- By Duncan Williams -
On December 14, 2009, the Senate Energy and Natural Resources Committee held a hearing to discuss the Nuclear Energy Research Initiative Improvement Act of 2009, introduced by Colorado Senator Mark Udall. Although still a bill, the legislation would authorize the United States Department of Energy to conduct research into, among other things, small-scale reactors that produce less than 300 Megawatts. Compact nuclear reactors are taking center stage in the global nuclear renaissance partly because they cost much less to build than the larger and more common conventional reactors in use today. By manufacturing the small reactors at one location, as opposed to shipping the components the reactor site to be assembled, allows manufacturers to produce large volumes of reactor components leading to assembly-line efficiencies never before seen in the nuclear power industry.
If the bill passes, the DOE would receive funding for research which will likely pour into the national laboratories across the nation in order to spur further research into the design of small reactors. However, several companies have already been developing small-scale reactors and have even submitted their design for certification by the Nuclear Regulatory Commission (NRC). One company, NuScale Power, is already positioning itself to become a major player in the miniature reactor marketplace.
The NuScale design was originally developed by Oregon State University and the Idaho National Engineering and Environmental Laboratory through a DOE funded program between 2000 and 2003. The original design was called the Multi-Application Small Light Water Reactor (MASLWR) and was based on tests using an experimental reactor built at Oregon State University. After making significant improvements on this initial design, the Oregon State University filed several patent applications in 2007. NuScale expects to submit the design certification for its 150 Megawatt reactor to the NRC in 2010.
The NuScale reactor design has various features which make it more compact. First, the nuclear fuel is located inside a reactor vessel, which is itself located inside another vessel called the containment vessel. This dual vessel design ensures the containment of the nuclear fuel in case of a catastrophic accident to the nuclear fuel.
Second, the steam generators are located inside the reactor vessel. In contrast, existing reactor plants locate the steam generators some distance from the nuclear fuel, which increases the overall size of the plant as well as requiring more materials (such as piping and valves) leading to and from the steam generators.
Third, instead of motor-operated pumps, the NuScale reactor system uses natural circulation to circulate the water throughout the reactor.
Recently published patent applications shed some light on how the NuScale reactor operates. For example, U.S. Patent Application No. 20090161812, published on June 25, 2009, discusses the basic design and safety features of the NuScale reactor. As can be seen in the diagram from the patent application, a containment vessel (54) is suspended in a pool of water (16) by one or more mounting connections (80).
The pool of water (16) acts as a protective cushion against earthquakes or other vibrational hazards as well as a reservoir of water to flood the nuclear fuel in the event of some catastrophic accident. A reactor vessel (52), made of stainless steel or carbon steel, is mounted inside the containment vessel (54). During normal operations, coolant (100) circulates throughout the reactor vessel (52), and no coolant (100) is located in the containment vessel (54). The patent application indicates that the containment vessel (54) is about 4.3 meters in diameter and 17.7 meters in height, while the reactor vessel (52) is about 2.7 meters in diameter and 13.7 meters tall. Design documents indicate that the height is proportionally more than the width in order to promote the natural circulation of the coolant (100).
During normal reactor operations, coolant (100) at the bottom of the reactor vessel (52) is heated by the fission process in the reactor core (6). The amount of heat produced by the fission process is adjusted by inserting or withdrawing neutron absorbing control rods (not pictured) into the reactor core (6). The control rods absorb neutrons used for fission, thereby reducing the fission process resulting in a lower output of heat. As the fission process heats the coolant (100), it becomes less dense and flows up the center of the reactor vessel (52) through the annulus (23) and the riser (24). As the coolant (100) cools down, the density increases causing the coolant (100) to flow down the outside of the annulus (23) returning once again to the bottom of the reactor vessel (52).
The patent application hints that the NuScale reactor is a boiling water reactor, because it mentions that the natural circulation may be enhanced by a “two phase condition” of the reactor coolant (100). The application suggests that some form of gas can be injected into the core (6) in order to supplement, or create, bubbles that promote natural circulation.
This patent application also describes the passive safety features of the NuScale reactor in the event of overpressurizing the reactor vessel (52). As can be seen in the diagram, if the reactor vessel (52) is overpressurized, a flow limiter (58) opens to allow coolant (100) to exit the reactor vessel (52) and flow into the containment vessel (54).
The hot coolant (100B) is cooled in the containment vessel (54) because the vessel (54) is immersed in a pool of water (16). As the coolant (100) condenses and cools at the bottom of the containment vessel (54), the coolant (100) then flows through a flow valve (57) back into the reactor vessel (52) via natural circulation. The patent application mentions that it takes approximately 2 hours and 38 minutes for 110 inches of coolant (100B) to accumulate in the containment vessel (54) in an overpressurization situation, allowing plenty of time for human intervention to correct the problem. Once the reactor vessel (52) returns to normal operating pressure, the flow limiter (58) shuts and normal operations can continue.
U.S. Patent Application No. 20090129532, published on May 21, 2009, describes how the NuScale reactor can be started up. The patent application indicates that the normal operating temperature of the reactor is 550 – 575 F, and the operating pressure is typically below 1500 psig. Before the fission process can be started, the reactor plant must first be heated up to this operating temperature and pressure.
To this end, a pressurizer system (55), consisting of electrical heaters located at the top of the reactor vessel (65), adds enough heat to initiate boiling of the water there. In order to prevent overheating, a spray valve located at the top of the reactor can be opened allowing cooler water to enter the reactor immediately reducing the temperature and pressure.
Heat can also be added to the reactor vessel (65) by way of a startup system (20), consisting of electric heaters, located just above the reactor core (6). The relatively cool water at the bottom of the reactor vessel is warmed by the startup system (20), causing the water to flow up through the shroud (22) and the riser (24). As the heated water reaches the top of the riser (24), it is cooled by a heat exchanger (26). The heat exchanger (26) consists of piping that is helically wound around the riser (24). Water flowing through the heat exchanger (26) is isolated from the coolant in the reactor vessel (65) and the two liquids never mix together. Instead, heat is transferred to the water in the heat exchanger (26) through the piping itself.
The water contained in the heat exchanger (26) turns to steam and is then directed to turbine-generators which then produces electricity. As the heat exchanger (26) extracts heat from the coolant, the temperature of the coolant lowers causing it to flow once again to the bottom of the reactor vessel. Once the reactor reaches normal operating temperature and pressure, the control rods are withdrawn from the reactor core (6) and the fission process adds enough heat to the system to continue the natural circulation of the coolant.
The NuScale reactor system is not without its disadvantages. In contrast to larger commercial reactor plants which can operate 30-60 years without refueling, the NuScale reactor must be refueled every 24-30 months. However, the compact vessel can simply be shipped to the manufacturing plant to be refueled instead of performing the complicated process at the reactor plant site. Additionally, although the NuScale reactor only produces 150 Megwattts of thermal energy (existing commercial reactor plants produce over 1000 Megawatts), multiple reactor vessels can be utilized to satisfy the required electrical demand of the power plant.
ConclusionThe NuScale reactor design marks a significant improvement in safety over existing commercial reactor plants. Since the NuScale reactor has no moving parts inside the reactor vessel, there is little chance of a mechanical failure occurring. No moving parts also means less periodic maintenance and resulting in more operational time. NuScale Power has attracted some of the top talent in the industry and therefore have the experience needed to meet all the NRC’s design certification goals.
For example, in February of 2009, Tom Marcille joined NuScale Power as its Chief Operating Officer. Mr. Marcille was previously the chief engineer for advanced reactors at Los Alamos National Laboratory where he managed the development of several reactor projects. NuScale’s Chief Technology Officer, Dr. Jose Reyes, currently serves as an IAEA technical expert on passive safety systems, and was previously a thermal hydraulics research engineer in the Reactor Safety Division at the NRC. Thus NuScale’s experience, as well as its contacts with the NRC and the National Laboratories, indicate that NuScale’s design is likely to succeed.
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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.
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As is, unfortunately, the case with many of the "Under the Hood" articles that he writes, Duncan Williams does not know what he's talking about:
"In contrast to larger commercial reactor plants which can operate 30-60 years without refueling, the NuScale reactor must be refueled every 24-30 months."
There is no "larger commercial reactor" in the world that can operate for 30-60 years without refueling. Large commercial reactors refuel on various schedules, usually ranging from 12-24 months. (There are "drawing board" designs for reactors that can operate for decades without refueling, but none of them have been deployed commercially.)
Also, the original MASLWR design was not "based on tests using an experimental reactor built at Oregon State University." It was developed at Oregon State, but the tests were carried out in a non-nuclear test loop, not an "experimental reactor."
Finally, the conclusion regarding improved safety is premature, at best. Since the NuScale reactor does not yet exist, there is no way to assess its actual safety performance in comparison to existing light-water reactors. It may ultimately offer improved safety performance, but right now, any such statement is mere speculation.
This is not meant to be disparaging of the NuScale concept. It may, in fact, be the "reactor of the future." But there's a long way to go before that potential may be realized.