Other Reactor Designs Part 2: CANDU in Detail

I ended the last part with an introduction to the Canadian Deuterium Uranium (CANDU) reactor design, a heavy-water moderated and naturally-enriched-uranium burning reactor. These two attributes alone make it a very different reactor from a typical light water reactor (LWR), but it has a few more features that make it a very distinct style.

• The natural uranium, since it isn't enriched, can undergo very little burnup before it loses the ability to be used as an effective fuel in the reactor.
• Its fuel assemblies are small hexagonal bundles that are stacked (well, lined up) with each other through assembly vessels, or "callandria", running through the large resevoir that makes up the heavy water moderator region. These callandria are pressurized at PWR operating temperatures, while the moderator region around them is not.
• The callandria are oriented horizontally, with control rods running both parallel and perpendicular. This along with the fuel bundles allows for "on-power" refueling, or the changing of fuel in the reactor while the reactor is operational. This makes up for the low burnup by having a very short fuel cycle, with automated systems removing the oldest fuel bundles at the end of the callandria and placing fresh, new bundles at the start.
• On power refueling causes somewhat of a non-proliferation concern, however. Taking nuclear fuel fresh out of the core will have a higher percentage of material ideal for dirty bombs or other dangerous practices.
• Unlike American LWRs that insist on a negative void coefficient (one of many terms to describe whether or not the reactor will shut itself down if things start to go very wrong), a CANDU generally has a slightly positive void coefficient. This is compensated for by additional safety systems built into the core and surrounding components.

While a lot of what the CANDU does is cool, the net result is a reactor that tends to cost a lot more than a typical LWR, while not producing as much power. This is, of course, less than ideal. However, the CANDU was born out of a perceived necessity by the Canadian government, as they believed they did not possess the means (at the time) to enrich uranium to the level needed to operate a typical LWR. As the ability to enrich has improved for them, they have managed to refit many CANDU facilities to run at higher enrichments, increasing lifetime and power level of the reactor while also making them more cost effective. In addition, the Advanced CANDU Reactor (ACR1000), the next generation of pressurized heavy water reactors, looks very promising.