The Telegraph (hattip Insty) turned the hype level to max in a recent article about the potential of thorium reactors. According to the headline, “Obama could kill fossil fuels overnight with a nuclear dash for thorium.” Against all odds, this is to happen in three to five years with a “new Manhattan Project,” and a “silver bullet” in the form of a new generation of thorium reactors. The author is so vague about the technologies he’s describing that it’s hard to avoid the conclusion that he simply doesn’t know what he’s talking about, and couldn’t be bothered to spend a few minutes with Google to find out. I’ll try to translate.
It’s claimed that thorium “eats its own waste.” In fact, thorium is very promising as a future source of energy, but this is nonsense. Apparently it’s based on the fact that certain types of thorium reactors actually could burn their own fuel material, as well as plutonium scavenged from conventional reactor waste and other transuranics, much more completely than alternative designs. This is certainly an advantage, but the fission products (lighter elements left over from the splitting of uranium and plutonium) would still be highly radioactive, and would certainly qualify as waste. Such claims are so obviously spurious that they play into the hands of opponents of nuclear power.
It is also claimed that “all (thorium) is potentially usable as fuel, compared to just 0.7% for uranium.” In fact, thorium is not a fissile material, meaning that, unlike uranium 235 (U235), which is the 0.7% of natural uranium the author is referring to, it cannot sustain a nuclear chain reaction on its own. It must first be converted to a lighter isotope of uranium, U233, which is fissile. In fact, the U238 that makes up most of the rest of the leftover 99.3% percent of natural uranium is “potentially usable as fuel” in that sense as well, by conversion to plutonium 239, also a fissile material.
The author is vague about exactly what kind of reactors he is referring to, lumping Dr. Carlo Rubbia’s subcritical design, which depends on a proton accelerator to provide enough neutrons to keep the fission process going, and molten fluoride salt reactors, which do not necessarily require such an accelerator. He claims that, “Thorium-fluoride reactors can operate at atmospheric temperature,” which they certainly could not if the goal were to generate electric power. I suspect that what he means here is that, unlike plutonium breeders, which require a high energy neutron spectrum to produce more fuel than they consume, thorium breeders could potentially use “thermal” neutrons that have been slowed to the point that their average energy, when converted to a “temperature,” would be much closer to that of the other material in the reactor core.
In any case, the design he seems to be so excited about is Dr. Rubbia’s “energy amplifier,” which, as noted above, would be subcritical, requiring a powerful, high current proton accelerator to keep the fission process going. It would do this via spallation, a process in which a copious source of the neutrons required to keep the reaction going would be provided via interaction of the protons with heavy nuclei such as lead, or thorium itself. This is the process used to produce neutrons at the Oak Ridge Spallation Neutron Source. Such reactors could easily be “turned off” by simply shutting down the source of neutrons. However, the idea that they would be inherently “safer” is dangerously inaccurate. In fact, they would be an ideal path to covert acquisition of nuclear weapons. Thorium reactors work by transmuting thorium into U233, which is the isotope that fissions to produce the lion’s share of the energy. It is also an isotope that, like U235 and Pu239, can be used to make nuclear bombs.
The article downplays this risk as follows:
After the Manhattan Project, US physicists in the late 1940s were tempted by thorium for use in civil reactors. It has a higher neutron yield per neutron absorbed. It does not require isotope separation, a big cost saving. But by then America needed the plutonium residue from uranium to build bombs.
“They were really going after the weapons,” said Professor Egil Lillestol, a world authority on the thorium fuel-cycle at CERN. “It is almost impossible make nuclear weapons out of thorium because it is too difficult to handle. It wouldn’t be worth trying.” It emits too many high (energy) gamma rays.
What Lillestol is referring to is the fact that, in addition to U233, thorium reactors also produce a certain amount of U232, a highly radioactive isotope of uranium with a half life of 68.9 years whose decay does, indeed, release potentially deadly gamma rays. It would be extremely difficult, if not impossible, to remove it from the U233, and, if enough of it were present, it would certainly complicate the task of building a bomb. The key phrase here is “if enough of it were present.” Thorium enthusiasts like Lillestol never seem to do the math. In fact, as can be seen here, even conventional thorium breeders could be designed to produce U233 sufficiently free of U232 to allow workers to fabricate a weapon without serious danger of receiving a lethal dose of gamma rays. However, large concentrations of highly radioactive fission products would make it very difficult to surreptitiously extract the uranium, and it would also be possible to mix the fuel material with natural or depleted uranium, reducing the isotopic concentration of U233 below that necessary to make a bomb.
With subcritical reactors of the type proposed by Rubbia, the problem of making a bomb gets a whole lot easier. Rogue state actors, and even terrorists groups if we “succeed” in coming up with a sufficiently inexpensive design for high energy proton accelerators, could easily modify them to produce virtually pure U233, operating small facilities that it would be next to impossible for international monitors to detect. There are two possible pathways for the production of U232 from thorium, both of which involve a reaction in which a neutron knocks two neutrons out of a heavy nucleus of Th232 or U233. Those reactions can’t occur unless the initial neutron is carrying a lot of energy as can be seen in figure 8 of the article linked above, the threshold is around 6 million electron volts (MeV). That means that, in order to produce virtually pure U233, all that’s necessary is to slow the incoming spallation neutrons below that energy. That’s easily done. Imagine two billiard balls on a table. If you hit one as hard as you can at the other one, what happens when they collide? If your aim was true, the first ball stops, transferring all its energy to the second one. The same thing can be done with neutrons. Pass the source neutrons through a layer of material full of light atoms such as paraffin or heavy water, and they will bounce off the light nuclei, losing energy in the process, until they eventually become “thermalized,” with virtually none of them having energies above 6 MeV. If such low energy neutrons were then passed on to a subcritical core, they would produce U233 with almost no U232 contamination.
It gets worse. Unlike Pu239, U233 does not emit a lot of spontaneous neutrons. That means it can be used to make a simple gun-type nuclear weapon with little fear that a stray neutron will cause it to fizzle before optimum criticality is reached. And, by the way, a lot less of it would be needed than would be required for a similar weapon using U235, the fissile material in the bomb that destroyed Hiroshima.
We’re quite capable of blowing ourselves up without Rubbia’s subcritical reactors. Let’s not make it any easier than it already is. Thorium reactors have many potential advantages over other potential sources of energy, including wind and solar. However, if we’re going to do thorium, let’s do it right.
UPDATE: Steven Den Beste gets it right at Hot Air. His commenters throw out the usual red herrings about the US choosing U235 and Pu239 over U233 in the Manhattan Project (for good reasons that had nothing to do with U233’s suitability as a bomb material) and the grossly exaggerated and misunderstood problem with U232. You don’t have to be a nuclear engineer to see through these fallacious arguments. The relevant information is all out there on the web, it’s not classified, and it can be understood by any bright high school student who takes the time to get the facts.