The DOE Looking to Microstructures and Nanostructures
- By Duncan Williams -
The United State government remains actively involved in the development of nuclear reactor technology. The Department of Energy (DOE) currently funds a network of national laboratories and universities that support the development of advanced nuclear energy production and fuel cycle technologies. This network spans from coast to coast, as seen in a figure from the “Fiscal Year 2009 Nuclear Energy Performance Plan” recently released by the DOE’s Office of Nuclear Energy.
Though much of the work done at these facilities is classified, recently published patent documents reveal that the DOE is looking into improving nuclear technology through the use of microstructures, and even nanostructures. First, I’ll discuss the microstructures described in U.S. Patent Number 7,521,007 (“the ’007 patent”), titled “Methods and Apparatuses for the Development of Microstructured Nuclear Fuels.” Then, I’ll discuss the nanostructures in U.S. Published Patent Application Number 20090080592 (“the ’592 publication”), titled “Swelling-Resistant Nuclear Fuel.”
MicrostructuresThe ’007 patent is currently assigned to the DOE, but was jointly developed with the University of California. Although there are many types of fuel sources, the ’007 patent refers to fuel pellets as the main fuel source. Fuel pellets are usually inserted into tubes, typically made of a zirconium alloy or stainless steel, in order to form fuel rods. The rods are then sealed and assembled into clusters for use in the core of the nuclear reactor. The fission process takes place inside these fuel pellets in the reactor core.
The fuel pellets discussed in the patent use uranium dioxide (UO2) as the fuel particles which is only a few micrometers wide. After coating each uranium dioxide fuel particle with a .5 – 5 micrometer layer of carbon, the fuel particles are then compressed together to form a fuel pellet. The thin coating on the fuel particles ensures that they are spaced within 10 micrometers from each other. This allows the fuel particles to be packed more densely in the fuel matrix, since previous fuel matrixes typically space the fuel particles over 100 micrometers from one another.
While compressing the fuel matrix to form a fuel pellet, the carbon coating on the fuel particles combine to form a carbon matrix. This carbon matrix captures many of the fission products released from the fuel particles during the fission process. Once emitted from the fuel particle, the fission products travel an average of 2-10 micrometers before coming to rest in the carbon matrix. The fission product remains in the matrix until the fuel pellet is recycled.
Upon reprocessing, the fuel pellet is dissolved in order to separate the reusable fuel particles from the fission products captured in the matrix. The reprocessed fuel particles can then be used in new microstructured fuels. Any of the current reprocessing methods in use today would effectively separate the fuel matrix components.
NanostructuresThe ’592 publication was originally owned by Lawrence Livermore National Security, LLC, but in 2008 was assigned to the Department of Energy. Note that this is a patent publication, and is not a granted patent. A patent application is published approximately 18 months after it is initially filed, but the patent owner can’t enforce it. The owner only gains full enforcement rights after the patent is issued. The U.S. Patent Office will only issue a patent once the application satisfies the dizzying number of strict statutory requirements. Even though the ’592 publication might not be issued as a patent, it does indicate how the Department of Energy is focusing its research.
The ’592 publication is currently assigned to the Department of Energy, but was jointly developed with the Lawrence Livermore National Security, LLC, at the Lawrence Livermore National Laboratory. The ’592 publication relates to fuel pellets that resist swelling which occurs in many fuel pellets during operation. During the fission of the uranium oxide fuel particle, gasses are released that remain inside the fuel pellet. Over time, these gasses collect and can cause swelling and local blistering of the fuel pellet. If the swelling becomes sever, the fuel pellet may rupture and release fission products and fuel particles into the reactor system.
Currently, a cladding is placed around the pellet in an attempt to further contain the fission products that are released as a result of the fission process. In anticipation of the release of the fission product gasses, a gap between the fuel matrix and the inner edge of the cladding is manufactured to allow for some swelling. Since fuel pellets are stacked against one another, the gaps inside each fuel pellet take up an appreciable amount of space that serves no purpose other useful purpose.
The ’592 publication describes several methods of creating nanoscale ligaments within the fuel matrix that act as an exhaust pipes for the fuel pellet. The ligaments allow the fuel pellet to retain the same volume throughout its operation, and allow the fission product gasses that would normally cause swelling to exhaust from the fuel pellet. The nanoscale ligaments also eliminate the requirement for a gap inside the fuel pellet, thus reducing the overall size of the core.
ConclusionAlthough the ’007 patent may cause more of the fission products to be trapped in the fuel pellet, the fuel pellet must still go through the time-intensive process of recycling. This includes cooling down for several years in a pool of water until the substance is cool enough to rework. This does not seem like a significant breakthrough in recycling fuel pellets. It does, however, seem to allow for an increase in the fuel particle density in the core, which seems like the biggest benefit.
As for the ’592 publication, the use of nanostructures in nuclear fuel is an exciting idea. Swelling of the fuel source has long been a safety issue in the nuclear industry. A fuel source that does not change its volume over time would be a significant advancement. But it is unclear as to where the fission product gasses would go once they were exhausted from the nanostructures in the fuel pellet. If these gasses are released into the reactor coolant, designed to transfer heat away from the reactor core, then the effect this would have on the remaining reactor system should be further studied.
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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.
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