It is constant. The uranium and the "moderators" are mixed together in the pebbles, kinda like ingredients in a cake. You can't see the eggs, but they are there................
I doubt that. Certainly it is true that the neutron moderators are part of the fuel pebbles, but those are NOT the control elements (which is what I take him to be referring to). There still has to be some kind of reactivity control device, and those are most likely to be regular old "control rods" made of a neutron absorber. The precise design and arrangement will obviously be different from a standard LWR or PWR, but there will still be a control system in place