Breeder reactors I have been told that I often start my posts by remarking that a friend asked me about such and such a topic. This is another one of those. Having read my earlier stuff on nuclear energy, someone asked me about breeder reactors, and whether they were perpetual energy machines. Well, they are not perpetual energy devices in any sense of the word, but they are a type of reactor which can produce more fuel than it consumes. I know that this sounds like perpetual motion, but it really is not, as the fuel is not produced out of nowhere, but rather is produced by converting an unusable (i.e. non- fissile) isotope of uranium into a useable (i.e. fissile) isotope of plutonium. Furthermore, unlike perpetual motion machines, they cannot run forever, since the plutonium does not in its turn produce more fuel, and so on ad infinitum. Anyway, let’s first look briefly at how normal reactors work, and all this will become clear: All nuclear reactors today generate energy through fission. Many people would love to invent fusion reactors, which are much cleaner, but for the moment no one knows how to maintain a sustainable but controllable fusion reaction. All we can do to date, insofar as fusion reactions is concerned, is build hydrogen bombs (for more on all this, please refer to my earlier posts). Anyway, fission is the process by which an atomic nucleus splits into two or more smaller (‘daughter’) nuclei, whilst simultaneously releasing some neutrons and some energy in the process. Source: http://www.visionlearning.com/library/module_viewer.php?mid=59 A diagram of a fission reaction involving uranium 235 (which is an isotope of uranium with 92 protons, and 143 neutrons in its nucleus). If you are not sure about what all this means, please refer to my earlier posts on atomic energy. The heat and electromagnetic radiation released during fission, is derived from the conversion of a small amount of matter into energy (the amount of energy released may be calculated using Einstein’s most famous formula: E = mc2). This energy can be used to boil water and thusly to drive steam turbines which are connected to electricity- producing generators. The method by which all of this is accomplished is by initiating a controlled but sustained chain reaction which is carefully damped so as to prevent it from running- away and becoming an atomic bomb, but yet is not so heavily attenuated as to cause the reaction to cease altogether. The driving force behind fission reactions are neutrons, and the secret to controlling and damping the reactions so they do not become dangerously excessive lies in absorbing a certain percentage of the freed neutrons (usually using a combination of both control rods made of silver-indium-cadmium alloy [80%, 15% & 5%], boron, hafnium, hafnium diboride or dysprosium titanate, which are inserted into or withdrawn from the rector core as needed, as well as boron compounds dissolved in the reactor coolant). Source: http://www.tutorvista.com/physics/nuclear-reactor-construction A schematic diagram of a nuclear power plant. Source: http://ec.europa.eu/research/energy/euratom/fission/microscope/reactors/index_en.htm As the control rods are withdrawn, the reaction proceeds and the core heats up. This hot core is continuously flushed with a fluid coolant in order to transfer the heat to the heat- exchanger / steam-generator, where it is then used to boil water. This steam, in its turn, is used to drive turbines which are connected to electricity-producing alternators. Source: http://www.xtimeline.com/evt/view.aspx?id=766694 This is what would happen if the reactor was not kept in check by means of control rods. You would have an exponentially-growing cascade of fission reactions (since every 235U nucleus which splits, releases 3 neutrons which could potentially split 3 further nuclei). This run-away series of chain-reactions would very quickly produce so much heat that the reactor core would blow up. This is basically how an atom / fission bomb works. The neutrons produced when a uranium nucleus splits then go on and slam into other uranium 235 nuclei and cause those to split in their turn, and so on. Now, in order for uranium 235 to fission efficiently, it needs slow-moving neutrons, but the neutrons which are produced in fission reactions are fast-moving. Thus, they are typically slowed down by a moderator substance (water, or helium usually) so as to enhance the number of fission reactions they can provoke, and thus work efficiently. In a breeder reactor by contrast, these fast neutrons are deliberately not slowed down. You see, although fast-moving neutrons may not be as efficient at provoking fission reactions in uranium 235, they can be readily absorbed by another isotope of uranium, uranium 238 (which is far more common than 235U), which then undergoes a series of nuclear reactions and eventually becomes plutonium 239. Source: http://www.green-planet-solar-energy.com/nuclear-power-information.html This is how uranium 238 becomes plutonium. A 238U nucleus, which contains 146 neutrons and 92 protons captures a fast neutron, and becomes uranium 239 (with 92 protons and 147 neutrons). This 239U however is unstable, and emits an electron (i.e. one of the neutrons in the nucleus changes into a proton and an electron), and becomes neptunium 239 (with 93 protons and 146 neutrons). This isotope of neptunium too is unstable though, and it in turn emits another electron, and becomes plutonium 239 (with 94 protons and 145 neutrons). 239Pu is also unstable, and highly fissile, but is not as unstable as 239Np, and this can be harvested from the reactor core and used to fuel other fission reactions, be it in new reactors, or in bombs. Thus, in breeder reactors, the coolant which is often used is liquid sodium, which is an inefficient neutron moderator so the neutrons produced by the fission reaction retain their high energies, and are thus easily able to initiate the conversion of uranium 238 (238U) into plutonium 239 (239Pu) via the intermediate reaction product neptunium 239 (239Np). The primary reason for using breeder reactors is that the vast bulk (~99.3%) of naturally occurring uranium consists of the relatively stable isotope uranium 238, and only a very little bit (~0.7%) of the unstable (and thus easily fissile) isotope uranium 235. In fact, there are also very, very small portions of other isotopes present (~0.005% of 234U for example), but these isotopes are irrelevant to our discussion. In order for a fission reaction to proceed, the naturally occurring uranium 238 needs to be artificially enriched with uranium 235, which, whilst easy enough to understand conceptually, is an extremely technologically complex and demanding process to actually implement (for more on this see my earlier posts). For nuclear reactors, typically, low-enriched uranium is used (containing ~3-8% 235U), whilst for making bombs, the enrichment can be extremely high (up to ~90% 235U), though to a large extent the requisite level of enrichment depends upon the specific design of the bomb being manufactured. Source: http://en.wikipedia.org/wiki/Enriched_uranium This is where breeder reactors come in: they can convert the unusable (non- fissile) uranium 238 into useable (fissile) plutonium 239. This is where the breeder part in their name comes from. There is no perpetual motion involved; in the process of producing energy from the fission of 235U, they also, coincidentally, convert a non-fissile element into a fissile one (the old alchemists dream realised by the way, the transmutation of one substance into another!). Once a breeder reactor has exhausted its uranium 235, the core can be removed and reprocessed to extract the plutonium 239, which can then be used to power other reactors, or to make bombs. Depending upon their design, breeder-reactors can actually make more fuel (up to 30% more) than they consume. Plutonium 239 is even more fissile than uranium 235, and in a typical (non- breeder) nuclear reactor, some percentage of the 238U in the core does get converted into 239Pu anyway, which then undergoes fission and contributes to the overall energy output of the reactor. In a breeder reactor, one of the design objectives, particularly if the aim is to produce fuel for nuclear weapons, is to prevent too much of the plutonium 239 from undergoing fission whilst it is still in the breeder reactor. Again, using fast neutrons helps to achieve this aim, as they provoke far fewer fission reactions than do slow neutrons. There are some serious issues with all of this however, as the process of extracting plutonium, which is highly radioactive, is fraught with potential danger, and requires careful handling, as well as proper public scrutiny and oversight. Source: http://en.wikipedia.org/wiki/Wikipedia:Picture_of_the_day/July_2009 In gun-type atomic weapons, the typical fuel is highly-enriched uranium, whereas in the implosion-type bomb design, the preferred fuel is plutonium 239.
Pages to are hidden for
"Breeder-reactors"Please download to view full document