Nuclear energy


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Nuclear energy

  1. 1. Nuclear energy Nuclear energy is generated by neutron bombardment of certain uranium or thorium isotopes (nuclides), which causes fission of the atom nucleus, a process which releases huge amounts of heat that can then be used to make steam and drive turbines for electricity generation. Nuclear energy provides 13 % of total electricity generation worldwide. In the U.S. it is 19 %, in Europe 30 % and in France 80 %. Electricity production by primary source World total electricity production = 2.3 TW (20 000 TWh) Data collection and presentation by Carl Denef, Januari 2014
  2. 2. Uranium is mined either in open pit, by standard underground mining or by in situ dissolving of the minerals and pumping the solution to the surface. In open pit mining, the ore is exposed by drilling and blasting and then mined by blasting and excavation. Workers need to stay in enclosed cabins to limit exposure to radiation. Water is extensively used to suppress airborne dust levels and in deep undergound mines to cool. There is less waste material removed from underground mines than open pit mines. However, this type of mining exposes underground workers to the high levels of radioactive radon gas, unless sufficient ventilation is installed. The naturally occurring oxide forms and not the uranium metals are used for safety (the oxide melting point is much higher than that of the metal and it cannot burn, being already in the oxidized state). Before use as fuel uranium is enriched. Today (2013) 437 nuclear power reactors are operational in 31 countries.[4] The total identified and probable (yet undiscovered) uranium resources are about 15 megaton, representing a power capacity of 235 TW. World production is ~60,000 tonnes/year generating ~380 GWelectric power (~1 TW primary heat power from the reactors) 2
  3. 3. Uranium235 (U235) (0.7% of all natural uranium) is used in the conventional nuclear reactors, called thermal reactors, using slow (low energy) neutrons. During operation there is both fission and the formation of new isotopes due to neutron capture, i.e. U236 and U238. Further neutron capture and beta particle decay generates Plutonium (Pu)239 Pu240 , Pu241, Pu242 and other transuranic or actinide nuclides. Pu239 and Pu241 are fissile. Some of the fission products have a high neutron absorption capacity, by which neutrons are removed from the reactor, making the nuclear reactor stand still. Typically after 3-5 years the spent fuel has to be removed to a final repository for storage as waste. Recycled fuel (MOX): Spent fuel can also be delivered to a nuclear reprocessing plant where 95% of spent fuel can be recycled to be returned as fuel (known as ‘mixed oxides’; MOX) in a power plant. MOX is made in the UK and France, and to a lesser extent in Russia, India and Japan. About 30 thermal reactors in Europe (Belgium, Switzerland, Germany and France) are using MOX as nuclear fuel. However, in many countries recycling is not done or prohibited by law to avoid that the Pu is used for nuclear weapon production. 3
  4. 4. U238 (99.3% of all natural uranium) is used as fuel in ‘fast breeder reactors’. Fast (high energy) neutron bombardement of U238 turns this isotope into several isotopes of plutonium. Two of these, Pu239 and Pu241, then undergo fission to produce heat. Whereas water under pressure is the coolant in thermal reactors, liquid metal (sodium, mercury, lead) is the coolant in fast breeder reactors, as they have a much higher boiling point than water. The high heat capacity provides thermal inertia against overheating. The outlet temperature of the reactor is 510–550 °C. So far, these reactors remain in the R&D phase, except for one in France, one in Russia and one in Japan. Both China and India are now building fast breeder reactors. In principle these reactors extract almost all of the energy contained in the fuel, decreasing fuel requirements by a factor of 100 compared to traditional U235 reactors. Moreover, they utilize uranium at least 60 times more efficiently than the U235 reactor. The great advantage of fast reactors is that they permit nuclear fuels to be bred from almost all the uranium-derived actinides in nuclear waste from conventional thermal reactors, including depleted uranium samples (remainder uranium after U235 enrichment), in this way strongly mitigating the actual nuclear waste problem. 4
  5. 5. A future source of nuclear energy could be Thorium232. It is used in a conventional thermal reactor. Neutrons turn Th232 into U233 and then cause fission of U233. Th232 is about 3.5 times more common than uranium in the Earth's crust. India has looked into this technology, as it has abundant thorium but little uranium reserves. The thorium reactor makes a closed cycle: Once started the reactor keeps working automatically (see Figure) 5
  6. 6. Coal 24-30 MJ/kg Natural Gas 38 MJ/m3 Crude Oil 45-46 MJ/kg Uranium235 - thermal reactor grade 500,000 MJ/kg Uranium238 – fast breeder grade 86,000,000 MJ/kg Typical Heat Values of Various Fuels The great advantage of nuclear energy plants is their very high energy density, constant power supply, small size of land use for installation and absence of CO2 emissions (at least after the uranium mining and enrichment phase and construction of the power plant). Nuclear energy saves the emission of about 2.6 billion tonnes of CO2 each year (compared with about 10 billion tonnes per year emitted from fossil fuel electricity generation).The Intergovernmental Panel on Climate Change (IPCC) has recommended nuclear power as a key greenhouse gas mitigation method that is currently commercially available. 6
  7. 7. There are serious safety and environmental concerns with U235 thermal reactor power plants.  Health issues: Because uranium ore emits radon gas, uranium mining can be a health hazard, unless adequate ventilation systems are installed.  Environmental issues: Nuclear power plants are almost always built near lakes, rivers and oceans because running a nuclear reactor requires a large amount of cooling water. A typical 1 GW nuclear reactor needs approximately 1500 m3 per minute and this warmer water is then discharged back into the local ecosystem causing adverse effects for the aquatic life.  Safety: The 1979 accident at Three Mile Island, the 1986 Chernobyl disaster, the 1995 Monju accident and the recent Fukushima nuclear disaster in Japan clearly demonstrated the potential catastrophic danger of nuclear power plants. This played a part in stopping new plant construction in many countries and to shut down facilities in some others.  Radioactive waste disposal: About 10,000 tonnes of highly radioactive nuclear waste is stored each year,[99] mainly at individual reactor sites (over 430 locations around the world). Of particular concern are Technetium99 (half-life 220,000 years) and Iodine129 (half-life 15.7 million years). Other products are unconverted U235 , plutonium and curium. About 95% of the depleted uranium is stored as uranium hexafluoride (UF6), in steel cylinders in open air close to enrichment plants. There is no consensus yet where to safely longterm store nuclear waste.  Misuse of fuel for nuclear weapon production (from plutonium)  Nuclear terrorism 7
  8. 8. The nuclear power debate There are multiple organizations which have taken a position on nuclear power – some are proponents, and some are opponents. Opponents • Friends of the Earth International, a network of environmental organizations in 77 countries.[183] • Greenpeace International, a non-governmental environmental organization[184] with offices in 41 countries.[185] • Nuclear Information and Resource Service (International) • World Information Service on Energy (International) • Sortir du nucléaire (France) • Pembina Institute (Canada) • Institute for Energy and Environmental Research (United States) • Sayonara Nuclear Power Plants (Japan) Proponents • World Nuclear Association, a confederation of companies connected with nuclear power production. (International) • International Atomic Energy Agency (IAEA) • Nuclear Energy Institute (United States) • American Nuclear Society (United States) • United Kingdom Atomic Energy Authority (United Kingdom) • EURATOM (Europe) • Atomic Energy of Canada Limited (Canada) • Environmentalists for Nuclear Energy (International) 8
  9. 9. Attempts to mitigate the nuclear radioactive waste problem • Reprocessing of the spent U235 fuel can potentially recover up to 95% of the remaining uranium and plutonium which can be reused as fuel (MOX). The radioactivity left consists largely of short-lived fission products, and its volume is reduced by 90%. Reprocessing is done in the UK and France, and to a lesser extent in Russia, India and Japan. However, it is not allowed in the U.S.[129] The Obama administration has disallowed reprocessing on the basis of nuclear weapon proliferation concerns.[130] • The nuclear waste problem can substantially be mitigated in the future by using fast breeder reactors that almost completely convert the nuclear fuel, leaving radioactive waste products in much smaller amounts for much shorter times (a few hundred years). • Used Thorium fuel also remains radioactive for only a few hundreds of years. • Fast breeder reactors also offset the present relative shortage of U235. • However, fast breeder reactors are much more expensive, the fuel needs to be more enriched, sodium used as a coolant can explode and burns in air and is very corrosive and nuclear proliferation concerns remain real. Thorium fuel products are claimed to be more nuclear weapon proliferation-resistant than other fuel products since Thorium produces fissionable U233 instead of fissionable plutonium. 9
  10. 10. 58 World mine production is about 60,000 tonnes per year, but a lot of the market is being supplied from secondary sources such as stockpiles, including material from dismantled nuclear weapons. The latter are, however, rapidly declining. Present production of new uranium fuels cannot follow increasing demands. Shortage (peak uranium) is imminent. Read more. In recent years 40 new nuclear power plants are being built or planned. With the presently functioning U235 thermal reactors at present consumption rates, the proved resources of U235 in the Earth crust, will be exhausted in about 60-80 years. Thus, a further expansion of thermal reactors looks unrealistic. On the basis of probability estimates that more resources will be found – be It at lower concentration and thus higher cost of mining and with more environmental damage – a period of 270 years can be covered at best. 10
  11. 11. Optimistic prospects Nevertheless several optimistic considerations have been advanced for nuclear energy use in the future: If recycling of Pu in spent uranium fuel becomes more widespread or U238 fast breeder reactors are used, nuclear energy could be generated for thousands of years. If U235 is extracted from phosphate mines (that are richer in uranium), up to 160,000 years could be covered. However, it is not shown to be economically feasible yet. Uranium is present in seawater in amounts of 3.3 parts per billion (4.6 x 109 tonnes – which is ~300 x the conventional reserves) and rivers bring uranium into the sea at a rate of 3.2 x 104 tonnes per year (~50 % of present consumption). At present however, extraction of uranium from the sea has only been tested at the laboratory scale and is very expensive. Read more. . However, new methods of affinity adsorption have recently been developed that may halve the cost. Read more. See also the book « The Ultimate Resource 2 ». Source: OECD, 2006b; OECD, 2006c. 11
  12. 12. • Direct use of nuclear fission energy is used to drive steam tubines to give locomotive force in some large ships and in nuclear submarines. • For small vehicles, however, a small energy carrier is needed. Under high temperature various procedures exist to create such carriers. Nuclear power plants produce an excess of heat that can be adopted to make the carrier. The following procedures are in the research and development phase: – Generation of syngas (hydrogen + CO mixture) from methane by steam methane reforming or from coal by coal gasification – Generation of hydrogen from water by electrolysis at high temperature (the electrolysis reaction is more efficient at higher temperature) or from water by thermochemical splitting. Nuclear power plant electricity can be used during periods of lower electricity needs (off-peak). – Generation of synthetic fuels from coal, natural gas, oil shale, or biomass. Since it requires large energy input, excess nuclear heat may be a welcome opportunity. World commercial synthetic fuels production capacity was over 240,000 barrels per day (38,000 m3/d) in 2009. 12
  13. 13. View other slide shows on nuclear energy