The generation and use of energy is central to the maintenance of organization of civilization. Life itself is a state of organization maintained by the continual use of sources of energy. For millennia, humanity has been using more and more energy to attain a higher standard of living and there is no reason to think that this trend has reached an end.

Therefore, we should embrace the idea that we need energy and ask ourselves which characteristics we would desire for energy generation. We should never waste energy or pass over opportunities to enhance energy efficiency, rather we should seek to generate energy in ways that are reliable, resilient, and have minimal environmental impacts.

It is very fashionable to believe that we are moving towards a future where all of our energy needs will be provided by sources that release energy intermittently 1, and that we will harvest, aggregrate, and distribute this energy along a “smart grid” of greatly expanded distribution. Such a future is simply not feasible; it will consume huge amounts of land and resources and threaten our industrial economy with structural unreliability. Nevertheless, this vision is promulgated not only by well-meaning individuals but also by fossil-fuel interests who know that their technology will be required to accommodate for the unreliability of these “natural” sources.

They know that stored energy sources, on the other hand, can be released and controlled at a desired rate. Examples of stored energy sources might be water behind a dam or the energy stored in the chemical bonds of a material. That chemical energy might have been stored within the last few years, like firewood, or it may have been stored millions of years ago like coal. Until 1939, mankind was unaware of the potential to release the energy that was stored billions of years ago in the nuclear structure of thorium and uranium. Now we realize that this stored energy might be the greatest and most valuable of all energy sources.

There are simple reasons to assert the primacy of nuclear energy sources over all others. First, nuclear energy is stored energy. It can be released under control and at a rate that we desire. It is not a form of flux energy where the rate of release is beyond our control and we can only hope to capture some of it. Second, the density of the stored energy is superlative. There is over a million times 2 more stored energy in a unit of nuclear fuel 3 than a unit of chemical fuel. This is because of the strength of the “binding energy” that is released in either case.  Stated otherwise, the energy released through fission of the nucleus has a million-to-one advantage over combustion of hydrocarbons.

Fission of natural uranium requires the construction of reactors that maintain high neutron energies (fast-spectrum reactors) throughout their operation. This is because the fission of plutonium-239 (the result of neutron absorption in uranium-238, the dominant natural isotope) does not produce enough neutrons to sustain the process unless it is bombarded by high-energy neutrons.

Fission based on natural thorium, on the other hand, is much easier because thorium’s absorption product (uranium-233) produces enough neutrons from collision with a slowed-down (thermal) neutron to sustain the fission reaction, given that the reactor is designed to be frugal with its neutrons. This feature, and the abundance of thorium worldwide, give thorium a profound advantage over the other nuclear fuels for sustained energy generation.

Thorium is about four times more abundant than natural uranium (U238 and U235) and hundreds of times more abundant than just the more rare isotope U235 used to fuel legacy nuclear reactors.  Thorium is abundant in the Earth’s crust and widespread across the United States and around the world.

For maximum safety, nuclear reactions should proceed in a thermal (slowed-down) neutron spectrum because only thermal reactors can be designed to be in their most critical configuration, where any alteration to the reactor configuration (whether through accident or intention) leads to fewer nuclear reactions, not more. Thermal reactors also afford more options for achieving negative temperature coefficients of reactivity (which are the basic measurement of the safety of a nuclear reactor). Reactors that require neutrons that have not been slowed significantly from their initial energy (fast-spectrum reactors) can always be altered in some fashion, either through accident or intention, into a more critical configuration that could be dangerously uncontrollable because of the increased reactivity of the fuel. Basically, any fast-spectrum reactor that is barely critical will be extremely supercritical if its neutrons are moderated in some way.

“Burning” uranium-238 produces a fissile isotope (plutonium-239) that “burns” inefficiently in a thermal (slowed-down) neutron spectrum and does not produce enough neutrons to sustain the consumption of uranium-238. In contrast, “burning” thorium-232 produces a fissile isotope (uranium-233) that burns efficiently in a thermal neutron spectrum and produces enough neutrons to sustain the consumption of thorium. Therefore, thorium is a preferable fuel, if used in a neutronically efficient reactor.

Achieving high neutronic efficiency in solid-fueled nuclear reactors is difficult because the fuel sustains radiation damage, the fuel retains gaseous xenon (which is a strong neutron poison), and solid fuel is difficult to reprocess because it must be converted to a liquid stream before it is reprocessed.

Fluid-fuel reactors can continuously strip xenon and adjust the concentration of fuel and fission products while operating. More importantly, they have an inherently strong negative temperature coefficient of reactivity which leads to inherent safety and vastly simplified control. Furthermore, decay heat from fission products can be passively removed (in case of an accident) by draining the core fluid into a passively cooled configuration.

Fluid-fueled reactors based on liquid fluoride salts have all the advantages of a fluid-fueled reactor plus they are chemically stable across a large temperature range, are impervious to radiation damage due to the ionic nature of their chemical bond. They can dissolve sufficient amounts of nuclear fuel (thorium, uranium) in the form of tetrafluorides in a neutronically inert carrier salt (lithium7 fluoride-beryllium fluoride). This leads to the capability for high-temperature, low-pressure operation, no fuel damage, and no danger of fuel precipitation and concentration.

The liquid-fluoride reactor is very neutronically efficient due to its lack of core internals and neutron absorbers; it does not need “burnable poisons” to control reactivity because reactivity can continuously be added. The reactor can achieve the conversion ratio (1.0) to “burn” thorium, and has superior operational, safety, and development characteristics.

Liquid-fluoride reactors can retain actinides while discharging only fission products, which will decay to background levels of radiation in ~300 years and do not require long duration (>10,000 year) geologic burial.

A liquid-fluoride reactor operating only on thorium and using a “start charge” of pure U-233 will produce almost no transuranic isotopes. This is because neutron capture in U-233 (which occurs about 10% of the time) will produce U-234, which will further absorb another neutron to produce U-235, which is fissile. U-235 will fission about 85% of the time in a thermal-neutron spectrum, and when it doesn’t it will produce U-236. U-236 will further absorb another neutron to produce Np-237, which will be removed by the fluorination system. But the production rate of Np-237 will be exceedingly low because of all the fission “off-ramps” in its production.

We must build thousands of thorium reactors to displace coal, oil, natural gas, and uranium as energy sources. This would be impractical if liquid-fluoride reactors were as difficult to build as pressurized water reactors. But they will be much simpler and smaller for several reasons. They will operate at a higher power density (leading to a smaller core), they will not need refueling shutdowns (eliminating the complicated refueling equipment), they will operate at ambient pressure and have no pressurized water in the core (shrinking the containment vessel dramatically), they will not require the complicated emergency core cooling systems and their backups that solid-core reactors require (because of their passive approach to decay heat removal), and their power conversion system will be much smaller and power-dense (since in a closed-cycle gas turbine you can vary both initial cycle pressure and overall pressure ratio). In short, these plants will be much smaller, much simpler, much, much safer, and more secure.

We submit for your consideration that the development of a thorium-fueled, liquid-fluoride reactor is a compelling and achievable goal with broad environmental and societal benefits. Flibe Energy has been created to bring this development to reality.

Notes:

  1. We could divide energy sources into two broad categories. There are those energy sources that represent stored energy that can be dispatched on demand, and there are those energy sources that represent the capture of energy that is being released by sources beyond our control. These latter sources of energy may be manifested in the blowing wind or the sunshine falling on an area on the Earth’s surface. They could be represented by the water flowing down a river or tidal waters flowing back and forth through a channel twice a day. They might also include the flux of energy coming from the interior of the Earth due to the decay of thorium and uranium–geothermal energy. What all these energy sources that are in flux have in common is that we have little or no control on their ultimate sources, and we are simply trying to opportunistically capture or extract some fraction of their energy. Whether we are successful does not change the balance of the energy that is there. The sun will shine on an area whether or not there is a solar panel to capture the energy or not. The wind will blow through a canyon whether or not a windmill captures some of the energy. Radionuclides within the Earth will decay and release geothermal heat whether or not we attempt to heat water with it and produce steam and drive a turbine. Because they appear to us to be renewed continuously by means beyond our control, we have come to call these energy sources “renewables”, but in reality, thermodynamics teaches us that there is no such thing as truly “renewable” energy. All energy release represents the depletion of some original energy source, and generally this energy source is the Sun, with the notable exceptions of geothermal energy and nuclear energy. We comfort ourselves with the knowledge that the nuclear fusion energy released by the Sun will continue for billions of years. We comfort ourselves with the knowledge that the radionuclides within the Earth will continue releasing geothermal heat for billions of years. We comfort ourselves with the knowledge that the Moon will be driving tides for billions of years. So perhaps we should call “renewable” energy sources “sustainable” energy sources, in that the rate at which the energy is being released is sufficiently small relative to the magnitude of the source to allow it to be released for many billions of years.
  2. Chemical energy is released when the electron configuration of atoms is rearranged through a chemical process (combustion, digestion, etc.) Electrons are bound to nuclei with binding energies measured in a few electron volts (eV). The protons and neutrons in an atomic nucleus, on the other hand, are bound with energies measured in millions of electron volts (MeV). Thus, rearranging the nucleus of an atom (through fusion or fission) releases roughly a million times more energy than chemical energy release.
  3. There are four basic nuclear “fuels” found in nature: deuterium, lithium, thorium, and uranium. Deuterium and lithium are useful in fusion while thorium and uranium are useful in fission. Deuterium is an isotope of hydrogen that is found wherever hydrogen is found (such as water). Lithium is a light metal found in the evaporate deposits left behind in high-altitude lakes. In a traditional fusion reactor, lithium is converted to tritium (another hydrogen isotope) and then fused with deuterium, releasing energy and additional neutrons. But fusion is fundamentally difficult because positively charged particles tend to repel each other strongly, and only extraordinary temperatures, magnetic confinement, and complicated engineering can coax them to fuse. Despite all this effort, the goal of practical, economical fusion energy is distant and perhaps unreachable, even if the physics can be conquered in the laboratory. Fission based on uranium or thorium, on the other hand, is much easier because neutrons are used to induce destabilization and splitting of the nucleus. The neutron is uncharged, so there is no magnetic repulsion to contend with in the fission process. No magnetic confinement or vacuum chambers are required either. Fission also generates unstable, neutron-rich fission products that seek stability through successive beta decay.
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