Yes. Plutonium is different from uranium. Uranium has really those two isotopes. And let me call them by their names, uranium-235 and uranium-238. That's simply two different isotopes of the same metal. 235 is fissionable. Plutonium, when it's produced in a reactor, the first isotope you get is plutonium-239. That comes from the uranium-238. Almost immediately after that isotope will absorb another neutron and become plutonium-240. At least, some fraction of it will. And that is a highly unsatisfactory isotope to the weapons designer, because that gives off a lot of neutrons itself, and makes it very difficult to trigger any kind of an explosion effectively. But it goes right on. It goes to plutonium-241, to plutonium-242, and that whole mixture of isotopes of plutonium is exactly what the bomb designer does not want. He wants pure plutonium-239. That comes from reactors that are specially set up to produce the isotope plutonium-239, and not all of the mixture of isotopes that come out of the nuclear reactor.