Nuclear 101: Spent Fuel Reprocessing

Welcome to the 12th segment of Nuclear 101. The previous segment discussed uranium enrichment, the process in which the fissile radioisotope U235 is separated and collected in concentrated form. Low enriched uranium (LEU) can be used for nuclear fuel or can be further enriched into highly enriched uranium (HEU), which becomes a serious weapons proliferation threat once enriched to about 90% U235. Generally the enrichment process is stopped once energy-grade LEU is reached and the product is used in fuel fabrication for nuclear reactors. Spent fuel is the byproduct of the generation of nuclear energy.

The recycling of spent fuel is called reprocessing. Since energy generation requires the use of fissionable material (the concentration of U235 in LEU), the once-enriched uranium is now depleted uranium and contains even less of the radioisotope U235 than natural uranium. Spent fuel consists of uranium (96% with about 1% of this being U235 that can be re-enriched), plutonium (1%), and high level nuclear waste (3%). Reprocessing is the chemical separation of the uranium and plutonium from the high level waste products for the purpose of generating additional energy.

PUREX

The most widely used reprocessing method is Plutonium and Uranium Extraction (PUREX). The PUREX process begins with spent fuel rods being chopped up into small pieces. These pieces are dissolved in hot concentrated nitric acid. The process that separates the now aqueous blend of plutonium and uranium from other materials occurs with the use of two counter-current streams of solvent chemicals. The first stream is nitric acid and the second is tributyl phosphate diluted in kerosene or dedocane. This highly toxic process continues with the evaporation (conversion into gas) of concentrated plutonium nitrate and uranium nitrate that then goes through a process of precipitation in which the materials are converted back into liquid. A calcination (heating) process converts the plutonium to PuO2 in powder form and the uranium to UO3 in powder form. 

The extracted uranium can then be re-enriched and sent once again for fuel fabrication. The extracted plutonium is sent for mixed oxide (MOX) fuel fabrication for nuclear reactors. Below is a video discussing the process at AREVA's La Hague Recycling Facility in France, the largest reprocessing facility in the world.

The Controversy of Reprocessing

Plutonium is sensitive to weapons proliferation. The radioisotope Pu239, which is the dominant isotope of plutonium extracted during spent fuel reprocessing, is considered weapons-grade in its current form without having to go through a lengthy re-enrichment process that the extracted uranium would have to go through to achieve a weapons-grade product. The International Atomic Energy Agency (IAEA) considers all plutonium, except that which is highly diluted, to be ‘direct-use’ material that can be used to manufacture nuclear weapons without transmutation or future enrichment. 

Note: Refer back to the Nuclear 101 segment on radioactive half-life to learn more about transmutation as well as how uranium can transmute into Plutonium- a process that occurs for a small portion of uranium within a nuclear reactor.

Reprocessing technologies were originally used for the purpose of extracting weapons-grade plutonium for nuclear weapons development.  The ability to generate electricity from fuel fabricated from plutonium was a later development. Relatively few countries reprocess spent fuel because the process can potentially be a proliferation risk. Russia, France and Japan are among the few countries that do reprocess. Proponents of spent fuel recycling, also called a closed nuclear fuel cycle, argue that the process can generate an estimated 25-30% more energy than a once-through cycle and leads to a great reduction in nuclear wastes. Opponents argue that the process is highly toxic, expensive, and the byproduct is too much of a proliferation risk, and therefore promoting the process is not in the best interest of international security.

Many new reprocessing methods have been developed to make reprocessing less dangerous. Some of these methods are currently being used on a small scale. These methods attempt to either dilute the plutonium to make it less proliferation sensitive or they attempt to only extract the uranium from the spent fuel by leaving the plutonium mixed in with other nuclear wastes. If you are interested in reading more about other reprocessing technologies, the World Nuclear Association has a more detailed description. 

Below is a very creative and well done video by an engineering student at the University of Texas about the controversies surrounding reprocessing.

Prime Minister Abe and President Obama

Current Event
Japan announced on March 24, 2014 at the Nuclear Security Summit at The Hague that an agreement had been reached for the nation to turn over an estimated 300kg of weapons-grade plutonium to the United States. Japan had purchased the material from the United States in the 1960s for research purposes. Fifty years later the stockpile had turned into a major point of tension between Japan and China due to the risk of the material being diverted for nuclear weapons development. The international community had raised serious questions regarding the security of the stockpile and considered it unnecessary for Japan to hold onto it any longer. Japan will share the costs with the United States to down-blend the material into reactor fuel. Japan also has an estimated 44 tons of plutonium stockpiled from previous reprocessing activities. Following the Fukushima incident, Japan had temporarily stopped nuclear power generation. The nation plans to restart reactors using MOX fuel fabricated from reprocessed materials. This current deal between Japan and the United States is hailed as a positive step toward securing proliferation-sensitive materials around the world and further strengthens international cooperation in nuclear security.

The next segment of Nuclear 101 will focus on nuclear waste storage and disposal, the final step in the nuclear fuel cycle.

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Sources:
International Atomic Energy Agency
Oak Ridge Laboratory
World Nuclear Association: Reprocessing
World Nuclear Association: Plutonium
"Japan-U.S. nuclear deal announced at Hague summit"
"Japan draws Chinese ire over $21-bn nuclear reprocessing plant"
"Japan to turn over large nuclear stockpile to the U.S."
"Reducing plutonium stockpiles"
"Japan to Transfer Plutonium Stockpile to the United States"_