Fissile materials have been in the news a lot in recent years – Iran’s uranium enrichment program, North Korea’s continuing nuclear weapons program, the on-going nuclear security summit, and so forth. Given all of this attention this seems like a good time to talk about what fissile materials are and how they’re produced – and when they become a potential security problem.
Defining some terms is a good place to start.
Fission is the process by which a heavy atom (typically U-235 or Pu-239) splits into two smaller atoms (fission products), usually after being tapped with a neutron. The fission process releases a lot of energy – 200 million electron volts (MeV) per fission, compared to a few electron volts per atomic bond broken during combustion.
Fissile materials are materials that will sustain fission. As noted above, the most common fissile materials are U-235 and Pu-239. Fissile materials are produced either in a nuclear reactor (in the case of Pu-239) or by processing uranium to enrich the concentrations of the fissile isotope, U-235.
Natural uranium is uranium in which about 99.2% of the atoms are U-238 and about 0.72% of the atoms are the fissile U-235.
Enriched uranium is uranium in which the fraction of U-235 has been increased; depleted uranium is that in which there is less U-235 than found in nature. Reactor-grade uranium fuel contains between 3% to 6% U-235 (some reactors use lower-enriched fuel, some use a little higher) while research reactors – used to produce radionuclides for research and medical purposes – use uranium that is up to 20% enriched. Anything higher than 20% enrichment is banned by the Non-Proliferation Treaty except for the existing nuclear weapons states – weapons-grade uranium is enriched to 90% and higher, almost pure U-235. Incidentally, the nuclear reactors on American submarines and aircraft carriers is higher than 90% enrichment.
So – one fissile atom (U-235) is found in nature, but in concentrations too scarce to support fission while the other (Pu-239) shows only the barest hint of a presence. Neither is naturally found in concentrations that will make even a nuclear reactor (unless we are going to use heavy water or graphite to help slow the neutrons down to make fission more likely), let alone nuclear weapons – making fissile materials takes a huge effort to either enrich the U-235 to useable concentrations or to fabricate the Pu-239. Both processes have been used – both seem to be in use in North Korea and have been used by other nuclear powers – and both use very different processes.
Uranium enrichment is probably the best place to start. Both uranium isotopes – U-235 and U-238 – are chemically identical so it simply isn’t possible to chemically process uranium to turn natural uranium into reactor fuel or nuclear weapons. The only difference is the fact that the U-238 atom is just a tad heavier than its lighter sibling. But this small difference is enough. When uranium is mixed with fluorine to make uranium hexafluoride gas (UF6) and then introduced into a rapidly spinning centrifuge the lighter 235UF6 floats on top (towards the center of the centrifuge tube) of the heavier U-238 hexafluoride. Very slightly enriched uranium is collected by siphoning off the topmost layer. Repeat this process over and over – the more times the better – gradually produces uranium enriched anywhere from reactor grade to weapons grade.
The basic science of uranium enrichment is deceptively simple. The difference in mass between U-235 and U-238 is not much of a “handle” for engineers to grab onto and the amount of enrichment at each step is almost trivially small. This is why it takes hundreds or thousands of centrifuges to produce a meaningful amount of uranium. Coordinating and controlling this enrichment “cascade” is fiendishly complicated. Not only that, but centrifuges work most effectively at high speeds – whirling at hundreds of thousands of RPM and generating outlandish centrifugal forces that simply tear apart all but the strongest materials. Requiring fantastically precise manufacturing, these centrifuges are prone to failure – especially in programs that are still developing expertise.
There are other ways of enriching uranium – gaseous diffusion, electromagnetic separation, thermal diffusion, to name a few – but virtually all of them rely on the same principle of using the tiny difference in mass to sort out uranium atoms into the fissile and the rest. This is what makes uranium enrichment so difficult – and it’s one of the factors that makes plutonium production so attractive.
Plutonium and uranium are closely related chemically – they both reside among the actinide elements on the lower bar that lies beneath the main body of the Periodic Table. But closely related is not identical – unlike U-235 and U-238, plutonium chemistry is just enough different from uranium to make it possible to chemically separate the two elements. So if one has a mixture of plutonium and uranium, one can chemically process the mixture to remove the plutonium without having to go through the entire enrichment process. But first, of course, we have to make the plutonium.
Plutonium is produced in nuclear reactors – preferably reactors using low-enriched uranium. Reactor fuel that has 2% (for example) U-235 has 98% U-238. That U-238doesn’t fission well, but it captures neutrons quite nicely. So a U-238 atom that intercepts a neutron from fission will turn into a U-239 atom. This atom has a fairly short half-life and it quickly decays to form Neptunium-239, which decays in turn to form Pu-239, which fissions quite nicely. So to make Pu-239 all we need to do is to “cook” U-238 in a neutron field and then wait for the U-239 to go through a few radioactive decays. This happens routinely in every nuclear reactor on Earth – in fact, a significant fraction of reactor energy comes from the fission of Pu-239, Pu-240, and other plutonium isotopes that are produced during this same neutron capture.
Once the plutonium has been produced it has to be separated from the rest of the elements in the reactor fuel, but this is not necessarily a straight-forward process. The problem is that the reactor fuel has been irradiated and has fissioned for however long it’s been in the reactor – it is dangerously “hot” and has to be given a chance to decay until radiation levels are safe. After several months of decay the fuel is chopped into pieces and dissolved, after which it can be chemically processed to extract the plutonium. The requires an elaborate chemical processing capability as well as the ability to work in hot cells using remote manipulators – but these are tried-and-true technologies that are easier and lower-tech than uranium enrichment.
The bottom line is that there are two main paths to producing fissile materials. We can take natural uranium and, through herculean efforts, increase the fraction of fissile U-235 to levels that will sustain fission or that will make a weapon. Or we can use nuclear reactors to bombard natural uranium with neutrons to produce plutonium that we can then chemically extract from the spent fuel. Both systems have a long and proven track record, both have been mastered by many nations, and both represent paths to nuclear weapons.