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  Cover Story  
  September 13,  2004
Volume 82, Number 37
pp. 31-35

Enhanced safety, improved economics, and simpler designs are the keynotes of the next generations of nuclear energy systems


New generations of nuclear energy systems are now in various stages of planning and development. The new reactors will feature so-called passive safety systems that do not require human intervention in the case of an accident. Some will operate at sufficiently high temperatures to produce hydrogen from water as well as electricity. Experts say the new systems will be more economical to build, operate, and maintain than current generations of nuclear reactors.

EVOLUTIONARY Four advanced boiling-water reactors, such as this one at the Lungmen Power Station, Taiwan, are under construction in Japan and Taiwan.
These new types of reactors are often described as "evolutionary" or "revolutionary." The evolutionary systems, known as generation III and III+ systems, have designs that evolved from the generation II fleet of reactors that were built in the 1970s and 1980s and continue to operate today. Generation III systems were developed in the 1990s and feature enhanced safety systems. They are more economical to build, operate, and maintain than the previous generation; two are currently in operation and another four are under construction. Generation III+ systems are evolving from the generation III systems but are not yet operational. They are actively under development and being considered in several countries for deployment over the next decade or so.

"Many current plants around the world have been in operation for several decades and will be decommissioned over the next 10 years or so," says Sue Ion, executive director of technology at British Nuclear Fuels (BNFL). "Evolutionary designs are intended to replace existing nuclear plants and to prevent sizable increases in carbon dioxide emissions in the future. The revolutionary designs aim to deliver safe, competitive, and sustainable energy in the longer term."

The revolutionary designs, known as generation IV systems, have revolutionary reactor and fuel cycle systems. They are being developed in parallel with the evolutionary generation III+ designs.

Six new designs were identified for further study by the Generation IV International Forum (GIF), which was initiated in 2000 by a group of nine countries: Argentina, Brazil, Canada, France, Japan, South Africa, South Korea, the U.K., and the U.S. Switzerland became a member of the forum in February 2002, and the European Atomic Energy Community joined in July 2003. The technologies were selected following the evaluation of 100 or so different nuclear energy concepts by more than 100 expert scientists and engineers from more than a dozen countries.

"Generation IV is an international initiative aimed at developing nuclear energy systems that can supply future worldwide needs for electricity, hydrogen, and other products," observes Hussein S. Khalil, director of the Nuclear Engineering Division at Argonne National Laboratory, in Illinois, and a member of the Generation IV Roadmap Project. "These systems are to be deployable no later than 2030 for providing competitively priced and reliable energy products while satisfactorily addressing nuclear safety, waste, proliferation, and physical protection concerns."?

At present, 441 nuclear power reactors operate in 31 countries, producing over 363 billion W of electricity worldwide, according to the World Nuclear Association. Another 30 reactors are under construction, and some 24 countries--including six that do not currently operate nuclear reactors--are planning or proposing to build an additional 104 reactors.

Nuclear energy may also be expanded toward the production of nonelectricity energy services such as hydrogen production, water desalination, and district heating, Khalil points out.

The 103 nuclear reactors currently in operation in the U.S. generate over 97 billion W of electricity--about 20% of the country's electricity.

"In the U.S., improved efficiency has in the past decade yielded the equivalent of some 20 new nuclear power plants," Khalil notes. "In 2001, the average operating cost of the 103 U.S. nuclear power plants was 1.68 cents per kilowatt-hour, second only to hydroelectric power among baseload generation options." Baseload is the portion of electricity generated that remains continuous and does not vary over 24 hours.

Experts say the new systems will be more economical to build, operate, and maintain than current generations of nuclear reactors.

FORECASTS INDICATE that the U.S. will need about 335 billion W of new generating capacity by 2025, according to the U.S. Department of Energy's (DOE) Office of Nuclear Energy, Science & Technology. This growth would require building and commissioning an average of 50 to 60 new power plants per year over the next two decades.

Western European countries generate around 35% of their electricity from nuclear power--more than from any other source. France and Belgium produce 78% and 55%, respectively, of their electricity from nuclear power. However, only one European Union country, Finland, is planning to build a nuclear reactor. France is considering the possibility of building a new generation of nuclear power plants. Belgium, Germany, Holland, and Sweden are planning to phase out existing plants. Austria, Denmark, and Ireland have stated policies against nuclear energy. Italy is dismantling its four plants following a vote against nuclear power in a 1987 referendum. Spain, which currently operates nine reactors, has a moratorium on constructing new plants. The U.K. is keeping its nuclear options open.

Russia has six nuclear plants under construction and is proposing to build eight more. China, India, Japan, South Korea, and Taiwan have extensive civil nuclear power programs: 17 reactors are being built, and another 70 are planned or proposed.

Ion points out that the U.K. pioneered the world's first commercial-scale nuclear reactors, Magnox reactors, in the 1950s. Magnox reactors employ natural uranium metal, which contains 0.7% of the fissile isotope uranium-235 and around 99.2% uranium-238. The fuel is encapsulated in an alloy of magnesium and aluminum. A graphite moderator surrounding the fuel slows down neutrons released by fission of uranium-235 so that they can collide with other uranium-235 nuclei, causing more fission and a nuclear chain reaction. Control rods made of boron steel, a neutron-absorbing material, are inserted into or withdrawn from the core to control the rate of reaction or to halt the reaction. Gaseous carbon dioxide is used as a coolant to transfer heat generated by the nuclear chain reaction in the reactor core to a steam turbine that generates electricity.

A total of 26 Magnox reactors were built in the U.K. Eight remain in operation, but they will be decommissioned by 2010. Nuclear reactors such as the Magnox reactors that were operational before the 1970s and made use of natural uranium are known as generation I reactors.

The generation II reactors of the 1970s and 1980s constitute most of the plants currently operating, notes Per F. Peterson, professor of nuclear engineering at the University of California, Berkeley. Almost 60% of these reactors are pressurized water reactors (PWRs), in which the pressurized water serves as a moderator and coolant. The fuel, ceramic uranium dioxide, is typically encased in long zirconium alloy tubes. The uranium-235 is enriched from its original 0.7% abundance to 3.5–5.0%.

TEST RIG This model of a power conversion system for the pebble bed modular reactor was designed and built by the Faculty of Engineering, North-West University, Potchefstroom, South Africa.

THE SECOND most common type of reactor is the boiling-water reactor (BWR). Currently more than 90 of these are operating throughout the world. BWRs "are similar to pressurized water reactors, except that the coolant water is allowed to boil, and steam passes from the top of the reactor directly to the turbine," Ion explains.

PWRs and BWRs are known as light-water reactors. The 33 CANDU (Canada deuterium uranium) pressurized water reactors currently in operation in Canada, on the other hand, employ heavy water (D2O) as a moderator and coolant. The reactors use natural uranium (0.7% U-235) dioxide as a fuel rather than enriched UO2.

The second generation of reactors in the U.K. are advanced gas-cooled reactors (AGRs). Like Magnox reactors, they use graphite as a moderator and CO2 as a coolant. The AGR fuel is enriched uranium (2.5–3.5% U-235) oxide pellets encased in stainless steel tubes that allow the reactors to operate at higher temperatures than the Magnox reactors.

The Russian-designed RBMK reactors are boiling-water reactors with graphite moderators. The reactor at the Chernobyl Nuclear Plant in Ukraine that disintegrated in a steam explosion in April 1986 was an RBMK reactor.

Reactors that use water or graphite as moderators to slow neutrons and sustain the fission chain reaction are known as thermal reactors. "Light atoms, such as hydrogen, deuterium, and carbon, slow neutrons down to thermal energies (below 1 eV)," Ion explains. "At these energies, the probability of a collision between a neutron and the fissile U-235 nucleus is around two orders of magnitude higher than that for the high-energy neutrons that are generated by fission."

In contrast, fast neutron reactors do not have a moderator and use fast neutrons directly to generate power. When configured to produce more fissile material than they consume, they are known as fast breeder reactors. The fuel rods contain a mixture of UO2 and plutonium dioxide. The coolants are liquid metals, usually sodium. The extra energy of fast neutrons increases the probability of fission occurring. "Fertile" isotopes, like U-238 in natural uranium, capture some of the neutrons, creating fissile isotopes such as Pu-239. Fast reactors can therefore use depleted uranium (uranium that has less than 0.7% of U-235) as a fuel.

"Fertile isotopes do not undergo fission but can instead capture neutrons and transmute into an isotope of another element which can undergo fission," explains Tim J. Abram, manager of advanced reactor systems at BNFL.

Several countries, including China, India, and Japan, have R&D fast breeder reactor programs. A fast breeder reactor in Russia has supplied electricity to the grid since 1981.

The first generation III system, a General Electric-designed advanced BWR, started operating in 1996 at the Kashiwazaki-Kariwa Nuclear Power Station in Japan, and another is now in operation. Two more are under construction in Japan and another two in Taiwan. The designs were certified by the U.S. Nuclear Regulatory Commission (NRC) in the 1990s.

8237cover_Ionat 8237cover_khalil 8237cover_magwood

IN MANY PARTS of the world, particularly in the U.S. and Europe, the overriding public concern relating to the future development of nuclear power plants is the issue of safety.

"Attaining safe energy is the most technologically important nut to crack if we are to achieve a sustainable high-technology civilization," comments Terrence J. Collins, chemistry professor at Carnegie Mellon University, Pittsburgh. "I think nuclear is the wrong way to go because it can never be safe," he says. "Yes, we can do it, but all we need is one serious accident or a sabotage incident, and the public will insist on another direction. All the invested effort will be wasted."

Even so, a vast amount of effort is going into enhancing the safety of advanced generation III+ reactor designs that are now evolving and the revolutionary generation IV technologies. They all incorporate what is known as passive safety systems.

"There are three primary goals for the safety of nuclear reactors," Peterson points out. The first is reactivity control, which is the process of stopping the fission reactions. Peterson notes that all Western power reactors incorporate reactivity control as an intrinsic feature of their designs. The Chernobyl reactor that caused the accident in 1986, on the other hand, did not, and instead relied on operating procedures, which were violated.

The second safety goal is to reliably remove decay heat, which is the heat generated by radioactive decay of the fission products that continue to be produced even after fission reactions stop. Decay heat, if not removed, can result in overheating and damage to the fuel. Failure to adequately remove decay heat contributed to the Three Mile Island accident in Pennsylvania in March 1979.

The third goal is to provide multiple barriers to contain the radioactive material. The barriers include the fuel cladding, the reactor vessel, and the containment building.

Generation I and II reactor safety systems are "active" because they rely on active electrical and mechanical control of equipment such as sensors, valves, pumps, accumulators, heat exchangers, and backup power supplies. The multiple parallel redundancies that are built into the designs add to the complexity of the systems and the construction and maintenance costs.

"In current nuclear power plants, decay heat removal under accident--loss-of-coolant--conditions is performed by active safety systems that consist of redundant and diverse sets of equipment capable of pumping water in to cool the reactor core," Peterson says. "Power for this equipment comes from redundant and diverse sources including large emergency diesel generators.

"In passive safety systems, decay heat removal occurs primarily by gravity-driven flows, using a combination of convection and phase change to remove and transport heat out of the reactor containment," he continues. Typically, the only power required to activate the systems is battery power to open valves and maintain power to instrumentation and control systems.

ONE EXAMPLE of an advanced reactor with passive safety systems is the economic simplified boiling-water reactor (ESBWR), which was developed by General Electric from its advanced boiling-water reactor design.

IMPACT RESISTANT The pebble bed modular reactor building is designed to withstand significant external forces such as aircraft impacts, explosions, or tornadoes. The reactor pressure vessel (left) and power conversion unit (right) are housed in a reinforced concrete structure.
"ESBWR is particularly interesting because it is the first light-water reactor design where the most important safety-related parameters for a large-break loss-of-coolant accident--the peak temperature of the metal cladding of the fuel and the peak pressure reached in the containment building--can be calculated on the back of an envelope," Peterson says. ESBWR is at the preapplication stage for NRC design certification.

"GE continues to be committed to the nuclear industry," comments Andrew C. White, president and chief executive officer of GE Energy's nuclear business. "As the sole remaining U.S.-owned nuclear vendor, we are enthusiastic about the potential for construction of new nuclear power plants both in the U.S. and globally. There is no other large, single-generation source for energy that is economic, reliable, and safe, while also helping to protect our air quality."

The Westinghouse AP1000 light-water reactor has also been submitted to NRC for full design certification. Westinghouse Electric, which is wholly owned by BNFL, built the first PWR in 1957.

AP1000 features advanced passive safety systems. The reactor has a modular design that will reduce construction times to as little as three years from the time the concrete is first poured to the time that fuel is loaded into the core, the company claims. AP1000 is capable of running on a full mixed oxide (MOX) core if required. MOX fuel consists of both uranium and plutonium oxides. It contains about 5% plutonium, which is the main fissile component of the fuel.

Ion points out that AP1000 has fewer components than conventional PWRs. For example, compared with a conventional 1,000-MW PWR, AP1000 has 50% fewer valves, 35% fewer pumps, 80% less pipe, and 85% less cable.

"Compared with the Sizewell B PWR [in England], the building volume of the AP1000 is about half in terms of concrete," she says. "And because of simplification of the design compared with conventional PWRs, we are able to significantly reduce the outage times and therefore the running costs of the reactor. The reactor can be on-line at full power for well over 90% of the time."

AP1000 and ESBWR are two of the eight evolutionary reactor design candidates considered by the Near-Term Deployment Group, which was organized by DOE as part of its Nuclear Power 2010 program. The program, which was unveiled by Secretary of Energy Spencer Abraham in February 2002, aims to build new nuclear power plants in the U.S. by the end of the decade. The 2010 program expects that the advanced reactor designs will produce electricity in the range of $1,000 to $1,200 per kilowatt of electricity.

THE CANDIDATES comprise three PWRs, three BWRs, and two high-temperature gas-cooled reactors (HTRs).

"One of the limitations of current light-water reactor technology is that the maximum temperature that can be achieved is about 350 °C," Ion explains. "This means that the thermal efficiency that can be achieved is limited."

The pebble-bed modular reactor (PBMR), which is currently planned for commercial operation in South Africa by around 2010, is an HTR. It uses helium as a coolant and graphite as a moderator. The South African government designated the PBMR project as a national strategic project. Current investors are Eskom, South Africa's power utility; the Industrial Development Corp. of South Africa; and BNFL.

"While a PWR operates at coolant temperatures of typically 340 °C, the PBMR is designed to achieve at least 900 °C," Ion continues. "This higher temperature will give a thermal efficiency of up to 44%, which translates into roughly one-third more output than a conventional PWR."

A PBMR reactor is essentially a large hopper filled with graphite pebbles, about 60 mm in diameter, each filled with thousands of UO2 fuel particles with diameters of less than 1 mm. Each fuel particle is coated with two layers of pyrolytic carbon, silicon carbide, and porous carbon. The coatings retains the gaseous fission products.

"Heat generated by nuclear fission in the reactor is transferred to helium that passes through the bed," Abram explains. "Helium is an inert gas, so it doesn't react with any of the components inside the reactor core. The helium moves on to compressors and, eventually, to a gas turbine that converts the thermal energy into electricity. The modular design means that the components are factory-made, so plants are quicker to assemble," he adds. The PBMR module can be used to generate power in a stand-alone mode or as part of a power plant that consists of up to 10 units.

The PBMR sets new standards in safety, not only through its design, according to Ion. The silicon carbide layer not only protects the fuel during storage and fission but also makes it extremely difficult for anyone to divert the fuel elsewhere, she says.

All six revolutionary nuclear reactor technology concepts identified for development by GIF operate at higher temperatures than the generation II and III reactors currently in operation. The new systems range from a supercritical-water-cooled reactor (SCWR), which operates at 510–550 °C, to a helium-cooled very-high-temperature gas reactor (VHTR), which has an operating temperature of 1,000 °C. SCWR is the only one of the six generation IV technologies that is cooled by water.

"The SCWR system uses a high-temperature, high-pressure, water-cooled reactor that operates above the thermodynamic critical point of water to achieve a thermal efficiency approaching 44%," Khalil notes.

Three of the six generation IV concepts are fast reactor systems that are cooled either by helium gas, lead, or sodium. All use depleted uranium as a fuel.

PRESSURIZED WATER The Westinghouse AP1000 reactor, as shown in this artist's impression, has a construction time of about three years.
A key aspect of these designs is how they deal with the high-level waste from fission reactions. This waste includes heavy nuclides--actinides such neptunium, americium, and curium--that remain highly radioactive for tens of thousands of years. The helium-, lead-, and sodium-cooled fast reactors are designed to have closed fuel cycles. The actinides are separated from the spent fuel and returned to the fission reactors.

The generation IV molten salt reactor also has a closed fuel cycle. The reactor is described as an epithermal reactor because the neutrons generated in the reactor have energies just above those of thermal neutrons. Uranium fuel is dissolved in a sodium fluoride salt that circulates through graphite channels to moderate the energies of the neutrons. Fission products are removed continuously, and the actinides are fully recycled. Plutonium and other actinides can also be added to the reactor along with depleted uranium.

SCWR is designed to be a thermal reactor in the intermediate term, using enriched UO2 as a fuel with a once-through fuel cycle. However, the ultimate goal is to build it as a fast neutron reactor with full actinide recycling.

VHTR has an open fuel cycle. It will employ enriched UO2 as a fuel, possibly in the form of the pebbles coated with a graphite moderator like those required for PBMR.

"The once-through cycle is the most uranium resource-intensive and generates the most nuclear waste," Khalil explains. "However, the amounts of waste produced are still quite small in both volume and mass compared with other energy technologies, and existing uranium resources are believed to be sufficient to support a once-through cycle well into this century.

"In the longer term, uranium resource availability could also become a limiting factor," he continues. "A challenge to long-term, widespread deployment of generation IV nuclear energy systems is to ensure they operate using fuel cycles that minimize the production of long-lived radioactive wastes while conserving uranium resources."

VHTR, helium- and lead-cooled fast reactors, and the molten salt reactor are all designed to generate electricity and also to operate at sufficiently high temperatures to produce hydrogen by thermochemical water cracking. At present, about 97% of hydrogen is produced from fossil fuels by steam reformation of methane. Around 3% is produced by electrolysis of water, but the electricity costs for the process are relatively high.

"The direct thermal decomposition of water is impractical, as it requires temperatures in excess of 2,500 °C," Abram says.

THERMOCHEMICAL hydrogen production, on the other hand, can be achieved at temperatures of less than 900 °C. One such process is the sulfur-iodine cycle, in which sulfur dioxide and iodine are added to water, resulting in an exothermic reaction that creates sulfuric acid and hydrogen iodide. At 450 °C, the HI decomposes to iodine (which is recycled) and hydrogen. Sulfuric acid decomposes at 850 °C, forming sulfur dioxide (which is recycled), water, and oxygen.

"The only feeds to the process are water and high-temperature heat, typically 900 °C, and the only products are hydrogen, oxygen, and low-grade heat," Abram explains. "Nuclear power is particularly well suited to hydrogen production by such a process because of its near-zero emissions."

DOE, although supporting research on several generation IV reactor concepts, is giving priority to VHTR technology, notes William D. Magwood IV, director of the DOE Office of Nuclear Energy and chairman of the GIF policy group. The technology is known as the Next Generation Nuclear Plant (NGNP).

"The NGNP would be able to make both electricity and hydrogen at very high levels of efficiency," Magwood says. "It would be deployable in modules that will better fit the highly competitive, deregulated market environment in the U.S. and would be extraordinarily safe, proliferation-resistant, and waste-minimizing.

"The base concept of the NGNP is that of a very-high-temperature, gas-cooled reactor system coupled with an advanced, high-efficiency turbine generator and an even more advanced thermochemical hydrogen production system," he continues. "We have very high expectations for this technology."

Within DOE's fiscal-year 2005 budget request of $30.5 million for the generation IV program, $19.3 million is budgeted for NGNP activities. The NGNP 2005 effort will be focused primarily on continuing concept design activities and on R&D activities related to fuels and structural materials for use at high temperature and high levels of radiation.

Khalil points out that the six generation IV systems exhibit diverse characteristics and benefits. In the long term, he suggests, it is unlikely that one particular reactor system will be the preferred means to meet all the generation IV goals and system applications. "Rather, a combination of reactor types is likely to be employed, forming a nuclear energy fleet in which each reactor type is used in the role that it fills best," he says.

The DOE strategy for nuclear energy is to deploy the first U.S. advanced light-water reactor with an evolutionary generation III+ design some time between 2010 and 2020. The aim is then to deploy the first commercial generation IV thermal reactor during the next decade and commercial generation IV fast reactors during the period of 2040–50.

  Chemical & Engineering News
ISSN 0009-2347
Copyright © 2004

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