that are close to critical. If you are interested in such a functionality for OpenMC, I’d encourage you to create an issue on our github repo.
Best,
Paul