Other Reactor Designs Part 1: Heavy-Water Moderated

I spent the entirety of this last weekend on our senior design project so I decided I may as well write a blog on it since it's currently difficult to think about anything else. So far for this blog, any time I've discussed a reactor it's been a light water reactor (LWR), or one that is moderated and cooled by normal, everyday water. But there are so many more reactor designs that the nuclear community have built or at least theorized in the quest for higher thermal efficiency, cost effectiveness, and filling different niches.

Relevant to my group's senior design project are reactors that are moderated by heavy water, or water molecules that have deuterium (an atom with a nucleus of one proton and one neutron) instead of hydrogen bound to their oxygen. A "typical" LWR uses its coolant as its moderator; that is, the flux of neutrons generated by the fuel is slowed by the moderator (which we want), and also reduced through absorption into the water molecules (which we don't necessarily want). Any neutron absorbed by the water is one that cannot be used to cause a fission and produce fuel. So, a higher enrichment of fuel is required in order to produce enough neutrons to compensate for those that are lost to absorption.

However, deuterium has a much lower cross section for absorption compared to hydrogen, as the deuteron already has an extra neutron from "naturally occurring" hydrogen and really doesn't want another one. So for heavy water, two thirds of your atoms have much smaller cross sections, greatly reducing the neutron absorption in heavy water. This allows reactors to be operated with lower enrichments even all the way down to naturally enriched uranium, saving money and resources that would normally go into the difficult and expensive process of enriching the reactor's fuel.

This design was first put forward by the Canadian energy industry in the form of the Canadian Deuterium Uranium (CANDU) reactor design. It was created out of a perceived necessity by the Canadian government and energy industry, as they believed they did not have solid access to enriched uranium, or the infrastructure to produce their own. The end result is a reactor that is large and pricey, but runs on naturally enriched uranium.