Principles of naval engineering - Historic Naval Ships Association

Chapter 24. -NUCLEAR POWER PLANTomany unstable nuclei emit an alpha particle.The results of alpha emission can be seenfrom the following equation:92 U'238 1"^* 90Th 234In the above equation, the parent isotopeof uranium (goU ) is a naturally occurring,radioactive isotope which decays by alphaemission. Since the A and Z numbers mustbalance in a nuclear equation, and since analpha particle contains two protons, we seethat the uranium has changed to an entirelynew element.The radioactive isotope of thorium(QQTh234) produced in the above reaction further(fecays by the emission of a beta particle symbol^) as indicated in the following equation:90 Th234__^^+g^Pa234The beta particle has properties similarto an electron.3 However, the origin of thebeta particle is within the nucleus ratherthan the orbital shells of an atom. It is postulatedthat a beta particle is emitted at anextremely high energy level when a neutronwithin the nucleus decays to a proton and anelectron (beta particle). When this phenomenonoccurs, the proton stays within the nucleusforming an isotope of a different elementhaving the same mass.A radioactive isotope may go through severaltransformations of the above types before reachinga stable state. In the case of 92U238there are a total of eight alpha particles andsix beta particles emitted prior to reachinga stable isotope of lead (82Pb206),The third manner in which a naturallyradioactive isotope may reach a more stableconfiguration is by the emission of gammarays (symbol y). The gamma ray is an electromagnetictype of radiation having frequency,high energy, and a short wave length. Gammarays are similar to X-rays in that the propertiesare the same. The distinguishing factorbetween the two is the fact that gamma raysare originated in the nucleus of an atom,whereas the X-ray originates from the orbitalelectrons. In general it can be said that agamma ray is of higher energy, higher frequency,and shorter wave length than an X-ray.Frequently an isotope which emits an alphaor beta particle in the decay process willemit one or more gamma rays at the sametime, as in the case of 27^'^ > ^^ isotopethat decays by beta emission and at the sametime emits two gamma rays of different energylevels. Some radioactive isotopes reach a stablestate by the emission of gamma rays only. Inthe latter case, since gamma rays have neithermass nor electrical charge, the A and Znumbers of the isotope remain unchanged butthe energy level of the nucleus is reduced.An important property of any radioactiveisotope is the time involved in radioactivedecay. To understand the time element, it isnecessary to understand the concept of halflife. Half-life may be defined as the timerequired for one-half of any given numberof radioactive atoms to disintegrate, thus reducingtheradiation intensity of that particularisotope by one-half. Half lives may vary frommicroseconds to billions of years. At timesan isotope may be said to be "short-lived"or "long-lived", depending upon its peculiarradio-active half-life. Some half-lives of typicalelements are:ggU^^S ^ ^ gj ^ jq9 yg^j.gggU^^^ = 7.13 X 10^ years88 Ra226 = 1620 yearsFrancis W. Sears and Mark W. Zemansky, Uni -versity Physics (3d ed.; Reading, Mass.: Addlson-Wesley Publishing Company, Inc., 1964), p.986.53 I^^^ = 6.7 hours84 Po214 ^ jo-6 seconds617