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Miniature Nuclear Reactors

Nuclear Power Industry News

Senator Mark Udall (D-Co) last week introduced a bill in the Senate titled “The Nuclear Energy Research Initiative Improvement Act of 2009,” which would authorized the Department of Energy to fund research in the field of compact, or miniature, nuclear reactors as an energy source.  However, many companies have already begun researching and designing smaller more compact nuclear reactors that can provide electricity for years to rural outposts, or third world communities.  Here is just a sample of some of the major players already in the marketplace.

French Atomic Energy Commission
On December 26, 2006, the U.S. Patent Office issued Patent No. 7,154,982, titled “Compact Pressurized Water Nuclear Reactor,” to the French Atomic Energy Commission (Commissariat a l’Energie Atomique).

Unlike most conventional nuclear reactor plants, the reactor described in this patent positions most of the reactor plant components inside the reactor vessel itself.  In conventional reactor plants, the reactor vessel houses only the nuclear fuel that drives the fission process.  Pumps, connected to the reactor vessel via metal piping, circulate water into the reactor vessel in order to absorb the heat produced from the nuclear fission process.  After the water is heated, it is pumped out of the reactor core travels through piping to a heat exchanger which removes the heat from the water.  A separate tank, known as a pressurizer, is connected to the reactor vessel and acts as a surge tank for the water in the vessel.  All of these components are external to the reactor vessel in a traditional reactor.

But as can be seen in the diagram taken from this patent, most of these components are located inside the reactor vessel.  The pumps (28), heat exchangers (42), and pressurizer (30), are all located inside the reactor vessel (10).  The pumps (28) circulate water down the sides of the reactor vessel across the heat exchangers (42) which cools the water.  The water then enters the bottom of the vessel (10) and flows upwards through the reactor core (14) which contains the heated nuclear fuel.  The fission reaction in the reactor core (14) is controlled by hydraulic control rods (40) which can be inserted into the nuclear fuel to absorb neutrons and effectively halt the fission reaction.  As water is heated, it rises up into the u-tubes (26), which heats water in the secondary closed space (24).  This water turns to steam and can then be used to rotate turbines to generate electricity.

Although the exact dimensions of the reactor are not disclosed, the patent indicates that the design is applicable to small and medium power nuclear reactors with an output of 600 Megawatts. This makes it one of the largest “compact” reactors in the marketplace.

Toshiba
Toshiba is currently designing a small 10 Megawatt nuclear reactor known as the “Super-Safe, Small and Simple” (4S) design.  Intended to be buried underground, this reactor can last 30 years before needing a fuel replacement.  Although the reactor design Toshiba submitted for approval by the Nuclear Regulatory Commission (NRC) shows the reactor uses enriched uranium-235 as a fuel, the patent owned by Toshiba covering this product indicates that plutonium can also be used.

The reactor described in U.S. Patent No. 7,139,352, titled “Reactivity Control Rod for Core,” is similar to the French design, since all the components of the reactor coolant system are located inside the reactor vessel.  As can be seen in a figure from this patent, the reactor core (2) contains the reactor fuel.  Electromagnetic pumps (13) circulate liquid sodium to the bottom of the core, then up through the reactor core (2) in order to remove heat created during the fission process.  The heated sodium rises up the center the reactor and enters the steam generator (14) which cools the liquid sodium causing it to circulate down to the bottom of the reactor core again.  The hot sodium flowing through the steam generator (14) is used to create steam which can then be used to rotate turbines to generate electricity.

Hyperion Power Generation
Los Alamos National Laboratory has granted an exclusive license to Hyperion Power Generation to license, manufacture, market and distribute a 1.5 meter nuclear reactor about the size of a hot tub.  Hyperion claims that just one of these reactors (25 Megawatts) has enough energy to power 20,000 average American homes for at least 5 years.

The only known patent application associated with this technology is U.S. Patent Publication No. 20080069289, published on March 20, 2008, and titled, “Self-Regulating Nuclear Power Module.”  As can be seen in a figure of this reactor from the patent application, the reactor core vessel (28) is filled with uranium hydride (UH3) fuel particles containing enriched uranium-235.  The entire structure seen in this diagram would itself be enclosed in a hydrogen gas atmosphere.

The properties of the uranium hydride itself serves to regulate the fission process in the reactor core.  The heat generated during the fission process causes the temperature to rise, which causes the hydrogen in the uranium hydride particle to detach from the fuel particle and to form hydrogen gas.  Since there are less hydrogen particles in the reactor core, the reactor core is less dense, and thus more neutrons leave the reactor vessel and are not used in the fission process.  This causes the reactor power level to go down, which reduces the temperature.  If the power level gets low enough, the temperature drops to a level which causes the hydrogen to once again bond onto the uranium hydride particles, increasing the density of the core.  Since the core is more dense, then neutrons are more likely to be used in the fission process resulting in reactor power going up.

In order to maintain the proper equilibrium of hydrogen in the enclosed atmosphere, the vessel (28) is surrounded by several layers of trays (22) containing depleted uranium or thorium.  Theses substances absorb and expel hydrogen much the same as the uranium hydride particles,  allowing the hydrogen to quickly reach equilibrium instead of constantly oscillating between too much and too little hydrogen.  Without the trays (22), the reactor would constantly oscillate attempting to reach an equilibrium.

Babcock & Wilcox
Babcock & Wilcox has formed a new entity, B & W Modular Nuclear Energy, LLC, to lead the development, licensing and delivery of its new 125 Megawatt “mPower” brand of nuclear reactors.  Although no published patent documents are yet available, the NRC’s website indicates that this reactor core uses enriched uranium-235 as fuel and will last approximately 5 years before needing replacement.  As seen in the diagram taken from the NRC’s website, this design  is similar to the French design which locates most of the reactor plant components inside the reactor vessel itself alongside the nuclear fuel.

NuScale Power, Inc.
NuScale is developing a 40 Megawatt reactor which can last 2 years before needing replacement.  A reactor utilizing this design has already been built and tested in cooperation with the laboratories at Oregon State University.  Similar to the other designs, NuScale places several reactor plant components inside the reactor vessel.  Unlike the other designs, the coolant inside of NuScale’s reactor circulates through natural circulation and does not require any pumps.  This design could give NuScale the advantage since less moving parts inside the reactor vessel means there is less of a chance that repairs will be needed.  However, this design also includes the traditional enriched uranium-235 nuclear fuel requiring the use of control rods to absorb neutrons in order to control the fission process.  Thus, NuScale’s design seems to be less automated than some of the other designs.  NuScale has applied for several patent applications relating to this design, but have not yet been issued from the U.S. Patent Office.

"Originally published on 11/4/2009 at www.nuclearstreet.com."