Electrical power production using a nuclear power source is accomplished through the heat produced by bringing greater than a specific amount of radioactive material together. Water is heated to create steam, which in turn spins turbines to generate electricity. Greater quantities or concentrations of nuclear material produce more heat. The higher the temperature of the heat source, the more efficient the production of electricity from it becomes. If the core becomes too hot, it is possible for core material to melt. If the reactor core is kept relatively cool, the energy production is not thermodynamically or economically efficient. Combinations of different fuels, fuel cladding, and coolants have been tested to optimize reactor cores for efficient power production while remaining very safe. Many different combinations are in use today, each with it’s advantages and disadvantages.
The lifetime of nuclear fuel
Nuclear fuels are elements that are found in nature that naturally emit energy. When they emit energy, they change into a different type of material which may or may not have the property of also being radioactive. In basic terms, nuclear power generation collects these materials and puts them into machinery that can transform their energy emission into electricity. The major steps in this process are mining the fuel, refining the fuel, shaping the fuel chemically and physically, using the fuel, and correct care of spent fuel (also known as nuclear waste).
The most common fuel for nuclear power generation is uranium. Uranium is naturally radioactive, and can be found in many places in the world in the form of ore. Many of the currently utilized uranium deposits — particularly in Saskatchewan, Canada– are of such high concentrations that robots must be used during mining and ore processing to protect miners and technicians from unnecessary exposure to radiation. Much care must be taken with mined uranium ore since it can be dangerous to human beings.
In the early history of uranium mining in Saskatchewan, it was believed that the dangers involved in mining were known. The contamination detection equipment used at the time did not take into account the fact that the miners would be breathing in particles from the air in the mine. The radioactive dust, which was not dangerous to the miner’s skin, was much more dangerous if breathed in. The negative health effects of this mistake were notable, so a change in detection and working conditions was required. Modern radiation detection equipment for use in a mine now also tests the air carefully to guard against accidents such as these.
Ore that is dug out of the ground has relatively low concentrations of uranium compared to what is needed for reactors. A process of refinement is used to remove all the other elements from the ore, leaving us with a very pure fuel for use in reactors, similar to how other metals or minerals are processed and smelted. For ease of use in nuclear reactors, uranium metal is often shaped into bars or pellets at this point.
There are two major types of uranium: Uranium-238 (U-238) is much more common, representing over 99% of naturally occurring uranium, while Uranium-235 (U-235) is comparatively rare, comprising approximately 0.72% of naturally occurring uranium. U-235 is the isotope that is needed for all current industrial nuclear power generation because of the fact that it can relatively easily undergo nuclear fission. Nuclear fission is the process by which an atom breaks apart, giving off a tremendous amount of energy in the process. Some types of nuclear reactors require the uranium to be enriched. The enrichment process increases the percentage of U-235 present in the fuel through chemical processes which attempt to remove everything that is not U-235. The less useful part of the uranium metal, comprised mostly of U-238 is removed and stored, it is called ‘depleted’ uranium. Depleted uranium has been used for military weaponry and other uses around the world. It is possible that reactors in the future will use U-238 as a fuel, since they will have the ability to burn it up as well as U-235. Some forms of nuclear bombs can make use of highly enriched uranium, but the current world nuclear nations rely on plutonium for their bomb material.
Use in Reactors
Bars or pellets made of uranium are fed into reactors as fuel assemblies and usually remain in the reactor for several months, sometimes for several years. Nuclear fuel naturally gives off energy faster when in close proximity to material like itself. Thus the more nuclear fuel there is nearby, the hotter the fuel gets. The goal of a nuclear reactor is to keep the fuel density below such a level that the fuel exceeds design temperatures, melting or otherwise damaging the fuel package. ‘Critical Mass’ is the state wherein a runaway nuclear chain-reaction is possible, and results in reactor damage and/or a meltdown. Reactors are designed and operated in such a way that critical mass cannot be reached. In addition, they are also required by law to have additional, independant, automatic backup systems to prevent the unlikely event. Nuclear weapons are designed to exceed critical mass very quickly when they detonate, releasing tremendous amounts of energy in an instant.
Nuclear fuel gives off heat and radiation which are absorbed by the reactor vessel and the surrounding heat transfer fluid (usually water). This superheated water is then used to drive a system similar to a steam engine. Heat engines, of which steam engines are a special type, create energy from a temperature difference between two reservoirs. Nuclear reactors are usually placed near large bodies of water because they need a cool reservoir to create the needed temperature contrast with the superheated water. The larger the difference in temperature between these two bodies of water, the more efficient the electrical production is. The water from each of the two sources is circulated in physically unconnected loops such that the water exposed to the reactor core never enters the cool reservoir even if an accident occurs.
Even after it is removed from the reactor, spent fuel continues to discharge dangerous levels of radiation. The faster nuclear materials decay into less active states, the faster it becomes harmless. Therefore the most dangerously radioactive materials decay to negligible levels of radioactivity in a few days. The waste can be potentially be dangerous for many years so it must be dealt with carefully. First, it is held in cooling ponds on the site of the reactor where it emits energy harmlessly into water for a number of months. Once the waste is emitting a sufficiently low amount of heat it is transferred to cement containers that dissipate the remaining heat output and shield the still significant radiation. Storage beyond this point has not been directly addressed by many nuclear nations but some choose to reprocess the fuel for additional use. Many techniques for addressing the waste problem have been developed, but few nations have committed to large-scale implementation of them due to low natural uranium costs and the additional costs associated with such implementations.
While being cooled, spent fuel being cooled slightly irradiates the water, increasing the concentration of a radioactive form of hydrogen called tritium. Tritium usually is the only radiation that leaves a nuclear plant and enters the natural environment. This low concentration emission is tightly regulated by nuclear nations and is generally considered to be of no risk to humans or the environment. The legal standards in Canada, for instance, require that the dosage to normal citizens be below the level of natural radiation that exists in the environment. Nations which have chosen not to reprocess often seal radioactive material in glass or cement casks. Improper cask design has resulted in leakage of dangerous material requiring clean-up and bad environmental publicity for countries that have chosen this path of waste stewardship.
A variety of techniques for reducing or reusing nuclear wastes exist in the world today. Some exist only at a technical small scale experimental level, others are utilized by countries such as Japan, France, and the UK. Some techniques raise concerns of nuclear weapon proliferation because plutonium is separated from the waste during one of the stages. Plutonium is a key component to creating a modern nuclear weapon. In general, such techniques are not used on the majority of waste because of a combination of cost concerns (reused fuel is more expensive than freshly mined fuel) and social proliferation concerns. Fears of ‘Proliferation’ refer to the possibility of the increase in the number of nuclear weapons in the world, especially in relatively unstable nations.
Large scale implementation of more advanced reprocessing techniques show promise of more thorough use of the energy stored in nuclear fuels and wastes, but are considered by many nations to be too expensive to develop or are undergoing further research.
On the research reactor scale, designs have been tested that turn normally inert or disadvantageous elements of fuel into elements that help the process rather than hinder it. ‘Breeder Reactors’ are called such because they can produce more useful fuel elements than they consume. This is allowed within the laws of physics because the majority of material in fuel loaded into a reactor is inert and is not fissionable by common reactor designs. These reactors are more complex, and the speed of research and implementation is slow for a variety of reasons including proliferation risks and the current low cost of newly mined uranium.
Breeder reactors that are designed to act upon fuel and waste from the uranium/plutonium fuel type require fast neutrons which have not been slowed by moderators such as water or graphite. This contrasts with the standard reactor which uses slow (thermal) neutrons and had much research invested for the development of nuclear weapons. Because of this, much research needs to be done to engineer around the properties of fast neutrons that differ from the slow neutrons we are have a large amount of experience with.
Not all nuclear material requires fast neutrons to breed new fuel. Thorium can be used as a breeder material with thermal neutrons. Thorium does not fission on its own as U-235 does, so a fissionable material must be used to carry out this process. Significant research would be required to commercialize breeder technology of this type, but in doing so many of the problems of the Uranium/Plutonium fuel cycle could be avoided, and many of the wastes existing as by-products of the current fuel cycle could be productively taken care of. For a google tech talk about an experimental thorium-cycle reactor that was built, click here.
A proliferation risk refers to the increased opportunities for nuclear weapons production when additional plutonium is produced by an operating reactor or separated from other nuclear fuel elements during reprocessing. Some fear that nuclear material will be stolen or sold for clandestine purposes and uses for weapons, and this underlies proliferation concerns.
These proliferation concerns often arise from the past misuse of reactors in the production and isolation of weapons materials. The most well understood and most widely implemented fuel reuse techniques are derived from the American weapons program. This may help you understand why weapons proliferation is such a concern, yet the nuclear industry continues to make use of this technology to get additional usage as an energy source from spent fuel.
Technology and techniques are being continually developed to reduce the proliferation risk or making existing techniques less costly to implement. Some of the more promising and interesting areas of nuclear energy research such as liquid fuels make proliferation risk management inherently more difficult, yet should not be discarded solely because of this.
Proliferation risk management is a controversial and sometimes polarizing topic, with widely differing viewpoints on both ends of the spectrum. Some scientists believe that not even a security guard should be required to secure nuclear materials, while others believe that the proliferation opportunity provided by even mining uranium is unacceptable. Both of these viewpoints are extreme and unlikely to be the best way to deal with the subject, a more moderate approach is rarely admitted by proponents of either extreme.
Nuclear proliferation management is something humans are capable of, and do on a daily basis. This essentially means keeping trustworthy accounting of the nuclear material coming in, being consumed or produced, and exiting a nuclear facility. The nuclear industry is currently the most tightly regulated industry in the world.
There are a number of international treaties and organizations tasked with creating regulations for the global production and use of radioactive materials. These groups are concerned with limiting the excess exposure of people and the environment to radiation. They also regulate and enforce rules about the sale of nuclear materials and technologies between countries such that weapons material and technology are not obtained by nations who have not signed non-proliferation treaties. Additionally, they also regulate the flow of nuclear material over international borders of existing nuclear nations.
The amount of radiation dose taken by people above the natural background level is regulated by a number of national level organizations, such as the Canadian Nuclear Safety Commission (CNSC) in Canada. Each nation sets its own standard, and often includes different limits for the general public and trained radiation workers.
The release of radioactive materials from nuclear power plants and fuel processing facilities is regulated by these agencies. The amount and type of radioactive materials released are kept such that the probable dose to the public will not exceed standards. The majority of nuclear facilities release no radioactive materials at all, and would only do so in a sizeable accident such as Three Mile Island.
The most regulated aspect of the nuclear and radioactive industry is the storage, creation and transport of potential nuclear weapon materials and trade of nuclear technologies. Nuclear regulators keep careful accounting of these procedures to confirm that no material is transfered to groups who are not allowed to have it.
The majority of reactors for electrical generation in use today are in the 600 – 1000 MW range. A typical north american house consumes between 1 and 2 kW on average. This means that a reactor could provide power to approximately three-quarters of a million homes.
Ordinary water under pressure is the coolant and moderator (reaction slowing material) of choice.
Safety concerns have lead to reactors with double and triple redundant safety systems that jump into action without human intervention, without need of external power. Such safety systems are designed to go to work in fractions of a second, as this is as fast as a nuclear reactor can move out of balance in a worst-case scenario.
Design lifetimes are in the 40 – 60 year range, varying both on reactor design and from plant to plant, depending on the quality of maintenance they have been given.
Most reactors burn enriched uranium in a once-through cycle, after which the spent fuel (waste) is stored indefinitely. This is economically popular because natural uranium is currently cheap and prevalent, while the cost of storage is less than the cost of reprocessing. It is likely that future reactors will be designed to utilized spent fuel material as the cost of new uranium increases and the demand for responsible waste stewardship comes to term. The reason this is economical is because only a tiny fraction of the fissionable material is actually used in a once-through cycle. The ‘spent’ fuel is extremely similar to new fuel. It differs only in some disadvantageous chemical imbalances that make it unsuitable for use in its current form. Chemical and reprocessing techniques can make thespent fuel into fuel that can be used in reactors.
A wide variety of reactor designs and test reactors exist, but are not used for commercial power production because they are not as cost effective as the designs that are currently in popular use. Reactor cost and construction time estimates decrease significantly as more reactors of a given design are rolled out.
Many reactors can also make use of thorium as a breeding fuel, but much less research progress has been made down this avenue because thorium fuel does not produce weapons grade plutonium. Thorium is many times more abundant than uranium, and a much higher proportion of thorium is an appropriate isotope for power production.
New designs take time to construct. There have been substantial cost overruns in the past with the construction of new designs. Some new designs have been rolled out to less than optimal performance in early years, and there is a risk of unforeseen difficulties with a particular design that are not seen until it enters full scale operation, sometimes for years. Regulators are highly skeptical of new reactor designs which implement previous unimplemented features. Regulators often work with the designers to further bolster the strength of safety systems.
Large centralized reactors are the only type of reactor to be competitive on a cost-per-watt basis with fossil fuel power. Because of this, nuclear reactors of this design are less suited to widely dispersed demand regions. Much of the power can be lost due to transmitting it over large distances, but this is an issue common to all large centralized power generators. High electrical transmission infrastructure construction costs are a fact of life in sparsely populated areas. Because of the large scale of a nuclear power plant’s output, a large amount of water is required to cool the plant. A large nearby river, lake, or ocean is required to meet this cooling requirement. A cooling tower can be used to ease this requirement, but are rarely substituted for the large water requirement. A cooling tower may still requires a significant amount of water, just less so than the standard system. Another option for cooling nuclear reactors is a dry cooling tower. Dry cooling towers use air cooling, and therefore do not evaporate water in the cooling process. The usage of dry cooling towers is notably more expensive than the standard cooling method.
Nuclear power generators emit less CO2 per kilowatt than many competing power generators, such as natural gas and coal. Nuclear power plants primarily produce CO2 emissions during the construction process and through the process of producing new fuel from natural uranium. Nuclear power generates a much smaller volume of waste than coal power while using a much smaller volume of fuel. This is somewhat mitigated by the potency of the nuclear waste, but management techniques such as storage and reprocessing exist to further utilize much of this waste as new fuel. While natural gas does not produce the solid waste volume of nuclear or coal, a significant amount of CO2 greenhouse gas are emitted per kilowatt power produced. Compared to other low carbon energy sources such as hydroelectric, wind and solar, a nuclear power plant has a significantly smaller land footprint. The key environmental issue of nuclear waste can be proactively dealt with by responsible governments, although it adds notably to the cost of the power produced. This strategy has been demonstrated as viable by France and Japan. Further research may make this process more cost effective, and alternative reactor designs could make the reprocessing of new nuclear waste less costly.
Nuclear power generation has only been possible in practice if large loans are guaranteed at low interest rates from governments. Additionally, the associated risk is shouldered by the government. This means that if there is ever a large-scale disaster, the government has to clean up after it. Nuclear power companies, though large, do not have the resources to deal with large scale accidents. Even if the risks of such an accident are small, a conscientious government would take steps to prepare for possiblities such as those, perhaps incurring a notable cost in doing so. Large amounts of capital need to be sunk in the project from day one. No money is gained back from a nuclear reactor for a few years, because it takes years to build a reactor and get it online. New designs take a long time to be finalized because of the capital scale and complexity of the projects. Very large companies and/or government funded research are required for this.
Much concern is centered upon managing the waste products of nuclear reactors. This problem can largely be designed around by reprocessing waste into fuel. Many countries feel they have good reasons for not doing this, such as concerns about nuclear proliferation. Other countries forgo more proactive waste management strategies based primarily on economic considerations. Such countries would likely only reprocess their waste if it becomes economically effective to do so through changes in the price of new fuel or the emergence of more efficient reprocessing technologies. Strong and vocal demand from citizens may also motivate governments to alter these long term strategies.
As with all mining ventures, uranium mining has a substantial effect on the local environment. Comparatively little uranium is mined for the amount of electricity that will be produced through its use, but this does not excuse what damage is done. Effective and efficient cleanup of a decommissioned uranium mine is crucial to minimizing the long term effects of the large amounts of mildly radioactive materials that are unearthed at a uranium mine. This can include such basic techniques as the burial of mine tailings back into the mine at the end life of the mine. This will minimize the amount of radioactive materials that are transfered into the ecosystem through plant and animal life on the decommissioned site.
Many nuclear power critics argue that uranium deposits are so limited that natural supplies will be depleted within a human lifetime. This is a complex issue because more technically advanced fuel cycles including reprocessing can extend the useful life of the uranium supply 100 fold using existing technology in use in places such as Japan and France. This will also likely be mitigated by the discovery of new uranium deposits as the field of uranium exploration is still in its infancy.
Further technical advances pioneered on a small scale reveal the ability to run alternative reactor designs that create more fuel than they use. This means that they can create many times as much energy as a current reactor design using the same amount of starting nuclear material.
Adaptations and Variants
Throughout the history of nuclear power a number of coolant types have been researched and implemented. Coolant choice can affect the safety, efficiency, simplicity of management, and fuel choice of a reactor design. Coolant types that have been used include CO2, water, heavy water, liquid salt, and even liquid lead.
The trend of nuclear reactor design has been to higher and higher energy output designs. The range of reactor electricity outputs ranges from 3 KW research reactors to the 1.65 GW European Pressurized Reactor. A number of new reactor designs under development return to smaller sizes to address the safety and complexity issues that arise in such large nuclear power plants.
As many fuel types have been experimented with as coolant types. Natural (un-enriched) uranium, metallic and oxide uranium fuels, mixed oxide (MOX) fuels including uranium and plutonium oxides, graphite coated oxide pellets, as well as liquid thorium fuels have all been tested or implemented in reactor designs.
A reactor type which utilizes fast instead of slow neutrons is known as a breeder reactor. Breeder reactors have the ability to produce more fissionable fuel than they consume. This is done through the neutron bombardment of inert elements of the fuel that converts them into fissionable materials. These reactor types are more difficult to operate than the typical reactor, but promising designs such as the liquid flourine thorium reactor (LFTR) have been demonstrated in past research.
American reactors use slightly enriched uranium fuel. They do not currently perform any reprocessing of their fuel for primarily political reasons. They are currently planning to store their spent fuel until a better plan becomes economical.
Enrichment, a cycle or two of reprocessing (because of economics, proliferation, social concerns, and technology at the time), long term storage of waste planned after that. Some European nations are researching further reprocessing techniques while others are looking at phasing out nuclear power all together.
Maximal reprocessing, heavy government investment in infrastructure to do this. Drastic reduction in waste produced at an increased cost. Japan buys the spent nuclear fuel from European nations like Britain and France for reprocessing into fuel.
Thorium is a fertile material, meaning that it can be converted into fissile fuel for a nuclear reactor. It has some properties that make it easier to reprocess. It is also much more abundant than uranium, especially in countries like India and China. Research has been done into using thorium in existing western designs such as the CANDU design with success.
Can burn a wide variety of nuclear fuels, including unenriched uranium or the breeding of fuel using thorium. These reactors could actually burn the waste of American plants for profit but hasn’t been done, likely because of capital cost of a plant to perform such a physical transformation of the fuel. CANDU reactors seem to have shorter lifespans than other types of reactors. A new design has been developed but has yet to be constructed.