Transuranic Waste

Transuranic waste is material that is contaminated with U-233 (and its daughter products), certain isotopes of plutonium, and nuclides with atomic numbers greater than 92 (uranium). It is produced during the reprocessing of spent fuel to separate plutonium for use in fabrication of nuclear weapons. This waste has been accumulating since the establishment of the Hanford Site in 1943. Currently there is 906,000 cubic feet of transuranic waste stored at Hanford. Their radioactivity levels are generally low, but since they contain several long-lived isotopes, they must managed separately. This classification is composed of isotopes with half-lives greater than 20 years and giving a total activity of greater than 100 nanocuries per gram of waste. Isotopes include plutonium-239, half-life 2.4 E4 yr.; americium-241, 433 yr.; americium-243, 7380 yr.; curium-244, 18 yr.; and curium-245, 8500 yr. A very long-lived half-lived isotope is also present, neptunium-237, 2.1 million yr.

Transuranic wastes give off very little heat, and most of them can be handled by ordinary methods not requiring remote control. For many years they were buried in shallow trenches, but since 1970 they have been placed in retrievable storage in large storage pools usually at the plant. Now the United States operates with a policy of the "once-through" fuel cycle. What this means is that each fuel assembly remains in the reactor for about 2 years, after which it is placed into the storage pools. Another policy that has been suggested is to recycle the uranium and plutonium in the storage pools. This is possible through reprocessing the spent fuel by methods similar to those used at Hanford during World War II. Plutonium can be blended with slightly enriched uranium to form a fuel called "mixed-oxide"(MOX), which is a combination of UO2 and PuO2. The uranium in spent fuel has a U-235 content higher than that of natural uranium. It can be returned to the isotope separation plant for re-enrichment and reuse.

There are several advantages to reprocessing, in contract to a once-through use of fuel. The valuable energy resources contained in the fissile plutonium-239 and plutonium-241 are made more readily available. By recycling the uranium and plutonium the amount of mining and milling of new uranium is reduced, which helps to conserve the uranium reserves. The waste volumes to dispose of are smaller for the fission products separated in reprocessing than for spent fuel. Also, the removal of plutonium and its subsequent consumption in a reactor eliminates it as a source of hazard in waste disposal. Finally, there is a good chance that rare strategic metals could be extracted from the fission products.

The only disadvantages include the extra cost for the reprocessing facility and its operation. Another is the radiation exposure to workers at this facility. The final disadvantage is that there will be an increase in the accessibility of plutonium, which could be stolen and sold to terrorists. The odds of this happening are not likely though considering the amount of security at power plants nowadays. Unfortunately reprocessing was banned as a national policy here in the United States by Jimmy Carter in 1977 because of the potential terrorist threat mentioned above. He also hoped that countries not yet having nuclear weapons would be discouraged from developing reprocessing facilities to obtain plutonium. However, countries in Europe such as France have been reprocessing fuel for years without any problems at all. President Reagan rescinded the ban in 1981 and urged the nuclear industry to resume commercial reprocessing. Financial problems and uncertainty about future government policy, however, have discouraged the industry from acting. To this day, reprocessing is not practiced here in the United States and the amount of spent fuel continues to climb.

One place that is currently handling transuranic wastes is Argonne National Laboratory. There they are developing a process which will concentrate the TRU isotopes in a noncorrosive solid phase that is acceptable for long-term storage until disposal at the waste isolation pilot plant (WIPP) or at another suitable facility. The process currently being studied involves coprecipitation using magnetite (FeO.Fe203), a mixture of ferric and ferrous oxides. The magnetite is formed by adding ferric and ferrous ions to the waste solution and then adding sodium hydroxide to make it alkaline. The magnetite precipitates, carrying the transuranic elements out of solution. For simple acid solutions, decontamination factors >10,000 for plutonium and americium have been achieved in direct-strike precipitation's. Multiple precipitation's are used to reduce the activity in the supernatant to <0.1 nCi/g. Once the resultant liquid is separated from the solids, it may be reclassified as low level waste and can be disposed of under existing regulations. The solids are then mixed with sufficient absorbent to absorb any remaining liquid and placed in storage. Click here to see a Department of Energy map of sites that store transuranic wastes, or click on the links below to visit those sites homepage.

Other Generating and Storage Sites

Rocky Flats Environmental Technology Site

Lawrence Livermore National Laboratories

Idaho National Engineering Laboratories

Los Alamos National Laboratories

Oak Ridge National Laboratories

Savannah River Site

Nevada Test Site

Mound, Ohio

 

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