Searete’s Traveling Wave Reactor
- By Duncan Williams -
One of the more revolutionary reactor designs in the nuclear industry today is Searete LLC’s traveling wave fast breeder reactor.
Instead of using highly enriched uranium or plutonium as nuclear fuel, the traveling wave reactor uses natural uranium (U-238), or depleted uranium from used fuel rods from other reactors. Since 99% of all naturally occurring uranium is U-238, the traveling wave reactor would eliminate the need for uranium processing plants. Even though little is known about the operational details of the traveling wave reactor, recently published patent applications fill in some of the details.
The patent applications discuss a class of nuclear reactors known as fast breeder reactors. Fast breeder reactors utilize high-energy fast neutrons to sustain the fission process. Fast breeder reactors create (or breed) a large amount of fissionable material as it is operating, and often creates more fuel than it consumes. During the fast fission process, uranium-238 absorbs a fast neutron and becomes uranium-239. The uranium-239 then decays into neptunium-239, which then decays into plutonium-239. The plutonium-239 created during the fast fission process can be either immediately used in the nuclear reactor for further fissioning, or it can be removed from the reactor and enriched for use in weapons.
Traveling Wave Reactor Technology A recently published patent application, Pub. No. 20090252273, describes a fast breeder reactor that can operate for 50-100 years without refueling or removing any used fuel from the reactor. It utilizes a process disclosed as a “nuclear fission deflagration wave” to harness energy from nuclear fuel. The application describes an energy wave moving at a “glacial” pace from one side of the nuclear fuel to the other, releasing energy and creating new fissionable material along the way.
The deflagration wave propagates across nuclear fuel much like a cigarette burns from the tip towards the filter. The glowing red embers in the cigarette represent the deflagration wavefront traveling across the nuclear fuel. However, unlike a cigarette, a significant amount of fissionable material is being created at the wavefront which can be immediately used to continue the nuclear fission process. In other words, the traveling wave reactor creates Pu-239 which it then immediately uses as fuel to further propogate the deflagration wave.
In a September (2009) interview with Nuclear News, a publication issued by the American Nuclear Society, John Gilleland, a named inventor in the patent application, stated that the deflagration wave should actually be visualized as two waves: a breeding wave (which produces the Pu-239) moving just ahead of a burning wave (which consumes the Pu-239).
In fact, the wavefront’s dual role as a producer and subsequent consumer of Pu-239 is precisely what controls the slow and steady advance of the deflagration wave. Whenever the wavefront stalls, Pu-239 begins to buildup as a result of the fission process. Once enough Pu-239 forms it will begin to fission resulting in the wavefront advancing in the direction of the unburned fuel. If the wavefront advances too quickly, it encounters nuclear fuel (U-238) that is not efficient at fissioning with fast neutrons and thus the wave slows down. Once enough Pu-239 builds up, fission increases once again and advances the wave towards the unburned fuel.
In order to launch the deflagration wave the application describes utilizing a nuclear fission igniter (300) as shown in the top view of a spherical nuclear fission fuel structure (310). The fission igniter contains highly enriched uranium-235, which is more likely to fission using thermal neutrons. A linear accelerator is then used to fire protons at some intermediary substance, which in turns produces neutrons which then bombard the fission igniter (300). Once the fission igniter is ignited, a deflagration wave (304) slowly begins traveling throughout the fuel structure (310). The fuel only needs to be ignited once - thereafter the deflagration wave will propogate until all of the fuel has been burned.
Nuclear Fuel for the Traveling-Wave ReactorAnother pending patent application owned by Searete, LLC, gives more detail about the nuclear fuel used in the wave reactor. U.S. Publication No. 20090252283, discloses a nuclear fuel foam containing many large voids spaced throughout the fuel cell. As shown in the figure from the patent publication, the closed-cell voids (80) are large enough to trap fission product gasses that are inevitably formed as a result of the fission process. Previous nuclear fuel designs manufacture a gap in between the nuclear fuel (70) and the surrounding wall (50) to allow for swelling of the nuclear fuel (70) due the release of fission product gasses. However, the buildup of fission product gasses so close to the outer edge of the nuclear fuel may result in a release of these gases into the surrounding area in the event of a nuclear accident. The inner voids eliminate this problem by allowing the gasses to gather within the voids inside the fuel itself, instead of near the skin of the nuclear fuel.
The voids also allow room for the foam itself to expand due to the intense heat it experiences during operation. Due to thermal expansion, the foam itself will expand and cause the nuclear fuel to exert pressure on the outer wall (50). By expanding into these voids instead of against the outer wall, outward swelling of the entire nuclear fuel is reduced.
The foam itself can be made of alloys or oxides of enriched uranium, plutonium, as well as the more abundant thorium. Most importantly, the foam can be made from uranium-238 or depleted uranium from used fuel rods.
Even though a deflagration wave reactor isn’t scheduled for completion until 2020, there does seem to be substantial benefit to using the more readily available, and naturally occurring, U-238 as a reactor fuel. In the interview with Nuclear News, Gilleland indicates that no one has yet made a deflagration wave, and has only been confirmed using simulation software. So it seems that this technology has many years to go before it becomes a physical reality.
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About Duncan WilliamsDuncan Williams graduated from the University of Florida in 1994 with a B.S. in Physics, and a minor in mathematics. Upon graduation, he was commissioned in the U.S. Navy where he completed training in the Navy’s Nuclear Propulsion program. He then served onboard an aircraft carrier, the USS Theodore Roosevelt, as a reactor control division officer. Onboard, he was responsible for the operation and maintenance of the electrical and mechanical components that make up the reactor control systems. This includes the control rod drive mechanisms, the reactor safety and emergency systems, the reactor coolant pump systems, and the ion exchangers. He also developed and implemented ship-wide reactor safety drills in order to educate sailors in reactor safety.
Duncan then transferred to the U.S. Naval Academy, where he served as a senior instructor teaching Thermodynamics to senior cadets. While serving as an instructor at the Naval Academy, Duncan attended night law school at the George Washington University Law School. After receiving his J.D. in 2004, he resigned his commission and began working as an intellectual property associate with Kenyon & Kenyon LLP. While at Kenyon & Kenyon, he drafted numerous patents relating to medical devices, electronic devices, telecommunications, as well as other technologies. He also has experience in all stages of patent litigation, and has represented numerous Fortune 500 companies in protecting their intellectual property rights. Duncan is currently an intellectual property associate at Blank Rome LLP.
If you have questions, comments, or know of a patent that you think Duncan should review E-mail Duncan Williams>> firstname.lastname@example.org
In ANS Nuclear News (September 2009) interesting interview with John Gilleland is available. The article, "On the traveling-wave reactor", contains also a description how does the traveling-wave reactor work. The interview can be downloaded here: www.new.ans.org/.../a_647
An animation, which shows how the traveling wave reactor design works, from Intellectual Venture Lab is here: intellectualventureslab.com
An idea the breeding-burning wavefront traveling in the core of TWR is very interesting concept, and the real construction of TWR will be very significant. More about that was presented in the descriptions of the patent US20090252273: Automated nuclear power reactor for long-term operation, link: www.freepatentsonline.com/.../0252273.html
If there will be the promise of prosperity in this nation ever again, its roots will stem from the use of nuclear power. Fossil fuels ushered in the 20th centuries prosperity, but are wholly inadequate to do the same in the 21st.
Thanks for your article.
You missed a key aspect of the design: control. While day to day operation may not require control rod use in the way a typical PWR would, you still need to be able to stop the process if something goes wrong. This can be done by dropping the neutron reflectors that surround the fuel, or by introducing control rods. I understood from John Gilleland's video posting that they intended to use B3C control rods, though how this would be implemented is not clear. Any information you have would be appreciated.
PS Your email doesn't seem to work.