Nuclear Fission and Fusion CfE Higher Physics – Unit 2 P&W

Learning Intentions Explain what is meant by alpha, beta and gamma decay of radionuclides Identify the processes occurring in nuclear reactions written in symbolic form State that in fission a nucleus of large mass number splits into two nuclei of smaller mass numbers, usually along with several neutrons State that fission may be spontaneous or induced by neutron bombardment

Rutherford Scattering In order to investigate the structure of the atom alpha particles were fired at a very thin gold foil in a vacuum. The angles through which the particles were deflected were measured. gold foil movable detector q alpha source beam of alpha particles

Rutherford Scattering Observations Most alpha particles passed straight through the foil with little or no deflection. A few particles were deflected through large angles. Some particles were even deflected backwards.

Rutherford Scattering Conclusions Even though the foil is about 100 atoms thick, the fact that most passed through shows that most of the atom must be empty space. Since some alpha particles bounce back, most of the mass and all the positive charge must be concentrated in a very small volume (the nucleus). These conclusions led to Rutherford’s model of the atom, in which the diameter of the atom is about 10,000 times that of the nucleus.

Atomic Structure Subsequent experiments have shown that the nucleus of the atom contains particles called protons and neutrons. Protons have a mass of 1 unit and a charge of +1. Neutrons also have a mass of 1 unit, but no charge. Orbiting the nucleus are electrons with negligible mass (~1/1840th that of a proton or neutron) and a charge of -1. Since atoms are electrically neutral, the number of electrons orbiting the nucleus is equal to the number of protons in the nucleus.

Atomic Structure p n e -1 ~0 electron 1 neutron +1 proton symbol 1 neutron +1 proton symbol charge mass number particle n p e

Atomic Structure Continued The number of protons in the nucleus is called the atomic number (A). Every element in the periodic table has a different atomic number. The total number of protons and neutrons is called the mass number (Z). A particular nucleus can be identified by both its atomic number and mass number: mass number Z A X symbol atomic number

Isotopes Different atoms of the same element can have different mass numbers (i.e. the same number of protons, but different numbers of neutrons). These are known as isotopes. For example, neon has two common isotopes: 20 10 Ne neon-20 10 protons and 10 neutrons 22 10 Ne neon-22 10 protons and 12 neutrons

Radioactive Decay Some nuclei are unstable and emit nuclear radiation (alpha, beta or gamma radiation) in order to achieve stability. This process is known as radioactive decay. A radionuclide (or radioisotope) is an isotope that decays radioactively. The original radionuclide is called the parent and the new nuclide produced is called the daughter.

Radioactive Decay Alpha Decay An alpha particle is a helium nucleus, with an atomic number of 2 and a mass number of 4. For example, the radioactive decay of uranium-238: 238 92 U 234 90 Th 4 2 He + alpha particle

Radioactive Decay Beta Decay A beta particle is a fast moving electron, which is created when a neutron decays into an electron and a proton. For example, the radioactive decay of lead-210: 210 82 Pb 83 Bi -1 e + b particle

Radioactive Decay Gamma Decay In gamma radiation, energy is lost by the nucleus in the form of electromagnetic radiation, but there is no change in the atomic number or mass number. Gamma radiation is a result of a reorganisation of the protons and neutrons in the nucleus.

Nuclear Fission Nuclear fission is the splitting of a large nucleus into smaller nuclei. Nuclear fission can occur spontaneously, but is usually induced by neutron bombardment. For example: + E krypton-92 barium-141 3 neutrons neutron uranium-235

Nuclear Fission (continued) 235 92 U 36 Kr 1 n + 141 56 Ba 3 The mass number and atomic numbers are both conserved during nuclear fission. However, when the total masses before and after the reaction are compared accurately, some of the mass appears to have been “lost”. This “lost” mass is known as the mass defect.

Nuclear Fission (continued) Learn This!! This “lost” mass is converted into kinetic energy according to Einstein’s equation: E = mc2 E = energy produced (J) m = mass defect (kg) c = speed of light = 3 x 108 ms-1

Learning Intentions State that in fusion two nuclei combine to form a nucleus of larger mass number Explain, using E = mc2, how the products of fission and fusion acquire large amounts of kinetic energy Carry out calculations using E = mc2 for fission and fusion reactions.

Nuclear Fusion Nuclear fusion is the joining of two smaller nuclei to form a larger nucleus. Nuclear fusion is the process occurring in stars. For example: hydrogen-2 (deuterium) neutron helium-3 + E

Nuclear Fusion (continued) 2 1 H 3 He + n As in nuclear fission, a small amount of mass appears to be “lost” during the reaction. Again, this mass is converted into kinetic energy according to Einstein’s equation (E=mc2).

Worked Example Calculate the energy produced when a uranium-235 nucleus of mass 390 x 10-27 kg is split by a neutron to produce molybdenum-98 with a mass of 163 x 10-27 kg and xenon-136 of mass 225 x 10-27 kg together with two more neutrons. (mass of neutron = 1.67 x 10-27 kg)

Worked Example U Mo n + Xe + n + n Mass before = U +n 235 U 98 Mo 1 n + 135 Xe + 1 1 n + n Mass before = U +n Mass before = 390 x 10-27 + 1.67 x 10-27 Mass before = 391.67 x 10-27 kg Mass after = Mo + Xe + 2n Mass after =163 x 10-27 + 225 x 10-27 + 2 x(1.67 x 10-27) Mass after = 391.34 x 10-27 kg

Worked Example (continued) Lost mass = mass before – mass after Lost mass = 391.67 x 10-27 - 391.34 x 10-27 Lost mass = 0.33 x 10-27 kg E = mc2 = 0.33 x 10-27 x (3 x 108)2 = 2.97 x 10-11 J

Learning Intentions Describe the principles of the operation of a nuclear fission reactor in terms of fuel rods, moderator, control rods, coolant and containment vessel Describe the coolant and containment issues in nuclear fusion reactors.

Parts of a nuclear reactor From national 5, you should remember the main parts of a nuclear reactor: Control rods Moderator Fuel rods Containment vessel Coolant

Parts of a nuclear reactor Control rods – dip in and out to absorb neutrons Fuel rods – contains uranium for nuclear reactions Containment vessel – thick concrete Moderator – slows down neutrons Coolant – water pumped around the reactor core to cool it.

Learning Intentions Describe the coolant and containment issues in nuclear fusion reactors.

Fusion Reactors In order for the reactions to take place reactors produce extremely high temperatures (up to 100 million Kelvin). This produces a plasma due to electrons being ripped from atoms. The plasma needs to be contained away from the walls of the container – as it will evaporate the walls and will be cooled itself. It also needs to be confined long enough to ensure more energy is extracted than absorbed.

Fusion Reactors The plasma can be: Contained and confined using a strong, toroidal electromagnets to produce a powerful magnetic field. Heated by inducing electric currents in it. Water can be used to extract the heat for power generation

Fusion Reactors