A Single Fluid Reactor with an inner zone that includes a solid neutron moderator, which can have through holes defined therein. This solid neutron moderator can have a relatively small diameter, which can range, in some embodiments, from less than one meter to about 1.5 meter. The solid neutron moderator effectively creates an inner zone with a neutron profile that is far more thermalized than if the solid neutron moderator were absent. The surrounding layer of salt surrounding this inner zone has a much harder neutron spectrum.