MOX fuel: Encyclopedia II - MOX fuel - Overview

MOX fuel - Overview

In every nuclear reactor core there is both fission of isotopes such as uranium-235, and the formation of new, heavier isotopes due to neutron capture, primarily by U-238. Most of the fuel mass in a reactor is U-238. This can become plutonium-239 and by successive neutron capture Pu-240, Pu-241 and Pu-242 as well as other transuranic or actinide isotopes. Pu-239 is fissile, like U-235. (Small quantities of U-236 and Pu-238 are formed similarly from U-235.)

Normally, with the fuel being changed every three years or so, most of the Pu-239 is "burned" in the reactor. It behaves like U-235 and its fission releases a similar amount of energy. The higher the burn-up, the less plutonium remains in the spent fuel, but typically about one percent of the spent fuel discharged from a reactor is plutonium, and some two thirds of the plutonium is Pu-239. Worldwide, almost 100 tonnes of plutonium in spent fuel arises each year. A single recycle of plutonium increases the energy derived from the original uranium by some 17%, and if the uranium is also recycled this becomes about 30%. With additional recycling the percentage of fissile Plutonium in the mix decreases requiring the total plutonium percentage to be increased.

Re-licensing precedes the introduction of MOX fuel into existing reactors. Often only a third to half of the fuel load is switched to MOX. The use of MOX does change the operating characteristics of a reactor, and the plant must be designed or adapted slightly to take it. More control rods are needed. For more than 50% MOX loading, significant changes are necessary and a reactor needs to be designed accordingly. The Palo Verde nuclear power plant near Pheonix, AZ was designed for 100% MOX core compatibility but has so far always operated on fresh slightly enriched uranium.

CANDU reactors could use 100 percent MOX cores. Atomic Energy of Canada Limited (AECL), reported to the US NAS committee on plutonium disposition that it has extensive experience in testing the use of MOX fuel containing from 0.5 to 3 percent plutonium. According to the AECL, CANDU reactors can use 100 percent MOX cores without physical modification.

Adapted from the Wikipedia article "Overview", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki