Mitsubishi’s US-APWR
- By Duncan Williams -
In order to meet the growing demand for electricity in the United States, many companies are researching new designs of reactors. But before a new reactor design can be built, an application must first be submitted to a government agency known as the Nuclear Regulatory Commission (NRC). The NRC regulates, among other things, the construction and operation of nuclear reactors in the United States, and is authorized to issue different types of licenses relating to new nuclear reactors.
For example, the NRC can issue what is known as a design certification for reactor designs that don’t drastically depart from previously approved designs. Once filed, the NRC reviews the application in order to verify that the technical assumptions are correct. On December 31, 2007, Mitsubishi Heavy Industries, Ltd. (MHI), submitted an application for a design certification for a reactor called the Advanced Pressurized-Water Reactor (US-APWR). This large reactor design is capable of producing 4,451 Megawatts of thermal energy. On August 29, 2008, MHI submitted its first revision to this design, which the NRC is currently reviewing.
However, a design certification does not authorize a reactor to be built and operated. In order to receive this authorization, an applicant must submit a separate application known as a combined license (COL) application. In a COL application, the NRC reviews the applicant’s qualifications, design safety, environmental impacts, as well as safety at the proposed reactor site. On September 19, 2008, Luminant Generation Company LLC submitted an application for a combined license to build and operate the US-APWR at Luminant’s Comanche Peak site near Glen Rose in Somervell County, Texas.
The drawing shown here was submitted to the NRC by MHI on August 29, 2008, in support of its design certification for the US-APWR. The drawing shows the overall design of the reactor vessel including fuel rods, control rods, as well as flow throughout the reactor. Each of these topics will be discussed in further detail below.
The hollow fuel rods in the US-APWR contain low-enriched uranium dioxide which is formed into pellets and stacked against one another inside the fuel rod. The fuel rod itself is made from an alloy of zirconium. After the fuel rods are loaded with uranium dioxide pellets, they are then filled with helium gas and sealed on both ends.
It takes 264 fuel rods to form one box-like fuel assembly, in a 17 x 17 square array. The fuel rods are held in place by a series of grid spacers located periodically throughout the length of the fuel assembly. The grid spacers in the middle of the assembly are made of Zircolay-4, while the grid spacers at the top and bottom of the assembly are made of Inconel 718.
The US-APWR design is similar to previous proven reactor designs, but includes various improvements relating to the flow of water through the reactor vessel. U.S. Patent No. 7,245,689, issued on July 17, 2007, describes a flow stabilizer located inside the reactor vessel which reduces the turbulence caused by the large volume of water flowing through the vessel during normal operations.
The arrows in the diagram from the patent show the flow of water through the reactor vessel (2). Reactor coolant pumps force water into the reactor vessel (2) via the inlet nozzle (3). The water then flows down between the reactor vessel (2) and the reactor core tank (4) via a downcomer portion (5). Water continues to flow down to the lower plenum (8) area and is directed upwards through the reactor core. The water then flows up through the reactor core (13) and absorbs the heat generated by the fuel assembly (15).
The water leaves the reactor vessel (2) via the outlet nozzle (17) and is directed towards a steam generator (not shown). The steam generator contains cooler water which is isolated from the heated water that just left the reactor vessel.
Heat is transferred to the cooler water causing it to boil and produce steam, which is used to spin turbine-generators and produce electricity. The now cooler water is then pumped back to the reactor vessel where the entire process begins again.
Due to the large volume of water flowing through the reactor vessel, a significant amount of turbulence occurs at the lower plenum (8) area at the bottom of the reactor vessel. Swirling vortices can form and disrupt the flow of water resulting in improper cooling of the fuel assembly. To address this problem, MHI has developed a special design for the lower connecting plate (30) located near the lower plenum (8).
This diagram from the patent is a top view looking down at the lower connecting plate (30). The water flowing to the bottom of the reactor vessel flows between the outer surface of the lower connecting plate (30) and the inner surface (9) of the lower plenum. The outer edge of the connecting plate (30) alternates between a curved shape (32) and a cut-off portion (33) every 45 degrees. The cut-off portion creates a larger area for the water to flow and thus more evenly distributes the water, resulting in reduced turbulence. The lower connecting plate (30) also has small holes (35) to allow for piping containing instrumentation (e.g., neutron detectors, temperature detectors, flow detectors). Larger holes (34) also allow for water to be distributed more evenly, further reducing turbulent flow.
Turbulent water flow is also a concern in the area surrounding the fuel assembly in the US-APWR. U.S. Patent Application No. 20090285347, published on November 19, 2009, describes a device which minimizes the wear on the control rods in the nuclear reactor caused by uneven turbulent flow. The purpose of control rods in a nuclear reactor is to absorb neutrons and intentionally disrupt the fission chain reaction. In order to sustain a fission chain reaction, neutrons emitted from the uranium fuel as a result of fission must be absorbed by other uranium atoms. By inserting control rods into a reactor core, the neutrons are absorbed by the control rods instead of a uranium atom – interrupting the fission chain reaction.
A diagram from the patent application shows the control rod assembly from the US-APWR. When the control rods are partially inserted in the reactor core, the reactor power output immediately goes down because not enough neutrons are available to sustain a fission reaction. Fully inserting the control rods results in shutting down the reactor.
As can be seen in the diagram, the control rods (4) are inserted into control rod guide tubes (3) through openings in an adapter plate (6) located at the top of the reactor vessel. The control rod guide tubes (3) are located in close proximity to the fuel rods (2).
Vibrations caused by the turbulent flow of water over the control rod guide tubes (3) and fuel rods (2) causes them to brush against one another, which has been known to cause unwanted abrasions.
To minimize these abrasions, MHI has designed an adapter plate (6) as seen in the diagram from the patent application. The patent application explains that this arrangement forces the water to flow towards the center of the fuel assembly, thereby stabilizing the control rod. This creates a force which counteracts any harmful transverse or vibrational motion of the control rod guide tubes (3).
U.S. Patent No. 6,636,580, issued on October 21, 2003, describes the basic design of each individual control rod.
As can be seen in the diagram from the patent, the control rod is made of a stainless steel cladding tube (11), which is hermetically closed at both ends by a top end plug (12) and a bottom end plug (13). The neutron absorber material (14) is pushed downward on the bottom end plug (13) by way of a spring (15).
Design documents submitted to the NRC indicate that the neutron absorber material is made from silver, indium and cadmium.
The ordinary diameter portion (14b) of the neutron absorber material has a larger diameter than the reduced diameter portion (14a) located at the tip of the control rod. This allows for a sleeve (16) made of stainless steel to be inserted between the tube (11) and the reduced diameter neutron absorbing material (14a).
The stainless steel sleeve (16) is less susceptible to thermal expansion and contraction due to sudden changes in temperature, thus reducing the stresses on the control rod tip when it is inserted into a reactor core with high operating temperatures.
This reactor design is not a huge departure from reactor designs previously submitted and approved by the NRC. Not withstanding the inevitable objections the NRC always issues regarding new reactor designs, it is likely that this design will eventually be approved by the NRC. The NRC has indicated that a final safety evaluation report will be issued relating to its design certification in September of 2011.
The final outcome of the design certification will greatly affect Luminant’s combined license application for the Comanche Peak Power Plant in Texas.
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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
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