And that is not the end of the story either. Current and planned reactors are nearly all based on using uranium enriched with 2 35U as a fuel. But there are reactors that can use 238U to make new nuclear fuel. These are known as breeder reactors or fast neutron reactors. Recall from Chapter 6 that 238U does not fis­sion when it is bombarded with neutrons—it is the 235U that absorbs the slow neutrons and undergoes fission. But 238U does have a very interesting property. It can absorb a fast neutron to turn into 239U, which rapidly p-decays into nep­tunium (239Np), which in turn rapidly p-decays to form 239Pu—a fissionable isotope. In fact, part of the power from a standard light water reactor2 comes from fission of 239Pu after it has been created. Since more than 99% of natural uranium is 238U, there is a very large amount of potentially usable uranium that can be converted to plutonium and then burned in a reactor. Use of breeder reactors in a nuclear fuel cycle would extend the supply of usable fuel by a factor of about 60-fold (54).

The design of a fast neutron reactor is quite different from a standard pres­surized water reactor or a boiling water reactor (see Chapter 5). Water is used as a moderator in a standard reactor to slow down neutrons so that they can effi­ciently cause fission in 235U, but a fast neutron reactor requires fast neutrons so no moderator can be used. Plutonium can be used as the fuel because it fissions after absorbing a fast neutron and it releases more neutrons on average than 235U. Uranium enriched to 20% or 30% 235U can also be used—it can fission with fast neutrons, but much less efficiently than with slow neutrons, so a higher concen­tration of 235U has to be used. A blanket of 238U is wrapped around the core of the reactor and some of the fast neutrons that escape from the fission of plutonium are absorbed by 238U to create more 239Pu. The net result is that more plutonium is produced than is burned in the core—hence the name “breeder" reactor. The blanket must be recycled to extract the new plutonium fuel, which can then be made into fuel pellets to be burned in a reactor to generate power and more fuel (54). Fast neutron reactors can also be built in a different design configuration to burn up plutonium isotopes and other transuranics that pose a problem for nuclear waste. In this case, the fast neutron reactors are called “burners” (55, 56).

There are some issues to consider with a fast neutron breeder reactor. Water can’t be used for heat transfer from the reactor to the turbine because it slows down neutrons. The most common alternative is liquid sodium, which does not interact with neutrons and has good heat transfer properties. The one downside is that it is highly flammable when exposed to air or water. Standard light water reactors operate at high pressures and temperatures, while a sodium breeder reac­tor operates at a high temperature but low pressure, since it remains liquid up to a temperature of 1,621°F. That makes it easier to prevent sodium leaks that might contact water or air. Another liquid metal that can be used for cooling and heat transfer is liquid lead, but it is highly corrosive. The design requirements and the need for recycling plants to recycle the fuel mean that a breeder reactor program is an expensive option. It will not be economically viable as long as there is an adequate supply of uranium for conventional reactors (54). A final consider­ation is that breeder reactors use enriched plutonium for fuel and generate more plutonium. This raises the risk of diversion of plutonium to terrorists or rogue countries who might try to make a bomb. Clearly, the breeder reactor fuel cycle would have to be tightly controlled. On the flip side, fast neutron reactors can be designed to burn up plutonium isotopes and other transuranics, greatly reducing the problem of long-term storage of spent nuclear fuel.

Is this technology just a dream for the future? In fact, there are over 400 reactor-years of experience with breeder reactors. The first breeder reactor was developed in the United States in Idaho at what is now known as the Idaho National Laboratory (INL). INL has a long and storied history as the lead national laboratory for the development of nuclear energy and development research. On December 20, 1951, the world’s first breeder reactor—the Experimental Breeder Reactor-I (EBR-I)—began operation at INL. It produced about 100 kW of elec­tricity and ran until 1964, proving that a reactor could be used to generate more fuel than it used. A second generation of breeder—the EBR-II—was also built at INL and operated for 30 years until 1994, producing 20 MWe and demon­strating that a breeder reactor could run for decades with no corrosion from the liquid sodium coolant at high temperatures of over 850°F. The EBR-II was also a research reactor that led to the integral fast reactor (IFR) (40).

The IFR was designed as a complete facility that included the reactor, a special kind of reprocessing (pyroprocessing), and fuel fabrication. It was intrinsically safe because the reactor core could not melt down. Even if there was a complete loss of power to operate pumps to circulate sodium, the core would heat up and cause expansion of the sodium, causing convection currents to circulate sodium. And since liquid sodium doesn’t boil until a very high temperature (883°C or 1,621°F), there is no risk of a steam explosion. Numerous tests showed that the reactor would just go subcritical and fission would halt. The IFR would have led to a full-scale commercial breeder reactor but it was killed by Congress in 1994, three years before it was to come online (57). As a result, the United States has no current breeder reactors in operation. However, GE-Hitachi has followed up on the IFR design and is developing the Prism fast neutron reactor that has two modules of 311 MWe each. It is not designed as a breeder but rather as a burner reactor that will burn up plutonium and other transuranics, reducing the problem of spent nuclear fuel (54).

France also has years of experience with breeder reactors in its Phenix program. The Phenix reactor was a sodium-cooled breeder reactor that had a capacity of 250 MWe and ran from 1973 to 2009, when it was shut down. A much larger 1.2 GWe breeder reactor—the SuperPhenix—was built from 1974 to 1981 but did not begin generating power until 1985 and then seldom operated at full capac­ity. It sparked enormous protests from environmentalists, including a march of 60,000 protesters in 1977 during construction and a rocket-propelled grenade (RPG) attack in 1982 attributed to the international terrorist Carlos the Jackal. During its checkered history, it was shut down more than it was operating, partly due to fixing technical problems but more often due to political and management issues. Power production was halted in December 1997 after 11 years of part-time operation and was not restarted due to a court ruling and a political decision by Prime Minister Lionel Jospin to close down the reactor (58). As part of the Generation IV nuclear reactor program, France is working on the design of the Astrid (advanced sodium technological reactor for industrial demonstration) and the French government is supporting its development. A final decision on con­struction is expected by 2017 (54).

A few other countries also have experience with fast breeder reactors. The for­mer USSR built the BN-600 sodium-cooled breeder reactor, which began operat­ing in 1980 and continues to the present. It produces a nominal 600 MWe and has one of the best operating records of any Russian reactor. A smaller version, the BN-350, was operated in Kazakhstan from 1972 to 1999, primarily to run a desalination plant. A new version, the BN-800, is under construction. Russia has sold two BN-800 breeder reactors that are under construction in China. Japan built a breeder reactor, the Tonju, in 1994, but a sodium leak led to its shutdown for 15 years until it was restarted in 2010. Germany and England also have some experience with fast neutron reactors (54).

So, the possibility of breeder reactors is not just a fairy tale but has a substantial history and is being worked on in numerous countries to both extend the supply of nuclear fuel and to burn up plutonium and other actinides. These reactors are part of the international plan for Generation IV reactors that are intrinsically safe. Uranium may not be a renewable energy, but it is a sustainable energy source for a very long time horizon.