Inside Hitachi's Advanced Boiling Water Reactor
- By Duncan Williams -
A recently issued patent (U.S. Patent No. 7,349,518), assigned to Hitachi, Ltd., indicates that many of the efficiencies found in the Advanced Boiling Water Reactor (ABWR), including its smaller size, is due to its reactor core design. Before we discuss how the ABWR attains its smaller size, we must first understand how the boiling water in the reactor core affects the reactor’s operation.
The ABWR is the next generation in the family of Boiling Water Reactors (BWRs). BWRs are the second most common type of nuclear reactor in use today, while Pressurized Water Reactors (PWRs) remain the most common. The main difference between the two designs is that the BWRs operate at a lower pressure than the PWRs. Even though the BWRs are highly pressurized (1000 – 1100 psia), they operate at only half of the pressure found in PWRs. The lower pressure in the BWRs allows the water circulating through the core to boil resulting in a mixture of water and steam surrounding the reactor core. Since steam does not carry heat away from the reactor core nearly as well as water, ABWRs are always monitoring and attempting to maintain a certain amount of water circulating in the reactor core to prevent overheating.
One of the improvements of the 1356 Megawatt ABWR over other BWRs is its smaller size. By reducing the height of the reactor core, Hitachi claims that the building surrounding the reactor requires less construction material as well as less time to build, thus lowering the overall construction costs. In fact, miniaturizing the reactor core size is a common goal for many of the new reactors being designed today. However, reducing the reactor core height alone poses a unique danger in BWRs as opposed to PWRs. Reducing the height of the reactor core in BWRs, without changing other parameters, reduces the amount of coolant in liquid phase in the reactor core, posing serious overheating problems.
Hitachi’s newly issued patent reveals that the secret to reducing the size of the core is in the new design of its fuel assemblies. The ABWR core contains approximately 720 hexagonal-shaped fuel assemblies. Fuel assemblies are made up of many individual fuel rods which are packed with neutron emitting material that drive the nuclear fission reaction in the core.
Figure 1 in Hitachi’s patent shows a cross section of one of its hexagonal fuel assemblies, which the patent refers to as a channel box (1). The fuel rods (6) at the outer corners of the channel box (1) are loaded with uranium oxide (UO2) at 4% by weight. Every other fuel rod (2) located along the outer perimeter has a slightly higher uranium oxide content (4.9% by weight), but also contains a neutron absorber - gadolinium oxide (Gd2O3) at 4.5% by weight. All other fuel rods in the core contain uranium oxide at 4.9% by weight. The patent goes on to describe many other variations of these percentages, but the pattern of the control rods remains the same for each variation.
The patent suggests that by using this specific arrangement of fuel rods, the height of the reactor core can be reduced by 1.8 meters while maintaining the same safety margin as other currently operating BWRs. The fuel rod arrangement also allows for a smaller 2.9 meter outer radius of the reactor core. Hitachi also indicates that both the fuel rods and channel boxes in the core are thinner than conventional BWRs. The overall result is that the building surrounding the core is reduced by 10 meters in height, thus greatly lowering construction costs.
Other important features of the ABWR are highlighted by the statements Hitachi made to the U.S. Patent Office while applying for the patent. For example, Hitachi distinguishes its reactor core over other patented reactor cores by pointing out that the ABWR contains at least one water channel running through the center of the reactor core. Hitachi even amended its patent claims to include this feature, which indicates that this is an important aspect to the ABWR.
Although the patent claims indicate that fissile plutonium is present in the ABWR core, Hitachi emphasized to the U.S. Patent Office that the ABWR is a burner type of nuclear reactor. A burner type of nuclear reactor uses U235 as its main fuel source, and necessarily produces a small amount of plutonium as a natural result of the U235 fission process. In contrast, a fast breeder reactor uses both U238 and Pu239 as its fuel source, and actually creates more plutonium in the reactor core than it uses during the fission process. Hitachi further emphasized to the U.S. Patent Office that its new design would create more plutonium in the core than previous designs of BWRs. Thus, it appears that another benefit of the ABWR core design is the increased production of plutonium akin to a fast breeder reactor.
Hitachi also emphasized that the fuel elements have a density of 2.8 to 4.5 kg/L at the time of fuel loading. Apparently, this characteristic is necessary in order to achieve the benefits of the a shorter reactor core height while maintaining the same safety margin as in other BWRs. Hitachi explains that maintaining this specific density in the core allows only 40% of the water in the reactor to boil, leaving the remaining 60% of water in the core to cool the reactor.
With this new design, GE-Hitachi is solving several problems facing the ever-shrinking nuclear reactor core designs. Smaller core designs are the way of the future, mainly because they are more palatable to the skeptical general public. For example, smaller nuclear reactors have less of a negative environmental impact in the event of a catastrophic failure, allaying the public’s fears of another Three-Mile Island incident.
Additionally, having several small reactors in a single plant reduces the chances of a power interruption in the event that one of the reactors is unavailable. Having multiple reactors allows the electrical load to be carried by the remaining reactors, so that there is greater continuity to the power grid. Much like the ever-shrinking size of computers, the miniaturization of the nuclear industry is underway and is here to stay.
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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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Your statement that "smaller reactors have less of an environmental impact in the event of a catestrophic failure".....is misleading. This entire article is about Hitachi's patent on a physically smaller reactor core and achieving the same 1356 MW electric output (as opposed to ~3900 MW Thermal reactor power) power rating. The environmental impact of a catestrophic failure is a function of the initial power of the reactor core.....not necessarily the physical size. In order to achieve this power level the same physical quantity of fissile material must be used as in the 'older' version of the ABWR core design. Hitachi has achieved a higher density core design, that may be slightly more efficient, but basically uses the same amount and enrichment level of U-235. Thus no reduction of environmental effects would be seen in a catestrophic failure of this design.