(A slightly technical post; but I find this subject pretty fascinating. This post is more-or-less an extension of my last post, “The Nuke Post” ; I’m going to assume that readers have read that first.)
As I alluded to in my last post and comments, whether breeder reactors are affordable or possible is perhaps a key to the question of the practicality and desirability of nuclear power as we go forward. So, the following is my take on the current state of the field, gleaned from quite a few different sources. Bottom line—1) they do exist and can be built, and 2) we probably can’t afford too many of them.
Breeder reactors are more appropriately called “fast reactors”, so I’m going to use that terminology. And to understand why they’re called fast reactors, we have to step back just a bit and look at “standard” nuclear reactors that are used to generate electricity, the vast majority of which are one form or another of a light-water reactor (LWR). (The “light” refers to regular water, instead of “heavy” water, the latter of which is a different molecular structure that doesn’t naturally occur in large quantities.) These reactors use fuel rods that are enriched to 3%-5% U-235, which are typically interspersed with graphite or carbon control rods, which can be slid in and out of the core to control the speed of the reaction. When they are slid out, the fissile elements (typically U-235) approach critical mass, with neutrons being released that cause, in turn, more neutrons to be released.
Now, in case you’re wondering why the chain-reaction doesn’t accelerate out of control—there are different degrees of critical mass, such as sub-critical, delayed subcritical, and prompt-critical. In most nuclear chain-reactions, some of the neutrons released have a delay that ranges from a few milliseconds to several minutes. In a power reactor this allows control rods time to control the reaction. Fuel in a power generation reactor could never reach a prompt-critical state (necessary for a nuclear explosion), even if the control rods were pulled and left out. The reactor would suffer a melt-down, or even a small explosion, but it wouldn’t be a nuclear explosion, per se. Nuclear weapons typically use plutonium, combined with conventional explosives, to achieve a prompt-critical state.
But back to light-water reactors—when a nuclear chain-reaction begins, the neutrons released have so much energy that they don’t tend to cause fission when they hit other U-235 atoms. They do this much better if they are slowed down, or “moderated”. Thus, the importance of water as a coolant, because water (light or heavy) is an excellent moderator. In LWR’s, water flows through the core, moderating the neutrons, and enabling them to more effectively split other U-235 atoms. A moderated neutron is sometimes called a “thermal” neutron, because its temperature (and therefore energy) has been reduced to that of the coolant. Thus, LWR’s are sometimes called “thermal reactors”.
So, that’s that. LWR’s are pretty standard, make quite a bit of electricity around the world, and, unfortunately, leave a lot of high-level nuclear waste behind, as much as 27 tons a year for a 1 GW reactor. Typically about 97% of the original uranium remains in a “spent” fuel rod, along with those fission products I discussed in the last post. In fact, that’s one reason the rod is considered spent—the fission products are all excellent neutron absorbers, and as they build up, the reactor works less and less well, until it has to be shut down and the rods replaced (typically done in thirds, each rod spending three years total in the reactor, but with a third of the rods replaced each year).
This form of operation is considered an “open fuel cycle”; the rods are used once and then disposed of. (In the U.S., no one wants them, so each reactor facility stores them, first in pools of water until their radioactivity subsides some, and then in casks on-site) Now, many consider this to be a total waste, as it is possible for the spent rods to be reprocessed and the fission products removed, and the remainder mixed with new uranium and reused. But, plutonium is typically separated out as part of reprocessing, and proliferation fears caused a ban on reprocessing during the Carter administration in the U.S.in 1977, and this ban still stands.
So, back to where we began, to fast reactors. They’re called fast reactors because, unlike all of those LWR’s, they don’t moderate the fast neutrons. By letting the neutrons keep their energy, they are much more likely to hit a fertile (but not fissile) U-238 atom, and transform it into fissile P-239. Thus, potentially making more fuel than they use. (Though it’s not some sort of free-lunch—they still need to be fueled with U-238). The cores have to use highly-enriched (17%-26%) uranium, or plutonium, (called “start-up fuel”) and then the core is surrounded with a “blanket” of fertile U-238 where the plutonium is made. That plutonium can’t be used for power until the blanket is reprocessed, though, but I’ll get to this is a bit. Continue reading