///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.////

I just looked up the CANDU system. Totally unlike anything I worked on which were Westinghouse and General Electric plants. But, as I said, I stopped working in nuclear plants in the 80s and i don't know what systems other manufacturers were using.   

////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.////

I know light requires enriched uranium but since I never worked at a lightwater facility, I don't know which is better. Heavy is cheaper, I know that.

///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.////

I never worked with boiling water reactors. Never heard of it. Every plant I worked at was always pressurized. In fact, I can't see how that could possibly work. Letting the water boil in the plants I worked at would be EXTREMELY dangerous. It wouldn't be allowed. We couldn't even let the water transfer heat through nucleate boiling which is extremely efficient because it was too close to boiling at that point and so was too dangerous to allow.