Nuclear power is back in vogue as a source of energy, thanks to revived interest in safe and less expensive reactor technologies.

Authored by:
Leland Teschler

Editor
leland.teschler@penton.com

Key points:
• Reactors using liquid-metal coolant can be thermally more efficient than light-water designs and potentially simpler to build as well.
• Modular light-water reactors that employ passive cooling systems have built-in safety measures and can be less complicated than conventional designs.

Resources:
The Babcock and Wilcox Co., www.babcock.com
Hyperion Power Generation Inc., www.hyperionpowergeneration.com
NuScale Power Inc., www.nuscalepower.com
TerraPower LLC, www.terrapower.com

There is an old joke sometimes told in the nuclear-power industry: Trying to sell nuclear power is like trying to sell anthrax. That’s because it has taken close to 30 years for the nuclear industry to get past the black eye it sustained from reactor emergencies at Three Mile Island in the U. S. and Chernobyl in the Ukraine.

But nuclear experts say the conditions that led to problems at TMI and the catastrophe at Chernobyl will never happen again. Chernobyl, for example, was built without a containment structure, considered a basic safety measure since the birth of the nuclear age. The problems at TMI arose both because its operators lacked training, and from the fact that its control room was configured as a more-complicated version of what you might find in a coal-fired generation plant.

The safety record of the nuclear industry has been exemplary since the dark days of the 1970s and 80s. Just two nuclear workers (in Japan) lost their lives in the 1990s because of accidents, and there have been no injuries recorded in the last 10 years. Nuclear-industry promoters like to contrast this record with that of the wind industry, where between 30 and 40 workers have lost their lives annually for the past several years.

All in all, nuclear proponents feel they have nothing for which to apologize when it comes to safety. And with a growing cry for carbon-free methods of generating electrical power, promoters say nuclear reactors deserve a larger role. Furthering this view is the advent of smaller reactors that promise to get past the other often-cited drawback of nuclear power: mind-numbingly high costs. Some of the new designs currently on the drawing boards are small enough to fit on a flat-bed truck and be mass produced to easily verified specifications. And one design addresses the growing problem of nuclear-waste disposal by using waste material from other reactors as its fuel.

The ability to make use of such nuclear-waste material (basically depleted uranium from conventional light-water reactors) is particularly important in the U. S., where there is no reprocessing of this material back into fuel. Consequently, an estimated 56,000 metric tons of depleted uranium sit in the U. S. The long-term storage of this material has become political.

In 2006, the research firm Intellectual Ventures, Bellevue, Wash., launched a subsidiary called TerraPower LLC to perfect the concept of a nuclear-waste-fueled reactor. TerraPower figures the stockpiles of waste represent enough fuel to generate $100 trillion worth of electricity with its reactor design. Conversely, the firm points out there is no need to enrich uranium for fueling its reactor design, thus eliminating an expensive step in the power-generation process.

TerraPower’s design is actually a refinement of an idea first conceived in the 1950s.
TerraPower calls it a traveling-wave reactor and has developed TWR designs for generating 300 MWe and ~1,000 MWe.

The reason TWRs can burn spent fuel from LWRs is that such material still contains a high fissile content. To get fission going, TWRs use a small chunk of enriched “starter fuel” such as Pu-239 from deactivated nuclear weapons. (Enriched fuel is basically that containing atoms which can easily be split in a chain reaction.) The small chunk of enriched material is used to initiate a slow-moving wave (hence the traveling wave moniker) in which neutrons produced by fission reactions in a power-producing region of the fuel then convert adjacent fuel from fertile isotopes (such as U-238) into fissile isotopes (such as Pu-239). In other words, the reaction wave breeds new plutonium fuel in front of it, and the wave proceeds as long as depleted uranium is supplied.

TWRs consume substantially less uranium than LWRs per unit of electricity generated, because TWRs have a higher thermal efficiency and higher fuel density. The higher thermal efficiency comes from the fact that TWRs use a liquid-metal coolant rather than water. Such reactors can operate at temperatures higher than LWRs because the coolant pressure does not increase with temperature. Current TWR designs employ liquid-sodium coolant at 600°C. The ability to operate the coolant system at ambient pressure greatly simplifies the reactor. There are fewer subsystems required, a plus for reactor reliability. A less-complex machine has a lower risk profile and a lower construction cost as well.

But commercial TWRs aren’t exactly just around the corner. TerraPower figures the first practical units could pass regulatory hurdles and be ready to go into service by the early 2020s, most likely outside the U. S.

Going modular
There are several other reactor designs being investigated that also employ liquid-metal cooling. One in this category is the Hyperion Power Module (HPM) conceived by Hyperion Power Generation Inc., Santa Fe, N. Mex. The coolant is lead-bismuth eutectic (LBE). The reactor uses uranium nitride as fuel contained in stainless-steel fuel pins.

Hyperion calls its reactor a modular design because it foresees the heat (power) extracted from the reactor being transferred for power conversion to a separate location within the plant. As with conventional reactors, the produced steam could then drive industrial processes or be used to convert the primary power into electricity using conventional turbine-generators. Configuring the reactor this way gives it portability, Hyperion says, so that a basic reactor module could conceivably be hauled around on a flat-bed truck.

Hyperion says its Power Modules would measure only 2.5 × 1.5 m (about the size of a backyard hot tub) and would produce about 25 MW each. The relatively small reactors would target remote and strategic locations. Rather than dealing with spent fuel rods, Power-Module users would instead ship the whole reactor module back to the factory for refueling.

The HPM core has three redundant shutdown systems in the event of a malfunction. Even so, the shutdown systems are less complicated than those in a LWR because the water used as coolant in such designs boils away quickly in an accident. This fact forces conventional reactors to incorporate several safety systems and operating personnel to prevent a core melt. Hyperion says there’s much less risk of liquid-metal coolant boiling away during a mishap, so such reactors don’t need the same level of safety and operational personnel.

In addition, power-conversion machinery can never get contaminated because the steam generator and power-conversion equipment are physically separate from the reactor.

Hyperion says its approach costs 30% less to build compared with conventional gigawatt-reactor installations ($1,400/kW compared with $2,000/kW). The company also says its reactors will be half as expensive to operate, based on costs for field generation of steam in heavy oil-recovery operations ($3/million Btu for Hyperion compared with
$7/million Btu for natural gas). Costs could drop even further if the reactors go into mass production, Hyperion says.

Not all modular reactor designs use liquid-metal cooling. One created by Babcock and Wilcox Co., Lynchburg, Va., is a conventional light-water reactor housed in a containment structure big enough to store the waste the plant generates throughout its 60-year life. Thus, the scheme eliminates the need for a separate storage facility. Called the mPower reactor, it has the capacity to provide 125 to 750 MWe or more for a 4.5-year operating cycle without refueling.

One benefit of using a conventional light-water-reactor design is that it can be certified under existing regulations. The reactor itself could be constructed in a factory and shipped by rail car to its final destination. All in all, Babcock and Wilcox thinks its approach could cut construction times for nuclear plants in half while making them inexpensive enough for even poor countries to afford them.

Another company trying to commercialize a modular light-water reactor design is NuScale Power Inc., Corvallis, Oreg. The original idea comes from Oregon State University, which developed a one-third scale, electrically heated test facility that replicates the entire system at temperature and at pressure. The full-size unit will be capable of producing 45 MW. It takes the form of modules, each having its own combined containment vessel, reactor system, and turbine-generator set.

NuScale says its power plants will be scalable so that a single generating facility could have just one or up to 24 units. The reactor pressure vessel contains the nuclear fuel, reactor, and the steam generators. Water in the reactor circulates using a convection process known as natural circulation. The water-circulation system doubles as a passive safety system because water circulates with no help from pumps or other mechanical devices. As with the Babcock and Wilcox design, the NuScale reactor module, consisting of the containment and its contents, can be fabricated at manufacturing facilities in the U. S. As a result, construction can take place in much less time than with conventional reactors.

The NuScale nuclear reactor and steam generator, also known as the Nuclear Steam Supply System (NSSS), is a self-contained assembly of reactor core and steam-generator tube bundles within a single pressure vessel. The NSSS and the passive-safety heat-removal systems sit within a steel containment structure.

NuScale says the NSSS parameters are much lower than those of a typical LWR, and the thermal rating of its reactor is several times smaller. Coolant pressure and steam pressure is about 50% lower than that of a typical LWR. The power-generation system is simpler as well. The entire turbine generator can be replaced with a spare unit for overhaul. Additionally, NuScale plants will use nuclear fuel assemblies similar to those in today’s commercial nuclear plants. The only difference is the length of the fuel assemblies (6 ft for a NuScale system instead of the traditional 12 ft) and the number of assemblies in the reactor.

NuScale figures it will be prepared to file for Design Certification of its reactor with the Nuclear Regulatory Commission next year. The NRC review process is expected to take about three years. Depending upon the time it takes the NRC to review the application, NuScale thinks its first nuclear facility might be operational sometime in 2018.

© 2011 Penton Media, Inc.