The problem of management and disposition of excess weapons plutonium must be considered in the context of the large world stocks of fissile materials. While all but a small fraction of the world's HEU is in military use, civilian stocks of plutonium are several times larger than military stocks and are growing much faster, by some 60 to 70 tons each year. Most of these civilian stocks, however, are in the form of radioactive spent fuel from the world's power reactors, from which the plutonium is difficult to extract. The difficulty of extracting this plutonium declines substantially as the radioactivity of the fuel decays over the decades after it leaves the reactor. Roughly 130 tons of plutonium have been separated from spent fuel for reuse as reactor fuel, of which some 80 to 90 tons remains in storage in separated form.
Plutonium customarily used in nuclear weapons (weapons-grade plutonium) and plutonium separated from spent reactor fuel (reactor-grade plutonium) have different isotopic compositions. Plutonium of virtually any isotopic composition, however, can be used to make nuclear weapons. Using reactor-grade rather than weapons-grade plutonium would present some complications. But even with relatively simple designs such as that used in the Nagasaki weapon—which are within the capabilities of many nations and possibly some subnational groups—nuclear explosives could be constructed that would be assured of having yields of at least 1 or 2 kilotons. Using more sophisticated designs, reactor-grade plutonium could be used for weapons having considerably higher minimum yields. Thus, the difference in proliferation risk posed by separated weapons-grade plutonium and separated reactor-grade plutonium is small in comparison to the difference between separated plutonium of any grade and unseparated material in spent fuel.
While plutonium and HEU can both be used to make nuclear weapons, there are two important differences between them. The first is that HEU can be diluted with other, more abundant, naturally occurring isotopes of uranium to make low-enriched uranium (LEU), which cannot sustain the fast-neutron chain reaction needed for a nuclear explosion. LEU is the fuel for most of the world's nuclear power reactors. In contrast, plutonium cannot be diluted with other isotopes of plutonium to make it unusable for weapons. "Re-enriching" LEU to the enrichment needed for weapons requires complex enrichment technology to which most potential proliferators do not have access, while separating plutonium from other elements with which it might be mixed in fresh reactor fuel requires only straightforward chemical processing. Thus, the management of plutonium in any form requires greater security than does the management of LEU.
Second, as noted earlier, in the current nuclear fuel market, the use of plutonium fuels is generally more expensive than the use of widely available LEU fuels—even if the plutonium itself is "free"~-because of the high fabrication costs resulting from plutonium's radiological toxicity and from the securityrol, including projects such as this.