It doesn't claim an anisotropic flux, it claims there are more neutrons in one place (downstream) than another (upstream).This is insane! The neutron flux will not be concentrated downstream, but will be more or less an isotropic source, with some fraction streaming upstream to the source.
What's the specific problem here? Sure, controlling that will be a challenge.The 427 MWt is equivalent to 122 3500 MWt LWR nuclear plants! Into 196 kg/s?!? Really!?