Research and Test Reactors

There are now close to 100 nuclear reactors that are used for research and test purposes at universities and at industrial or government-operated installations in the United States, compared to about forty nuclear power plants. Although the amounts of nuclear fuel used in research and test reactors are typically only a few kilograms, many of these reactors use high-enriched uranium for fuel. In the plants where research reactor fuel is fabricated, high-enriched uranium is often present in metallic form and sufficient quantities for making several fission bombs. Plutonium or uranium–233 is generally not used for fuel in these types of reactors. A few research or test reactors, however, use low or intermediate-enriched uranium and therefore make plutonium in their cores. The total quantities of plutonium made this way are very small compared to the quantities made in nuclear power plants, however.

Two types of research reactors comprise most of those now operating in the United States: those using fuel similar to that used in the Materials Testing Reactor (MTR), and a type of training, research, and isotope production reactor called the TRIGA. Both types are water-cooled and operate at thermal power levels ranging from a few kilowatts to a few megawatts. (They are not used to produce electricity.) They both use intermediate or high-enriched uranium (20 percent or 90–95 percent uranium–235) for fuel. Typical core loadings are three to six kilograms of contained uranium–235. The actual amounts depend on the power level and other design features that may differ considerably between reactors.

MTR-type fuel assemblies consist of flat plates of uranium-aluminum alloy. The plates are separated by channels for water used both as a moderator and a coolant. TRIGA fuel elements consist of a homogeneous, solid hydride of uranium zirconium alloy (about 8 percent uranium and 92 percent zirconium) pressed into aluminum or stainless steel-clad cylinders about one inch in diameter. Neither type of fuel, even if it uses high-enriched uranium, could be used for making a fission bomb without separating the alloy materials. This would be somewhat easier to do with MTR-type fuel than TRIGA-fuel but, in both cases, it would be an easier task than any chemical separations of plutonium from diluted fuel materials.

Would-be fission bomb makers would have to steal several entire cores of fresh fuel assemblies for either type of reactor to have enough