Nuclear

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Contents

Overview

Nuclear power is the common name for electrical power converted from heat produced by Fission. It is presently one of the major sources of electricity, being respnosible for 15% of the world's electricity and 20% of the electricity produced in the United States. Nuclear power is considered "carbon free" or "carbon neutral", as the operation of a nuclear power plant does not in any significant way affect the carbon balance of the Earth's ecosystem. Nuclear power is also considered "non-renewable" in that the production of new nuclear fuel, ultimately, requires the explosion of a star, an event not likely or desirable on Earth.

Because of nuclear power's extremely high energy density - even with the relatively inefficient burnup of fuel and the moderate turbine efficiency - the actual mass of the produced spent fuel is very low - about 30 tonnes of high level nuclear waste (long-lived actinides and fission products), and 800 tonnes of depleted uranium at a 1,000 MW power plant. This is in comparison to the ~850,000 tonnes of CO₂ produced in a year by a 1,000 MW coal plant burning 380 tonnes of coal per annum, or the ~425,000 tonnes of CO₂ per year produced by natural gas.


Current Useage

About 20% of U.S. electricity is generated using nuclear power. In France 79% of electricity is generated with nuclear power, in Belgium 60%, in Switzerland 42%, in Sweden 39%, in Spain 37%, in Japan 34% and in the United Kingdom 21% of electricity is generated with nuclear power.

The percentage of electricity generated from nuclear power varies greatly from state to state within the U.S., on a percentage basis, from highest to lowest, the percentage of total net power generation from nuclear power by U.S. state is (with number of plants): Vermont(1) 85.3%, New Jersey(4) 74.5%, New Hampshire(1) 62.4%, Connecticut(2) 61.6%, South Carolina(7) 58.2%, Illinois(11) 54.4%, Pennsylvania(9) 44.0%, Virginia(4) 43.5%, New York(6) 38.2%, California(4) 37.2%, Arizona(3) 36.6%, North Carolina(5) 34.2%, Nebraska (2) 33.3%, Massachusetts(1) 31.2%, Minnesota(3) 30.2%, Arkansas(2) 29.3%, Georgia(4) 28.6%, Alabama(5) 27.1%, Maryland(2) 26.9%, Mississippi(1) 25.9%, Kansas(2) 21.8%, Wisconsin(3) 21.0%, Louisiana(2) 20.3%, Florida(5) 18.9%, Michigan(4) 16.6%, Texas(4) 12.7%, Missouri(1)11.7%, Ohio(2) 11.6%, Iowa(1) 9.8%, Washington(1) 5.4%.

The following states do not generate any electricity from nuclear power (with percentage of power from coal shown behind each listing): Alaska 3.7%, Colorado 87.5%, Delaware 80.2%, District of Columbia 0%, Hawaii 0%, Idaho 0%, Indiana 98.2%, Kentucky 96.6%, Maine 0%, Montana 4.9%, Nevada 64.5%, New Mexico 88.5%, North Dakota 93.0%, Oklahoma 63.9%, Oregon 8.2%, Rhode Island 0%, South Dakota 37.9%, Utah 95.0%, West Virginia 99.3% and Wyoming 97.2%.

Some states which use nuclear power use very little coal to generate electricity. California, Connecticut, and Vermont get less than 1% of their electricity from coal. New York gets 5.5% of its electricity from coal and Washington gets 3.4% of its electricity from coal.

In warm states, the percentage of electricity generated with nuclear power is lower in the summer, since nuclear power is better suited to baseline power needs than to meeting peak demands.

Fission

In nuclear fission, an atomic nucleus absorbs a neutron and reacts by separating into two smaller nuclei (fission products) and an average of 2-3 neutrons, often accompanied by gamma rays. Fission is highly exothermic with an energy density of ~23,300 kilowatt hours per gram of fissile material. This is due to the fact that the parts of the original nucleus escape the reaction site extremely fast, and as they bump into other nuclei, this velocity is translated into average kinetic energy, aka, heat. By comparison, the combustion of coal produces roughly 7 kilowatt hours per gram of fuel.

When, in a controlled system, every fission event produces exactly one more fission event, that system is called "critical". For a fission reactor to remain critical, it must carefully manage its neutron budget such that about one of every 2-3 neutrons produces another fission event. Modern reactors do this by first designing the reactor such that excess heat reduces the neutron absorbption - and thus, reaction - rate. The term for this is a "negative thermal coefficient of reactivity".

There are two basic types of fission, based on the velocity of the neutrons that instigate the fission reaction: Thermal fission and Fast fission. Thermal reactions generally produce fewer neutrons than fast reactions, however, it is more difficult for a fast neutron to be absorbed by a fissile nucleus for fission.

In thermal fission, a neutron must be slowed down to "thermal" speeds, or speeds that closely approximate the kinetic energy of the surrounding material. This is normally done using a moderator - a media in which a neutron can not only bounce around, but give up large amounts of its kinetic energy. Examples are normally light atoms, like hydrogen and carbon.

At thermal speeds, most fissile materials present large targets to neutrons - they have a large "neutron cross-section". This allows reactors utilizing the thermal spectrum to produce a lot of power in a small space, but the lower rate of neutron production means that the neutron budget is more limited than in a fast reactor.

Alternative targets for fission neutrons include reactor parts (normally chosen for their high ratio of neutron reflection or moderation to absorption), fission products, and nonfissile actinides. When a nonfissile actinide absorbs a neutron and subsequently decays to a fissile nucleus, it is said to have been "bred".

In current generation nuclear reactors, nuclear fuel is "spent". Spent fuel is nuclear fuel that has built up enough in the way of fission products to adversely affect its neutron budget - about 1% conversion, or "burnup". At this point, the nuclear reaction can no longer be sustained in this fuel, and it must be replaced with a fresh bundle.

Fuels

There are three readily available fissile materials for nuclear power: Uranium-233, Uranium-235, and Plutonium-239.

The most commonly used fuel is Uranium-235. Natural uranium is mostly the breedable Uranium-238 (which, when it absorbs a neutron, breeds to Plutonium-239) with less than a percent Uranium-235 content. To make nuclear fuel pellets, the U-235 is "enriched", or concentrated, such that there is ~5% U-235 and ~95% U-238. This results in the production of about 7 grams of high-purity U-238, aka, "Depleted Uranium" for every gram of fuel produced.

All isotopes of plutonium are synthetic, being produced by relatively short exposures of depleted uranium to high neutron flux, followed by PUREX reprocessing (described below). Plutonium-239, as used as nuclear fuel, comes largely from nuclear weapons, via the Megatonnes to Megawatts program, the START treaty, and other such disarmament treaties.

Uranium-233 is not presently used in nuclear reactors, but is being considered, as the relatively abundant element thorium can be bred to U-233.

Reprocessing and Proliferation Concerns

Spent nuclear fuel consists of a mix of elements, usually with a break down of about ~1% fission products, ~4% U-235, ~0.6% fissile actinides, ~0.2% nonfissile actinides, and the balance, ~94.2% U-238. The reprocessing of spent nuclear fuel involves the removal of the fission products from the fuel pellets. The actinides may remain in the fuel pellets, as they can be bred with the U-238 in most reactors, and ultimately fissioned.

PUREX, the current prevailing technology for nuclear reprocessing, is short for "Plutonium-Uranium Extraction". Without going into too much detail, PUREX converts a stream of spent fuel into three streams: plutonium, uranium, and fission products. An emerging technology, pyroprocessing, is able to separate the fission products from the actinides without differentiating the actinides or separating out the plutonium specifically.

Reprocessing of spent nuclear fuel can potentially reduce our streams of what we consider nuclear waste in at least two major ways. First, because the bulk of spent nuclear fuel ends up going back into reactors as new fuel, the mass of actual waste is significantly reduced (in practice, by a factor of about 60). Second, because many of the fission products have decay chains that end in valuable commodities (medical radioisotopes and rare earth metals), some of the waste stream can be further diverted. Lastly, most of the radioactivity from spent fuel is from the bred actinides - which are removed from the waste stream - and the remaining fission products decay, as an aggregate, to background radiation levels in 300-500 years, rather than the 250,000-1,000,000 years cited for the disposal of spent nuclear fuel.

There is presently a moratorium on nuclear reprocessing in the United States, requiring it to be stored. The nominal process for this is that nuclear waste is vitrified - that is, embedded in leaded glass - and stored in large "dry casks", or concrete and steel containers.

Current plans in the United States are to dispose of all high level nuclear waste in the nation at the Yucca Mountain, Nevada site, but political and court opposition has held up this approach. More recently, a judge held that a plan to maintain the site for the next 10,000 years was not careful enough. Low level nuclear waste consists of just about anything, such as employee uniforms, which has had tangential exposure to radioactivity, and depleted uranium tailings. This is far more voluminous, but remains dangerous for a far shorter period and presents a far lower risk to the public.

Opponents of Yucca Mountain are concerned that it is located so close to a major population center (Las Vegas), that transporting high level nuclear waste to Yucca Mountain poses a serious risk, and that ground water could be contaminated in ways whose risk is underestimated by current plans. Proponents of Yucca Mountain would argue that, whatever its faults, it is a far better place to store high level nuclear waste than in the cooling pools and dry cask storage facilities at the 104 nuclear power plants across the nation, which were often not designed with comprehensive environmental studies.

Contamination at former fuel production and processing sites such as Fernald, in Ohio, and the Hanford Site in Washington State, are often used to demonstrate the risks in on-site storage and processing of radioactive compounds.

Proliferation

The risk that nuclear materials in reactors will be used by terrorists or rogue states to create nuclear weapons. This is a particular concern in the Soviet nuclear industry where a great deal of nuclear fuel is unaccounted for. This concern is largely directed to the use of nuclear power in nations such as Iran and North Korea whom the United States does not trust.

There has also been misplaced concern in the United States over the reprocessing via PUREX, in that an isolated plutonium stream may represent a proliferation risk. However, the plutonium in spent fuel, or "Reactor-grade" plutonium is only about 50% Pu-239. The remainder is about half-and-half Pu-240 and Pu-241. Pu-240 has a high spontaneous fission rate, and Pu-241 decays to Am-241, also with a high spontaneous fission rate. Any significant mass of reactor-grade plutonium is, therefore, extremely dangerous to handle, and wholly unsuitable for a weapon.

Accidents

The Chernobyl disaster, in April 1986, is considered the worst nuclear power plant accident in history. Chernobyl was a thermal spectrum reactor, cooled by water, and moderated by graphite. During an unauthorized test of an attempt to solve another safety issue, the water coolant boiled off in reactor 4. Because the primary moderator was graphite, a boil-off meant that the reactor suffered a heat spike (mainteined reactivity in absence of cooling), and subsequently the core breached, exposing the graphite to air, igniting it. Since there was no containment vessel for the reactor itself, the resulting fire sent radioactive fallout into the atmosphere.

The direct fatality count was 50, all of which were reactor staff. It is estimated that there may ultimately be a total of 4,000 deaths attributable to the accident, due to increased cancer risk.

The loss of coolant accident at Three Mile Island, unit 2, occurred in 1979, and is considered to be the worst case scenario for its reactor type: water moderated and cooled thermal reactor. A mechanical fault and human error contributed to this accident, which resulted in a partial meltdown, a radiation release, and the permanent shutdown of TMI-2. There were no fatalities, and the estimated effect of the radiation release is one to two additional cancer deaths in the 10-mile radius around TMI.

Politics

The "NIMBY", or "Not In My Backyard" phenomena drives much of the debate about nuclear power and nuclear waste. Many people are afraid of nuclear power and don't know the facts one way or the other, and hence don't want it anywhere near them.

Mining

The mining of uranium as nuclear fuel is done in various ways: open pit, underground, and heap and in-situ leaching. Open pit and underground mining of uranium are normally part of other mining targets, such as copper, gold, silver, and rare earths mining. The leaching techniques are more specific to uranium, and have a lower overall footprint, using mostly sulfuric and nitric acids to "leach" out the urnaium from the surrounding rock. There are concerns about spread of radioactive material with any form of uranium or thorium mining.

The distribution of uranium in the earth's crust is log-normal, with an average concentration of about 3ppm. At the present cost and consumption of uranium, we have about 80 years of Uranium, however, there is a 300-fold increase in the amount of uranium recoverable for each tenfold decrease in ore grade. As such, this fact only means that the cost of uranium will increase by a factor of 10 in 80 years. Nuclear power proponents claim that the energy density of uranium, the small contribution of fuel cost to the overall cost of nuclear power, and the expectation of expanding nuclear recycling and use of alternative nuclear fuels imply that this is not a large issue.

Technology

At present, most nuclear power plants generating power in the world use Light Water or Heavy Water reactor designs. In a light water reactor, water is pressurized until it is dense enough to act as a moderator for thermal-spectrum fission. In a heavy water reactor, this is also necessary, but to a far lesser extent - but with the disadvantage that heavy water is far more expensive to obtain.

Liquid metal cooled fast reactors are also in production, though significantly less common.

Many small, modular reactors are in the process of getting NRC approval in the United States. These are in the 50-300 MW range, promise very low prices per watt, and are largely designed for "drop-in replacement" of fossil fuel electrical plants.

The proposed next generation of nuclear reactors is based on leveraging the neutron budget to continuously breed fuel from the existing reactor's fuel stream.

The Hyperion power module, currently in development by Hyperion Power Generation, LLC, is a uranium fueled, lead-bismuth-cooled, fast reactor with good theoretical power characteristics.

The Travelling Wave Reactor, currently in development by Terapower, Inc. is a simple design: a canister of breedable material with a starter of fissile material capped on one end to start the reaction. The geometry of the canister is such that, as the starter fissions, it breeds a wave of fissile material ahead of it, maintaining the reactivity of the reactor. Such devices would come shipped with a 60 year supply of fuel - most likely U-238. It is speculated that such a reactor would be controlled strictly by the insertion of cadmium (neutron absorbing) rods. Early calculations place the burnup of such a reactor at 20% of the input fuel.

The Liquid Fluoride Thorium Reactor, presently in development by Teledyne Brown, is a homogenously fueled molten salt cooled and moderated reactor. In this reactor, fissile material is dissolved in a fluoride salt and cycled through a core chamber with a critical configuration. Around the core chamber, a blanket of the same salt, but containing thorium instead of a fissile, absorbs about half of the neutron flux from the core. Upon absorption of a neutron, thorium breeds to uranium-233, which is fissile. The U-233 is separated from the blanket salt by fluoridation and added to the fuel stream. As the fuel stream fissions, fission products are produced; these are continuously removed from the reactor, enabling effectively 100% burnup. LFTR is also a higher temperature reactor, which can enable better efficiency in converting heat into electricity. Because the fuel, coolant, and moderator are all the same material, this reactor tends to self-regulate, as reactivity decreases sharply as the fuel stream is heated.

See Also

External Links

Alternatives

The alternatives to nuclear power are either other nonrenewable energy sources like

Or renewable energy sources like

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