|1959 Civil Defense Publication|
The news this week, like the news last week and the week before, is full of the slaughter of ordinary people at work, markets, and play. Sometimes the perpetrator is just a lone wacko and at other times he (it’s usually a he) voices some ideological justification. If I have to choose, I prefer the wackos. They openly enjoy killing for its own sake, which, while unacceptable, at least has a certain purity to it. Far worse are those who claim to be pursuing the greater good or to be avenging some wrong, whether real, imagined, or open to interpretation. It is always ideologues and moralists who commit mayhem on the largest scale, and who feel justified doing it.
It is well that the deadliest munitions are so hard to come by. There is little doubt there are individuals and groups who would use NBC (nuclear-biological-chemical) weapons if they could. Here and there they already have, e.g. the anthrax attacks in the US in 2001 and the sarin gas attack on the Tokyo subway in 1995. Fortunately, the N part of that triad remains the domain of states, at least for now. Yet, no individual or group can commit violence on the scale that a state can, and that brings us to the issue of Non-proliferation. This, too, is in the news lately thanks to a controversial agreement currently under debate. Whatever one’s opinion on it, underlying the debate is concern about an older agreement.
The Treaty on the Non-Proliferation of Nuclear Weapons (NPT) went into effect in 1970 and originally was set to expire in 1995. It is, in essence, a bargain in which the signatory nuclear powers agree to share nonmilitary nuclear technology with states that agree to forgo the development and acquisition of nuclear weapons. It was reasonably successful during the next 25 years. The fear in 1970 was that there would be some 30 mutually hostile nuclear armed states by 1995 – a frightening scenario. In the event, only India, not a signatory to the treaty, officially acquired nuclear weapons in that time period. Unofficially, it is assumed two others, Israel and South Africa, developed nukes secretly before 1995, though South Africa dismantled its nuclear arsenal in 1989. One might think the upcoming expiration would have been a major news story in the early 1990s, but, as those old enough will recall, it wasn’t. Perhaps this was just as well, for the NPT in 1995 was extended indefinitely without any political brouhaha about it. In the US at least it was barely mentioned outside of the most wonkish publications and PBS talk shows.
I do remember Henry Kissinger on one of those shows discussing the matter with other guests (whose names I don’t remember) however. He confused at least one of those guests by talking almost exclusively about the global distribution of nuclear power reactors. When the person objected that the issue was weaponry, not electric power, he responded, “That’s what we’re really talking about, aren’t we?” and continued with what he was saying. This didn’t appear to clear up anything for his co-panelist, but, of course, he was right.
There is no big design secret to a fission bomb. The basics are widely known and plans have appeared on the internet. The obstacle to building one is the production of fissile material, either Uranium-235 or Plutonium-239; the former needs to be at least 90% pure and the latter more than 93% pure to be weapons-grade. The path to U235 involves no esoteric technology. The US succeeded in the 1940s, but it was a huge and massively expensive industrial undertaking. It still is. Natural uranium is 99.28% U238, which is not bomb material; this is why it is perfectly legal for a private individual to own 15 pounds (7kg) of natural uranium. 0.71% of natural uranium is U235 and the rest consists of other isotopes. (U233 would work in a bomb too, but it is even rarer than 235.) U238 and U235 are so nearly identical that teasing out the latter, even with large multiple centrifuges, is a long, tedious, and repetitive process. This is the process presently used in Iran, supposedly for reactor fuel; reactor-grade uranium needs to be enriched only to 3% U235. The advantage to uranium over plutonium is that once you have enough weapons-grade uranium, the bomb design and manufacture are simple. Smack two subcritical pieces of U235 together quickly to create a single critical mass. That’s basically it. The first uranium atomic bomb ever built used a gun to shoot one piece into the other, and this design has been copied by others. Every single one has worked on the first try.
Few countries are able or willing to invest the time, resources, and fortune into refining weapons-grade uranium. There is, however, a shortcut to nuclear weapons if you have a reactor. A portion of the U238 in the fuel rods of reactors will convert to plutonium in the course of normal operation. Though extracting Pu239 from the rods is neither easy nor safe, it is well within the capability and financial resources of relatively modest industrial powers. It is why protocols for the disposition of spent fuel are a major component of the NPT, and it is why Kissinger was right. The drawback to plutonium is that it is impossible to eliminate all Pu240 contamination from the desired Pu239 end product. Pu240 is a highly unstable neutron emitter that makes a simple gun design for a bomb infeasible; the two pieces would start to interact at a distance before critical mass was achieved and the bomb would fizzle. For this reason, an implosion is used instead; a Pu sphere of subcritical mass is compressed to a critical density. The lenses and timing in this sort of device need to be flawless or the sphere will just deform rather than explode. Building a functioning weapon of this design is more of a challenge, but meeting it is still quicker than refining uranium.
Fusion weapons, which use fission devices as triggers, are much more complex than pure fission designs, of course. Nevertheless, each of the first five nuclear states succeeded in building fusion devices less than a decade after its first fission test. China took 32 months.
Since 1995, Pakistan and North Korea have tested and deployed nuclear weapons, bringing the total number of acknowledged or presumed nuclear armed states to 9. Besides these 9, there are 22 other countries that operate nuclear power reactors. All of those 22 countries have the industrial potential to extract plutonium from fuel rods. It is possible – even likely – that some of them already have in place the facilities for doing so, if only as a precaution. This is why a breakdown of the NPT could add a dozen or more nuclear weapons states to the current list in perhaps as little as a year or two. The risk is not negligible.
Unsurprisingly, it is not legal for private individuals to own plutonium. Oddly, there is an exception. The isotope Pu238 is not practical bomb material but it makes a marvelously long-lived power source for batteries. The Voyager space probes, now entering interstellar space, rely on Pu238 batteries. So do some 20 remaining people (out of several hundred) who in the 1960s and 1970s received Pu238 powered pacemakers, which last a lifetime. They’re allowed to keep them, though by law the plutonium from the pacemakers must be sent to Los Alamos when the users die.
It’s a shame you can’t trust people with any isotope of plutonium. Folks once dreamed of atom powered cars, planes, homes and, yes, spaceships that never needed refueling. All of them are entirely possible as a matter of engineering. It’s a nice dream. Regrettably, for human reasons rather than technical ones, a nightmare is more credible.