Energy Multiplier Module
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The Energy Multiplier Module (EM2 or EM squared) is a nuclear fission power reactor under development by General Atomics. It is a modified version of the Gas Turbine Modular Helium Reactor (GT-MHR) and is capable of converting spent nuclear fuel into electricity and industrial process heat, without separative or conventional nuclear reprocessing.
The EM2 is an advanced modular reactor expected to produce 265 MWe (500 MWth) of power at 850 °C (1,600 °F) and be fully enclosed in an underground containment structure for 30 years without requiring refueling. The EM2 differs from current reactors because it doesn't use water coolant but is instead a gas-cooled fast reactor, which uses helium as a coolant for an additional level of safety. The reactor uses a composite of silicon carbide as cladding material and beryllium oxide as neutron reflector material, another level of safety because the ceramics can handle higher temperatures. The reactor unit is coupled to a high-efficiency direct-drive helium gas turbine which in turn drives a generator for the production of electricity. Use of a gas turbine allows a heat conversion efficiency much greater than conventional steam turbines currently in use.
The nuclear core design is based upon a new conversion technique in which an initial “starter” section of the core provides the neutrons required to convert used nuclear fuel, thorium or depleted uranium (DU) into burnable fissile fuel. First generation EM2 units use uranium starters (approximately 15 percent U235) to initiate the conversion process. The starter U235 is consumed as the used nuclear fuel/DU or used nuclear fuel/thorium is converted to fissile fuel. The core life expectancy is approximately 30 years (using used nuclear fuel and DU) without refueling.
Substantial amounts of valuable fissile material remain in the EM2 core at the end of life. This material is reused as the starter for a second generation of EM2s, without conventional reprocessing. There is no separation of individual heavy metals required and no enriched uranium needed. Only unusable fission products would be removed and stored.
All EM2 heavy metal discharges could be recycled into new EM2 units, effectively closing the nuclear fuel cycle, which minimizes nuclear proliferation risks and the need for long-term repositories to secure nuclear materials.
Economics and workforce capacity
The expected cost advantages of EM2 lie in its simplified power conversion system, which operates at high temperatures yielding approximately 50 percent greater efficiency and a corresponding one-third reduction in materials requirements than that of current nuclear reactors.
Each module can be manufactured in either U.S. domestic or foreign facilities using replacement parts manufacturing and supply chain management with large components shipped by commercial truck or rail to a site for final assembly, where it will be fully enclosed in an underground containment structure.
The EM2 utilizes used nuclear fuel, also referred to as “spent fuel” from current reactors, which are light water reactors. It can tap an estimated 97% of unused fuel that current reactors leave behind as waste.
Spent fuel rods from conventional nuclear reactors are put into storage and considered to be nuclear waste, by the nuclear industry and the general public. Nuclear waste retains more than 99% of its original energy; the current U.S. inventory is equivalent to nine trillion barrels of oil - four times more than the known reserves. EM2 uses this nuclear waste to produce energy.
By using spent nuclear waste and depleted uranium stockpiles as its fuel source, a large-scale deployment of the EM2 is expected to reduce the long-term need for uranium enrichment and eliminate conventional nuclear reprocessing.
Conventional light water reactors require refueling every 18 months. EM2’s 30-year fuel cycle minimizes the need for fueling handling and can reduce the proliferation concerns associated with refueling.
Energy safety and security
EM2 utilizes passively safety systems designed to safely shutdown using only gravity and natural convection in emergency conditions. Control rods and drums are automatically inserted during a loss of power incident via gravity. Natural convection flow is used to cool the core during whole site loss of power incidents. No external water supply is necessary for emergency cooling. The use of silicon carbide as a safety-enhanced fuel cladding in the core ensures no hydrogen production during accident scenarios and allows an extended period of response when compared to the use of Zircaloy metal cladding in current reactors, which are reactive and not as heat resistant as ceramics in EM2.
Underground siting in a silo improves safety and security of the plant to terrorism and other threats.
The EM2 is similar to the helium-cooled fast neutron spectrum reactor, referred to as the Gas-Cooled Fast Reactor (GCFR). This concept has some attractive characteristics, but some have expressed concerns regarding design safety. To achieve its objectives of high nuclear fuel utilization with relatively compact size, the reactor must operate with very high power density and with very little material in the reactor core that can absorb heat during a severe accident. If not properly designed, this type of reactor could undergo a very rapid meltdown during a Loss-of-coolant accident (LOCA). Some argue that this is a substantially less safe alternative to modern commercial water-cooled reactors.
The EM2 introduces additional safety and practical engineering challenges beyond the conventional GCFR. The EM2 fuel is an unproven concept and is expected to vent (release) its radioactive fission products while the reactor is operating, which essentially eliminates the fuel as a barrier to radioactivity release and defeats the concept of defense-in-depth to radioactivity release required by the U.S. Nuclear Regulatory Commission. The EM2 proponents also claim the reactor core can last up to 30 years without requiring refueling. Proving a new nuclear fuel can last this long without significant levels of failure is practically impossible, especially from a nuclear regulatory licensing perspective. Furthermore, this type of fuel cycle can represent a significant risk for proliferation of nuclear fissile material. The EM2 core is fueled with large quantities of depleted uranium which converts to weapons-grade plutonium long before the end of its claimed 30 year fuel cycle. In fact, according to a study performed by Princeton University (Glaser et al., Nuclear Technology, Vol. 184, October 2012, pp 121-129), an EM2-type reactor sized to produce 200 MWe will produce about 750 kg of super-grade plutonium (> 95% of the fissile isotope Pu-239) within about 5 years, which is enough plutonium for about 70 nuclear weapons. Conventional light water reactors (LWRs) (and the MHR discussed below) are much more resistant to proliferation, with the plutonium in spent fuel containing about 60% Pu-239 after about the normal 3 to 4 years of irradiation. In terms of safety and proliferation risks, the EM2 is an unacceptable nuclear reactor concept, especially for commercial deployment in a post-Fukushima world and increased concerns over nuclear proliferation.
General Atomics was once the industry champion of the world's safest reactor concept, a modular, helium-cooled thermal neutron spectrum reactor, sometimes referred to as a Modular Helium Reactor (MHR). In contrast to the EM2, this reactor concept has a large quantity of material in the core that absorbs heat and prevents the reactor fuel from reaching meltdown temperatures, even if all of the coolant is permanently lost. Unfortunately, the senior management at General Atomics abandoned the MHR in favor of EM2, and has stuck with this strategy even in the aftermath of the Fukushima accident. Some proponents of the EM2 have falsely claimed the EM2 has the same inherent safety characteristics as the MHR as part of their dishonest marketing strategies.
Japan has the high temperature engineering test reactor (HTTR), which is an operational, engineering-scale prototype of the MHR. It has been used to demonstrate the inherent safety characteristics of the MHR, including a series of safety demonstration tests just a few months prior to the Fukushima accident. Perhaps the events that occurred in Japan can lay the foundation for developing, demonstrating, and commercializing a next generation of nuclear power with inherent safety. International collaboration among the U.S., Japan, and other nations on the MHR would provide a relatively quick path for achieving this goal. More information on the HTTR is available at:
Other countries, including S. Korea and China are actively pursuing MHR technology. China has initiated construction on a commercial-scale MHR plant and will host the next international conference on MHR technology in 2014:
- American Association for the Advancement of Science
- Nuclear Energy Institute
- Nuclear power
- Nuclear safety in the United States
- Economics of new nuclear power plants
- United States Department of Energy
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- Official website
- Presentation about the EM2 reactor at the Department of Nuclear Engineering, University of California-Berkeley, ustream video Previous presentation