Overview
In every uranium-based nuclear reactor core there is both fission of uranium isotopes such as uranium-235 (235
92U), and the formation of new, heavier isotopes due to neutron capture, primarily by uranium-238 (238
92U). Most of the fuel mass in a reactor is 238
92U. By neutron capture and two successive beta decays, 238
92U becomes plutonium-239 (239
94Pu), which, by successive neutron capture, becomes plutonium-240 (240
94Pu), plutonium-241 (241
94Pu), plutonium-242 (242
94Pu) and (after further beta decays) other transuranic or actinide nuclides. 239
94Pu and 241
94Pu are fissile, like 235
92U. Small quantities of uranium-236 (236
92U), neptunium-237 (237
93Np) and plutonium-238 (238
94Pu) are formed similarly from 235
92U.
Normally, with the fuel being changed every three years or so, most of the 239
94Pu is "burned" in the reactor. It behaves like 235
92U, with a slightly higher cross section for fission, and its fission releases a similar amount of energy. Typically about one percent of the spent fuel discharged from a reactor is plutonium, and some two thirds of the plutonium is 239
94Pu. Worldwide, almost 100 tonnes of plutonium in spent fuel arises each year. A single recycling of plutonium increases the energy derived from the original uranium by some 12%, and if the 235
92U is also recycled by re-enrichment, this becomes about 20%. With additional recycling the percentage of fissile (usually meaning odd-neutron number) nuclides in the mix decreases and even-neutron number, neutron-absorbing nuclides increase, requiring the total plutonium and/or enriched uranium percentage to be increased. Today in thermal reactors plutonium is only recycled once as MOX fuel; spent MOX fuel, with a high proportion of minor actinides and even plutonium isotopes, is stored as waste.
Existing nuclear reactors must be re-licensed before MOX fuel can be introduced because using it changes the operating characteristics of a reactor, and the plant must be designed or adapted slightly to take it; for example, more control rods are needed. Often only a third to half of the fuel load is switched to MOX, but for more than 50% MOX loading, significant changes are necessary and a reactor needs to be designed accordingly. The Palo Verde Nuclear Generating Station near Phoenix, Arizona was designed for 100% MOX core compatibility but so far have always operated on fresh low enriched uranium. In theory, the three Palo Verde reactors could use the MOX arising from seven conventionally fueled reactors each year and would no longer require fresh Uranium fuel.
According to Atomic Energy of Canada Limited (AECL), CANDU reactors could use 100% MOX cores without physical modification. AECL reported to the United States National Academy of Sciences committee on plutonium disposition that it has extensive experience in testing the use of MOX fuel containing from 0.5 to 3% plutonium.
The content of un-burnt plutonium in spent MOX fuel from thermal reactors is significant – greater than 50% of the initial plutonium loading. However, during the burning of MOX the ratio of fissile (odd numbered) isotopes to non-fissile (even) drops from around 65% to 20%, depending on burn up. This makes any attempt to recover the fissile isotopes difficult and any bulk Pu recovered would require such a high fraction of Pu in any second generation MOX that it would be impractical. This means that such a spent fuel would be difficult to reprocess for further reuse (burning) of plutonium. Regular reprocessing of biphasic spent MOX is difficult because of the low solubility of PuO2 in nitric acid.
Read more about this topic: MOX Fuel