Breeder Reactor Energy

If the operational conditions and design of a reactor are adjusted to maximize the amount of Pu produced it is possible to operate the reactor to produce more fertile isotopes than were originally used to start the reactor. This operational mode is called the breeder reactor. The breeder reactor can greatly extend the amount of potential energy available from uranium because it is possible to use the 99.3% U238 present in natural uranium, as fuel. It is also possible to use thorium (Th ) in a breeder reactor to produce fertile U233. The use of the breeder reactor will extend the lifetime of the nuclear fission energy source to several hundred years. 120,121,122

In the discussion of the current class of nuclear burner reactors, no mention was made of recovering the residual fuel present in the exhausted fuel elements. Depending on many factors involved in the operation of the reactor the fuel elements may contain from 5% to 30% of the original U235. In addition, some of the neutrons produced by the fission of the U235 are captured by U238 converting it to

117 Bebbington, William P., "The Reprocessing of Nuclear Fuel", Scientific American, Vol. 235, No. 6, December 1976, Page 30

118 Cohen, Bernard L., "The Disposal of Radioactive Waste from Fission Reactors", Scientific American, Vol. 236, No. 6, June 1977, Page 21

119 Johnson, Jeff, "Up From the Dead", Chemical & Engineering News, Vol. 79, No. 36, September 3, 2001, Page 29

120 Weinberg, Alvin M., "Breeder Reactors", Scientific American, Vol. 202, No. 1, January 1960, Page 82

121 Bump, T. R., "A Third Generation of Breeder Reactors", Scientific American, Vol. 216, No. 5, May 1967, Page 25

122 Seaborg, Glenn T. and Bloom, Justin L., "Fast Breeder Reactors", Scientific American, Vol. 223, No. 5, November 1970, Page 13

U239. U239 has a half-life of a little over 23 minutes and decays to neptunium 239 (Np239) with a halflife of 2.3 days. Np239 decays to plutonium 239 (Pu239), Pu239 has a half-life of 24,000 years. Pu239 is a fertile isotope and will fission when struck by neutrons. It can be used as the fuel for a reactor in the lit "yyc Q

same manner as U . In a state-of-the-art burner reactor, the residual fertile U and Pu represent a smaller source of energy than the original U235 present in the fresh fuel rod. This coupled with the current policy of not reprocessing used fuel elements results in the relative short lifetime for the burner fuel cycle.

In the burner fuel cycle, the uranium is cycled one time through the reactor. All the depleted fuel

235 239

elements and any remaining fertile U or Pu is stored. In the breeder reactor, design and operating parameters are be adjusted to promote the production of more fertile isotopes than are consumed in the reaction. This is the substance of breeder reactor technology.

In an energy system based on breeder reactors, it is necessary to process the used fuel elements to recover the fertile elements and discard the waste products. The difficulty with this approach lies in the handling of the uranium, plutonium and highly radioactive nuclear fission products.

The used fuel rods, removed from the breeder reactor, must be chemically processed to separate the fertile materials from the fission products. To achieve this chemical separation the fuel rods are dissolved in strong acid. As in working with the raw elements, great care must be taken to prevent a critical mass of U and Pu239 to build up during the separation operation. Failure to prevent a critical mass during processing is the reason the workers were killed in Japan in 1997.

The valuable fertile elements are recovered from the acid solution by extraction with an organic solvent. The acid residue, containing the extremely radioactive fission products, is processed to convert the waste into a stable solid form. The fission product waste, in a very concentrated form, is stored for ultimate disposal. This waste represents a different problem than the waste from current burner reactors. Because of the chemical concentration step there is less total mass of material. The same concentration process that reduced the mass of the waste concentrates the radiation produced into a smaller more intense package. This waste is so radioactive that it gets hot and must be actively cooled or diluted to prevent meltdown. Safe storage and disposal methods are very difficult to design.

It has been suggested that the high level waste be packaged and used for heating. From the standpoint of thermal conservation, this is an excellent concept. Packaging the high level waste and protecting, the surrounding from the particles and gamma rays it would emit appears to be an insurmountable barrier to this use of high-level waste in most heating applications.

The plutonium produced by the breeder reactor presents a problem in weapons' proliferation. Separation of fertile U235 from natural uranium requires very expensive, complex and costly isotopic separation plants. These plants can only be built by large wealthy nations that have a strong technology base. When built, they are very difficult to hide because of their size, energy requirements and waste heat output. Fertile Pu539 and U233 can be separated from fuel elements by relatively simple chemical methods that do not require the complex isotope separation. If the breeder reactor cycle is used widely there will be a lot of Pu"* and IT" flowing in the energy infrastructure. The more there is, the more difficult it will be to prevent its theft by terrorists. The amount of Pu239 required for the fabrication of a bomb is less than 20 kilograms. Plutonium has a specific gravity of 17. Because of the high specific gravity, a 1.2-liter block will contain 20 kilograms. This relative small size and weight of the materials necessary for the construction of a bomb make protection from theft more difficult. 123

The use of the breeder reactor energy generation system can provide civilization with a reliable source of energy for several hundred years. It will also present civilization with an unprecedented waste disposal problem in the form of highly radioactive fission waste, and a greatly increased problem of protecting itself from the theft of fertile material suitable for the fabrication of bombs. Removing the waste from the earth and disposing of it in space can solve the waste disposal problem. 124 While technically feasible and potentially able to protect the earth from any chance of contamination, it will require a space launch capability of greater reliability.

Renewable energy sources do not couple well with the user because of the mismatch between the best time for production of energy and the peak demand time. Nuclear fission cycles have a mismatch for a different reason. One problem with renewable energy sources is erratic production that does not match the demand curves. The nuclear reactors operate best at a constant flat output level. They are difficult to turn up and down to match, the demand curves. As with renewable sources, an energy storage medium is useful to match the output to the user. Fission burner and breeder reactor energy systems can provide appropriate energy for space heating, process heat and the generation of electricity. Neither has inherent a portable energy source suitable for storage of energy to power transportation.

On balance, a breeder-fission based energy system would appear to be sufficiently robust to provide an energy source for the replacement of fossil fuels. The attendant problems of weapons proliferation and fission product disposal are significant barriers to its use.

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