josepp-

"I'm asking if pellets of Uranium are encased in nickel rods how do neutrons from the moderator reach them since the moderator is making no contact with the fuel? And how is the process of fission controlled in such a system?"

I don't know the answer to that for sure without refreshing my aging tiny brain with research. My understanding is that the fuel rod casings are not impermeable to whatever (gamma rays and alpha and beta particles) are being emitted by the breakdown of the uranium pellets. The boric acid has the capacity to absorb some of these emissions which helps moderate the rise in temperature.

It's also my understanding that the Japanese reactor is a light water reactor rather than a heavy water reactor such as we have festering in New Brunswick, Canada. The fact that it is also a boiling water reactor rather than a pressurized water reactor like most of the reactors in the States is another factor to consider. I'm not sure if it makes the reactor more safe or less safe. Well, any idiot realizes now that the Fukushira reactor is not safe now.

Charley Noble