Energy Multiplier Module

The Energy Multiplier Module (EM2 or EM squared) is a nuclear fission power reactor under development by General Atomics.^[1] 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.^[2]

Schematic diagram of a helium-cooled reactor with a gas turbine-generator

Design specifications

The EM2 is small modular reactor expected to produce 265 MW_e (500 MW_th) of power at 850 °C (1,600 °F) and be fully enclosed in an underground containment structure. 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. The reactor uses a composite of silicon carbide as cladding material and beryllium oxide as neutron reflector material. 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.^[3] First generation EM2 units use uranium starters (approximately 15 percent U235) to initiate the conversion process.^[4] 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.^[5]

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.

Nuclear waste

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.^[6] 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.

Non-proliferation

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.^[7]

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.^[8] 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 cladding in current reactors.

Underground siting in a silo improves safety and security of the plant to terrorism and other threats.

The EM2’s high operating temperature can provide process heat for petrochemical fuel products and alternative fuels, such as biofuels and hydrogen.^[9]

Criticisms

Due to the design, EM2 introduces safety and practical engineering challenges beyond conventional reactors. The EM2 fuel is an unproven concept and is expected to vent (release) its radioactive fission products while the reactor is operating, which changes the typical dynamic of the sealed fuel rod as the primary barrier to radioactivity release. This type of vented fuel design in not present in current generation reactors and may present a challenge to get approval for use though the U.S. Nuclear Regulatory Commission. The EM2 proponents also claim the reactor core can last up to 30 years without requiring refueling. This includes use of silicon carbide cladding that is still under development, although it is in some ways similar to the TRISO coating. Proving a new nuclear fuel can last this long without significant levels of failure will need to be successfully demonstrated before it could be licensed and used. Furthermore, this type of fuel cycle can, like all present Light Water Reactors, potentially represent a risk for proliferation of nuclear fissile material. According to a study performed by Princeton University,^[10] an EM2-type reactor sized to produce 200 MWe and fueled with U-238 will produce about 750 kg of super-grade plutonium (> 95% Pu-239) within about 5 years. While this material is then burned as fuel in the typical 30 year high burn up cycle, if the country operating the EM2 used a 5 year low burn-up schedule, significant quantities of weapons grade plutonium could be extracted.

Prior to EM2, General Atomics was promoting a modular helium-cooled thermal neutron spectrum reactor, sometimes referred to as a Gas turbine modular helium reactor (GT-MHR). In contrast to the EM2, this reactor concept cannot be used to burn up the minor actinide fraction of spent nuclear fuel/other reactor's nuclear waste, but has a large quantity of prismatic graphite blocks in the core that would absorb heat, conferring thermal inertia, and thus help slow the rise in temperate in the reactor fuel from reaching meltdown temperature values, even if all of the coolant is permanently lost. In reactors of this type without additional materials in the reactor core to absorb heat during a severe accident such as a loss of coolant accident, this type of reactor would undergo a very rapid meltdown. EM2 designers claim their reactor is designed to overcome this inherent risk by using a direct, passive cooling mechanism that automatically engages in accident events. Additionally, both GT-MHR and EM2 make use of silicon carbide cladding for additional safety margins.

See also

• 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

References

10. Wald, Matthew L.(Sept. 24, 2013). Atomic Goal: 800 Years of Power From Waste". New York Times. Retrieved 10 December 2013.

17. St. John, Alison (May 21, 2012). A better nuclear power plant? KPBS News. Retrieved 12 September 2012.

External links

• Official website
• Presentation about the EM2 reactor at the Department of Nuclear Engineering, University of California-Berkeley, ustream video Previous presentation