Under The Hood With Duncan Williams - Thorium Power’s Seed-Blanket Thorium Reactor
Thorium Power’s Seed-Blanket Thorium Reactor
- By Duncan Williams -
nuclearstreet.com
The ever increasing global demand for energy is driving new research into alternate fuel sources for nuclear reactors. Most nuclear reactors in operation today use uranium dioxide (UO2) as a fuel source, which includes a large initial amount of enriched uranium-235. During reactor operations, the uranium-235 is converted to plutonium as a result of fission. The resulting plutonium can be removed from the reactor and undergo further processing to eventually be used in nuclear weapons. Many corporations are developing nuclear reactors that use a fuel source other than uranium in order to eliminate this plutonium production.
One of those corporations is Thorium Power, Ltd., now called Lightbridge Corporation. Thorium Power is developing reactors that limit the production of plutonium by utilizing a thorium fuel cycle. No plutonium is produced as a result of the fission of thorium. However, thorium itself is too stable to sustain a fission reaction, and therefore requires some form of enriched uranium or plutonium in the reactor core to fuel the fission process during its initial operation. Thorium Power is focusing on reactor cores which are made of enriched uranium or plutonium (the seed) in the center of the core, enshrouded in thorium oxide (the blanket) which is placed along the outer boundary of the reactor core. So even though Thorium Power’s design utilizes thorium, there is still a significant amount of enriched uranium (or plutonium) in its reactor core, which will fission into plutonium. But the plutonium produced in Thorium Power’s reactor will be “denatured,” or of an extremely low quality, rendering it unsuitable for use as in atomic weapons.
It turns out that the fission of thorium produces particles that contaminate the otherwise reactor-grade plutonium produced from the fission of uranium. During the thorium fuel cycle, thorium-232 captures a neutron and eventually fissions into uranium-232. The presence of uranium-232 in the spent fuel would lower the quality of any plutonium in the core, making it much less desirable for use in atomic weapons. For this reason, Thorium Power has been busy researching, refining, and patenting thorium-based reactor core designs that can be used in most conventional pressurized water reactors in operation today.
Thorium Power owns five issued patents relating to thorium “seed-blanket” reactors. For example, one of its patents, U.S. Patent No. 6,026,136, issued on February 15, 2000, titled “Seed-blanket reactors,” provides details relating to the layout of the reactor core. Figure 2 from this patent shows a cross-sectional diagram the overall reactor core. As shown in the diagram, the core consists of an inner seed region (18) surrounded by a blanket region (20). The seed region contains seed fuel rods (22) made of a uranium-zirconium alloy, which includes both uranium-238 as well as enriched uranium-235. The patent explains that the seed fuel rods should contain approximately 20% uranium-235 and 80% uranium-238. Figure 2 also shows that several fuel cells in the seed region have no fuel and contain only reactor coolant.
The blanket region (20) contains blanket fuel rods (26) made from a thorium-uranium oxide mixture. The patent explains that the blanket fuel rods should have approximately 20% enriched uranium-235. Enriched uranium is present in the blanket region in order to help sustain a fission reaction during the initial operating phase of the reactor when the thorium is unable to sustain a fission reaction by itself. After the reactor has operated for some time, enough fission products have been produced in the core that, in conjunction with the thorium, a self-sustaining fission process is possible.
Thorium Power’s new patent application, U.S. Patent Publication No. 20090252278, describes a similar reactor core design. Similar to all of Thorium Power’s designs, the reactor core described in this application can be used in most pressurized water reactors in operation today. In fact, the application explicitly mentions that the design can be used in the Russian VVER-1000 reactor plant.
As can be seen in Figure 1 from the application, the reactor core is made up of individual hexagonal cells. The cells that line the outer boundary make up the blanket area of the reactor core. These reflector assemblies (4) are made of thorium oxide, or plutonium. Alternatively, they can include a mixture of water and metal (preferably the same type of metal used for the reactor core). Unlike the seed area in the patent discussed earlier, the seed area in the new application does not contain water tubes.
Figure 5 shows a cross-sectional view of a fuel assembly from the seed region of the reactor core. As can be seen in this diagram, a displacer (17) made of zirconium runs through the center of the fuel rod. The kernel (14) portion of the fuel rod is made of enriched uranium, or reactor-grade plutonium. A cladding (16) made of a zirconium alloy surrounds the kernel (14), and acts as a containment barrier separating the nuclear fuel from the reactor coolant flowing on the outside of the cladding (16).
One of the key characteristics of Thorium Power’s reactor design is that the three-lobed fuel assembly spirals upwards, much like a spiraling staircase. This increases the surface area of the fuel element in contact with the coolant, resulting in more efficient heat removal from the three-lobed fuel assembly. In addition, the spiral design promotes natural circulation in the event the coolant pumps stop the flow of coolant to the reactor core.
Should the pumps stop, the coolant entering the bottom of the reactor would heat up resulting in decreased density. Since hot water is less dense that cooler water, the hot water will begin to rise by traveling up the spiraling fuel assembly.
Thus, the spiral design of the fuel assembly promotes forced convection of coolant in the event of a loss of all coolant pumps.
Thorium Power’s design shows promise in that the reactor uses less enriched uranium in the core than conventional reactors, and replaces it with thorium. Since there is less uranium in the core initially, then less plutonium will be produced as a result of fission. In addition, Thorium Power’s design denatures any plutonium in the core, making it unfit for use in atomic weapons.
This denaturing is caused by the creation of uranium-232, which is extremely difficult to separate from the remaining spent fuel. In fact, the decay of uranium-232 produces byproducts that emit extremely high energy gamma rays, making any reprocessing of the spent fuel a challenge.
Although the denatured plutonium is not suitable for atomic weapons, it does contain extremely toxic substances which would be suitable for use in a “dirty-bomb.” The greatest danger posed by a dirty-bomb is not the explosion itself, but instead comes from the spread of radioactive material emitting radiation over a large area. Thus, even though Thorium Power’s design does not produce reactor-grade plutonium suitable for atomic weapons, the toxicity of the spent fuel indicates that this design is far from being proliferation-resistant.
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About Duncan Williams
Duncan 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 |
