On February 9th, 2012, the U.S. Nuclear Regulatory Commission (NRC) approved the first nuclear power generation plant construction plans in the more than three decades since the scare at Three Mile Island in 1979. The new reactors, which will be located in Waynesboro, GA, and operated by Atlanta-based Southern Co. (NYSE:SO), has since suffered delays and increases in construction costs but two new reactors at the Vogtle Electric Generating Plant should be producing nuclear energy by 2020.

There were six combined license applications for new nuclear power plant construction projects under review by the NRC as of August 11th, 2015; combined licenses give a company the right to construct a nuclear power plant and then operate the facility for a period of 40 years, a period that can be increased in length by filing a renewal application. Three have already been issued: the one for the Vogtle plant and then others for new reactors at the Fermi plant in Newport City, MI, as well as two new reactors at the Virgil C. Summer Nuclear Station in Jenkinsville, SC. Proposed nuclear power plant construction sites run through a wide swathe of the eastern half of America starting from northeastern Pennsylvania and New Jersey and traveling down through the Dallas-Fort Worth region of central Texas.

Over the past few weeks, we’ve been taking an in-depth look at the issue of expanding the use of nuclear power plants for generating clean electricity here in America. This summer, the Japanese government began the process of recommissioning nuclear power reactors to produce electricity just a few short years after a series of explosions at the Fukushima Daiichi power plant caused major evacuations and widespread fears of radiation contamination. This decision points out an important aspect of nuclear power: that despite doomsday fears of a nuclear meltdown, the history of nuclear power has had a remarkably low rate of failure. We’ve also taken a look back at Fukushima Daiichi as well as the disasters at Three Mile Island and Chernobyl to see how the fate of the past can be avoided in the future.

The current state-of-the-art in nuclear power plant reactor design and construction is known as Generation III+. These reactors are essentially safer versions of the Generation III reactors that began operation in 1996 starting with the Kashiwazaki-Kariwa commercial plant in Japan. Generation III+ reactors must operate within very strict safety guidelines. The power plant’s structure must be durable enough to withstand a plane crash without releasing radiation. Power plants must operate for periods of 60 years. The grace period after reactor shutdown, during which time no human intervention is required, must be 72 hours. Further, the risk of core melt accidents must be low enough that a risk assessment analysis returns a calculated core damage frequency (CDF) of 1×10-4 per year.

Many Generation III reactors are configured as light water reactors, which use a source of normal water to serve as both the coolant and the neutron moderator for the reactor. Five of the applications submitted to the NRC for new nuclear power plant construction in America would have utilized the Generation III light water reactor design known as Advance Passive 1000 (AP1000), a power plant configuration sold by the Westinghouse Electric Company. This reactor has a 157-fuel-assembly core designed for a nominal net electrical output of 1,110 megawatt electric (MWe). According to information provided by Westinghouse, the plant has a CDF rating which is 1/20th the maximum CDF currently permissible by the NRC, meeting our country’s current minimum requirement for nuclear power plant safety many times over. In the event of a major failure, such as a coolant pipe break, the AP1000 incorporates passive safety-related systems which utilize natural physical forces, including laws of gravity, compressed gases and natural circulation, to prevent overheating of the core and the containment structure without any operator involvement or the use of active components that could malfunction. The NRC concluded its final review of the AP1000 designs in August 2011.

Another Generation III light water nuclear reactor design which has been proposed for multiple nuclear power plant projects is the Economic Simplified Boiling Water Reactor (ESBWR) developed by GE-Hitachi Nuclear Energy. This design, certified by the NRC in October 2014, offers a 1,520 MWe electric output and GE-Hitachi’s website says that it has the lowest CDF of any current Generation III or III+ reactor at 1.7×10-8 per year. The use of one ESBWR power plant to generate electricity could prevent the emission of about 7.5 million metric tons of carbon dioxide annually that would be created to generate the same amount of electricity from conventional means. The ESBWR design is a simplified version of previous boiling water reactors which utilizes 25 percent active components such as pumps, motors and valves. Passive safety systems incorporated into ESBWR design include an isolation condenser system that transfers decay heat into the atmosphere, a passive containment cooling system which uses a series of six heat exchangers to condense steam into water in the event that a pipe breaks, as well as a gravity driven cooling system that passively injects cooling water into the reactor if there’s a loss of coolant.

Currently undergoing the design certification process with the NRC is the U.S. Advanced Pressure Water Reactor (US-APWR) developed by Mitsubishi Nuclear Energy Systems. This design configuration, an adaptation of Mitsubishi’s 24 pressure water reactors which it has built in Japan, has an industry-leading thermal efficiency of 39 percent and would operate in the 1,700 megawatt class. Emergency core cooling design elements include a high-performance accumulator system and fast-start gas turbine generators that provide immediate backup power to safety and cooling systems in the event of an electrical failure. The electrical output of US-APWR is increased compared to previous Mitsubishi designs while the number of reactor core fuel assemblies has been decreased, enabling more power generation from less fuel. One application for a US-APWR has been submitted for the Comanche Peak facility in central Texas.

Other than ESBWR and AP1000, the only nuclear power reactor design type which has been chosen for multiple combined license applications in the U.S. is the EPR, formerly known the Evolutionary Power Reactor, pioneered by Areva (EPA:AREVA). The EPR is a 1,650 MWe reactor which offers electricity production at costs of up to 10 percent compared to other light water reactors. Safety measures include an isolation compartment for molten core and the plant facility’s design, which is resistant to both aircraft crashes and seismic events. This reactor is designed to operate for a service life of up to 60 years. Innovative aspects of the EPR include steam generators designed to increase the reactor’s thermal efficiency as well as a neutron reflector surrounding the core that prevents neutron leakage and increases the reactor pressure vessel’s lifespan by limiting irradiation.

Light water reactors are the only type of Generation III or III+ power plant for which American combined license applications have been submitted, but there are other Gen III reactors being constructed in the world. Heavy water reactors use heavy water, or water in which a substantial amount of hydrogen has been replaced by deuterium, and they have been pursued for use in Canada, India and China. However, heavy water reactors often bring international monitoring concerns because these types of plants are capable of producing weapon-grade plutonium. In China, there’s also one construction project underway for a high-temperature gas-cooled reactor (HTGR), a nuclear reactor design which uses no water at all for coolant but regulates temperature with the use of a graphite core structure which takes many days to reach maximum temperatures even if there are no auxiliary cooling systems available. Fast neutron, or fast breeder, reactors have been operating for decades but have also been upgraded to Generation III design standards. These plants can also produce the plutonium necessary for nuclear weapons but it can also utilize nuclear power plant byproducts typically seen as waste that takes millions of years before returning to safe radiation levels.

The international nuclear community is currently underway on the next evolution in nuclear power plants, which will be known as Generation IV. Development of the new standards is being pursued by the Generation IV International Forum, which includes 13 member states including the U.S. Nuclear reactors being developed for this generation of nuclear power include a few varieties of fast reactors, such as lead-cooled fast reactors or sodium-cooled fast reactors, as well as supercritical water-cooled reactors and very high-temperature gas reactors. Molten-salt reactors are another type of Gen IV reactor which is currently the development focus of a company spun off from research at the Massachusetts Institute of Technology. These reactors, which will represent significant upgrades in economics, safety, reliability, sustainability and proliferation-resistance, are expected to begin deployment sometime during the 2030s.