Hi Thomas,
Tony Scudiero from NVIDIA did some initial work on a GPU port as part of the DOE FastForward program, but it was never completed. Take a look at his presentation here:
http://on-demand.gputechconf.com/gtc/2014/presentations/S4170-put-neutron-transport-sims-nuclear-reactions.pdf
Monte Carlo on GPUs is an active research area, and I don’t think there’s any consensus on what the “right” path forward is yet, especially for neutron transport. There have been more successes to-date with porting photon transport to GPUs due to the fact that cross sections are much easier to handle. Berkeley is active in this area as is RPI. I’m sure there are others as well.
Best regards,
Paul