Among North Korea's feather-ruffling moves this week came the claim that the country has reprocessed 8,000 spent nuclear fuel rods. What does rod reprocessing have to do with building fresh nukes?
It's basically the poor man's way of obtaining plutonium, the substance most prized for the fabrication of nuclear weapons because of its relative stability. The fuel rods in question are leftovers from North Korea's reactors, which were ostensibly built for nonmilitary purposes. Such rods are initially filled with enriched uranium—that is, uranium that has a relatively high content of the fissile uranium-235 isotope. Naturally occurring uranium is approximately 99.3 percent uranium-238, which doesn't do the trick when inserted into a reactor. Enriched uranium has been "purified" in order to up the uranium-235 percentage to about 5 percent.
When a fuel rod is made, enriched uranium is baked into inch-long pellets and inserted into metal tubes. In the belly of a reactor, the atoms of this fuel split apart, releasing tremendous amounts of energy in the form of heat. That heat turns the surrounding water into steam, which in turn pushes a turbine. During this process, the uranium-238 converts into plutonium-239, which is ideal for weapons production. However, since civilian reactors are designed to keep pumping until all the fuel's spent, the morphing doesn't stop there. Some of those plutonium-239 atoms absorb an additional neutron and become plutonium-240. The new isotope isn't fissile and thus not what military engineers crave.
After a year or two, the fuel rods are tapped out. They're often transported to a nearby water tank, where they cool down for a while—the irradiation process creates so much heat that rods need one to three years to become sufficiently chilly. That's when reprocessing can commence. The simplest approach is to dissolve the rods in nitric acid, a technique known as the Purex Process. The end result is approximately 96 percent uranium, 1 percent plutonium, and 3 percent assorted toxic byproducts. (The ratio of Pu240 to Pu239 in the resulting plutonium depends on a variety of factors, including the percentage of uranium-235 in the pellet and the length of time the rod was used.) Proponents of nuclear energy argue that recycling the rods reduces waste, since the plutonium and uranium can be reused as fuel.
The downside is that the resulting plutonium can also be used for more nefarious projects. The U.S. Department of Energy officially defines "weapons-grade plutonium" as that containing at least 93 percent of the fissile plutonium-239 isotope. (The rest can be nonfissile Pu240.) But even crude "reactor grades"—those that contain less than 80 percent Pu239—can still pack a wallop. There's also no way of knowing whether the North Koreans operated their reactors specifically to minimize the conversion of plutonium-239 to plutonium-240, which would produce truly menacing nuclear material.
Explainer thanks the World Nuclear Association.