The Nuclear Power Industry Generating Electricity via Nuclear Energy

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Let us now visit a nuclear power plant. Let us step inside. No, this is not a giant teakettle, but it certainly acts like one. Nuclear power plants and teakettles share a common characteristic. They boil water, and produce steam. With steam, a kettle can whistle to indicate that it's ready. The steam in a nuclear power plant turns a turbine that spins a generator that produces electricity that lights our homes when we flip a switch. That's it. Generation of electricity is the name of the game.

A nuclear power plant (NPP) uses steam to generate electricity, just as coal-, oil-, and gas-fired power plants do. The difference between a fossil fuel power plant and a NPP is the method used to heat the water that produces steam and, of course, the fact that NPPs do not produce greenhouse gases. In a NPP, uranium is the fuel used in place of coal or oil. However, unlike coal or oil, uranium doesn't burn; it fissions. Also, as noted earlier, although atoms of the same element have an equal number of protons in their nuclei, they can have different numbers of neutrons. These different forms of the same element are called isotopes. The isotope for reactor fuel is uranium-235, which releases heat via fission: the splitting apart of a heavy atom into two atoms having slightly less mass. Time out for fission.

Enrico Fermi, an Italian physicist, conducted experiments in 1934 showing that neutrons could split atoms. When he bombarded uranium with neutrons, the elements he got were lighter than he expected. Four years later, Otto Hahn and Fritz Strassman, conducting experiments in Germany, fired neutrons into uranium and were similarly surprised to obtain barium, a much lighter element, half the atomic mass of uranium. Lisa Meitner in Copenhagen, working with Niels Bohr and Otto Frish, was quick to realize that the lighter elements that both Fermi and Hahn and Strassman found, fit perfectly with Einstein's mass/ energy equality as expressed in his iconic formula, E = mc2. She showed that the lost mass had changed to an unprecedented amount of energy and heat, which proved that fission had occurred, and also was another confirmation of Einstein's work.

Natural uranium consists of 0.72% of 235U, 99.27 % 238U, and a trace of 234U. The 0.72% of 235U is not sufficient to produce a self-sustaining critical chain reaction in the type of nuclear reactors used in the United States, which require uranium fuel enriched to 2.5-3.5%.

The 103 operating reactors in the United States are either pressurized-water reactors (PWRs) or boiling -water reactors (BWRs). Other than the difference in pressure, both consist of four major components: nuclear fuel, control rods, coolant (or moderator), and shielding.

The essential factor in fissioning is the enormous amount of energy released as a consequence of the slight difference in mass that Einstein' s equation indicates, namely, that mass and energy are interchangeable. The fissioning of a single 235U nucleus releases about 200 million electronvolts (MeV) of energy—far more energy than the reaction of burning coal or oil to heat water. As shown in Figure 6.6, when a neutron strikes an atom of 235U, it releases heat and 2-3 neutrons, each of which is now available for the fission of three more 235U nuclei. In the next stage, about 9 neutrons (3 from each of the three fissioned uranium nuclei) are released. As long as more neutrons are being released, the fissioning continues as a chain reaction. In this way, many billions of 235U nuclei can fission in less than a second, with the release of huge amounts of energy in a short time.

This heat - producing fissioning is generated and controlled in the reactor, which contains the core, control rods, and coolant. The reactor in a NPP performs the same function as a boiler in a fossil fuel plant, but no combustion gases are produced by a NPP.

Boiling-water reactors heat water in the core and allow the water to boil directly into steam, which goes directly to a turbine. A pressurized- water reactor uses water under pressure to cool the reactor and transfer heat. The heated water transfers its heat energy to a secondary system where steam is produced and is then piped to a turbine. The core of the reactor, the heart of a NPP, contains the uranium fuel, which is formed into ceramic-coated pellets

Neutron 9 • Neutron

Neutron 9 • Neutron

U-235

U-235

Neutron

Figure 6.6. The process of nuclear fission; high-energy neutrons can split the nucleus of heavy uranium atoms, releasing lighter atoms and great amounts of energy. (Figure courtesy of the Department of Energy.)

about 3 inch in diameter and a bit more than a 1 inch long. Each uranium oxide (UO2) pellet releases about the same amount of energy as does a ton of coal or 200 gallons of oil. Indeed, that's one of the great benefits of nuclear power, the tremendous amount of coal and oil not needed. These energy-rich pellets are stacked end to end in 12-14-foot-long rectangular arrays, and just under ¿-inch-diameter zirconium alloy tubes. These fuel rods, shown in Figure 6.7, are arranged into bundles of 225 or more, making up a fuel assembly. Reactor cores can contain 150-800 assemblies, weighing over half a ton.

Fission occurs within the assemblies, and is controlled by the neutron-absorbing cadmium control rods. As neutrons move swiftly (one-fifth the speed of light), they must be slowed if they are to strike 235U nuclei, rather than simply flying through the rods. The coolant, in this case water, also acts to moderate neutron speed.

When the control rods are raised out of the core, fission increases, producing more heat. As the rods are lowered, fission decreases along with heat production in the coolant flowing between the fuel assembly rods. The coolant pre-

ROD CLUSTER CONTROL TOP NOZZLE

CONTROL ROD

GRID

BULGE JOINTS

GRID SPRING

BOTTOM NOZZLE

CONTROL ROD

GRID

BULGE JOINTS

GRID SPRING

HOLD DOWN SPRING

THIMBLE TUBE

MIXING VANES

DASHPOT REGION

DIMPLE

Figure 6.7. Views of the control rods: (a) a labeled schematic diagram showing all the elements; (b) a close-up view showing the size of a bundle; (c) a view showing the tubes in place.

HOLD DOWN SPRING

THIMBLE TUBE

MIXING VANES

DASHPOT REGION

DIMPLE

THIMBLE SCREW

Figure 6.7. Views of the control rods: (a) a labeled schematic diagram showing all the elements; (b) a close-up view showing the size of a bundle; (c) a view showing the tubes in place.

vents the core reactor from becoming too hot and also carries heat away from the reactor to the steam generator.

In a pressurized-water reactor, the piping system that contains the coolant is called the primary side. The separate system of piping where steam is produced is the secondary side - These systems do not mix. The heated primary-system water flows through the steam generator tubes, which are surrounded by the cooler water of the secondary system. The steam generator is the link between the two systems. In the PWR, a pressurizer maintains the primary side at high pressure to prevent boiling, yet permits water temperatures to reach 600°F (315°C). Since primary-system water is far hotter than secondary-system water, it easily boils the secondary-system water to steam, which is then piped to a turbine whose shaft spins furiously, driving the connected generator,

which churns out electricity. After spinning the turbines, the steam condenses back to water, then returns to the reactor, and the cycle begins again. A reactor coolant pump keeps the primary water circulating in the closed primary system.

Because the fission process is radioactive, barriers are built in to protect against release of radioactivity. As noted, the uranium oxide fuel is formed into ceramic pellets, sealing in the radioactive compound. The fuel pellets packed into the zirconium rods prevent release of fission products.

When a new nuclear power plant starts up, a neutron source such as plutonium is added to provide neutrons to initiate the chain reaction. When a reactor is shut down for refueling, there are neutrons in the reactor that can kick-start the chain reaction. The core, as shown in Figure 6.8 , where fission occurs, is placed in a shielded 450-ton, eight-inch-thick steel reactor vessel. The reactor itself is housed in the containment building with its multiple layers of protection noted in the figure (lower right). Although NPPs produce no carbon dioxide or other greenhouse gases, they do produce waste in the form of spent nuclear fuel. Every 12-18 months NPPs are shut down for refueling. One -

fourth to one-third of the oldest fuel assemblies are replaced. After 3-4 years in a reactor, the uranium pellets are no longer sufficiently energized to power the chain reaction, but still hot enough to be harmful. This waste, referred to as spent fuel, or high-level waste, must be managed appropriately, as it contains radioactive cesium, strontium, technetium, and neptunium. Given their various half-lives, some will remain radioactive for several years, while others will be radioactive for millions of years. If we were to take all the spent-fuel assemblies produced to date in the United States by the 103 operating NPPs, which generate 20% of the country's electricity, and stack them side-by-side, end-to-end, the assemblies would cover an area of about the size of a football field to a depth of about 15 feet. Not all that much. But how to store them safely?

When spent fuel is initially removed from a reactor, the assemblies are placed on racks in a 40 - foot- deep pool of water contained in a steel -lined concrete basin (e.g., as shown in Fig. 6.8, upper left), where the water cools the fuel. After it has cooled for about a year, the fuel is moved to dry, 18-foot-high storage casks with 9-inch-thick skins of carbon steel, which hold about 18 tons each (similar to those shown in Fig. 6.8 , lower left). These casks are either placed upright on concrete pads, or stored horizontally in concrete bunkers awaiting removal to a permanent storage facility. In September 2005, after a protracted struggle, the Nuclear Regulatory Commission voted to issue a

Valve Koch Glish
Figure 6.B. Energy sources for US electricity and spent-fuel statistics. {Source: Office of Civilian Radioactive Waste Management.)

license to "private fuel storage," to build and operate a used nuclear fuel storage facility at Skull Valley, Utah, on the Goshute Indian Reservation, which the Goshutes have tried to obtain for the past 10 years. This wind-swept land of sage and scrub, 50 miles west of Salt Lake City, Utah, will become the country's largest bunker for highly radioactive waste, until a final resting place is established—as for example, at Yucca Mountain, Nevada, which we shall deal with shortly [32].

Radioactive waste can also be low-level waste. Approximately 90% of the radioactive waste produced around the world is low- l evel, but contains no more than 5% of all the radioactivity in low and high levels combined. This type of waste consists of lightly contaminated trash and debris, such as paper, clothing, cleaning materials, metal and glass, and tools used in commercial and medical nuclear industries. Many countries bury their low- l evel, short-l ived waste in protected shallow trenches, or concrete-lined bunkers. Most low-level waste decays away to natural background levels in months or several years. In the United States, low-level waste is sent to disposal sites, such as Barnwell, South Carolina, and Richland, Washington, licensed by the Nuclear Regulatory Commission (NRC). Each state, or group of states, is responsible for disposing of and managing this waste. At these sites, the NRC requires that emissions not exceed an annual dose to any member of the public of 25 mrem to the whole body, 75 mrem to the thyroid, or 25 mrem to any other organ. Actual public exposures are far less than the NRC limits.

Since the mid-1940s spent fuel has accumulated throughout the country, stored in temporary facilities at some 125 sites in 39 states, located in urban, suburban, and rural areas, most near large bodies of water. Over 160 million people live within 75 miles of temporarily stored nuclear waste. Current storage methods are safe, but these above-ground facilities are not meant for long-term storage and will not withstand rain, wind, sun, and other environmental risks for the tens of thousands of years that they are expected to remain hazardous.

The international scientific community has determined that the best option for permanent storage is underground, and that deep, geologic disposal is technically feasible and will protect the public, provide security, and protect the environment. Think Yucca Mountain.

Yucca Mountain, in Nye County, Nevada, 90 miles northwest of Las Vegas, has been studied by the U.S. Department of Energy (DOE) and scientists in university geology departments for 20 years, without agreement. Nevertheless, in 2002, Congress approved President Bush's recommendation of Yucca Mountain as a suitable site for the DOE to construct and operate a geologic repository to safely and permanently dispose of 50,000 metric tons of high-level/spent nuclear waste currently stored at 72 sites across the country. To construct and operate the 1000-foot-deep repository, DOE must obtain a license from the Nuclear Regulatory Commission. As part of the license application, DOE must demonstrate an effective quality assurance program that ensures its safe construction and operation, while protecting public health [33].

DOE must also prove to the EPA 's satisfaction that radiation will be safely contained for 10,000. At this time, DOE's plan to have waste shipped to Yucca Mountain by 2010 may be unrealistic, given changes to its plan by the state of Nevada, and the NRC's line-by-line scrutiny of DOE's proposal. However, with pressure mounting for cleanup of the many sites, and for safe, permanent disposal, Congress may force the issue. Safe passage to Yucca Mountain would then become an issue as a safe, dependable transportation system becomes a crucial link. With primary responsibility for regulating the safe transport of radioactive materials in the United States, the Department of Transportation (DOT), has been working with the NRC to set design and performance standards that must be met for a transportation package or container to be certified [34].

Of overriding importance is the fact that during the past 40 years more than 3000 shipments of spent nuclear fuel have been transported over our highways, waterways, and railroads, with a safety record of no fatalities, injuries, or environmental damage caused by the radioactive cargoes. Contributing to this successful record are the rugged, dumb-bell-shaped containers that have been developed. As shown in Figure 6.9, these are heavy, sealed, thick-walled, steel structures that safely confine the spent-fuel assemblies. These casks are considered to be the must robust containers yet developed by the transportation industry. They are also designed to shield a train shipment, on the rails and buffer cars, both fore and aft. Container design and integrity must demonstrate protection against radiologic release under the following hypothetical accidents:

Figure 6.9. The rugged dumbbell-shaped containers used for transporting spent-fuel rods.

• A 30-foot free fall on to an unyielding surface

• A puncture test allowing the container to free-fall 40 inches onto a steel rod, 6 inches in diameter

• An 8-hour immersion under 3 feet of water

For spent fuel, an undamaged cask must be subjected to a one-hour immersion under 660 feet (200 meters) of water [35].

On arrival at Yucca Mountain, waste will be transferred to permanent disposal containers. Management of nuclear waste is an issue that typifies scientific uncertainty and complexity and that can easily polarize people. Its extremely long-term character raises questions of intergenerational concern, and deals with questions of energy sufficiency versus long-term security. As there are no "right" answers to those ethical questions, how can the needs of the current generation be accommodated with those that follow? One option may be to reconsider a 300,000-year repository for a more modest, shorter plan, venturing that the following generation will find better ways of managing the fuel. So, with their anticipated life of a hundred years, temporary storage casks will contain cooler fuel. Recall that after 150 years, 5 half-lives, 32nd of both cesium and strontium's initial potency will be left. But that 100-150 years will be far from the decay requirements of the heavier isotopes. Nevertheless, time does reduce heat and radiation. Although uranium could be recovered by reprocessing, current reprocessing costs are prohibitive, given the low cost of new uranium. But things could be different in 100 years. If in a 100 years the world has run out of oil, and burning coal would be too climate-damaging, nuclear power would be in high demand, and there would be need for 5001000 additional NPPs—or, depending on the state of fuel cells and other alternate energy sources, there would be little need for nuclear power. So, why not use the casks for the next 50-75 years, then decide what's next? By that time new storage technologies will surely have been developed, allowing greater confidence in the type of storage that would be suitable for several hundred thousand years. There is even current experimental evidence that transmutation would effectively render high-level waste benign. Particle accelerators, used to produce isotopes for medical use, could be used to fire subatomic particles into high-level waste, changing long-lived radioactive elements to short-lived waste. Creativity is something we excel at. Before too long, the storage problem, too, will be solved [ 36]—and with the next generation of nuclear reactors, far less waste is anticipated.

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