Nuclear Fission Reactors: Boiling Water and Pressurized Water Reactors

DOUGLAS K. VOGT Lawrence Livermore National Laboratory Livermore, California, United States

from hydrogen enriched in deuterium are called heavy 1. Electricity Production from Light Water Reactors water. light water reactor (LWR) Any reactor that is cooled and 2. Pressurized Water Reactors moderated by ordinary or light water. Both pressuri- 3. Boiling Water Reactors zed water and boiling water reactors are light water 4. Ship Propulsion reactors. pressurized water reactor (PWR) A reactor that operates at high pressure (typically 42000 psia). At this pressure, PWR coolant water does not experience significant Glossary boiling. Reactor coolant is heated by thermal energy from nuclear fission, resulting in a temperature increase. boiling water reactor (BWR) A light-water-cooled and - Heated coolant flows to a heat exchanger with lower B moderated reactor that operates at a pressure of 1000 pressure water (B1000 psia) on the ‘‘nonreactor side.’’ pounds per square inch at atmospheric pressure (psia). The lower pressure water is converted to steam. The At this pressure, reactor coolant is converted from steam from the steam generator flows to a conventional liquid water to steam by thermal energy from nuclear turbine-generator system, where it is converted to fission. The boiling occurs within the reactor core. electricity. Steam from the core of a BWR flows to a conventional turbine-generator system, where it is converted to electricity. decommissioning At the end-of-life stage of a nuclear power plant, the facility contains highly radioactive Scientists in the former Soviet Union and in the West components. As part of decommissioning, these components, including the reactor and other plant areas, are who developed nuclear weapons in World War II and decontaminated to reduce the amount of radioactivity. in the early days of the Cold War realized that the Some components will be removed from the reactor site tremendous energy produced by nuclear fission could and buried in an engineered disposal site. Pressurized be tapped either for direct use or for generating water, boiling water, and naval reactors are decommis- electricity. It was also clear that nuclear fission sioned. energy would allow development of compact, long- high-level radioactive waste repository An engineered lasting power sources that could have various facility that isolates spent fuel or the radioactive waste applications, including powering ships, especially from spent fuel reprocessing, safeguarding the environ- submarines. From their roots in naval nuclear power ment. programs, light water reactors have become an light water Hydrogen has two stable isotopes, (1) protium important source of electric power in many coun- (99.985% natural abundance), which has a nucleus consisting of one proton, and (2) deuterium (0.015% tries. Light water reactors built over the past 30 years natural abundance), which has a nucleus consisting of continue to generate electricity, and new reactor one proton and one neutron. Water molecules formed designs, the advanced boiling water reactor and the from water with natural abundance hydrogen (99.985% advanced pressurized water reactor, are planned for protium) are called light water. Water molecules formed operation in the 21st century.

1. ELECTRICITY PRODUCTION Laboratory and General Electric Company were FROM LIGHT WATER REACTORS developing the boiling water reactor (BWR). A small prototype BWR, Vallecitos, generated electricity in The first nuclear reactor to produce electricity (albeit Pleasanton, California from 1957 to 1963. Dresden- a trivial amount) was the small experimental breeder 1, which was designed by General Electric, was the reactor (EBR-1) in Idaho, in the United States. EBR-1 first large commercial BWR (200 MWe). Dresden-1 started operating in December 1951. In 1953, was constructed 50 miles southwest of Chicago, President Eisenhower proposed his ‘‘Atoms for Illinois and operated from 1960 until 1978. Peace’’ program, setting the course for civilian In 1964, the first Soviet land-based PWR power nuclear energy development in the United States. reactor began power operations in Novovoronezh However, it was the U.S. effort under Admiral (Volga region). This reactor was a new 210-MWe Hyman Rickover that led to the development of the pressurized water reactor known as a VVER (an pressurized water reactor (PWR) for submarine use. acronym in Russian for veda-vodyanoi energetiches- As part of the PWR development, Westinghouse ky reaktor, meaning ‘‘water-cooled power reactor’’). Electric Corporation built a land-based submarine In the Murmansk region of the Arctic Northwest, a rector prototype at the National Reactor Testing slightly bigger VVER with a capacity of 440 MWe Station near Arco, Idaho. The prototype started was constructed at the Kola Nuclear Power Plant. operation in March 1953, and in 1954, the first This became a standard Russian PWR design. nuclear-powered submarine, the USS Nautilus, was By the end of the 1960s, electric utilities world- launched. In 1959, both the United States and the wide were placing orders for PWR and BWR units former Soviet Union launched their first nuclear- of 400 MWe to more than 1000 MWe. Other powered surface vessels. countries have built primarily PWR and BWR types, The success of the submarine prototype reactor so that today, 65% of the world nuclear electric led to the U.S. Atomic Energy Commission decision generating capacity is PWR and 23% is BWR. to build the 90-megawatt electric (MWe) output Overall, 16% of the world’s electricity now comes Shippingport demonstration PWR near Pittsburgh, from nuclear energy. Throughout the world, there Pennsylvania. Shippingport started operation in are 438 commercial nuclear generating units with 1957 and operated as a commercial nuclear power a total capacity of about 351,000 MWe. Worldwide, plant until 1982. In the United States, Westinghouse LWRs generated over 2,000,000,000 megawatt- Electric Corporation designed the first fully commer- hours (MWh) of electricity in 2001. Figure 1 shows cial 250-MWe PWR, Yankee Rowe, which started the location of nuclear power plants worldwide operation in 1960 and operated until 1992. As the that are under construction, in operation, or under- PWR was being developed, Argonne National going decommissioning. Figure 2 summarizes the

Europe Russia 60°N North America

Asia 45°N

30°N

Africa 15°N East Asia West Asia 0°

South America 15°S

30°S

45°S

120°W90°W60°W30°W0° 30°E 60°E 90°E 120°E 150°E

FIGURE 1 Location of nuclear power plants worldwide. Map from the International Nuclear Safety Center (Argonne National Laboratory). Nuclear Fission Reactors: Boiling Water and Pressurized Water Reactors 335

United States WA France NH MT ME ND VT Japan MN OR Germany MA ID SD WI NY MI RI Russian Federation WY CT Ukraine PA NV NE IA NJ UT South Korea CO IN OH DE WV Sweden CA KS IL VA MD MO KY Spain NC AZ NM TN Belgium OK SC AR MS Bulgaria AL GA Taiwan TX LA Slovak Republic Switzerland FL Czech Republic Hungary Finland Years of Number of Average China commercial operation reactors capacity (MDC) India 0−9 2 1134 South Africa 10−19 47 1092 Mexico 20−29 55 779 Brazil United Kingdom FIGURE 3 Locations of operating light water reactors in the Pakistan United States as of December 2000 (there are no commercial Slovenia reactors in Alaska or Hawaii). MDC, Maximum dependable Netherlands capacity. This map and additional data are available on the Armenia Internet at the Web site of the U.S. Nuclear Regulatory 020406080 100 120 Commission (http://www.nrc.gov). Power plants FIGURE 2 Number of light water reactor nuclear power plants worldwide.

Containment structure Steam line number of operating LWR nuclear power plants in Control rods Steam Reactor Generator each country. vessel generator To minimize dependence on fossil fuel imports, Pump Reactor several countries have placed significant dependence Turbine Cooling tower on nuclear energy from LWRs. The following Condensor Pump cooling countries generate more than 30% of electricity water output using LWRs: France, Belgium, Slovak Repub- lic, Ukraine, Sweden, Bulgaria, South Korea, Hun- FIGURE 4 Schematic view of a pressurized water reactor. gary, Slovenia, Switzerland, Armenia, Japan, Finland, and Germany. Eight countries have manu- active material from the reactor under accident factured large LWRs for electricity generation: the conditions; a steam-powered turbine-generator that United States, France, Japan, Germany, Russia, produces electricity; and a condenser and cooling South Korea, Sweden, and the United Kingdom. towers for heat rejection. Figure 3 shows the 32 states with operating LWRs in The nuclear steam supply system includes nuclear the United States. fuel that is configured as the ‘‘reactor core’’ in a reactor pressure vessel. Reactor coolant water is pumped through the reactor core and heated. Heated water passes through tube-and-shell heat exchangers 2. PRESSURIZED WATER REACTORS called ‘‘steam generators.’’ On the reactor side of the steam generator, the water enters the inside, or tube 2.1 Plant Design Characteristics side, of the steam generator, passes through, and Figure 4 provides a schematic view of a pressurized eventually returns to the reactor through a closed water reactor power plant. The power plant systems piping system, to be reheated. On the turbine- include a nuclear reactor and steam generator (called generator side, the water passes over the outside, or the nuclear steam supply system), which produce shell side, of the steam generator tubes and is heated. steam using energy from nuclear fission; a reactor Heat transfer through the tubes provides the energy ‘‘containment system,’’ which limits release of radio- to convert liquid water to steam on the shell side of 336 Nuclear Fission Reactors: Boiling Water and Pressurized Water Reactors the steam generator. Moisture (entrained liquid water) is removed from the steam and the steam flows to a conventional steam turbine. The steam turbine is coupled to a conventional electric generator. After steam passes through the steam turbine, it passes through another heat exchanger called a ‘‘condenser,’’ where the steam is condensed to liquid water. The condensed water from the steam turbine is then pumped through heat exchangers called ‘‘reheaters’’ and is returned to the steam generators in a closed piping loop. The rejected heat from the condenser system is discharged to the environment by evaporative cooling from cooling towers. The electric generator output is supplied to a local transformer yard that is connected to the local electrical grid. The reactor containment building, turbine-generator, and cooling towers are the promi- nent buildings visible at most nuclear power plant sites.

2.2 Fuel Design Characteristics There are numerous PWR nuclear fuel assembly designs. However, most PWR power reactor fuel designs will be conceptually similar to the fuel assembly schematic shown in Fig. 5. The PWR uranium dioxide (UO2) fuel pellets are nominally 0.375 inch in diameter and 0.5-inch-long solid cylinders. From 200 to 300 fuel pellets are loaded into hollow zirconium fuel tubes called fuel rods. A column of fuel pellets, typically 12 feet (144 inches) long, is loaded into a fuel rod. The PWR fuel tube is typically 150 inches in length. The nominal 6-inch length of the fuel rod that does not contain fuel pellets is called the ‘‘plenum.’’ The plenum provides a space to accommodate fuel pellet axial thermal expansion during operation and the fission product gases that are released during power operations. A FIGURE 5 Schematic view of a pressurized water reactor fuel spring is inserted into the top of the fuel rod to hold assembly. the fuel pellets down in the fuel tube during transportation to the reactor and end caps are the top and bottom of the fuel assembly are end welded onto the upper and lower ends of the fuel fittings. The end fittings are typically machined from rod. Fuel rods are inserted into spacer grids that are stainless steel and provide structural strength to the fabricated from a zirconium or inconel alloy. Fuel fuel assembly and help control inlet and outlet flow rods are held in place in the spacer grids by small leaf from the reactor core during operation. In the reactor springs. Control rod guide tubes replace fuel rods pressure vessel, 120 to 193 fuel assemblies are typically at 20 to 25 fuel rod locations. The spacer arranged into an approximately cylindrical geometry. grids are welded to control rod guide tubes. Some Fission of uranium dioxide fuel in the reactor core fuel assemblies may have a central fuel rod location is the heat source in a nuclear reactor. Fuel typically used for in-core instrumentation. Depending on fuel operates in a reactor for 3–5 years. During this time, assembly design, from 196 to 289 fuel rod locations the equivalent of 3–5% of the uranium atoms initially are available in a PWR fuel assembly. Fuel assembly loaded into the reactor fission, leaving fission products designs normally arrange fuel rods in a square array in the fuel. Plutonium and other actinides are also with 14, 15, 16, or 17 fuel rod locations on a side. At produced by neutron capture. At the end of its Nuclear Fission Reactors: Boiling Water and Pressurized Water Reactors 337 power-generating life, fuel is discharged from the TABLE I reactor and replaced with fresh fuel. The discharged Pressurized Water Reactor and Fuel Design Parameters fuel is called ‘‘spent nuclear fuel’’ or ‘‘spent fuel.’’ Though some fission products in spent fuel are stable Design parameter Typical value (nonradioactive) or decay to nonradioactive isotopes, Thermal power 3411 MWt spent fuel remains radioactive for very long periods of Electrical power 1100 MWe time following discharge from the reactor. Spent fuel Reactor pressure 2250 psia generates decay heat and penetrating radiation (high- Reactor coolant inlet temperature 5601F energy g rays and b particles). Spent fuel is stored at Reactor coolant outlet temperature 6201F reactor sites for several years after discharge, typically Reactor coolant flow rate 140 Â 106 lb m/hr 5 or more years. There are two methods of disposition Number of fuel assemblies in core 193 of spent fuel. It is either shipped (in shipping packages Number of steam generators 4 designed for radioactive material) to a nuclear fuel Steam turbine inlet temperature 5411F reprocessing facility for recovery of residual uranium Steam turbine inlet pressure 970 psia and plutonium or shipped to a high-level radioactive Fuel rod array 17 Â 17 waste repository for disposal. Table I provides a Fuel rods per assembly 264 summary of key design characteristics of a typical Control rod locations per assembly 25 pressurized water reactor. Fuel assembly cross section 8.426 Â 8.426 inches Fuel assembly length 160 inches 3. BOILING WATER REACTORS Fuel rod outer diameter 0.36 inch Fuel rod clad thickness 0.0225 inch 3.1 Reactor Design Fuel pellet outer diameter 0.3088 inch Fuel pellet length 0.53 inch Figure 6 provides a schematic view of a boiling water Fuel assembly weight 535 kg reactor power plant. The power plant systems Fuel assembly uranium weight 423 kg include a nuclear steam supply system, which produces steam using energy from nuclear fission; a reactor ‘‘containment system,’’ which limits release of radioactive material from the reactor under Reactor building accident conditions; a steam-powered turbine-gen- (Secondary containment) Inerted drywell erator that produces electricity; and a condenser and (Primary containment) Electricity Main steam lines Turbine generators cooling towers for heat rejection. to Reactor switchyard The nuclear steam supply system includes nuclear core fuel (discussed later) that is configured as the ‘‘reactor core’’ in a reactor pressure vessel. Reactor coolant water is pumped through the reactor core Feedwater Condenser and heated. At the BWR system operating pressure, Control rods pumps liquid water is converted to steam as the water is heated. Moisture is removed from the steam in a Torus moisture separator located in the reactor pressure vessel above the reactor core. The steam flows out of FIGURE 6 Schematic view of a boiling water reactor system. the reactor building to a conventional steam turbine. The turbine-generator, condenser, and cooling towers for a BWR are similar to those for a PWR. to 300 fuel pellets are loaded into hollow zirconium fuel tubes called fuel rods. A column of fuel pellets, typically 150 inches long, is loaded into a fuel rod. 3.2 Fuel Design Characteristics The fuel tube is nominally 160 inches in length. The There are numerous BWR nuclear fuel assembly nominal unfilled length of the fuel rod that does not designs, but most BWR power reactor fuel designs contain fuel pellets is called the ‘‘plenum.’’ The will be conceptually similar to the fuel assembly plenum provides a space to accommodate fuel pellet schematic shown in Fig. 7. The BWR uranium axial thermal expansion during operation and fission dioxide (UO2) fuel pellets are nominally 0.4 inch in product gases that are released during power diameter and 0.4-inch-long solid cylinders. From 200 operations. A spring is inserted into the top of the 338 Nuclear Fission Reactors: Boiling Water and Pressurized Water Reactors

TABLE II Boiling Water Reactor and Fuel Design Parameters

Design parameter Typical value

Thermal power 3579 MWt Electrical power 1150 MWe Reactor pressure 1050 psia Reactor coolant inlet temperature 5331F Reactor coolant outlet temperature 5511F Reactor coolant ﬂow rate 104 Â 106 lb m/hr Number of fuel assemblies in core 800 Steam turbine inlet temperature 5401F Steam turbine inlet pressure 965 psia Fuel rod array 8 Â 8 Fuel rods per assembly 62 Water rod locations per assembly 2 Fuel assembly cross section 5.4 Â 5.4 inches Fuel assembly length 176 inches Fuel channel thickness 0.12 inch Fuel rod outer diameter 0.483 inch Fuel rod clad thickness 0.032 inch Fuel pellet outer diameter 0.41 inch Fuel pellet length 0.41 inch Fuel assembly weight 220 kg Fuel assembly uranium weight 183 kg

FIGURE 7 Schematic view of a boiling water reactor fuel during operation. A BWR fuel assembly has a square assembly. cross section fuel channel that encloses the fuel assembly on all four sides. The fuel channel limits the flow of water from one fuel assembly to adjacent fuel fuel rod to hold the fuel pellets down in the fuel tube assemblies in the reactor core. In the reactor pressure during transportation to the reactor and end caps are vessel, 400–800 fuel assemblies are arranged into an welded onto the upper and lower ends of the fuel approximately cylindrical geometry. rod. Fuel rods are inserted into spacer grids that are As with the PWR, BWR fuel typically operates in fabricated from a zirconium or inconel alloy. Fuel a reactor for 3–5 years. The general characteristics of rods are held in place by small leaf springs in the BWR and PWR spent fuels are similar. BWR spent spacer grids. Depending on the fuel assembly design, fuel is stored on-site and is eventually shipped off-site from 36 to 81 fuel rod locations are available in a for reprocessing or to a high-level radioactive waste BWR fuel assembly. Most fuel assembly designs repository for disposal. Table II provides a summary arrange fuel rods in a square array, with 7, 8, or of key design characteristics of a typical boiling 9 fuel rod locations on a side. Depending on the fuel water reactor. assembly design, several fuel rod locations contain an open-ended zirconium alloy rod to allow water to flow the length of the fuel assembly as water boils in 4. SHIP PROPULSION the rest of the reactor core. This open-ended rod is known as a ‘‘water rod.’’ At the top and bottom of Nuclear power is particularly suitable for ships, the fuel assembly are end fittings. The end fittings are which need to be at sea for long periods without typically machined form stainless steel and provide refueling, or for powerful submarine propulsion. structural strength to the fuel assembly and help Today, over 150 ships are powered by small nuclear control inlet and outlet flow from the reactor core reactors. The United States Navy operates some 100 Nuclear Fission Reactors: Boiling Water and Pressurized Water Reactors 339 nuclear ships. Though most nuclear-powered vessels In contrast, nuclear propulsion has proved both are submarines, they range in type from icebreakers technically and economically feasible in the Soviet to aircraft carriers. Arctic. The power levels and energy required for The development of nuclear propulsion marked icebreaking, coupled with refueling difficulties for the transition of submarines from slow underwater other types of vessels, are significant factors. The vessels to warships capable of sustaining 20–25 knots icebreaker Lenin was the world’s first nuclear- while submerged for months. The Nautilus led to the powered surface vessel and remained in service for parallel development of further submarines, powered 30 years, though new reactors were fitted in 1970. It by single pressurized water reactors, and an aircraft led to a series of larger icebreakers, the Arktika class, carrier, the USS Enterprise, powered by eight reactor launched beginning in 1975. These vessels have two units in 1960. A cruiser, the USS Long Beach, reactors and are used in deep Arctic waters. The followed in 1961 and was powered by two of these Arktika was the first surface vessel to reach the early reactor units. Remarkably, the Enterprise North Pole. For use in shallow waters such as remains in service. Over time, standardized reactor estuaries and rivers, shallow-draft Taymyr-class ice- designs for naval propulsion were developed and breakers with one reactor are being built in Finland built by both Westinghouse and General Electric. and fitted with their nuclear steam supply system in The U.S. technology was shared with Britain; Russia. They are built to conform to international nuclear-powered ship development in France, the safety standards for nuclear vessels. former Soviet Union, and China proceeded sepa- Naval reactors are pressurized water types, which rately. differ from commercial reactors producing electricity The Soviet Union built 245 nuclear submarines in that they have high power density in a small between 1950 and 1994. The largest submarines are volume and therefore run on highly enriched the Typhoon class, powered by twin 190-MWt PWR uranium (420% uranium-235) and long core lives, reactors. The Soviet Union developed both PWR and so that refueling is needed only after 10 or more lead-bismuth cooled reactor designs, the latter using years. New cores are designed to last 50 years in highly enriched (490%) fuel. Eventually, three gene- carriers and 30–40 years in submarines. Decommis- rations of Russian submarine PWRs were utilized, sioning nuclear-powered submarines has become a the last entering service in 1987. At the end of the major task for both the United States and Russia. Cold War, in 1989, there were over 400 nuclear- After removing the spent fuel, normal practice is to powered submarines operational or being built. cut the reactor section from the vessel for disposal in Some 250 of these submarines have now shallow land burial as low-level waste. been scrapped and some on order were canceled, due to weapons reduction programs. Russia and the United States had over 100 each; the United SEE ALSO THE Kingdom and France had less than 20 each and China had 6. The United States is the main navy FOLLOWING ARTICLES with nuclear-powered aircraft carriers; both the United States and Russia have had nuclear-powered Nuclear Engineering Nuclear Fuel: Design and cruisers. Russia has eight nuclear icebreakers in Fabrication Nuclear Fuel Reprocessing Nuclear service or being built. Fusion Reactors Nuclear Power Economics Development of nuclear merchant ships began in Nuclear Power, History of Nuclear Power Plants, the 1950s but has not been commercially successful. Decommissioning of Nuclear Power: Risk Analysis The U.S.-built NS Savannah was commissioned in Nuclear Proliferation and Diversion Nuclear 1962 and decommissioned 8 years later. It was a Waste Occupational Health Risks in Nuclear Power technical success, but not economically viable. The Public Reaction to Nuclear Power Siting and German-built Otto Hahn cargo ship and research Disposal facility sailed some 650,000 nautical miles on 126 voyages in 10 years without any technical Further Reading problems. However, it proved too expensive to operate and was converted to diesel. The Japanese Duke Power Company (1996). ‘‘McGuire Nuclear Station Updated Final Safety Analysis Report, May 1996.’’ Duke Mutsu was the third civil vessel. It faced technical Power Company, Charlotte, North Carolina. and political problems. These three vessels used International Atomic Energy Agency (IAEA). (2001). ‘‘Energy, reactors with low-enriched uranium fuel. Electricity and Nuclear Power Estimates for the Period up to 340 Nuclear Fission Reactors: Boiling Water and Pressurized Water Reactors

2020.’’ Reference Data Series No. 1, July 2001. IAEA, Vienna, Uranium Information Center (2002). ‘‘Outline History of Nuclear Austria. Energy.’’ Nuclear Issues Brieﬁng Paper # 50, February 2002. Mississippi Power and Light Company (1995). ‘‘Grand Gulf Uranium Information Center, Melbourne, Australia. Final Safety Analysis Report, December 1995.’’ Mississippi Uranium Information Center (2002). ‘‘Nuclear-Powered Ships.’’ Power and Light Company (Entergy Corporation), New Nuclear Issues Brieﬁng Paper # 32, June 2002. Uranium Orleans. Information Center, Melbourne, Australia.