By Gail H. Marcus

Only a few years ago, most experts would have written off nuclear power. It seemed too expensive, too unpopular, too risky.

But if anything, nuclear power today is enjoying a kind of renaissance, both in the United States and around the globe. The renewed interest stems from practical concerns about the need for more baseload power, a desire for greater national energy security, and concerns about the long-term impact of fossil fuel emissions. Nuclear power also fits in well with recently articulated national goals for developing a hydrogen economy. And nuclear power plants have demonstrated considerably improved performance of late.

Not only are new reactors being built (though not in the United States), but a number of countries have banded together to develop a true next-generation nuclear system. This international consortium has studied designs for a new generation of reactors, and has selected six for further research. These designs promise better economics, greater safety, improved resistance to proliferation, and a more sustainable fuel cycle than nuclear power has known to date.

These reactors would constitute the fourth generation of reactor design. Nuclear reactor technology has already gone through three complete generations in design. The first, called Generation I, evolved from the early demonstration reactors in the middle of the last century. The 103 commercial reactors currently operating in the United States and most of the commercial reactors operating in the world are Generation II. Most of these reactors use so-called light water technology: They are moderated (the neutrons slowed down to achieve an energy conducive to fissioning the uranium atoms in the fuel) and cooled with ordinary water. Other Generation II reactor technologies use other coolants and moderators; for example, many of the reactors in the United Kingdom are graphite moderated reactors cooled by carbon dioxide, and Canada's reactors are moderated and cooled by D2O, or heavy water.

Generation I: Shippingport, Pa., became home to world's first large-scale reactor in 1957.

A more advanced generation of reactors—Generation III—has been developed, but only a few have been built and operated, none in the United States. Three light water reactor designs (developed by General Electric, Westinghouse, and Combustion Engineering), have been certified by the U.S. Nuclear Regulatory Commission. They could be deployed in the United States, but no company has come forward to build one.

This could soon change. Shifts in the economics of power production as well as other factors have revived the possibility of nearer-term deployment of existing technology—either Generation III, or Generation III with some relatively minor enhancements to improve the economics (sometimes termed Generation III+)—in the United States. In addition, the Department of Energy has started a public-private partnership called Nuclear Power 2010, which is aimed at addressing the possible institutional factors (such as the licensing process and financing) that may be impeding the deployment of additional nuclear power in the United States.

Gas-Cooled Fast Reactor System The GFR is a fast-neutron-spectrum helium-cooled reactor. The reference reactor is a 288-MW electric system with an outlet temperature of 850°C using a direct Brayton cycle gas turbine for high thermal effi- ciency. A variety of core configurations are possible, including prismatic blocks and pin- or plate-based fuel assemblies.

NP 2010 has adopted the goal of achieving a utility decision by 2005 to deploy at least one new reactor in the United States within a decade. By necessity, the initiative is not designed to be an R&D program; instead, it is aimed at technologies that can be available in the very near term, such as evolutions of the Generation III designs already certified by the U.S. Nuclear Regulatory Commission.

The government funding is intended to ensure that the first commercial organizations that apply for NRC licensing and certification will not pay added costs because they are the first to test the new licensing process. The funding will not continue once the initial reactors are licensed.

At this point, the next move is largely up to industry. Although legislation is pending for federal funding for loan guarantees and other incentives for construction, the government will not select the technology or build a plant unilaterally. Companies must be ready to make a substantial economic commitment and to select from among several promising technologies to receive funding. To date, although the interest appears to be growing, industry leaders have indicated that the economics are not yet right.

There is also a longer-term approach to reactor design. By the late 1990s, a consensus emerged that, in the longer term, significant technological improvements would be needed to overcome the high costs and the public concerns about nuclear power.

To address this problem, the DOE began working to develop a significantly different kind of new reactor that would incorporate considerable improvements in economics, safety, and other matters. This new generation of reactors, Generation IV, would need strong international collaboration, and should have an international market.

Lead-Cooled Fast Reactor System The LFR is a fast-spectrum lead or lead/bismuth eutectic liquid metal-cooled reactor with a closed fuel cycle. Options include a range of plant ratings, including a "battery" of 50 to 150 MW of electricity that features a very long refueling interval, a modular system rated at 300 to 400 MW, and a large monolithic plant option at 1,200 MW. (The term "battery" refers to the long-life, factory-fabricated core, not to any provision for electrochemical energy conversion.) The fuel is metal or nitride-based.

And since considerable advances in materials, fuels, and other reactor technologies would be needed for this initiative to succeed, it would likely take several decades to develop this new design.

This long-term program, called the Generation IV Nuclear Energy Systems Initiative, is designed as a substantial research and development effort. Its goal is to develop a new generation of reactor systems to be available between 2010 and 2030. This program is integrated with several other initiatives in the DOE, and therefore can be regarded as not just a reactor development program, but rather a program to develop a full nuclear energy system, and to meet the need for energy in many forms.

A very important element of the Generation IV program is the international nature of the effort. The Generation IV International Forum (GIF)— currently made up of Argentina, Brazil, Canada, France, Japan, South Africa, South Korea, Switzerland, the United Kingdom, and the United States—has developed common evaluation criteria, identified all potential designs, and used the criteria to select the most promising designs for further development. More than 100 technical experts, drawn from all the GIF countries, as well as from the international bodies such as the Nuclear Energy Agency of the Organization for Economic Cooperation and Development, the International Atomic Energy Agency, and the European Union, were involved in this activity.

Molten Salt Reactor System The MSR features a circulating molten salt fuel mixture with an epithermal spectrum. The fuel is a circulating mixture of sodium, zirconium, and uranium fluorides, flowing through graphite core channels. The reference plant has a power level of 1,000 MW of electricity.

Some 100 different reactor designs were identified and evaluated. The designs ranged from ones that were really Generation III+ to a few that were radically different from known technologies. Some of the more speculative designs—such as a concept involving a gaseous-core reactor and a concept involving direct conversion of the thermal energy of the reactor to electricity, without the need for a turbine—would likely exceed the development period considered to be the bounds for a Generation IV reactor, and were eliminated.

The designs were judged on the basis of sustainability, safety and reliability, economics, and proliferation and security. Sustainable systems, for instance, would use fuel effectively and address the long-term burden of nuclear waste. Designs were judged on their economic potential by looking not only for a life cycle cost advantage over other sources of energy but also a level of financial risk comparable to other energy investments. And in an age of increasing concern about terrorism and nuclear weapons proliferation, the experts weighed the designs' ability to withstand attack and resist diversion or theft of materials that can be converted into weapons.

Sodium-Cooled Fast Reactor System The SFR features a fast-spectrum sodium-cooled reactor as well as a closed fuel cycle. There are two options: A 150- to 500-MW reactor with a metal alloy fuel, supported by a fuel cycle based on pyrometallurgical processing, and a 500- to 1,500-MW reactor with mixed oxide fuel, supported by a fuel cycle based upon advanced aqueous processing.

At the end of the process, six concepts were recommended for further development: the gas-cooled fast-spectrum reactor, the lead alloy-cooled reactor, the molten salt reactor, the sodium-cooled fast-spectrum reactor, the supercritical water-cooled reactor (thermal or fast spectrum), and the very-high-temperature gas-cooled reactor. Those concepts include a variety of coolants—water, gas, liquid metal, and molten salt—as well as both thermal (that is, moderated) and fast spectrum (unmoderated) designs.

With a robust R&D effort, most of those concepts could be developed and deployed by the year 2020. And each is aimed at meeting projected power needs in the mid-21st century. For example, several concepts— most prominently, the very-high-temperature gas-cooled reactor—have a higher output temperature and are therefore attractive for process heat applications. These concepts also would be well-suited to produce hydrogen in quantity and at an attractive price. Nuclear power currently is one of the most attractive means of large-scale production of hydrogen, and therefore is a critical element of the Bush administration's hydrogen initiative. There is currently legislation pending that would authorize construction of a demonstration VHTR for the production of hydrogen within a decade.

Generation II: The Quad Cities Generating Station in Cordova, Ill., is a light-water reactor that is owned and operated by Exelon Nuclear.

Although large reactors have dominated in industrial countries, less-developed countries with limited grids and regions with dispersed needs may find that smaller reactors match their needs better. The small "battery" version of the lead fast reactor, in particular, is expected to be well-suited to limited infrastructures. The gas fast reactor, very-high-temperature reactor, and sodium fast reactor are mid-size reactors that can also meet some of the needs of smaller grids. Some design concepts envision the sequential construction of a number of smaller units on one site to achieve an economic benefit through better matching of the supply with the growth in demand.

Supercritical-Water-Cooled Reactor System The SCWR is a high-temperature, high-pressure water-cooled reactor that operates at the thermodynamic critical point of water (374°C at 22.1 MPa, or 705°F at 3,208 psia). The supercritical water coolant enables a thermal efficiency about one-third higher than current reactors, as well as a simplification of balance of plant because the coolant does not change phase and is directly coupled to the energy conversion equipment. The reference system is a 1,700-MW reactor with an operating pressure of 25 MPa.

The designs also vary in terms of fuel cycle. The United States currently uses a once-through fuel cycle—that means no recycling of the fuel. Four of the six design concepts, on the other hand, feature a closed fuel cycle; this makes a greater use of the available energy from the fuel through the burning of heavy, long-lived radiotoxic constituents of spent fuel and the more efficient use of reactor fuel through recycling.

Now that the six reactor designs have been selected, the 10 GIF countries are in the process of beginning collaborative research programs. The initial phases of the research are intended to help identify the most attractive designs among the six selected. Then that smaller number of concepts would be developed further. And, in time, there may be international collaborative demonstration facilities. GIF will monitor the research progress at all stages and periodically evaluate the results to set the course for further activity.

Very-High-Temperature Reactor System The VHTR is a graphite-moderated, helium-cooled reactor with a once-through uranium fuel cycle. The reference reactor is a 600-MW thermal core with an outlet temperature of 1,000°C. The reactor core can be a prismatic block or a pebble-bed core. The high temperature enables applications such as process heat or hydrogen production via the thermochemical iodine-sulfur process.

The establishment of cooperative research programs will mark the first time the 10 countries involved in the GIF effort will have to commit substantial funding to this activity. Further, the group undoubtedly will have to confront a number of difficult problems about how to structure the research agreements. For example, the 10 countries have substantially different interests and objectives. Resource-poor nations such as Japan are focusing on the sodium-cooled reactor, with its significant potential for recycling of spent nuclear fuel in the near future. The United States, on the other hand, is presently most interested in the very-high-temperature gas-cooled reactor because it seems to have the best potential to support the development of a hydrogen economy.

Nevertheless, the countries have thus far shown a remarkable commitment to pursuing Generation IV technology, and have been able to compromise to balance their individual interests with their interest in reviving progress on this important technology. The challenges ahead are so great—and the stakes so large—that continuing this spirit of cooperation is critical if nuclear power is to play a significant role in this new century.

Gail H. Marcus is principal deputy director of the Office of Nuclear Energy, Science, and Technology in the U.S. Department of Energy, and a past president (2001-02) of the American Nuclear Society.



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