An inertial fusion energy (IFE) power plant consists of a target production facility (or target factory), target injection and tracking systems, the laser, a fusion chamber, and a power conversion system.
In the plant, many pulses of fusion energy per second (typically 10 to 20) would heat a low-activation coolant, such as lithium-bearing liquid metals or molten salts, surrounding the fusion targets. The coolant in turn would transfer the fusion heat to a turbine and generator to produce electricity. A NIF-scale laser operating at this repetition rate would produce more than 1,000 megawatts (MW) of electricity for the grid—enough to power a city the size of San Francisco.
The NIF laser operates at only a low repetition rate (a few shots per day), due to the need to carefully configure the research experiments and allow the facility's optical components to cool down between shots. The requirement to operate an IFE power plant at high pulse repetition rates requires the adoption of a new set of technologies:
Experiments on NIF will demonstrate ignition and energy gain—producing more energy from fusion than the laser delivers to start the process. For IFE, a gain between 50 and 100 is needed in order to minimize the portion of generated electric power that has to be recirculated within the plant to operate the laser.
The target factory must produce a continuous supply of high-quality targets at an acceptable cost—typically about one million targets per day at about 25 cents per target. The cost of the materials in the target is only a few cents; the majority of the cost arises from the production equipment and labor, as with most other mass-manufactured products. Detailed conceptual design studies for IFE target factories have been completed by LLNL and General Atomics, a participant in the National Ignition Campaign.
Many types of targets are being considered for laser IFE, including indirect-drive (like those that will be shot on NIF), direct-drive (currently being tested on the OMEGA laser at the University of Rochester), and advanced designs including fast ignition and shock ignition (see Target Fabrication).
In all cases, the deuterium–tritium (DT) fusion fuel is contained in a spherical fuel capsule, held at low temperature so that it forms a layer of DT ice on the capsule's inner surface. The capsules are made of carbon, beryllium, or carbon–hydrogen polymers. NIF will be able to test full-scale targets for an IFE plant, using the as-manufactured products, and so directly underwrite the fusion scheme being adopted.
In direct-drive, the capsule is directly irradiated by the laser beams. In indirect-drive, the capsule is placed inside a hohlraum—a centimeter-scale, can-shaped container made with high-atomic-mass materials such as gold and lead with holes in the ends for beam entry.
In NIF, targets are held in place at the center of the chamber A prototype gas gun target injection and tracking experiment at General Atomics showed that the accuracy needed for indirect drive targets can be achieved.and the beams are aligned to a fixed position for each shot. For IFE, targets will be injected at speeds greater than 100 meters a second and tracked in flight to provide data to a real-time beam-pointing system needed to assure the precise illumination required to achieve ignition and high energy gain. Target injection experiments using gas guns have been conducted at General Atomics, and tracking and engagement studies are under way. The required accuracy appears consistent with the use of existing technology and acousto-optic deflection technology to steer the laser beams onto the target.
While the total energy of the laser will be comparable to, or even less than, that of NIF, the IFE laser must operate at 10 to 20 shots a second, depending on the target yield per shot and the desired electricity output of the power plant. The vast majority of research on high-repetition-rate lasers for high-energy applications is focused on diode-pumped solid-state Lasers (DPSSLs), building on more than 35 years of experience with solid-state lasers leading up to NIF.
In addition to the ability to operate at high repetition rates, key considerations for the IFE laser include high efficiency (greater than 10 percent), low cost (to keep the price of electricity competitive with other energy options), long-life optics, high reliability, and low maintenance costs.
The Mercury laser projectat LLNL has been used to develop laser technology to meet these requirements, along with operational experience from the Laboratory's Atomic Vapor Laser Isotope Separation (AVLIS) facility and a range of other bench-top test facilities. AVLIS demonstrated the ability to operate high-power lasers running continuously for extended durations (10 years) at high availability (99 percent). Lessons learned from Mercury, combined with new developments in laser architecture and continuing improvements in components such as diode arrays and materials science, have defined the path to the design and construction of a beamline capable of driving an IFE power plant.
Each fusion target releases a burst of fusion energy in the form of high-energy neutrons (about 70 percent of the energy), X-rays, and energetic ions.
An artist's rendition of a LIFE power plant driven by diode-pumped solid-state lasers. Experiments at the National Ignition Facility will help provide the proof of concept for laser fusion power plants such as LIFE.The fusion chamber includes a meter-thick region that contains lithium (as a liquid metal, molten salt, or solid compound). This serves two purposes. First, it absorbs the fusion output energy, heating up to about 600 degrees Celsius. A heat exchanger is then used to drive a super-critical steam turbine cycle to produce the electricity. Second, this lithium "blanket" produces tritium through reactions with the fusion neutrons. This allows a closed fuel cycle to be formed, in which the power plant itself produces a key component of its own fuel. The other part of the fuel, deuterium, is extracted from seawater (roughly one part in 6,700). The deuterium found in just 45 liters of water, combined with the amount of lithium found in a single laptop battery would (allowing for inefficiencies) produce 200,000 kilowatt-hours—roughly 20 to 30 years' worth of electricity for someone living in Europe or the United States.
A key issue for the chamber is the survival of the innermost wall (first wall) that is exposed to intense heat and radiation from the target's X-rays, ions, and neutrons. Another key issue for the fusion chamber is intra-shot recovery: the conditions inside the chamber (such as vapor and droplet density) that must be recovered between each shot to the point that the next target can be injected and the laser beams can propagate through the chamber to the target.
Recent work at LLNL, in conjunction with partners across the United States, has produced an IFE power plant design known as LIFE (Laser Inertial Fusion Energy)to tackle these issues in a self-consistent manner using available materials and technologies.
By flowing a coolant through the fusion chamber at a steady rate, the pulsed fusion energy can be extracted at a constant rate and delivered to the power conversion system, which converts the thermal power to electric power. When liquids such as lithium metal or molten lithium salts are used for tritium breeding, the liquid is generally circulated as the primary coolant for the fusion chamber. When solid breeders such as lithium–aluminate (LiAl02) are used, high-pressure helium serves as the chamber coolant. In either case, the primary coolant circulates through heat exchangers that power electric power equipment. The efficiency of the power conversion systems depends on the outlet temperature of the primary coolant, which is limited by the materials used in the construction of the chamber. Conversion efficiencies of 40 to 50 percent are now available for the temperatures produced from fusion, using technology developed for modern coal plants. Some work has also been done on concepts for converting a portion of the target energy output directly to electricity.
An advantage of IFE is that the subsystems described above can be developed and tested separately, and often at lower cost than fully integrated facilities. The IFE power plant, however, requires successful integration of all the components with careful attention to the interfaces and the impact of design choices for one system on the others.
Artist's rendering of the exterior of a LIFE power plant.
Inherent safety and environmental sustainability are key benefits of fusion. Key characteristics include:
The consequences of accidents significantly beyond the design basis are well within regulatory limits, and represent a paradigm change from the dangers posed by nuclear power or gas transmission pipelines.
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