For a number of years, the United States government has chosen not to have an energy policy. Because of the scarcity of energy, we now are paying a high price. Today, average citizens are learning the difficult lesson that energy plays a much more significant role in their lives than they ever realized, and conditions will get worse before they improve. Between now and 2050, worldwide demand for electricity will double. Although new developments have been made in renewable power, the bulk of this need will be met with either coal-fired or nuclear-reactor power plants. Unfortunately, power plants take years to build; with nuclear types taking the longest.
As the cost of oil skyrockets, worldwide demand increases and supplies decrease. All forms of energy production are increasing as well; the cost to mine and ship coal has doubled in the past year. At the same time, the percentage of greenhouse gases in the atmosphere continue to increase, while public awareness of the effects of global warming changes the way we think about natural resources. Electricity is the form of energy we most depend upon and this dependence will only increase with time. In the world of tomorrow, electricity will charge electric cars batteries, produce hydrogen cheaply for fuel cell-powered vehicles, and possibly heat buildings in order to reduce the dependence on fossil fuels.
The question is where will all this power come from? At best, the electricity generated from wind, solar, and hydroelectric will supply 20% of our need—possibly even less 50 years from now, which leaves coal and nuclear fuels as the primary source for energy. The big problem with coal-fired generators, however, is the release of radiation, airborne mercury, and carbon dioxide (CO2) into the atmosphere. As a clean source of energy nuclear fission reactors will generate the needed power for the next 50 years. After that, it's hoped that fusion reactors (which combine small atoms to make bigger ones instead of breaking heavy atoms apart into smaller ones) will solve the energy needs.
Mark Peters, deputy to the associate laboratory director at Argonne National Laboratory, Argonne, Ill., says the public has three concerns about nuclear reactors: they must be safe from extreme forces of nature, such as tornadoes and earthquakes, and man, as well as secure from terrorist attacks; they must not leak radiation; and highly radioactive spent fuels must be recycled, not stored.
The greatest cost of producing electricity from nuclear power plants is the initial cost of construction. The goal is to build better plants with longer life expectancies on a shorter construction cycle. David Matthews, director of the division of new reactor licensing at the Nuclear Regulatory Commission (NRC), Washington, D.C., says that before the Three Mile Island (TMI) accident in 1979, it took, on average, a little more than five years to build a new nuclear power plant following issuance of a construction permit by the NRC. After the accident, the process took more than 11 years, making the it prohibitively expensive. The NRC is reducing the permitting time by precertifying standardized power plant designs submitted by reactor manufacturers such as AREVA, General Electric, Mitsubishi, and Westinghouse. These certified designs then can be built at locations around the country, while owners apply for a combined construction and operating license (COL). This process is expected to lead to a reduction in construction time, in contrast to the lengthy delays experienced during the post-TMI period, because the design is expected to be essentially complete before the start of construction. The design life for reactors built in the 1970s and 1980s was 40 years. However, most of these plants have performed well and have been recertified for an additional 20 years. The current thought is that most will win certification for an additional 20 years after that, making the cost of producing electricity very reasonable. New reactor facilities may start with certifications of 40 years but with a design life of 60. Given the advancements in concrete technology, it should be possible to specify 100-year and longer service lives; this is also done with structural concrete bridges today. It means that the extended service life of a reactor will depend more on improvements to the steel vessels that contain the nuclear reaction.
Currently the United States has 104 operating nuclear power plants, generating 20% of our electricity (90% of Chicago's power is from nuclear power plants). However, 80% of France's power is from its 59 nuclear power plants. The French company AREVA, Bethesda, Md., now leads the world in nuclear power plant construction.
How it all started
On Dec. 2, 1942, a group of 50 scientists led by Enrico Fermi gathered in an old squash court under the football stadium at the University of Chicago to witness the first controlled nuclear chain reaction. After CP-1 (Chicago Pile #1) as the project was called, the U.S. government, under the direction of the Army Corps of Engineers, organized the “Manhattan Project” to develop the first bomb.
In 1946, the Department of Energy (DOE) chartered Argonne National Laboratory to be a primary energy research center, administrated by the University of Chicago. Argonne, being a direct descendant of the Manhattan Project, would focus on all forms of energy research. The primary goal at the beginning was to develop a nuclear reactor for the production of electricity, which was achieved in 1951 with the construction of a research reactor at its Idaho facility—the world's first nuclear reactor power plant. In 1954, Russia became the first country to actually connect a nuclear power plant to an electrical grid. From that time forward, countries all over the world have built nuclear power plants.
After the TMI accident construction of new power plants stopped in the United States due to complicated regulation, increased financial risk to owners, market forces, and negative public sentiment. The 1986 Chernobyl plant meltdown in the Ukraine slowed down, but did not stop, construction for the rest of the world.
By the time of the Chernobyl accident, attitudes about nuclear power in the United States were mostly negative. The four fundamental fears included: nuclear power was too dangerous for man to experiment with, there would be more plant meltdowns and people would be exposed to radiation, there was no safe place to store nuclear wastes, and fuel from power plants could find their way into the wrong hands to make bombs.
Types of reactors
Nuclear fission reactors all work the same. Atoms with large numbers of protons and neutrons in their nucleus are split by neutron bombardment into smaller atoms—a process called fission. The splitting of these atoms releases large amounts of energy in the form of heat used to turn water into steam. This drives a steam turbine, which turns a generator to make electricity. Radiation also is released in the fission process, requiring containment and protection.
The reactors in nuclear power plants are defined by the generation of their design. Generations include I, II, III, III+, and IV (see Figures 2 and 3). Generation II reactors were built in the 70s and 80s, with Generation III+ being the next power plants to be built in the United States. At the present time, Toshiba is the only company building a Generation III (in Japan) and AREVA is the only one building Generation III+ plants—one in Finland and one in northern France. Generation III and III+ include refinements to Generation II and III designs. Christopher Grandy, a department manager for the Nuclear Engineering Division at Argonne, says the most significant changes involve greatly improved passive safety features (that requires less human intervention), fewer components, and simpler construction.
Compared to Generation II, Generation III and III+ power plants have several advantages. They cost less to build through standardized designs, reduce permit times, and lower construction costs. Service lifetimes are longer and components more reliable too. But increased safety is the primary emphasis, featuring more redundancy in safety systems and the inclusion of passive systems for Generation III+ plants for cooling reactors in the case of an unlikely event.
One of Argonne's principal roles is to research next generation reactors and make them practical for commercial use. Currently, research on Generation IV reactors is occurring; construction of these generators can be expected in the United States in about 20 years. These designs will be significantly different than previous generations and provide additional benefits. For instance, they will operate at much higher temperatures, making it possible to heat water high enough to separate it into hydrogen and oxygen, providing hydrogen for fuel cell generators and vehicles. Generation IV breeder and “actinide burner” reactors will reuse fuels from conventional Boiling Water Reactors (BWR) and Pressurized Water Reactors (PWR), greatly reducing waste. Reprocessing plants, which also should be built at this time, will recycle spent fuels and decrease waste material's radioactive half life to less than 200 years. Using German technology, there is one “pebble bed” high-temperature gas-cooled GenerationIV plant under construction atthe present time in South Africa.
Where concrete comes in
Much of the concrete in a reactor is for protection from all types of natural and man-caused disasters. Structural concrete walls protect against tornadoes, hurricanes, fire, and even the possible impact of a full-sized commercial jet. The engineered structurally reinforced concrete basemats of a reactor are typically 10 to 18 feet thick, provide seismic protection, and serve as the reactor vessel's foundation. Concrete, sometimes using borate and hematite shielding aggregates, prevents radiation from reaching beyond the vessel area.
Concrete requirements
It's been 30 years since the United States undertook construction of nuclear power plants and there have been many technical advancements to concrete in the interim: replacing some portland cement with pozzolans, the development of new admixtures, SCC, and the use of well-graded concrete mixes, to name a few. For good reason, the nuclear industry has always taken a conservative position regarding the inclusion of new products. So the question is whether some of these advancements will be used in next generation plant construction. Using SCC, for example, could greatly facilitate concrete placements at locations with rebar congestion. Dan Magnarelli, a construction manager for AREVA in the United States says they take advantage of improvements in concrete for plant constructions in Europe and hope to do the same when construction starts in the United States. This will include designing high-performance concrete (HPC) mixes with pozzolans to manage thermal cracking for mass concrete placements, using admixtures to facilitate placement while keeping lower water-cement ratios, and the possible use of SCC in some locations—especially where there is significant congestion of steel reinforcement.
Ray Ganthner, the senior vice president for plants deployment for the U.S. market for AREVA, says that the primary function of concrete in a nuclear power plant is safety. In the case of AREVA's EPR, a 4½-foot-thick reinforced concrete shield building that protects reactor components from outside forces is built over another 4½-foot-thick post-tensioned concrete containment building with a steel liner to protect the outside from anything that could happen inside the containment. The design of concrete mixes primarily will focus on limiting cracking and increasing durability. Here are some of the goals for concrete properties for reactor construction:
- Compression strength. Depending on where concrete is placed, requirements typically will fall between 4000 to 7500 psi.
- Flexural strength. Increased strength will be especially important for slab on ground and elevated slab placements at soil sites.
- Durability. Designed service lives for nuclear power plants are approaching 100 years. Concrete mixes will be designed to meet this requirement.
- Permeability. In some areas of a reactor, low permeability concrete will be important.
- Water-cement ratio (w/c). In order to control shrinkage, w/c ratios will be closely monitored.
- Thermal cracking. A lot of mass concrete requires cracking to be controlled primarily though mix designs using pozzolans.
- Consolidation. There must be good consolidation around steel reinforcement and this will be managed partly by mix design.
- Finish. Concrete finishes are more important in the areas of a plant where there is significant potential for contamination.
The construction process
From the context of present day, it's hard to imagine that the Generation II nuclear power plants were constructed without the technology that we've come to rely on today. The question is how so many critical systems were threaded together in the nuclear power generation stations of the 70s and 80s and how have they worked so well for so long?
In some ways, that effort involved building advanced prototypes, most with considerable custom work in both design and construction. As the power stations neared completion, they went through an exhaustive engineering process. Hundreds of engineers labored onsite resolving field problems and reconfirming design assumptions.
Today, we find ourselves on the threshold of a new generation of nuclear power station construction, equipped with new tools and a new approach. With this new approach, the NRC will issue a COL to allow both construction and operation, which should shorten the overall time for bringing a plant on line. By basing the licensing process on certified designs, the main focus can be on adapting the housing for the reactor and critical systems to a particular geographic location. This round of plant building will focus on construction meeting specific design codes.
Among the new tools, the most striking are those in the information technology field. Consider this, as Generation II plants reached completion, the move from mainframe computing to PCs had barely begun and the Windows operating system was yet to become a household name. The internet was in its infancy, dependent on dial-up connections. Yet, this was the environment in which the Generation II plants came online.
Today, computer hardware is no longer a limiting factor, with more computing power available in a single PC than ever before. Software, which provides the real power behind the hardware, also has become far more sophisticated. Engineers labored through design calculations for the Generation II plants with relatively simplistic models and what now seem like rudimentary assumptions. But in 1984, the first finite element analysis programs became available for PCs, beginning the revolution in how computer analysis was executed. The computing power routinely employed in today's engineering and design is at least one quantum leap ahead of that used in the previous generation. Add to that the benefits of reliable, high-speed networking and the future begins to look quite bright for construction of nuclear power facilities.
Beyond structural analysis, engineers and others involved in power plant construction today are taking advantage of additional technologies, such as 3-D modeling. Design programs have evolved from stick models to realistic renderings including geometric and material properties; some also model the effects of time. Another facet has been advances in data acquisition making 3-D laser scanning a great asset for modeling tools. According to Geoff Jacobs, vice president of strategic marketing for Leica Geosystems HDS, San Ramon, Calif., importing scan data into CAD programs initially created difficulties. Design programs struggled with the “point cloud” data provided by laser scanners. Today, he says, various plug ins process the input allowing designers to work with scanned data within CAD programs, greatly increasing both the accuracy of a model and the speed it can be assembled.
Constantine Petropoulos, manager of the civil/structural group in Sargent & Lundy's Nuclear Power Technologies, Chicago, says 3-D computer modeling will have a major impact on construction of Generation III+ facilities. The company has been a consultant to the power industry for more than a century and already has seen how such computer modeling can be used for interference checking to eliminate field problems ahead of actual construction. He and lead structural engineer John McLean agree that 3-D modeling will enable far more system design to be completed prior to construction. “We can design the location of component supports with great certainty,” McLean says. That makes it possible to move away from the wide use of post-installed concrete anchors in favor of more embedded plates, simplifying support installation and reducing possible adverse effects on the concrete.
Sophisticated design tools are making it possible to use a more modular approach to construction. Petropoulos says the core of many plant systems—electrical and mechanical modules—will be assembled and tested offsite, then brought in, installed, and interconnected. Although this has always been the case for the containment vessels, most other system assembly occurred piecemeal.
Structural work benefits from a modular approach, says McLean, citing the example of rebar mats for the massive foundation slab for the containment building. For one type of reactor, just the upper portion of the basemat reinforcement consists of five layers of closely spaced #11 and larger rebar. To avoid working within the limitations of the excavated area, the mat can be assembled and inspected in sections in a staging area nearby, then lifted into place with a high-capacity crane for final connection and concrete placement.
Scheduling and project management tools have made quantum advancements since the last round of nuclear construction. Such tools increasingly have been employed on projects of all sizes and will streamline the construction of Generation III+ plants. They also will help in the overall quality control efforts and provide both regulators and the public with safety assurances.
New communication systems will speed Generation III plant construction. In the 1980s, faxing was slow and of dubious quality, drawings were disseminated on microfiche, and document control was an issue. Today, digital imaging is used to make documents readily exchangeable and sophisticated document control tools enable sharing, control access, and ensure that the most up-to-date version is used.
Materials and field techniques
Throughout the previous generations and into the future, concrete is a reliable mainstay in nuclear power plant construction, says Ganthner. He notes that without concrete the construction simply would not be as robust as it needs to be.
Looking to the future means using advances in concrete technology, including admixtures for better placement in difficult conditions—such as using SCC in areas with heavy rebar congestion—and water-reducing admixtures that help in strength development. Supplementary cementitious materials, such as fly ash, slag, and silica fume, likely are to be used in concrete mixes developed for Generation III+ construction, both for their performance characteristics and environmental benefits. Placement techniques and the understanding of how to control the heat of hydration in mass concrete have improved since the era of Generation II construction. Modern computers simplify quality control in concrete production, delivery, and documentation.
Construction also will be simplified by using embedded plates for attaching system components to the concrete structure. Having more complete designs earlier in the process will make this possible and eliminate much of the need for field designs based on as-built measurements of conflicting systems. Put simply, things should fit together better. Of course, that makes construction sequencing and on-time delivery of components far more important.
Newer and better field layout tools using lasers, internal computing power, and integration with design systems will help contractors achieve new levels of accuracy in the field. Building structures to better meet design criteria still will require great care but take far less effort. Additionally, reinforcing steel locations can be laid out more accurately and checked by more modern tools. Rebar congestion will be reduced by the use of mechanical couplers and welded connections.
In the years since Generation II plants were designed, advances have been made in the field of anchorage to concrete resulting in better anchor qualification testing and assessment procedures, predictor equations and design guidelines. Both concrete expansion anchors and undercut anchors, which have been developed since the last generation of nuclear construction. Although their numbers may be smaller in new facilities, post-installed anchors provide flexibility in locating attachments to concrete and continue to play a critical role in construction.
As redundant safety measures are built into new facilities, the concrete finish in certain areas of the plant will be critical, says Ganthner. This will provide one more layer of protection in the event of any spill within the plant, he says, ensuring that contamination does not enter into the concrete. Forming systems developed in recent years, as well as perhaps SCC, will help speed construction and meet finish requirements without added difficulty. Although structural concrete quality has been increasing, it's probably a good thing that it has. Ganthner reports that metallurgical advancements for reactor vessels make them more resistant to neutron radiation damage. Vessel improvements and increased concrete durability will extend reactor life.
Tomorrow's power
With increasing costs for construction materials, especially steel, and energy, it's hard to estimate the cost of new reactor construction—probably in the $6 billion range. At present the NRC has received 13 permit applications for nuclear reactor plant constructions for the United States, with a total of 34 applications anticipated by 2012. Concrete will remain a key component with more than 400,000 cubic yards for an average sized plant.
The energy crisis in the United States will become more critical before it becomes better. We need to formulate a systematic solution—it may take 10 years to solve some of our current energy problems.
Nuclear safety
Some fear that fuel from nuclear power plants can be used to make bombs. But today's reactor fuels are only 5% enriched uranium. Bombs require very highly enriched uranium, which is difficult to achieve. Nuclear fuels in a power plant can't explode either.
Since the Three Mile Island and Chernobyl accidents, significant public fear has existed about nuclear reactors, believing that coal-fired generation plants are safer. But there were no unsafe exposures to radiation at Three Mile accident or at any other nuclear power plant with the exception of Chernobyl, which was improperly built without a containment structure. There are zero deaths per year resulting from nuclear power accidents but as many as 24,000 premature deaths each year involving coal, plus an additional 60,000 neurological problems for infants worldwide associated with airborne mercury from burning coal. It takes 1,000,000 pounds of coal to produce the same amount of electricity that 1 pound of nuclear fuel does. Nuclear power plants release nothing into the atmosphere, while coal-fired plants rank among the highest sources of air pollution (including CO2).
Generation III+ power plants vastly improve the safety and efficiency of nuclear power plants. Concrete is a major safety mechanism, protecting plants during seismic activity, weather events, possible enemy attacks, and the possibility of radiation leaks into the atmosphere. When we begin construction of Generation IV plants, we will be able to recycle spent fuels and manage wastes effectively, as well as produce hydrogen inexpensively.
