The Philosopher's Stone

My favorite lab course at RPI was Radiochemistry, which was the second part of a two semester sequence. The first course was on the theoretical underpinnings of nuclear and radiochemistry. I didn't take it, but talked my way into the second course after an interview with the two lead professors that convinced them that I knew the material well enough. The two were Overman and Clark, who had written our textbook, called simply Radiochemistry, and a little googling tells me that, yes, it was a widely used text.

My favorite lab work in the course was neutron activation analysis, which is very, very cool. After exposure to neutrons, many elements become radioactive, having at least one isotope that does so after neutron absorption. This is especially true of the heavier elements, which tend to have a lot of isotopes and a lot of energy shells in their nuclei. Most of the artificially created radioisotopes have a gamma spectrum associated with them, so if you measure their decay using a multichannel gamma spectrometer, you get a "fingerprint" of that isotope. Repeated measurements also give you decay rate information, and, putting all that together, you can usually get a pretty good quantitative trace element analysis of the original sample. Also, because only a few atoms absorb neutrons, and the product isotopes are usually fairly short lived, it's a non-destructive technique that can be repeated many times using the same sample.

The neutron source we used was called a "bomb" as I recall, not because it could explode, but because it was big and heavy and looked like laboratory "bombs" that contained compressed gas. What our N source contained was, if memory serves, beryllium and cobalt-60, the latter explaining the heaviness of the device. It was very thickly shielded.

Under high energy radiation, beryllium becomes a neutron source. Our source exploited the gamma-neutron reaction, which splits off a neutron from Be-9; the resulting Be-8 is unstable, and goes to two alpha particles. There is also an alpha-neutron reaction that gives a neutron and carbon-12. The early "initiators" on nuclear bombs used this reaction, with polonium as the alpha source, because it doesn't emit any gamma radiation, so it only worked after the Po and Be were brought close together, reducing the likelihood of pre-detonation.

Both boron-11 and deuterium also undergo the gamma-neutron reaction, though not as efficiently as beryllium. Californium-252 is also sometimes used these days as a neutron source, as it undergoes a fairly rapid rate of spontaneous fission.

There are commercial neutron sources that use nuclear fusion as the neutron source. Many of these are basically small particle accelerators that aim a beam of deuterons at a target that has tritium adsorbed onto it. A sufficient number of fusion events occur to make this a viable source of high energy neutrons for industrial applications. The D/T reaction has a characteristic energy (around 14 MEV), which is useful for deep imaging applications. Some of these type of neutron sources are quite small.

Larger installations often use full bore particle accelerators to create fusion, and sometimes the deuterium-deuterium reaction is used. This gives a lower energy to the resulting neutron, but deuterium is much cheaper and easier to work with than tritium, tritium being both radioactive and a proliferation hazard.

Another interesting neutron source is the Farnsworth Fusor, or rather, its descendants. These are "inertial electrostatic confinement" fusion generators, and examples of them have even shown up in high school science fairs. Usually, the rate of fusion is quite low; I've seen numbers for quasi-amateur builds that produce maybe a million fusion events per second. The radiation hazard from attendant X-rays is greater than the neutron hazard. However, I've also seen some advanced laboratory results suggesting output as high as 10^14 events per second which is respectable and dangerous. Once you get to this level of output, radiation damage to the electrical components becomes important, an indication of just how difficult the fusion problem is, since you're still orders of magnitude from practical power production.

If you want a really high neutron flux, the usual method of production is a nuclear reactor. Reactor-based high energy neutrons are usually obtained in a reactor with a fast core but a moderated outer shell that achieves criticality, thereby sidestepping the safety issues that come into play with fast reactors. However, there are "pulsed" reactors that rely on changes in neutron cross section with temperature to create "inherently safe" designs. One such design was described by Freeman Dyson in his book Disturbing the Universe.

Reactors only produce "fission spectrum" neutrons, of course, although for fast reactors the spectrum is shifted toward higher energies. If you really want a big flux of high energy neutrons, failing some major breakthrough in fusion technology, you want a "spallation source."

Spallation is based on the fact that, if you hit a heavy nucleus with a very high energy proton, you get a neutron "splash" effect (this would be the "liquid drop" model of the nucleus, we're using here). The Spallation Neutron Source in Oak Ridge, Tennessee runs a high power (over a megawatt) proton beam at about 1 Gev (1000 Mev) and gets about 30 neutrons per proton in the beam. The neutron spectrum of the output peaks at around 10 Mev, but some of the neutrons have much higher energies.

The Oak Ridge facility uses mercury as the spallation target, with the indication that liquid targets are more robust to the sort of shocks that a pulsed accelerator beam produces. There were some designs from an old USSR program that used a eutectic mix of lead and bismuth for a similar purpose, heated to liquefy the metal.

Remember that I noted above that reactors are sometimes used as neutron sources. One reactor design is to use a "subcritical" fast reactor and drive it to power production via a spallation beam. If, for example, the criticality of the reactor is 0.95, meaning that it is only 95% of the way to self-sustaining, then any neutron introduced into the reactor core will induce a reacton chain of about 20 more neutrons. Thus, a spallation source plus a sub-critical fast reactor can be a copious source of fast neutrons. Moreover, it's overall power balance will be positive; it will generate considerably more power than it consumes.

I've noted before that fast neutrons are a modern equivalent of the "Philosopher's Stone," able to transmute elements, and able to convey, well, not eternal life, but eternal death to those exposed. It's the transmutation aspects that have caught some interest. Fast neutrons will fission all transuranic elements, so, properly run, there is no plutonium et al. remaining after an accelerator driven reactor fuel cycle has run its course. In fact, an accelerator-driven reactor system can be designed to run on nuclear waste remaining from other reactors. It's also been suggested that other long-lived waste products, like technetium-99 and iodine-129 be transmuted to shorter-lived isotopes, taking the nuclear waste disposal problem from a time scale of millennia to a matter of years, or perhaps centuries if you don't want to transmute the cesium and strontium waste isotopes.

It sounds great, doesn't it? Nuclear energy without the waste disposal problem? So what's the catch?

Well, there are a couple of technical catches, such as the fact that even the Oak Ridge facility doesn't have the power to drive a full closed-cycle system. But that's a technical matter, and I have no doubt it's solvable. There's also the fact that such a system requires on-site fuel reprocessing, to extract the transuranics and other long-lived isotopes from the waste stream. That's a chemical engineering problem, and we don't have much experience with designing chemical processes that are totally closed cycle. More accurately, trying to do so has always resulted in some leakage, plus the occasional outright accident.

Still, it might be possible to get the thing to work well enough, technically speaking.

But there's a deeper problem, and that has to do with social and economic systems and ideology. As I've said before, nuclear energy is inherently "socialistic" in the sense that it requires government level planning and operation at every step of the way. Yes, an accelerator-driven nuclear power system would produce a substantial power surplus—at enormous initial capital cost. A government can pick up that tab and take that kind of risk; corporations could raise the money (the estimated price tag for a ADS is on the order of $20 billion, but would probably be notably higher, given NIMBY concerns, etc.) but are simply not trustworthy when it comes to high public risk endeavors. Corporations take risks to enhance profits. It is up to government to regulate corporations' risk taking, but the ascendance of Conservative Movement ideology in this country has degraded the regulatory process to such an extent that one simply can't trust the regulatory function of government. The NIMBY folks are not being mere obstructionists. They are being realistic.

It's said that there was a time, in the early days of explosives manufacturing in Europe, when the owners of an explosives company were required to live on site. Ask yourself how many corporate executives would situate themselves and their families next door to any nuclear reactor site.

ADS systems are well-suited for thorium fuel cycle nuclear power, and India has a lot of thorium, but not much uranium. And if China ever decides to curb its greenhouse gas emissions, ADS systems would look very attractive. Naturally socialistic, remember? Very much in the Chinese tradition. The two countries look like natural competitors in this particular game.

The U.S. does not. The oil and gas men are still in charge, willing to expend trillions for neo-colonial wars and the perpetuation of various sorts of privilege. The idea of spending government money on the creation of actual industrial capital is ultimately foreign to them. So here, as in so many other endeavors, the U.S. will not be on the cutting edge of technology. It is no longer up to the job.