Unit 16 Nuclear Reactions and Nuclear Power

1. Unit 16Nuclear Reactions and Nuclear Power

2. - - - 16-1 + + + + + + - - - Elementary Particles • Elementary particles are vital in nuclear reactions. • The only atomic particles that play a part in nuclear reactions are the protons and the neutrons; electrons do not play a part in nuclear reactions. • We use the following notation in order to describe the specific element we are considering in a nuclear reaction. • In this notation, X represents the element, Z represents the atomic number (number of protons), and A represents the atomic mass number (the number of protons and neutrons) of the element.

3. 16-2 Nuclides • This notation is very important because it allows us to represent different isotopes of an element X. • For a given atom, carbon for instance, nuclei are found that contain different numbers of neutrons even though they contain the same amount of protons. • Here are the symbols of some different isotopes of carbon. • They are all Carbon because they all have 6 protons; however, they have different numbers of neutrons. • These “different” carbons are known as Isotopes of each other. • The carbon to the left is known as “Carbon 12” because it has six neutrons and six protons. • The carbon to the right is known as “Carbon 16” and has six protons and ten neutrons.

4. 16-3 Isotopes • For the nuclides below, determine the number of neutrons and protons.

5. 16-4 Atomic Radii • Use the formula below in order to find the atomic radii of the following Nuclide.

6. 16-5 Particle Masses • Nuclear masses are specified in unified atomic mass units or amu • The atomic mass of a neutron is 1.008665 u. • The atomic mass of a proton is 1.007276 u. • A neutral hydrogen atom , which has an electron and a proton but no neutron, has a mass of 1.007825 u. • We will use the mass of a neutral hydrogen in the place of the mass of a proton. • The mass of a nucleus is known to be 4.002602 u. • Compare this mass to the masses of the appropriate number of protons and neutrons combined. • What did you discover? • In the question above, you found that the actual mass was less than the mass of its constituent parts. • What do you think happened to the “missing mass?”

7. 16-6 Binding Energy • The difference in masses discovered on the previous slide is known as the total binding energy of the nucleus. • This energy represents the amount of energy that must be put into the nucleus in order to break apart its protons and neutrons. • For a given nucleon , the total binding energy, Eb, is • Find the amount of energy put into the following nuclide in order to break it apart.

8. 16-7 Alpha Decay • Alpha decay is one of several types of nuclear decay. • In alpha decay a parent nucleus is broken apart to yield a daughter nucleus and an alpha particle. • The general equation for alpha decay is as follows. • An alpha particle is a neutral helium atom. Daughter Parent Alpha

9. 16-8 Q-Value • In alpha decay, the masses of the daughter nucleus and alpha particle combined are less than the mass of the parent nucleus. • The “missing mass” is converted into the kinetic energy of the alpha particle and the daughter nucleus. • This “missing mass” or released energy is known as the disintegration energy. • It is also known as the Q-value for the particular parent nucleus. • The equation used to determine the Q-value is • Find the daughter nuclide due to alpha decay and the Q-values or the nuclides below. • Go to http://atom.kaeri.re.kr/ton/nuc2.html in order to find the mass of the daughter nuclides.

10. 16-9 Beta Minus Decay • Beta decay is another of several types of nuclear decay. In beta decay a parent nucleus is broken apart to yield a daughter nucleus and a Beta particle. • A Beta particle is either an electron (e-) or a positron (e+). • If an electron (also known as a -) is emitted during the decay process, then the decay process is known as a “Beta minus decay.” • The general equation for beta minus decay is as follows. • The underlying reaction is the conversion of a neutron into a proton, electron, and an anti-neutrino. Neutron Proton

11. 16-10 Neutron Proton Beta Plus Decay • If a Positron (also known as a +) is emitted during the decay process, then the decay process is known as a “Beta plus decay.” • The general equation for beta plus decay is as follows. • The underlying reaction is the conversion of a proton into a neutron, positron, and a neutrino.

12. 16-11 Electron Capture • In electron capture, an orbital electron is captured by the nucleus, combines with a proton, and forms a neutron and a neutrino. • The general equation for electron capture is as follows. • The underlying equation for electron capture is the conversion of a proton and an electron into a neutron and a neutrino. Neutron Proton

13. 16-12 Nuclear Fission • Fission may be spontaneous or may be induced. • There are many radioactive elements occurring in nature that will undergo fission on their own. • Such fissions are known as spontaneous fissions. • Stable atoms can be forced to undergo the fission process. • The first step is to make them unstable by impregnating the nucleus of the stable atom with an extra neutron. • Once this neutron is added, the atom becomes unstable and excited. • An excited atom will break itself apart (fission). • Such a fission is known as an induced fission. • On the next slide we will consider the fission process that takes place in most nuclear reactors: that of U-235.

14. 16-13 Neutron Proton Nuclear Fission • Fission (breaking apart) is caused when a free neutron impregnates an atom. • This collision causes the atom to become excited. • An excited atom is unstable. • As nothing in nature wants to remain excited, this atom eventually splits apart. • The end products are two daughter atoms and two or three other free neutrons. • Watch the fission process below.

15. 16-14 Fission of U-235 • A slow neutron is absorbed by a U-235 Nuclide producing an excited U-236*. The apteryx denotes an excited nuclide. • The U-236* promptly breaks apart into two daughters and either 2 or 3 neutrons. In this example 2 neutrons are produced. There are many different possible combinations of daughters. • The total reaction, shown as a single step, is as follows.

16. 16-15 Balancing Fission Reactions • You must end all fission reactions with the same number of protons and neutrons as were present in the reactants of the fission. • We can use this conservation rule to determine one daughter when the second is known. • Determine the missing daughter in the induced fission reaction of U-235

17. 16-16 Nuclear Power Plant Components Ha A E Ga F B C Hb D Ja K Gc Hd Jb M Hc

18. 16-17 Reactor Pressure Vessel • Reactor pressure vessel – contains the primary loop coolant. • This coolant is heated by the high kinetic energy nuclear particles released during nuclear fission. • The energy is transferred (heat) from kinetic energy of the nuclear particles to kinetic energy of the water molecules. • Since Temperature is the average kinetic energy of a substance, the temperature of the primary water rises greatly. Return

19. 16-18 Primary Coolant • Described in detail above in part A. • Serves to transport the energy from the reactor core (C) to the steam generator (K). • What type(s) of heat transfer (convection, conduction, and/or radiation) occur when the water carries the energy from the reactor core to the steam generator? Return

20. 16-19 Reactor Core • Site of the nuclear reaction in which Uranium – 235 is bombarded by neutrons. • This bombardment subsequently causes the fission process to occur releasing the energy of the atom and transferring it to the primary coolant water. Return

21. 16-20 Primary Coolant Loop • Transports the primary coolant, which carries the energy produced by the reactor core, from the vicinity of the reactor core to the steam generator and back into the Reactor Pressure vessel. • What type of heat transfer is employed by the Primary Coolant Loop? • Ga – Hot water traveling from the reactor pressure vessel to the steam generator. • Gc – Cold water traveling from the steam generator back to the reactor pressure vessel. Remember, the words “hot” and “cold” are relative. Both Ga and Gc are at temperatures exceeding the boiling point of water at atmospheric pressure. Return

22. 16-21 Steam Generator • Uses the energy stored in the high temperature primary coolant to heat and vaporize the secondary coolant. Return

23. 16-22 Pumps • Ensures the continuous flow of both primary and secondary coolant. • If the pumps fail, then the power plant will lose cooling capabilities and a meltdown will result. • This situation is bad, bad, much! Return

24. 16-23 Steam Turbine • Converts the energy stored in the form of the steam’s kinetic energy and turns it in to work (the spinning of the turbine blades). • The rotation of the blades turns a shaft that is connected to an electric generator. Return

25. 16-24 Electric Generator • Converts the rotational motion (work) of the turbine into electrical energy Return

26. 16-25 Secondary Coolant Loop • Transports super heated steam to the steam turbine (E) where it turns the turbine blades. • The steam then leaves the turbine and passes to the condenser where it is “cooled” and condensed back into water. • From the condenser the “cooled” water is pumped back to the steam generator and begins the cycle again. • Ha – Super heated high-pressure steam • Hb – Low-pressure steam. • Hc – Low-pressure water. • Hd – High-pressure water Return

27. 16-26 Tertiary Coolant Loop • Provides “cold” water to the condenser. • This water removes energy from the steam causing the steam in the secondary loop to condense back into water. • Ja – cold water • Jb – Hot water leaving the condenser. This water is often used to preheat any water to be added to the secondary coolant loop. This additional water is found in a pre-heater (not shown). The secondary loop is prone to coolant loss, especially within the turbine. This coolant must be periodically replaced. Return

28. 16-27 Steam Condenser • Removes energy from steam and changes it back into water. Return

29. 16-28 Nuclear Power Plant Operations – The Core • The core of a nuclear power plant consists of the moderator (water), the fuel (U-235), and the control rods. • The control rods control the nuclear reactions within the core. • The control rods are positioned in spaces within the fuel. • When the control rods are slowly withdrawn, the nuclear reaction which generates heat to turn water into steam begins.

30. 16-29 Nuclear Power Plant Operations – Control Rods & Fuel Cells • The control rods are “neutron eaters.” • They are made of a material that will readily absorb neutrons. • When inside they fuel, they eat most of the neutrons produced by spontaneous fissions thereby keeping the reactor core from going critical. • However, some neutrons still make it past the rods causing other induced fissions. • When the rods are withdrawn, more neutrons are able to cause other induced fissions. • As a result, many more neutrons are produced causing the reactor to go critical.

31. 16-30 Nuclear Power Plant Operations • A Ha E Ga A F C K Hb B D Ja Gc Hd Jb Hc

32. 16-31 Meltdown! • A meltdown in a nuclear power plant is very bad. • A melt down could occur when there is a problem in the nuclear plant. • For example, say a water pump goes bad. • When the pump goes bad, the water near the core turns to steam. • This water is needed in order to cool the core. • Once the cooling water is lost, the fission process “goes wild.” • The core melts but the U-235 continues to fission. • It melts through the core and continues to melt into the Earth. • However, the main killer is the contaminated steam that gets into the Earth’s atmosphere. You are dead!

33. 16-32 Reactor Scram • Fortunately, the engineers who design the plants include inherent, automatic safety procedures. • The primary safety procedure is known as a reactor scram. • Watch what happens in this protected plant as the same error occurs. • When the pump failed, sensors in the plant detected and increased water temperature and a decreased flow in the plant. • These sensors triggered an automatic response which caused the control rods to rapidly slam into the fuel. • As a result, the fission process was halted, and the water did not boil away from the core. • In the US, all of our plants are built with these safety components; however, other countries are not as safety conscious.

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35. 16-1 A • A