Introduction to Nuclear Energy
More than 15% of the world's electricity is generated from nuclear power. The United States, France, and Japan are currently the global leaders in this technology. According to the International Energy Agency, as of 2007 there were 439 nuclear power reactors operating in 31 countries. the size of reactor units has grown from 60 MW in the 1950s to more than 1300 MW, with corresponding economies of scale in operation. Through out the development of civilian nuclear power, many hundreds of smaller reactors have been built, both for naval use (primarily in submarines) and as neutron sources, yielding enormous expertise in the engineering of small nuclear units.
How Does Small-Scale Nuclear Energy Work?
Conventional nuclear technology is considered a mature technology. Significant progress is also being made in the development of small-scale, self-contained, sealed nuclear reactors, which can essentially operate as a ‘battery’ to supply energy in the form of electricity and/or heat. These modern, small reactors for power generation are expected to be much more simple in design, be economical for mass production, and have reduced siting costs. They are also designed for a high level of safety in the event of malfunction and can be built independently or as modules in a larger complex, with capacity added incrementally as required. The International Atomic Energy Agency (IAEA) defines ‘small’ as reactors under 300 MW. To put this in perspective, ‘small’ is over 25% higher than the current peak power demand in the greater Fairbanks area (on the GVEA grid).
Already operating in a remote corner of Siberia are four small units at the Bilibino co-generation plant. These four 62 MWt (thermal) units are an unusual graphite-moderated boiling water design with water/steam channels through the moderator. They produce steam for district heating and 11 MWe (net) electricity each. They have performed well since 1976, much more cheaply than fossil fuel alternatives in the Arctic region.1
Within a nuclear reactor, a controlled nuclear chain reaction takes place. These chain reactions take place in the nuclear reactor core, which contains fuel rods, filled with Uranium-235, and control rods, filled with substances that readily absorb neutrons, such as boron, hafnium, or cadmium. In nuclear fission, a slow-moving neutron is absorbed by U-235 to create U-236. U-236 is struck by a stray neutron, releasing U-235, lighter elements, stray neutrons, and energy, initiating the continued chain reaction. Control rods absorb excess neutrons to prevent the reaction from escalating out of control (See Figure 2).
Water, the moderator contained in the reactor core, absorbs the heat released by the continual nuclear fission reaction and is converted into steam, which drives turbines that generate power. Some nuclear reactors also use graphite as a moderator.
Challenges in Nuclear Energy
There are several important challenges with nuclear power that would affect Alaska projects. Nuclear power is very expensive, current reactors are generally too large for Alaskan needs, small reactors are still under development, and will likely have high costs associated with installation and permitting in Alaska. The poor transportation infrastructure makes the construction of units difficult and makes compliance with current Nuclear Regulatory Commission regulations difficult. There are also environmental concerns associated with nuclear power including fears about damage to a unit leading to a release of radioactive material and storage of radiative waste created by the plant. Furthermore there are security concerns about nuclear proliferation that are inherent to all projects.
Nuclear Energy in Alaska
Small nuclear reactors are an intriguing emerging technology option for Alaska. Unlike conventional reactors, these nuclear ‘batteries’ are designed to be delivered to the site, installed with the generator system, and operated for the prescribed life (typically 5-30 years). After this time period, the fuel assemblies are removed and returned to the manufacturer, and the reactor assembly is refueled or shipped to disposal intact.
This type of fueling protocol allows plants to be simpler and less expensive to design and build. For designs that have no on-site spent nuclear fuel, the security requirements are reduced. The safety systems are passive and highly reliable without maintenance. The plants emit no greenhouse gases and can be small enough to be buried to minimize security issues. The power plant could be transported by barge in modules and installed in a building, with an excavation for the reactor vessel and containment system as deep as 100 ft deep.
There are a number of potential applications for these nuclear ‘batteries’ in Alaska. One of the most obvious would be to supply power for remote mines where diesel power would otherwise need be imported at high cost. Six of this type of reactor have been proposed for the Alberta oil sands region to provide heat to facilitate separation of oil from the sands. Power generation for remote communities is another potentially attractive application. The community of Galena has been working with Toshiba on obtaining a reactor for a number of years, and several other Alaskan communities have expressed interest in this technology. Galena is interested in a 10 MW reactor system, the 4S; designed by Toshiba to provide power and heat to the community. The city has passed a resolution supporting the installation of this reactor.
Conventional nuclear power plants have has been operational since the 1950s. Most conventional reactors are much too large to be compatible for use in Alaska due to low energy needs loads in most rural areas. While current reactors are too large, the technology exists to make smaller reactors that could be applicable to use in rural areas with lower energy needs. In fact, currently the technology exists to build units that could be applicable to some larger village needs.2
There are currently reactors that are of an applicable size, for example, the reactors used on some naval vessels are of the correct size, however they use highly enriched weapons grade uranium, not permitted for civilian use because of proliferation fears. Additionally reactors used for research purposes also exist, however they do not create electricity, nor are they permitted for use in that capacity. Small systems are being designed that could offer Alaskan communities viable generation potential, however these systems are pre-commercial and are not permitted for use in the United States and it could be years before reactors become available for installation in Alaska.3
Small Modular Reactors (SMR)
Small Modular Reactors are self contained units that produce less than 300MWe. These reactors are designed to be used in banks of several units, and have built in fail safe and shutdown devices. Many of these reactors are in a sealed containment vessel, and then buried underground. This could potentially lower the needed staff for a reactor. In the Small‐Scale Modular Nuclear Power: An Option for Alaska? Report, five different reactors are identified as potential units with Alaskan implications.
Toshiba 4S Unit
The Toshiba 4S unit (Super-safe, small, simple) has two designed capacities, 10MW and 50MW. This is a liquid-metal fast neutron reactor which uses sodium as a coolant allows the reactor to run at hotter temperatures that would vaporize water, and build pressure that would be cause the system to be under high pressure. Sodium boils at higher temperatures, allowing the reactor to be unpressurized.This reactor is designed to be encased in concrete and be 30 meters underground, with no direct access to the reactor and its uranium- zircon fuel rods. The operator and the steam turbines will be located in a separate building, located above ground. This reactor is designed to be fully operational for thirty years before it requires being refueled. It is designed to need fewer operators and security personnel than conventional reactors because the reactor will inaccessible. Furthermore, unlike other Small Modular Reactors (SMRs) The fuel rods last 30 years meaning that there would be no storage of dangerous radioactive materials onsite in pools or other storage facilities. Toshiba plans on seeking preliminary Nuclear Regulatory Commission (NRC) permits in 2012. This is the reactor that was proposed to provide power to Galena.
Hyperion Power Module
Like the Toshiba 4S, the Hyperion Power Module reactor would also be a liquid metal reactor, however the coolant used is different than that in the Toshiba unit. Rather than sodium, the Hyperion reactor uses lead-bismuth eutectic (LBE) which operates at ambient pressure. This reactor is designed to either produce 70MWt (thermal megawatts) or 25MWe (Megawatts electricity). Like the previous reactor, the hyperion is a self contained unit, sealed in a containment vessel, further sealed in a concrete vault and buried underground. The reactor is connected to LBE pipes that cycle an intermediate coolant which is used for creating steam that will be used for power generation. The reactor unit itself is sealed at the factory and transported as a single unit. It comes with enough fuel encased to last between 8 and 10 years. In 2010 a demonstration project was begun that is due to come online in 2017. Like the Toshiba unit, Hyperion plans on submitting a application for approval to the NRC in 2012.
This reactor is based off current commercial reactors that use Light water reactor (LWR) designs. This unit has been designed as a 45 MWe modular system, meaning it is meant for installation with several units. It is a prefabricated unit that bundles reactor and the steam generator into a single containment vessel which is 60x14 feet. This reactor uses uranium that is enriched to less than 5% and requires fueling every 24 months. It is passively cooled by naturally circulating water that relies on heating and cooling, because there are no pumps that can fail, this is a safer unit. This unit could become operational as early as 2018.
Babcock & Wilcox mPower
Like the NuScale Unit, this is a LWR unit. While it has a 60 year life, it uses uranium that is enriched to less than 5%, meaning that it must be refueled every 4.5 years. The spent fuel from this reactor would be stored onsite. Like the NuScale unit, this unit comes pre-fabricated in a vessel that is 12x72 tall that is meant to be shipped by rail to the installation site. It has a 125 MWe capacity.
GE - Hituchi Nuclear Energy Power Reactor Innovative small module (PRISM)
This fast reactor is, like the Toshiba unit, a liquid metal reactor cooled by liquid sodium. It uses recycled uranium, plutonium and zirconium. This reactor has a safety features that allows it to shut down in 1/5th of a second in an emergency situtation. The liquid sodium transfers heat to heat exchangers to water, which powers a steam turbine. Like other SMRs, this is designed to be below grade within a containment vessel atop an earthquake isolation device, which is designed to prevent damage to the reactor should a serious earthquake occur.
Pebble Bed Modular Reactor
This 80 MW reactor operates at very high temperatures, and is cooled by helium. It is fueled by 360,000 “Pebbles” that cycle through the reactor over three months and can be reused up to ten times. 165 MW reactor would produce 32 tons of waste per year, which would be stored onsite the 40 year operational life of the reactor. The helium coolant transfers heat to a steam generator through as it is circulated by a blower. The manufacturer is expected to apply for permitting in 2013.
Links and Resources
- Nuclear in Galena
- Galena Power Plant Wikipedia Article
- World Nuclear Association. "Small Nuclear Power Reactors." Helpful information on small nuclear reactors.
- U.S. Nuclear Regulatory Commission
World Nuclear News
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