Naturally occurring uranium is composed of two isotopes1, U235 and U238. U238, being the longer lived isotope comprises 99.28% of the total in naturally occurring uranium. In current generation Light Water Reactors (LWRs) the U235 is the fissile isotope that sustains the chain reaction. While there is U238 fission, it is relatively less common in a LWR. Some excess neutrons are absorbed in U238 converting it to U239 which then beta decays first to Np239 then Pu239. The Pu239 is fissile, and can continue the reactor’s chain reaction. In LWRs, the amount of Pu239 produced per fission is less than the amount of U235 consumed, so only a small fraction of the available U238 is used as a fuel. This fraction can be improved with fuel reprocessing, but the best that can be achieved with LWRs is about 3-4%. Another feature important to LWRs is that the uranium must be enriched in the U235 isotope for the chain reaction to be sustainable. 2 In typical commercial LWRs, the uranium enrichment typically runs in the range of 2-5%. Enrichment is accomplished in uranium enrichment plants that are committed for this purpose.
Breeder reactors are called such because they produce more fuel than they consume. They typically use Pu239 as the fissile isotope and U238 as the “fertile” isotope. Since less Pu239 is consumed in power production than U238 that is converted to Pu239, the Pu239 inventory gradually increases in the reactor core during operation of the plant. The “breeding ratio” is the ratio of fertile atom conversion to fissile atom consumption and is typically around 1.2-1.3 in a well designed reactor. The “doubling time” is the number of years of full power operation necessary to double the inventory of the fissile isotope, and is approximately 10-20 years. As a result of breeding, breeder reactors can effectively make use of at least 60% of the U238 in natural uranium rather than the 3-4% that is used in LWRs.3 Thus about 20 times more power can be extracted per pound of natural uranium in a breeder reactor than can be extracted in a LWR. The fact that each pound of uranium becomes more valuable means that uranium bearing ores that cannot be economically utilized in LWRs become a resource for breeder reactors. There are vast resources of low grade uranium ore available in the U.S. The same Marcellus shale that is currently being exploited for natural gas production contains about 25 ppm of uranium,4 far below the concentration that would be economic for LWRs, but potentially useful for breeder reactors. Using the domestic shale, there is sufficient uranium in the U.S. to power fleets of breeder reactors supplying basically the entire nation’s energy needs for thousands of years. This uranium resource picture is further elaborated upon in Appendix 8.
Demonstration is not a problem for the LMFBR. The world’s first LMFBR, the Experimental Breeder Reactor 1 (EBR-1) in Idaho, was also among the first nuclear plants of any kind to produce usable electric power on December 20, 1951. EBR-1 was followed by EBR-2, the Southwest Experimental Fast Oxide Reactor (SEFOR), the Enrico Fermi Atomic Power Plant (Fermi-1), and the Fast Flux Test Facility (FFTF) in the US. All preliminary design, extensive detailed design, licensing through to the award of a Limited Work Authorization, and major component fabrication was completed on the Clinch River Breeder Reactor Plant (CRBRP) prior to its termination in 1983. In addition, four sodium-cooled reactors that were not LMFBRs were built and operated in the U.S., the Sodium Reactor Experiment (SRE) in southern California, the Hallam Nuclear Generating Station in Nebraska, and two intermediate spectrum naval reactors, the S1G prototype built in Schenectady and the S2G installed in the Sea Wolf submarine.
Abroad there were two LMFBRs built in the U.K., three in France, two in Germany, two in Japan, and six in Russia. The range of sizes includes one, the Superphénix in France that was a full commercial sized plant at over 1200 MWe. The Russians completed a 600 MWe plant in 1980 that continues to operate and an 880 MWe plant in 2016 for approximately $2B that is also operating. The design of a follow-on 1200 MWe plant is underway in Russia. Extensive development of LMFBR technology has occurred at the Idaho National Engineering Laboratory and the Liquid Metal Engineering Center in the U.S. as well as at similar centers in the U.K., France, Russia, and Japan.
Since a key advantage of breeder reactors is that they produce more fissionable material than they consume, it is possible to design them so that they require only infrequent refueling. This is a capability that has not been well capitalized on by worldwide breeder reactor development to date and will be treated extensively in this paper. It is a capability that may be of considerable interest to utility company users and it opens a door for alternative design approaches.
Currently LMFBR development in the U.S., France, the U.K., and Japan is all but halted. There is continuing activity in Russia, India, Korea and China, but the energetic worldwide development of the technology so much in evidence in the 60s and 70s has all but ceased.
endnotes:
1 There is a small fraction (0.006%) of U234 in natural uranium.
2 It is possible to design reactors that can operate with natural uranium as the fuel, but they must be moderated with an isotope that has a low absorption cross section such as carbon or heavy water. The Canadian reactors which are moderated and cooled with heavy water are examples of this approach.
3 Some fraction of U238 is inevitably lost in reprocessing and fuel fabrication.
4 Bank, Tracy L., Trace Metal Chemistry and Mobility in the Marcellus Shale, University of Buffalo
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