MOX Fuel From Weapons-Grade Plutonium
- By Duncan Williams -
On February 25, 2010, the U.S. National Nuclear Security Administration (NNSA) announced that it entered into an inter-agency agreement with the government owned Tennessee Valley Authority (TVA) in order to evaluate the use of a nuclear fuel made from surplus weapons plutonium. This announcement comes on the heels of President Obama’s recently submitted FY 2011 Budget Request, which included a 25.8% increase in funding to the NNSA.
One reason for the increase in funding relates to an agreement between Russia and the United States, initially signed in 2000, known as the Plutonium Management and Disposition Agreement. According to this Agreement, each country agreed to dispose of, or immobilize, 34 tons of surplus weapons-grade plutonium. Initially, both sides began dismantling nuclear weapons and simply stored the nuclear material. However, concerns on both sides began to grow regarding the ability to quickly reassemble the nuclear material into weapons should the need arise.
This resulted in both sides initiating efforts to use the surplus nuclear material in nuclear reactors, which would physically tranform the nuclear material into a material that is unsuitable for use in nuclear weapons. To this end, a facility called the Mixed Oxide Fuel Fabrication Facility (MFFF), located at the Savannah River Site in South Carolina, is currently being constructed which will convert weapons-grade plutonium into nuclear fuel that can be used in existing U.S. commercial reactor plants.
The Savannah River Site has played an important role in the development of nuclear power in America. Construction first began at the Savannah River Site in 1950 by E.I. duPont de Nemours and Co., under the auspices of the Atomic Energy Commission. Since then, many different experimental reactors and nuclear fuels have been manufactured there. In the 1990s, the Savannah River Site began converting liquid nuclear waste into a solid glass form for long-term storage and disposal.
Although the Savannah River Site has been processing nuclear liquid waste for storage purposes, the construction of the MFFF is necessary to give it the capability to manufacture fresh nuclear fuel from plutonium. The MFFF is currently being constructed by Shaw Areva MOX Services and is scheduled to be completed in November of 2012. The design of the 600,000 square foot MFFF is based on Areva’s La Hague and Melox fuel treatment facilities in France.
The MFFF will manufacture a fuel known as mixed oxide (MOX) fuel. While conventional nuclear fuel is made from an oxide of uranium, MOX fuel is made from a mixture of uranium and plutonium oxides. Specifically, MOX fuels are made from a mixture of uranium-238 and plutonium-239 isotopes. Plutonium-239 is created in all reactor plants that utilize uranium as a nuclear fuel as a natural result of uranium-238 capturing a neutron. Plutonium created in this manner is called reactor-grade plutonium, and contains a relatively small amount of the plutonium-239 isotope.
In contrast, the plutonium found in nuclear weapons contains greater than 90% of plutonium-239, and is known as weapons-grade plutonium. Using the weapons-grade plutonium in a reactor converts it to reactor-grade plutonium and also deposits several impurities in the plutonium that are a result of the fission process. Thus, using weapons-grade plutonium in a reactor not only provides electrical power, it renders it unfit for use in nuclear weapons.
An announcement on the NNSA’s website indicates that MOX fuels will first be deployed at two existing reactor plant sites - the Sequoyah Nuclear Plant located near Chattanooga, Tennessee, and Browns Ferry Reactor Plant in northern Alabama. TVA is also pursuing the construction of two new Westinghouse AP1000 reactors at its Bellefonte reactor site near Hollywood, Alabama, which would be capable of operating with 100% MOX fuel. MOX fuel is widely used throughout Europe, as well as in Japan and Russia. Although several American companies have experimented with the use of MOX fuel stretching back to the 1970s, there is currently no domestic commercial reactor plant that uses MOX fuel. Recently, Duke Energy had begun experimenting with MOX fuel in several of its reactors, but last year announced it would no longer pursue this technology citing a breakdown in contract negotiations with Shaw Areva MOX Services.
In order to utilize MOX fuel, it is possible for existing reactor plants to replace the conventional uranium fuel assemblies with one or more MOX fuel assemblies. Using both forms of fuel in the same reactor poses several engineering challenges. For example, the MOX fuel operates at a higher temperature than the uranium dioxide fuel. This means that the MOX fuel assemblies undergo fission at a faster rate, and therefore release more fission product gases than the uranium dioxide fuel assembly. These fission product gases are contained inside the fuel assemblies, causing the internal pressures in the MOX fuel assemblies to increase at a faster rate than the uranium dioxide fuel assemblies, requiring the MOX fuel assemblies to be replaced more frequently.
Another problem with using both MOX fuel assemblies and uranium dioxide fuel assemblies together in the same reactor core is that the MOX fuel assemblies are more likely to absorb thermal neutrons than the uranium dioxide fuel assemblies. This is because plutonium isotopes are generally more neutron absorbent than uranium isotopes. Since the uranium dioxide fuel assemblies emit a large amount of thermal neutrons, the MOX fuel assemblies must be carefully positioned in the core in order to prevent them from absorbing too many thermal neutrons, which would result in unwanted power peaking in the MOX fuel assemblies. Power peaking can also be avoided by placing neutron absorbing material inside the reactor core so that some of the thermal neutrons are absorbed there instead of the highly-absorbent MOX fuel assemblies.
Solutions to these design problems are proposed in a patent publication owned by Westinghouse - the manufacturer of the nuclear reactor at the Sequoyah Nuclear Plant. U.S. Patent Publication No. 20100054389, published on March 4, 2010, describes a MOX fuel pellet that is arranged inside hollow tubes called fuel rods. As can be seen in the diagram from the patent publication, the hollow fuel rod (66) contains pellets (78) stacked on top of one another. The pellet (78) itself has a hollow center core to allow for the build up of fission product gases as a result of reactor operations.
The hollow center also reduces the amount of nuclear fuel contained in each pellet, which in turn lowers the overall operating temperature of the pellet during normal reactor operations. The fuel rod (66) also provides for a second plenum (90), which can further accommodate the release of fission product gases. This extends the period of time the fuel rod can be utilized before it needs to be replaced.
In order to absorb excess neutrons in the reactor during its operation, the patent publication describes replacing some of the MOX fuel pellets with pellets made from a neutron absorbing material, such as a gadolinium oxide. The patent publication also suggests that gadolinium particles may be added to either the MOX fuel pellets or the uranium dioxide fuel pellets in order to evenly distribute the neutron absorbing material throughout the reactor core.
Another patent publication, U.S. Patent Publication 20100040189, published on February 18, 2010, and owned by Commissariat A L’Energie Atomique, describes placing erbium in the cladding of the fuel rods containing MOX fuel. Contrary to Westinghouse’s patent publication, the French publication indicates that mixing gadolinium directly into the fuel can contaminate the fuel production lines, leading to inadvertent doping of gadolinium in all of the fuel pellets. In addition, gadolinium has different thermal properties than the MOX fuel pellets, as well as the uranium dioxide pellets, which can lead to local hot spots wherever the gadolinium particles are located.
Instead of using gadolinium to absorb neutrons inside the fuel rods, this publication describes placing another neutron absorbing element, erbium, in the cladding of the fuel rod itself. By homogeneously mixing erbium into the cladding of the fuel rod, more fuel can be loaded into the reactor and, therefore, more power can be produced during normal reactor operations. The isotope of erbium formed after it absorbs a neutron is not radioactive, and therefore has the advantage of not creating any additional radioactive waste.
These two conflicting patent documents indicate that more research is needed in order to fully develop MOX fuels made from nuclear weapons. Although the idea of using MOX fuels made from spent nuclear fuel is well established, the notion of using MOX fuels made from nuclear weapons has been largely unexplored. Areva’s MOX fuel plant in France only recycles used nuclear fuel and does not have the capability to process fuel made from nuclear weapons. The MOX fuel that will be produced at the MFFF will contain plutonium that is much more fissile than the MOX fuel produced at the French facility.
Thus, the MFFF is truly the first of its kind and is an important step in reducing the number of nuclear arms in a manner that will benefit the American people. But much more testing needs to take place in commercial reactors throughout America in order to explore the proper design of this new type of MOX fuel in conventional reactors. For example, more research must be devoted to determining what types of neutron absorbing material should be used in conjunction with MOX fuels. The agreement between the NNSA and TVA is an important first step in conducting the research needed to fully develop these new MOX fuels.
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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>> duncan@nuclearstreet.com