(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.
As mentioned in the last post, these aspects of fast reactors have all sorts of advantages; it makes way more fuel available (all naturally-occurring uranium could be used), and the spent fuel is truly spent, containing only fission products that have dramatically shorter radioactive half-lives. Fast reactors can also be set to burn up transuranics, depleted uranium, or even weapons-grade plutonium.
And, fast reactors are a reality—as many as 20 have been built in various forms around the world, dating back to the 1950’s and 60s. But, there are a number of reasons why we don’t have a whole bunch of fast reactors making electricity and creating more fissile fuel than they consume. First, fast reactors use much smaller cores, and they have to run dramatically hotter than LWR’s. In addition, they can’t be cooled with water, because water is a neutron moderator, and would slow the neutrons down. (Another aside here—everyone says “cooled”, but that’s the same loop that pulls the heat from the core to use it to make electricity, so it’s really part of the power cycle, in all of these reactors). So, most fast reactors use sodium or lead as coolant. Both are transparent to neutrons, and as such don’t moderate them, but lead is highly corrosive at high temperatures, and sodium burns when exposed to air and water, sometimes explosively. (A few other designs exist that are gas-cooled, using helium or nitrogen, but nearly all fast reactors that have been built have used either sodium or lead). So, that’s drawback number one—a very hot core that has to be cooled with difficult materials, some potentially explosive.
Drawback number two—the higher the breeding ratio (or “burn-up”) that a fast reactor can achieve, the less power it tends to make, because the neutrons are off making plutonium instead of making heat. Additionally, scaling up a fast reactor tends to reduce its efficiency, as larger cores tend to trap neutrons and keep them from hitting the blanket. So, most fast reactors have relatively small power outputs, if they are used for power at all, typically 250-300 MW. Compare this to a typical LWR, which might make 1 GW. These two factors together—the difficulty of the sodium cooling, combined with the lower output, are the real reason we don’t see fast reactors. If they cost twice as much, but only make a quarter of the power, and then have more down-time, then the results are capital costs that can be ten times that of a LWR, per unit of power actually produced. Until and unless uranium is very, very expensive, such plants just aren’t likely to work commercially.
Now, drawback number three—fast reactors have to be refueled fairly often. This has made uptime a problem for commercial reactors like France’s two Fenix plants. Fast reactors use up their smaller, richer cores more quickly that LWR’s, whereupon they have to be stopped, cooled, and refueled. In liquid metal reactors, the sodium or lead then solidifies, which makes starting a fast reactor more difficult that starting a LWR. THEN (notice, a whole long string of difficulties here), the fuel in the blanket and the core have to be reprocessed. If this is done in the same facility, then it is referred to as a “closed-fuel cycle” plant, or an “Integral Fast Reactor”, or IFR. Reprocessing is complicated, and can be done in several ways, most involving liquids and organic solvents. Along the way, plutonium is isolated, which is why it is considered by some (but not all) to be a proliferation risk.
Then, in another example of no-such-thing-as-a-free-lunch, fast reactors can be operated on a spectrum from burning up nearly all the fuel in the core, and therefore making it disappear (useful for getting rid of weapons-grade plutonium or high-level waste from LWR’s), to the other end, where they don’t do that but make lots of plutonium in the blanket. At that first end of the spectrum they’re referred to as “burners”, and at the other end as “breeders” (another reason that the term “fast reactor” is more appropriate than “breeder reactor”). Unfortunately, they can’t typically do both at the same time. So if you want all those great burner advantages, you have to forego the creation of excess plutonium.
Now, all of this so far has been about fast reactors that exist. Those other Gen. IV fast reactors that I mentioned in my last post, like the molten salt reactor where the fissile materials flow with the salt, or the Liquid-Flouride Thorium Reactors (LFTR’s), don’t currently exist, and as far as I know haven’t been built, even in experimental form. Some countries are pursuing these designs, but the technical challenges portend to make them even more expensive than ordinary fast reactors, which already tend to be too expensive to be workable.
So, two last things. First, while I don’t currently view nuclear power as a wise path forward, for all the reasons I keep mentioning, I do think that there is a place for fast reactors to be used as burners. The U.K. is currently considering building GE’s new S-PRISM designs to help it deal with its 120 tons of near-weapons-grade plutonium at Sellafield, and the U.S. is prepared to let GE build one as a demonstration plant at the Department of Energy’s Savannah River site . These plants will produce some power, though at high cost; they are really better viewed as a way for countries to pay a price to burn up high-level waste of various kinds. Russia is taking a similar path, having recently announced that it will convert its BN-600 to a burner configuration to help burn up Russia’s 40 tons of plutonium left over from the Cold War.
And, lastly—these developments in the nuclear industry are occurring against the backdrop of steadily dropping prices for solar and other renewables. Nuclear prices have tended to go up and up, while the price of solar has tended to go down and down. If these trends continue, nuclear power is likely to remain technologically fascinating and yet not common. And that’s a result I can live with.
Links– Great technical info on fast reactors— “Fast Neutron Reactors” on the World Nuclear Association website.