There is one more possibility for the future of nuclear power based on another natural material that is three times more common on earth than uranium—the element thorium. The Red Book estimates there are over 2.3 million tonnes of identified thorium resources, and over 6 million tonnes are likely available (47).

There has been little interest in mining thorium because it is not currently used in reactors, so there is undoubtedly far more available than is known. Natural tho­rium consists of a single isotope, 232Th. The problem is that it is not fissile itself— that is it cannot be used directly in a reactor—but it has to be converted into 233U in a reactor by capturing a neutron and undergoing a couple of rapid p decays. This is analogous to the process in which 238U—which is not fissile—absorbs a neutron in a fast neutron reactor and produces 239Pu. The 233U that is produced is fissile and can then be burned in the reactor or recycled and formed into fuel pellets for a new reactor.

There are several advantages to using thorium compared to uranium in reac­tors. A major advantage is that essentially all of it can be converted to fissile 233U, whereas only 0.7% of uranium is used (the fraction of 235U in natural uranium). It does not require enrichment—a major cost in producing uranium fuel. The nuclear waste produced is also more manageable, since little 239Pu is produced (it takes seven sequential neutron absorption events to transmute 232Th into 239Pu). That makes it much easier to deal with the shorter-lived fission products.

Several types of reactors can be used to convert thorium to 233U, including widely available boiling water reactors and CANDU reactors, made in Canada, which use heavy water as a moderator. Generation III high temperature gas reac­tors (HTGR), which use helium gas both as a coolant and to drive the turbine, are well-suited for incorporating thorium as a fuel. A novel new type of reactor—the molten salt reactor—is in the design stage. Both thorium and uranium would be incorporated in molten salt that would provide both the fuel and also the heat transfer to the turbine. The 233U that is produced from the thorium would be con­tinually extracted from the molten salt as the reactor is operating (59).

In reality, the design for a molten salt reactor was accomplished early in the history of nuclear power (in 1959) at Oak Ridge National Laboratory under the leadership of Alvin Weinberg. Since water is a moderator that slows down neu­trons and 233U requires fast neutrons to fission, a molten salt was used for cooling the reactor and boiling water to run a turbine. Lithium fluoride and beryllium fluoride had excellent properties as carrier salts into which thorium fluoride and uranium fluoride could be dissolved. An experimental reactor was built by 1965 and was operated for several years, though it used only uranium as a fuel, not tho­rium. But Weinberg lost the political and technological battle for reactor design to Admiral Rickover’s pressurized water reactor, and the thorium reactor design faded into obscurity. It is now being resuscitated by a dedicated band of true believers led by Kirk Sorensen who believe that liquid fluoride thorium reactors (LFTR) can solve the world’s energy problems (60).

While no reactors are currently operating on a thorium cycle, several have oper­ated for a number of years in the past. In the United States, the first experimental Peach Bottom reactor near Harrisburg, Pennsylvania, was a high temperature gas reactor using thorium and highly enriched uranium (HEU). It had a capacity of 40 MWe and ran from 1967 until 1974. It served as the precursor to a more ambi­tious 330 MWe HTGR near where I live—the Fort St. Vrain reactor—the first and only reactor to be built in Colorado. The Fort St. Vrain reactor operated from 1976 to 1989 but was shut down because of numerous operational problems that made it too expensive for the utility to manage. Germany operated a thorium high temperature reactor from 1983 to 1989. Its fuel consisted of small “pebbles” of thorium and HEU that constantly moved through the reactor. It is the precur­sor of the pebble-bed reactor that Germany was building in South Africa until construction halted recently. India is the country that is most vigorously pursuing thorium as a reactor fuel because it has little uranium but substantial resources of thorium (59, 61, 62).

SUMMARY

The quest for uranium began with little regard for safety of miners or the general public. However, in spite of the reality that underground miners were exposed to high levels of radon and many did develop lung cancer, modern rules for mining have greatly diminished the dangers to the public. Even the tailings from mining and milling had few if any actual health effects on the public. The shift toward ISR mining in recent years—for those sandstone deposits that are amenable to this type of min­ing—has greatly reduced the environmental issues associated with uranium mining.

The uranium fuel cycle includes mining, milling, enrichment, making fuel pellets that are burned in a reactor, and then dealing with the spent nuclear fuel. Much of the expense (and CO2 production) in the fuel cycle comes from the energy-intensive process of enrichment using the old gaseous diffusion technol­ogy. The new gas centrifuge technology dramatically reduces the energy neces­sary to get enriched uranium, however. In an open nuclear fuel cycle, such as that used in the United States, the spent nuclear fuel is simply stored, but in a closed nuclear fuel cycle, the plutonium and even the remaining 2 35U can be reused in reactors to get much more power out of the fuel. A number of other countries already do that, including France, Russia, Japan, Great Britain, and Germany.

Plenty of uranium exists to power the current 432 operating reactors world­wide for at least another century. Even if there is a dramatic expansion to more than double the number of reactors in a nuclear renaissance, there are still ade­quate known resources for powering these reactors for their design lifetime. With the future (but already proven) technology of fast neutron reactors, new fuel can be “bred” that would give adequate fuel for a thousand years or more. Not only that, but these reactors could be used to burn up plutonium and actinides to reduce the potential for nuclear terrorism. And the possibility exists to develop a thorium-based nuclear reactor economy that could coexist with the uranium-based nuclear reactors.

Clearly, there are adequate resources and technology for nuclear power to end the world’s reliance on coal and reduce greenhouse gas emissions from electric power generation. The question is, will we do so?