Nuclear Decay

4. Nuclear Decay • Chemical reactions all involve the exchange or sharing of electrons, they never have an influence on the nucleus of the atom.  • Nuclear reactions involve a change in the nucleus.  • There are forces in the nucleus that oppose each other, the "Strong" force holding Protons and Neutrons to each other and the electrostatic force of protons repelling other protons.  • Under certain arrangements of protons and neutrons the electrostatic force can cause instability in the nucleus causing it to decay.  It will continue to decay until it reaches a stable combination.

5. Stability Belt • This graph shows the stable nuclei in red.  There are several things to notice: • There are no stable nuclei with an atomic number higher than 83 or a  neutron number higher than 126. • The more protons in the nuclei, the more neutrons are needed for stability.  Notice how the stability band pulls away from the P=N line. • Stability is favored by even numbers of protons and even numbers of neutrons. 168 of the stable nuclei are even-even while only 4 of the stable nuclei are odd-odd. (This can't be seen from this graph due to its small size and lack of detail.)

6. Radioactive isotopes • Unstable nuclei, called radioactive isotopes, will undergo nuclear decay as it becomes more stable.  • There are only certain types of nuclear decay which means that most isotopes can't jump directly from being unstable to being stable.  It often takes several decays to eventually become a stable nuclei.  

7. Types of Radioactive Decay  • When unstable nuclei decay, the reactions generally involve the emission of a particle and or energy.  • Coming slides are describing the types of nuclear decay. • Notice that for each type of decay, the equation is balanced with regard to electric charge (atomic number) and total number of nucleons ( mass number).  • In other words, the total atomic number before and after the reaction are equal.  And the total mass number before and after the reaction are also equal.

13. Happens to nuclei with  Z>83The 2 p+ 2n loss brings the atom down and to the left toward the belt of stable nuclei.

14. Alpha particles Alpha particles leave a trail of liquid droplets, artificially colored green in this photograph, as they pass through a supersaturated vapor in a detector known as a cloud chamber

18. Happens to nuclei with high neutron:proton ratioA neutron becomes a proton causing a shift down and to the right on the stability graph  

20. Generally accompanies other radioactive radiation because it is the energy lost from settling within the nucleus after a change. Since gamma rays do not affect the atomic number or mass number, it is generally not shown in the nuclear equation.

21. Production of gamma rays

25. Happens to nuclei with a low neutron:proton ratioA proton becomes a neutron causing a shift up and to the left.  Always results in gamma radiation.

27. Happens to nuclei with a low neutron:proton ratioA proton becomes a neutron causing a shift up and to the left.  Always results in gamma radiation

29. This graph shows all the trends of decay and the band of stable nuclei.  There are some exceptions to the trends but generally a nuclei will decay following the trends (in multiple steps) until it becomes stable.  For example 92U238 will go through 8 alpha emissions and 6 beta emissions (not all in order) before becoming 82Pb206 The steps a nuclei follows in becoming stable is called a radioactive series.  The series for 92U238 is shown below as an example.

31. The natural radioactive decay series for 92 U 238 (uranium series)

32. Half-life • The half-life is the length of time required for half of any given amount of an element to decay into another element. • For example, if one begins with a gram of carbon-10, 20 seconds later only half a gram will remain, after 40 seconds only a quarter gram will be left, after 60 seconds an eighth of a gram, after 80 seconds one sixteenth of a gram, and after 100 seconds have elapsed from the beginning of the experiment, only one thirty-second of the original carbon-10 will remain (left). • Because it decays so fast carbon-10 is not found in nature, although it can be observed as the product of some nuclear reactions.

33. Half-life • The half-lives of the five unstable isotopes of carbon differ greatly. Whereas half of any starting quantity of carbon-16 decays to nitrogen-16 in three quarters of a second, the same reaction for carbon-14 decaying to nitrogen-14 takes nearly six thousand years. Even so, carbon-14 is not a stable isotope, and will disappear in time. • This slow decay of carbon-14 is the basis of a widely used dating method for archaeological materials. As long as any organism is alive, its carbon atoms are being exchanged continuously with the atmosphere. Plants and animals release into the atmosphere during respiration. Plants use atmospheric during photosynthesis to make carbohydrates, and animals obtain these carbon atoms by eating the plants. • A constant ratio of carbon-14 to stable carbon-12 is maintained in the atmosphere because of the continuous production of new carbon-14 by reactions with high - energy neutrons in the upper atmosphere.

36. Mass and Energy: Nuclear Reactions With nuclear reactions, the energies involved are so great that the changes in mass become easily measurable. One no longer can assume that mass and energy are conserved separately, but must take into account their interconversion via Einstein's relationship, E = mc2 If mass is in grams and the velocity of light is expressed as c = 3 x 1010 cm /sec , then the energy is in units of g cm2/sec2 , or ergs. A useful conversion is from mass in amu to energy in million electron volts (MeV):1 amu = 931.4 MeV

37. Binding energy • What holds a nucleus together? If we attempt to bring two protons and two neutrons together to form a helium nucleus, we might reasonably expect the positively charged protons to repel one another violently. Then what keeps them together in the nucleus? • The answer, is that a helium atom is lighter than the sum of two protons, two neutrons, and two electrons. Some of the mass of the separated particles is converted into energy and dissipated when the nucleus is formed. • Before the helium nucleus can be torn apart into its component particles, this dissipated energy must be restored and turned back into mass. Unless this energy is provided, the nucleus cannot be taken apart. This energy is termed the binding energy of the helium nucleus.

48. Control rods • These are made of steel containing a high percentage of material which can absorb neutrons, e.g. boron. • Control rods are pushed into the core of the reactor. They control the amount of reaction and hence the amount of heat energy being produced. • In emergency they can be used to shut down the reactor completely.

49. Coolant • The nuclear reaction produces heat (parallel to the burning of coal or oil in a conventional power station). The coolant carries this heat away.  • The coolant is taken  by a pipe to the steam generator where water is boiled. This  is a heat-exchange process and it thereby lowers the temperature of the coolant which is then returned to the core to collect more heat. • Typical coolants are water, carbon dioxide gas, liquid sodium. • In a Boiling Water Reactor (common in Finland, Germany, India, Japan, Mexico, Netherlands, Spain, Sweden, Switzerland, Taiwan and USA) which operates at about 300 oC water is both the moderator and the coolant. • A Pressurized Water Reactor (PWR) operates at a slightly higher water temperature of approximately 320 oC. • A Gas-cooled Reactor operates at still higher temperatures. • In a Metal-cooled Reactor, usually a Breeder reactor, sodium or a sodium-potassium alloy is the coolant. Sodium leaves the region of the fuel at about 800 oC.

50. Fuel • Typical fuels are uranium in the form of uranium or uranium dioxide; and plutonium 239P. Uranium dioxide is preferred to the metal as it has a higher melting point. • Natural uranium contains 0.7% 235U. This has to be increased to about 3% to be more useful in a nuclear reactor. • Fuel rod temperature under normal operating conditions is about 760 oC. The ceramic-fuel rod melts at about 2870 oC. • When a slow moving neutron collides with the uranium nucleus several different reactions may occur. One example is the following: • 23592U + n --> 14436Kr + 8956Ba + 3n + heat energy • If there is sufficient uranium (known as the critical mass) these product neutrons collide with further uranium nuclei and more reaction occurs. This escalating reaction is known as a chain reaction. To prevent this reaction getting out of hand and producing too much heat and causing meltdown the neutrons have to be absorbed by control rods. • The world's first nuclear reactor was a natural reactor at Oklo in Gabon. • NB 1. Since the percentage of 235U in the fuel is too low the nuclear reactor cannot turn into an atomic bomb. • 2. A pound (0.45 kg) of enriched uranium produces as much energy as approximately one million gallons (4.5 m litres) of burning petrol (gasoline).