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Energy and the New Reality, Volume 2: C-Free Energy Supply Chapter 8: Nuclear Energy L. D. Danny Harvey firstname.lastname@example.org. Publisher: Earthscan, UK Homepage: www.earthscan.co.uk/?tabid=101808.
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Energy and the New Reality, Volume 2:C-Free Energy SupplyChapter 8: Nuclear Energy L. D. Danny Harveyharvey@geog.utoronto.ca Publisher: Earthscan, UKHomepage: www.earthscan.co.uk/?tabid=101808 This material is intended for use in lectures, presentations and as handouts to students, and is provided in Powerpoint format so as to allow customization for the individual needs of course instructors. Permission of the author and publisher is required for any other usage. Please see www.earthscan.co.uk for contact details.
Outline • Basics of nuclear physics • Fuels and reactions inside a nuclear reactor • Types of nuclear power reactors • The nuclear fuel chain • Safety • Nuclear weapons and terrorism risks • Cost • Embodied energy and GHG emissions • Operational constraints • Current capacity, future scenarios
Nuclei and isotopes • A given chemical element has a fixed number of protons in its nucleus (number of protons = number of electrons) • A variable number of neutrons is possible, resulting in different isotopes of an element • Protons and neutrons together are called nucleons • The number of protons in the nucleus is called the atomic number, while the number of nucleons is called the mass number
Superscripts and subscripts in front of the chemical symbol are used to represent the mass number and atomic numberIn 126C, for example, 12 is the mass number and 6 is the atomic number.Because the element name and atomic number are redundant, it is common to just write 12C instead of 126C.
Forces in a nucleus • Electric force – repulsion between protons, varies with 1/distance2 • Nuclear force – attraction between any two nucleons (even having the same charge), varies much more strongly with distance • Can overcome the electric repulsive force at distances comparable to the radius of a nucleus • Thus, neutrons, by providing extra nuclear forces, act as a glue holding the protons in the nucleus together
Neutron:proton ratio and stability of nuclei • As atomic number increases, the ratio of neutrons to protons required for stability of the nucleus increases • Nuclei with one or two less or one or two more nuclei are unstable – they eventually decay • If a heavy nucleus splits (fissions) into the nuclei of two lighter elements, it will often have too high a neutron:proton elements (depending on how the heavy nucleus splits), so the fission products will themselves often be unstable
Half lives of unstable nuclei • In a collection of unstable nuclei of a given isotope, not all the nuclei decay at once • Rather, it is observed that half of the nuclei existing at any given time will decay within a fixed length of time called the half-life • Thus, if the half life is 10 years and we start off with 1000 nuclei of a given isotope, there will be 500 left after 10 years, 250 left after 20 years, 125 left after 30 years, and so on
Kinds of particles emitted during the radioactive decay of an unstable nucleus • Alpha particles, consisting of 2 protons and 2 neutrons (as in the nucleus of He) • Beta particles (electrons) • Neutrons When alpha particles are emitted, the nucleus drops down by two atomic numbers. When a beta particle is emitted, a neutron in the nucleus turns into a proton, so the nucleus moves up by one atomic number. Gamma rays (very short wavelength, energetic electromagnetic radiation) are emitted when a nucleus drops from an excited to a ground state (perhaps following absorption of a neutron)
Fission (splitting) of a nucleus • Can occur spontaneously (i.e., with no external stimulus) • Can also occur as a result of the absorption of a neutron – such nuclei are said to be fissionable • Can even occur (with some probability) when a neutron of arbitrarily low energy strikes the nucleus – such nuclei are said to fissile • When fission occurs, additional neutrons are released that can then sustain a fission chain reaction (and further neutrons will be released when the fission products themselves decay) • Some nuclei can absorb neutrons but without splitting – such nuclei are said to be fertile
Fuels (or potential fuels) and decay reactions in a nuclear power reactor • Uranium (the overwhelmingly used fuel) • Plutonium (from recycling of spent fuel) • Thorium (usable in principle)
Natural Uranium • 0.0055% U-234 (234U) • 0.7205% U-235 • 99.275% U-238 • All three isotopes are radioactive
U-235 is fissile, and a sample reaction is235U + n → 236U → 92Kr + 141Ba + 3nThe fission products (92Kr and 141Ba here) and neutrons travel at high velocity, adding some of their kinetic energy to the atoms or molecules of the material through which they travel, heating itNote that, in the above, 1 neutron is absorbed and 3 are emitted. If one of the 3 that are emitted is subsequently absorbed by another 235U, the reaction will be self sustaining.
U-238 is fertile, and the reactions that occur are238U + n → 239U → 239Np + ß239Np → 239Pu + ßPu-239 is both fissile and fertile – it can either absorb another neutron (followed by emission of a beta particle, just like 238U above), or it can split into two lighter elements (with a half life of 24,000 year) and emit more neutrons as it splits.The absorption of a neutron by 238U is the first in a sequence of transmutation reactions, shown in the next figure.
Figure 8.3. Transmutation reactions beginning with U-238. Source: Modified from Wilson (1996, in The Nuclear Fuel Cycle, From Ore to Waste, Oxford University Press, Oxford)
To sum up, the reactions occurring inside a nuclear reactor are primarily: • Absorption of neutrons by 235U, producing 236U that then fissions and releases further neutrons • Absorption of some of the above neutrons by 238U, producing 239Np and 239Pu through the sequential emission of two beta particles (electrons) • Transmutation of 239Pu into successively heavier elements (the transuranic elements) through absorption of neutrons and emission of beta particles
Thorium as a fuel • Thorium is fertile, meaning that it can absorb neutrons without splitting (like U-238) • It must be used in combination with U-235, which serves as the neutron source • The end result is to produce U-233 (which does not occur naturally), which can be easily separated from the spent fuel and fed into another reactor as a fuel in a closed cycle • This would increase the energy that can be derived from a tonne of mined U by 85% compared to the usual once-through use of uranium • Significant new technologies would need to be developed to use the uranium-thorium cycle
Sustaining nuclear decay reactions • Neutrons released by fission of U-235 after it absorbs a neutron move so fast that they have little chance of being absorbed by another U-235 (at least one neutron must be absorbed to sustain the process) • Thus, the neutrons must be slowed down using a moderator (water, heavy water, or graphite) – which absorbs energy from the neutrons without (ideally) absorbing the neutrons
Stabilizing nuclear decay reactions • For the reaction not to grow exponentially out of control, exactly one neutron from each fission of U-235 must cause another fission • This ratio is maintained by inserting control rods between the uranium fuel rods – the more that are inserted, the more neutrons that are absorbed before they can cause another fission
Thermal reactors • Neutrons that have been slowed down with a moderator are called thermal neutrons, and reactors using them are called thermal reactors • Those that use water as a moderator require that the uranium fuel (which is mostly U-238) be enriched in the fissile isotope U-235 (from about 0.7% to 3-4%). These are called light-water reactors • Reactors that use heavy water as a moderator (namely, the CANDU reactor) do not require enriched uranium. These are called heavy-water reactors.
Fast and fast breeder reactors • Neutrons that have not been slowed down are called fast neutrons. They can still be used if the reactor has a high enough density of fissile material. This requires fuelling a reactor with U-233, U-235, or Pu-239. These reactors are called fast reactors. • Pu-239 is a natural choice, since it is produced anyway in thermal reactors • If the Pu-239 content exceeds 10-20%, more Pu-239 will be created through absorption of neutrons by U-238 than is consumed during the fission that releases the neutrons, so these reactors are called fast breeder reactors
Fast and fast breeder reactors(continued) • By repeatedly cycling nuclear fuel through fast breeder reactors, almost all of the U-238 in uranium (which accounts for > 99% of U) can be used as a fuel. Otherwise, only the U-235 (0.72%) serves as a fuel • The problem – Pu is ideal for making nuclear weapons. Vast amounts (1000s of tonnes) would need to be separated from spent fuel for recycling. Only 1-2 kg are needed to make a bomb.
The elements produced with atomic number beyond uranium are called transuranic elements. Each of them is unstable and will eventually fission into lighter elements. Uranium and the transuranic elements, along with thorium, are referred to as actinides (they form a special series in the periodic table after the element actinium)
Measures of nuclear radioactivity • Becquerel (Bq) – 1 Bq = a rate of one decay per second • Curie (Ci) – the rate of decay of one gram of radium 1 Ci = 3.7 x 1010 Bq • Gray (J/kg) or rad (100s of erg/gm) – amount of energy deposited per unit mass of living tissue • Sievert (Sv) or rem – the energy deposited (grays or rads) times a factor that accounts for the different amounts of damage caused by different kinds of radiation
Sources of nuclear radiation: • Emission of beta particles during the transmutation of an actinide to an element with a higher atomic number (as in Fig. 8.3) • Emission of neutrons during the fission of an actinide • The fission products themselves, which are released with high velocity and are large, so they are quite damaging • Emission of beta particles during the eventual decay of the fission products themselves • Emission of alpha and beta particles produced by four different radioactive decay chains that proceed spontaneously without the absorption of a neutron
Fission products of concern • Iodine-131, 8-day half life, becomes concentrated in milk, absorbed by thyroid gland. Of greatest concern for first few weeks after a potential nuclear accident • Stronium-90, 29-year half life, mimics calcium, becomes concentrated in bones • Cesium-137, 30-year half life, 6% of fission products, mimics potassium, distributed throughout body
Radioactive decay chains • Thorium series (Th-232 to Pb-208) • Uranium series (U-238 to Pb-206) • Actinium series (Pu-239 to Pb-207) • Neptunium series (Pu-241 to Tl-205) The uranium series is shown in Fig 8.4
Figure 8.6 Sources of radio-activity from spent LWR fuel Source: MIT (2003, The Future of Nuclear Power: An Interdisciplinary MIT Study)
Partial summary so far: • Heat is produced inside a nuclear reactor from collisions of energetic particles produced by radioactive decay with the atoms of the material forming the reactor, or by the absorption of gamma rays • The kinds of radioactive decay are - the lighter nuclei produced by fission of U-235 (the predominant fissioning material) or by fission of transuranic elements (produced by neutron absorption) - beta particles emitted during the decay of transuranic elements that build up (mostly plutonium) - beta particles from the decay of the fission products themselves - gamma rays emitted following neutron capture by U-235, U-238, or the transuranic elements
Once the fuel has been removed from a nuclear reactor, • The reactions involving absorption of neutrons by 235U and 238U largely cease, as does the production of transuranic elements • The sources of radioactivity in spent nuclear flow are -during the first year, the decay of fission products, with those having half lives of hours to days -one year after removal, the radioactivity has dropped to 1.3% of that at the time of removal, and is dominated by the decay of fission products with half lives of around 30 years (primarily Sr-90 and Cs-137) - thereafter, the decay of transuranic elements (especially Am-241, Pu-240 and Pu-239) dominates (until 100,000 years after removal) -finally, radioactivity from the U and Np series (which initially increases over time) dominates
Nuclear Reactor Technologies • Boiling-water reactor (a LWR, thermal reactor) • Pressurized-water reactor (another LWR, thermal reactor) • CANDU HWR (also a thermal reactor) • High-temperature gas-cooled reactor (HTGR), a fast breeder reactor.
Figure 8.7 Overview of nuclear powerplant technologies Source: van Leeuwen (2007, Nuclear Power- The Energy Balance, Ceedata Consultancy, Chaarn, Netherlands, www.stormsmith.nl)
Figure 8.8a Boiling-water light-water reactor Source: Wolfson (2003, Nuclear Choices: A Citizen’s Guide to Nuclear Technology, MIT Press, Cambridge)
Figure 8.8b Pressurized-water light-water reactor Source: Wolfson (2003, Nuclear Choices: A Citizen’s Guide to Nuclear Technology, MIT Press, Cambridge)
Figure 8.8c Liquid-metal fast breeder reactor Source: Wolfson (2003, Nuclear Choices: A Citizen’s Guide to Nuclear Technology, MIT Press, Cambridge)
Nuclear technology generations • Generation I: still a few in operation • Generation II: accounts for most of today’s reactors, based on military research of the 1940s and 1950s • Generation III: about 20 different designs are under development. Mostly incremental improvements from Generation II, but can still take decades to develop • Generation IV: 6 advanced concepts under development, most involving a closed cycle with reprocessing of spent fuel on site to separate and use plutonium. These exist only on paper at present, and would take 2-3 decades to develop.
Steps in the Nuclear Fuel Chain • Mining and milling of primary uranium, production of tailings waste • Enrichment of primary uranium, done by converting U to gaseous form and using centrifuges or membranes under pressure, creates a stream of depleted uranium (DU) waste. About 7 kg of natural uranium (0.7% U-235) are needed to produce 1 kg of U enriched to 3.6% U-235, with U-235 depleted to 0.2% in the DU stream • Use of nuclear fuel (the burn-up is the amount of heat produced per kg of fuel) • Possible reprocessing of spent fuel, generating lots of liquid and gaseous wastes • Isolation (“disposal”) of spent fuel and/or reprocessing wastes
Fuel chain step 1: Separation of uranium from ores • Uranium occurs as oxides in uranium ores • The proportion of uranium ores containing U is quite small (ranging from 0.03% to 18.0% in commercial operations, but averaging only 0.2%) • The mass of ore that must be processed per unit mass of uranium, and the associated rock waste, is given by the reciprocal of the ore grade • Thus, for 0.2% grade ore, 500 tonnes of ore must be processed to obtain 1 tonne of U • However, in open-pit mines, up to 40 tonnes of rock might be excavated per tonne of ore that is extracted
The recovered ore is crushed and leached with sulfuric acid or alkaline fluids in order to separate out the uranium • The final product is called yellow-cake (U3O8) • 85% of the radionuclides in the original ore end up in the wastes, which are called tailings. • Management of the tailings (due to their radioactivity and toxicity) will need to continue essentially forever (several 100,000 years)
Figure 8.9: World uranium extraction techniques in 2007 Source: NEA/IAEA (2008, Uranium 2007: Resources, Production and Demand, OECD Publishing, Paris)
Figure 8.10: Yellowcake – U3O8, produced from milling of the U ore followed by leaching from the crushed ore. Source: www.wise-uranium.org
Fuel chain step 2: Enrichment of uranium in U-235 • Light-water reactors require the uranium fuel to be enriched in U-235 (from 0.7% in natural uranium to 3-4%) • This requires converting the uranium to a gaseous form (UF6), and using either gaseous diffusion through membranes under pressure, or centrifuges, to create 2 streams – one enriched in U-235 and the other depleted in U-235 • The depleted uranium has to be stored somewhere essentially forever
Figure 8.11 Waste canisters containing depleted uranium,produced during the enrichment of natural uranium in U-235 Source: www.wise-uranium.org