Nuclear Energy: Nuclear Decay - Energy and the Environment | PHSC 1014, Study notes of Physics

Download Nuclear Energy: Nuclear Decay - Energy and the Environment | PHSC 1014 and more Study notes Physics in PDF only on Docsity! ESA21: Environmental Science Exercises Nuclear Energy: Nuclear Decay Introduction The Nucleus Almost any phrase that has the word “nuclear” in it has a bad reputation. The term conjures up images of mushroom clouds and radioactive mutants. It is interesting to note that in the 1940’s and 50’s, the term that applied to energy derived from the decay radioactive material was atomic energy. This term was somewhat correct, since the energy was coming from the breakdown of the atom. It was not until later that the more appropriate term nuclear energy was used as more people began to understand that the energy was coming from the breakdown of the nucleus of the atom. The picture at the right is a popular image of the atom. While not a true depiction of the appearance of a real atom, it does mirror most of the features of it, such as a central nucleus composed of positively- charged protons and neutrally-charged neutrons that is orbited by negatively-charged electrons. The classic approach to discussing the atom is to tell students that it is held together by electrostatic attraction between the protons and electrons. But this is only part of the picture. What rarely gets discussed is what is holding the nucleus together, as there is not electrostatic force whatsoever holding the protons to the neutrons. The forces that are binding the neutrons to the protons are the strong and weak nuclear forces. Unlike the electrostatic force, which is relatively simple to explain (like charges repel and unlike attract), the strong and weak nuclear forces are rather complicated in that the discussion of them involves quarks, gluons, and vector bosons and their interactions. However, intimate knowledge of these forces is not required in order to understand nuclear energy, and therefore, we will leave it to the reader to research this subject as they desire. For the purposes of this discussion, we will merely note that the forces do exist, that they are important over very short distance scales (10-15 m and less), and that they bind neutrons to protons under certain conditions. Figure 1: Simplified diagram of an atom Isotopes If an atom is to be neutral, it must have the same number of electrons as it does protons, i.e. the total amount of negative charge must equal the total amount of positive charge (elements are defined by the number of protons in the nucleus). Is there some type of rule like this that applies to the number of neutrons that an atom has? It turns out that the answer is “No”. The complicated nature of the strong and weak nuclear forces plus the numerous configurations that could be used for storing protons and neutrons in a nucleus means that there is no given rule for the numbers of neutrons an element has. In fact, most elements have multiple numbers of neutrons that can be stored in the nucleus. Atoms that have the same number of protons, but different numbers of neutrons, are known as isotopes of an element. Not all of these isotopes are stable. For most naturally occurring elements, there is at least one isotope that is stable. The picture to the left shows three different isotopes of hydrogen. The first of these, which is what we normally think of as hydrogen, is called hydrogen-1 and m Figure 2: Bohr models of hydrogen, deuterium, and tritiu 1 is stable. The other two isotopes are called hydrogen-2 (deuterium) and hydrogen-3 (tritium). Deuterium, which is found in nature in about 1 in every 6500 atoms of hydrogen, is stable, whereas tritium, which is much rarer, is unstable and will undergo radioactive decay, if given enough time. Other examples of isotopes with which you might be familiar are carbon-12 (6 protons and 6 neutrons), which is stable, and carbon-14 (6 protons and 8 neutrons), which is unstable. Carbon-14 is one of the radioactive isotopes that is used to determine the age of biological fossils. There are some elements for which there are no stable isotopes. An example of this would be uranium. If given enough time, all forms of these elements will decay. Isotopes of a particular element behave the same chemically. That is to say, molecular compounds that can be made with one isotope of an element can be made in the same way with any other isotope of that same element. For example, water (H2O) can be constructed using hydrogen-1, hydrogen-2 (deuterium), or hydrogen-3 (tritium). The water molecules made from each of these will all look, taste, and feel like water. The only physical difference between them will be that water molecules made with deuterium and tritium will be heavier and denser than the one made with hydrogen-1. The one made with tritium will differ in one other way: they will also be radioactive. Radioactive Decay Methods of Decay When an atom decays, it can do so via one of three natural methods. These methods are known as alpha, beta, and gamma. In alpha decay, the unstable nucleus ejects an alpha particle, which is composed of two neutrons and two protons. Another way of stating this is that the nucleus decays by ejecting a helium-4 nucleus. In beta decay, the nucleus ejects a beta particle, which is either an electron (beta minus) or a positron (beta plus). At first glance, this would seem to be wrong, as a nucleus is comprised of protons and neutrons and contains no electrons or positrons. But they are produced in the nucleus whenever a neutron decays into a proton, an electron, and a neutrino or a proton decays into a neutron, a positron, and a neutrino. The last way of decaying is via gamma decay, which is when electromagnetic radiation is given off by the nucleus as the protons and neutrons become more tightly bound. There is another way for a nucleus to decay, though this method usually involves the actions of humans. A nucleus can be forced to break apart if it is hit by particles from outside of the nucleus. This was discovered by Ernst Rutherford in the early 1900’s when he bombarded nitrogen-14 (7 protons, 7 neutrons) with alpha particles (helium-4) to produce oxygen-17 (8 protons, 9 neutrons) and a proton. The easiest method for doing this type of decay is to bombard the nucleus with neutrons. Since the neutrons have no net charge, they are not repelled electrostatically away from the nucleus like a proton would be, and thus, do not require large energies in order to strike it. This will be important in our next activity, where we discuss the operation of a nuclear power plant. Half-Life Versus Activity If left on their own, unstable isotopes will decay in an exponential fashion. That is, given a large enough quantity of an isotope, the same percentage of them will decay in the same amount of time. This means that in a given year, the number of isotopes that decay, divided by the original number of isotopes, will be the same quantity. Rather than listing this fraction for isotopes, though, we often turn the issue on its head and discuss the length of time it takes for a certain fraction of the material to decay. In particular, we list the amount of time that it takes for half of the isotopes to decay, which is called the half-life of the substance. For example, iodine-131 has a half-life of 8 days. If one were to start with 100 kg. of it, after 8 days, they would only have 50 kg. After another 8 days, they would have 25 kg left, and so on and so forth. Eventually, the amount of iodine-131 would become so small that it would no longer obey exponential decay, at which time we would have no way of determining when a particular amount of the substance would decay. This is not to imply, though, that the fraction of the substance that will decay in a given time period is not important. It is very important, as this quantity is related to the activity, which is the number of decays that 2 corresponds to having an amount of radioactive material that can fit in the plant that has a high activity. As we saw in last week’s activity, a high activity requires either a large amount of a radioactive substance and/or an isotope with a short half-life. The physical limits on the amount of available isotopes and on the size of the power plant put constraints on the amount of material that can be used. Therefore, to get the required level of activity, a substance with a relatively short half-life is needed. This presents a problem, as substances with short half-lives are rarely found in nature in large abundance. This should not be surprising. Given the fact that the Earth is approximately 5 billion years old, any short half-life material that was originally present would have completely decayed by now. New isotopes are being created all of the time, as we found out last week, and some of these might have a short half-life. However, by the time that the material is found, mined, refined, and put into a power plant, a significant fraction of the material will have decayed. Thus, using natural decay of radioactive isotopes is not a very useful means for running a nuclear power plant. One could use bombardment of nuclei in order to break the isotopes apart and generate energy. As we pointed out last week, the easiest method for achieving this is to bombard the nuclei with neutrons, as their electrostatic neutrality means that there will be no force of repulsion from the protons in the nuclei. This does solve the problem of having enough nuclear material with a high enough activity to run the plant, as we can break apart even stable isotopes with this method. But it does create another problem. In order to bombard the material, we will have to input energy (separating neutrons from other material and accelerating a beam of them onto the nuclear material), which means that our overall efficiency will be less than what we desire. Chain Reactions We could get around this problem if we had a substance that produced free neutrons as a result of its being bombarded with neutrons. In other words, we need a substance that produces the catalyst (bombarding neutrons) from a reaction that was caused by the catalyst. It turns out that we are in luck, in this regard, as there is one naturally occurring isotope that is abundant enough to run a power plant. When uranium-235 (U-235) is bombarded with low energy neutrons, its nucleus will fragment into several parts, with neutrons being amongst them. A typical reaction for U-235 (one of many possibilities, all of which produce neutrons) is The whic meti This the r that only inter reac neut to ke has after diffe uran relea prod caus neutron + uranium-235 barium-144 + krypton-89 + 3 neutrons + 173.3 MeV of energythree neutrons that are released by this reaction are free to bombard three other uranium-235 nucle h would then decay into barium and krypt i, on fragments with up to 9 more neutrons and about three s the amount of energy being released. o y of ent , the chain reaction would look something like the picture t ight. There are two features about this chain reaction require further discussion. The first of these is that it is slowly moving neutrons that have a high probabilit acting with the uranium-235 nuclei and causing a tion. Therefore, a neutron moderator, which slows the rons down, is needed in a nuclear power plant in order ep the chain reaction occurring. The second feature to do with the number of neutrons that will be pres several different decays. While there are many rent possible decay reactions that could take place ium-235 nucleus will average about 2.5 neutrons sed in each one. This means that each reaction will uce more neutrons than what were there initially, thus ing more reactions to take place at the next stage. If Figure 5: U-235 undergoing a chain reaction 5 this is allowed to go on for some time, there will be so many neutrons around ready to react that too man decays will start taking place, which will release too much energy and cause the material to melt do For this reason, control rods, which are made from materials that readily absorb neutrons, need to be in the system in order to limit the number y wn. of reactions that can take place at any given time. Basic Reactor Design Besides having neutron moderators and some method for controlling the number of neutrons in the reactor, a nuclear power plant also has to have some way of transferring the thermal energy of the nuclear material to water in order to create steam to power a turbine. This can be done in a number of ways, depending upon how safe you wish to make the reactor and how much energy you wish to create. In the U.S., we have two basic designs for reactors. A boiling water reactor (BWR) places the hot nuclear material directly in the water where steam is created. This expanding steam is used to turn a turbine, which is connected to a generator that creates electricity. After the steam has passed through the turbine, it is cooled by a heat exchanger until it condenses back to hot water, and is then pumped back into the reactor chamber. A pressurized water reactor (PWR) looks similar to a BWR, except that the reactor is sitting in water that gets extremely hot, but is not allowed to turn to steam by the pressure applied to the chamber. This extremely hot water is used to heat another chamber of water where steam is generated. A diagram of a PWR is shown in Figure 6 below. Figure 6: Schematic of a pressurized water reactor While these are the two designs used in the U.S., they are far from being the only nuclear reactor designs. Other countries have experimented with different designs, with varying degrees of success. For instance, the former Soviet Union used an RMBK design that had carbon as its neutron moderator and water as a coolant that was passed through the reactor chamber in pipes, i.e. the fuel rods were not sitting in water. This reactor was the one that was involved in the famous accident at Chernobyl. To learn more about the different styles of reactors, check out the second link below. Additional Reading The following website leads to the U.S. Nuclear Regulatory Commission, which seeks to protect the public health and safety, as well as the environment, from the effects of nuclear reactors, materials, and waste products. This site provides information on reactors that are in operation in the U.S. and on the materials and waste that are involved in the process. 6 Nuclear Regulatory Commission Topic: U.S. Nuclear Regulatory Commission Summary: Links to information about U.S. reactors, nuclear materials, and nuclear waste Link: http://www.nrc.gov/ The next website is a privately owned website that is maintained by Joseph Gonyeau, a consulting nuclear engineer for the IAEA and the DOE. This site contains a wealth of information and links regarding the entire fuel cycle process. Of particular interest is the information on the various nuclear reactor designs found around the world. Virtual Nuclear Tourist Topic: Nuclear Reactor Designs Summary: Private informational website about nuclear energy. Check under Plant Designs for information about different reactor types. Link: http://www.nucleartourist.com/ References 1 http://www.llnl.gov/seaborginstitute/training/radiation_misuse_2.pdf, June 9, 2003. 7