Energy Multiplier Module

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.

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. 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.

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. 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.

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 versus metal 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.

Criticisms

EM2 is a helium-cooled fast neutron spectrum reactor, often referred to as a Gas-Cooled Fast Reactor (GCFR), designed for high nuclear fuel utilization within a relatively compact size, less than half the size of current reactors, as a result it can be factory fabricated, making it quicker and far less expensive to build. However in reactors without other heat absorbing materials in the reactor core to absorb heat and thus to confer thermal inertia during a severe accident such as a loss of coolant accident, this type of reactor would undergo a very rapid meltdown during severe accidents, however the EM2 is apparently designed to overcome this by using a direct, passive cooling mechanism to ensure that heat is always removed in such events. In addition, EM2 introduces additional technology development and practical engineering challenges that are inherent in the high degree of innovation that EM2 brings to the table. This includes innovating a fuel form that although similar to the tried and true TRISO coating, in that it is similarly coated with Silicon Carbide, the cladding is also a vented design that is to remove radioactive fission products while the reactor is operating to achieve its long core life. This might present a challenge to get this design through the U.S. Nuclear Regulatory Commission. General Atomics claims the reactor core can last up to 30 years without requiring refueling -- a safety feature that seals the core for decades, compared to current reactors that require opening the core for refueling approximately every 18 months. This claim needs to be validated by testing in the right test environments to prove a new nuclear fuel can last this long.

EM2 - An Honest Assessment

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 would be challenging, especially from a nuclear regulatory licensing perspective.

Furthermore, this type of fuel cycle can, like all present Light Water Reactors, potentially represent a significant risk for proliferation of nuclear fissile material. If the country operating the EM2, fuels the EM2 blanket with large quantities of depleted uranium, it would convert to weapons-grade plutonium if the country operated the reactor in a low burn-up schedule, as opposed to the intended 30 year, high burn-up cycle. 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 and fueled with U-238 will produce about 750 kg of super-grade plutonium (> 95% Pu-239) within about 5 years.

General Atomics was prior to the EM-2, 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 safely 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. The senior management at General Atomics abandoned the MHR in favor of EM2. Some proponents of the EM2 have claimed the EM2 has the same inherent safety characteristic of the MHR as they both have a Silicon Carbide cladding material covering the fissionable fuel, however they diverge in similarities when it comes to thermal inertia as the EM2 core would not contain any moderating graphite, as by definition, it is a fast neutron reactor design.

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 and it's all important fuel cladding material, 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:

http://www.jaea.go.jp/04/o-arai/nhc/index.html

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:

http://www.inet.tsinghua.edu.cn/htr2014/

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

External links

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