Before we begin, we have to ask the question of what renewable actually means. Most sources that people regard as ‘renewable’ are in fact solar sources. They get their energy from the sun. Wind is caused by the heating of the earth’s atmosphere by the sun. Fusion power, one of the forms of nuclear power, is what powers the sun. Fusion is generally regarded as the ultimate version of renewable energy. For more information on fusion power specifically, see our published fusion articles or our introduction to fusion power production.
While it is technically true that uranium is not a renewable resource like wind or solar energy, uranium has some interesting properties that cause some proponents to compare it to renewable resources. Very close comparisons can also be made with fusion technologies. Both fission and fusion transform mass into energy. When fission fuel comes out of a reactor, it is slightly lighter than it was at the beginning.
Fission currently releases about 16,000 times the energy per unit mass than burning coal does1 . The more advanced fission techniques that we discuss later in this article will unlock the energy from an even higher percentage of the fuel mass. By unlocking the nuclear potential of uranium, it seems possible that we can release a total of up to 54 times as much energy from our nuclear fission fuels than we currently do. In this case the nuclear waste volume should be small as compared to currently implemented nuclear fuel cycles. Additionally, these techniques would allow us to unlock the nuclear potential of thorium resources, which are estimated to be three times as abundant as uranium.
Types of nuclear materials
Before we can dig into this discussion, we need some terminology.
Fissile materials are isotopes that can sustain a nuclear chain-reaction with low energy neutrons. These are the materials that produce most of the energy in nuclear reactors. Some fissile materials are: uranium-233, uranium-235, plutonium-239, and plutonium-241. The only one of these that occurs naturally on the earth is uranium-235. This is why uranium-235 is crucial to the development of nuclear technologies. It is the only naturally-occurring fissile nuclear fuel.
There is a slightly larger category of materials, called fissionable, which includes all isotopes that can undergo fission. Some fissionable isotopes, such as uranium-238, only fission when the neutron striking is of high energy. All fissile materials are also fissionable, but not all fissionable materials are fissile. The category ‘fissionable’ contains the category ‘fissile’ as a subset.
Fertile materials are isotopes which are not normally fissile, but can be made so through a reaction known as neutron capture. For instance, when uranium-238 absorbs a neutron, this triggers a series of reactions that turn it into plutonium-239, which is a fissile material. Similarly for thorium-232, the naturally occurring isotope of thorium. Thorium captures a neutron and then undergoes a series of reactions to eventually become uranium-233, which is a fissile isotope. This path from thorium-232 to uranium-233 depends on the nucleus not absorbing another neutron during the time that it is undergoing these reactions. This is an important detail because it forms part of the basis for breeder reactors based on thorium fuel. For more information on this, see the Energy from Thorium website.
Reactor designs exist that can use fertile materials like uranium-238 or thorium-232 as fuel. These reactor designs are referred to as ‘breeder‘ reactors, because they can transmute fertile materials into fissile materials. This means they can ‘create’ fuel out of isotopes that in their original state are not useful for nuclear reactors. The interesting fact is that, when carefully designed and operated, breeder reactors can in fact transform more fertile material into fuel than they use in their operation. This means that a functioning breeder reactor is actually producing useful nuclear fuel that can be used elsewhere.
In the current incarnation of fission power, we are squandering our fissile material resources by running our reactors on a cheap supply of fissile materials such as U-235 and Pu-239. The fissile resources we are currently squandering are necessary to fuel the initial cycles of the breeder reactors we hope to use at the core of a more effective future nuclear fission industry. We should be using our fissile material resources to help us utilize our vastly greater fertile nuclear resources. This is crucially important to the long-term availability of fission power because the vast majority of the energy potential for fission is locked away in fertile materials that require breeder reactors in order to be effectively used.
Reprocessing technologies mean that, in effect, we can continue to use the same fuel material several times because each cycle through the reactor only releases a small fraction of the energy that the fuel contains. In a nuclear fuel cycle that includes reprocessing, most of the material in spent fuel is reusable in subsequent fuel. The extra detail is that we need to do work on the fuel in order to separate out isotopes that poison the nuclear process.
This additional work costs money, which today drives the price of reprocessed fuel above the cost of fuel from freshly mined uranium. It is expected that uranium deposits will be harder to find in the future, and more costly to extract. This may cause the price of reprocessing to end up below the cost of newly created uranium fuel.
Ideal fission = renewable?
For some people, breeder reactors and reprocessing technologies sound suspiciously like “Mr. Fusion” from the movie Back to the Future II. As a result, these individuals might claim that fission reactors are a renewable form of power. These individuals may be jumping the gun a little bit.
These advanced nuclear fission techniques would grant us the capability of producing large amounts of reliable power for centuries to come2 . Breeder reactors still need research and prototypes before they can be deployed on a large scale. Reprocessing currently entails an additional expense above the current ‘once through’ fuel cycle. Reprocessing may someday soon be less expensive than ‘once through’. It may also solve some of the major issues with the currently employed fission fuel cycle by burning up our currently existing waste.
Not as renewable as fusion…
In its ideal future form, the nuclear fission industry would be able to turn fuel mass into energy with relatively high efficiency. This would make it somewhat similar to power from nuclear fusion. Fusion depends on the lightest and most readily available elements in the universe. The fuel source for fusion, even just on the earth, is expected to be able to last billions of years. In short, fusion power would allow humanity to tap into the most abundant source of energy in the universe that we are aware of.
In contrast, fission power, even it its ideal future state, will be relying on fuels that are vastly more rare than the fuels for fusion. However, fission fuels do occur naturally on the Earth in fairly significant quantities. The long term supply for nuclear fission fuels on the planet earth is estimated to last several thousand years at current consumption levels, assuming that we adopt reprocessing as well as develop and implement breeder reactors.
…but still pretty good
Fission cannot guarantee fuel availability on the time scales that fusion can. Still though, thousands of years is a very long time. For the purposes of planning our energy grids today, we generally employ planning that reaches several decades into the future. We are unaware of electricity plans that look ahead more than a century in any really practical sense. On the scale of centuries, our entire power system, including generators, transformers, and transmission infrastructure, is likely to be completely replaced. With focused research, it seems extremely likely that nuclear fission can become a practical energy resource that we can rely upon for hundreds to thousands of years. With support and research we can deliver to the next generation a valuable nuclear energy resource rather than our society’s current mess of underutilized nuclear fuel, steadily accumulating waste, and weapons proliferation concerns.
It is sensible to be critical of fission for its current mis-management of waste and sometimes relatively high costs. It is also sensible for us to be very careful that our nuclear materials do not end up being used in weapons. It does not make sense however to stifle research into fission technologies simply because their fuel source will only last thousands of years. Some of these technologies may be very useful for humanity, and it makes sense for us to support them. If fission proves to be a cost-effective and safe power resource for centuries to come, then it will contribute immensely to humankind’s prosperity. We believe that our society should support the deep investigation of nuclear fission’s potential for clean, safe and affordable power.
This article is part of our nuclear myth and fact project.
- Energy Density and Waste Comparison of Energy Production. Nuclear Fissionary. Accessed October 12th, 2010. [↩]
- Nuclear Fission Fuel is Inexhaustible. This is a scientific paper that concludes that uranium can power our society at current levels for around 10,000 years. [↩]