1 / 25

18 IONISING RADIATION AND RISK Quantifying risk

18 IONISING RADIATION AND RISK Quantifying risk. Review properties of alpha, beta and gamma radiation Quantify risk associated with exposure to ionising radiation. Starter: On the paper, record a fact about alpha, beta or gamma radiation. Pass it on around the group.

meagan
Download Presentation

18 IONISING RADIATION AND RISK Quantifying risk

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 18 IONISING RADIATION AND RISKQuantifying risk • Review properties of alpha, beta and gamma radiation • Quantify risk associated with exposure to ionising radiation Starter: On the paper, record a fact about alpha, beta or gamma radiation. Pass it on around the group. Starter: Define the term risk.

  2. Question The activity of an alpha source is 2 x 106 Bq. An alpha particle carries 5 MeV of kinetic energy. (a) Calculate the absorbed dose in Grays (J kg-1) for a 60 kg person exposed to this source for 30 minutes. (b) What is the dose equivalent in Sievert? (Quality factor for alpha radiation = 20) (c) How does your answer to (b) compare with the annual dose equivalent of 2000 μSv? (d) The best estimate of the probability of developing a radiation-induced cancer is about 5 % per Sievert. What is the risk, expressed as both a percentage and as a fraction, of developing a cancer due to this exposure?

  3. The half thickness of aluminium Use these data to determine the half thickness of aluminium towards beta radiation : Thickness (mm) Corrected count rate (Bq) 0.1 1304 0.2 1150 0.3 1011 0.7 506 0.8 413 0.9 401 1.0 305 2.0 10

  4. Modelling radioactive decay • Model the process of radioactive decay, including decay series • Explain practical consequences of radioactive equilibrium Starter: A sample containing 0.5 g of uranium 238 has an activity of 6200 Bq. Use this information to show that the half life of uranium 238 is about 4.5 x 109 years. (Molar mass of U-238 = 0.238 kg mol-1; Avogadro’s number = 6 x 1023 mol-1)

  5. Алекса́ндр Ва́льтерович Литвине́нко Alexander Litvinenko: former KGB agent who criticised the Russian government. In November 2006 he was admitted to hospital in London with suspected heavy metal poisoning. He died three weeks later from radiation poisoning. It turned out that an ex-colleague had spiked his drink with a lethal dose of polonium-210. Polonium-210 has a half life of only 138 days, so it cannot be stored for long periods. It can only be made in nuclear reactors, none of which is in the hands of terrorists or criminals. You don’t need to be Sherlock Holmes to work out who had him killed.

  6. Nuclear binding energy • Explain how the stability of a nucleus arises from the Einstein mass-energy relation • Calculate stability of specific nuclides Starter: Why doesn’t a nucleus just blow apart due to repulsion between positively-charged protons?

  7. The stability curve for nucleons • Analyse data to plot a graph of binding energy per nucleon against nucleon number • Interpret fission, fusion and radioactive decay in the context of the graph • Calculate which nuclear processes are spontaneous Starter: Do Q4-6 on p235 of textbook. Note: Nuclear, not atomic, masses are given so you don’t need to subtract electron masses.

  8. A nuclear decay can occur spontaneously if energy is released in the process. Q1. Write down the equation for alpha emission from silicon 28, using the subscript-superscript notation, identifying the daughter nucleus formed. Q2. Use the following nuclear mass data to determine if alpha emission from silicon 28 can occur. Nucleus Mass / u Silicon 28 27.97693 Daughter 23.98505 Helium 4 (α) 4.00260

  9. Decay sequences • Investigate radioactive decay chains • Explain significance of radioactive equilibrium in the context of decay chains Starter: Q1. What is the most likely mode of decay for the nucleus oxygen-15? Explain your choice and write a balanced nuclear equation for the decay (Hint: the stable isotope of oxygen is oxygen-16). Q2. What is the nuclide formed when plutonium 241 undergoes the following sequence of decays: βαα βαα

  10. The thorium series The thorium series starts from the nuclide Th-232, which decays by alpha emission. The decay sequence from thorium is α, β, β , α, α, α, α, β, β, α. Draw a chain which show all the nuclides created in this decay series. The nuclide Bi-212 in this series can also decay by alpha emission followed by beta emission. Add these decays to the series.

  11. Fission • Explain how self-sustaining fission is achieved and controlled in a nuclear reactor • Calculate the energy released by a single fission event and the rate of fission needed for practical power generation Starter: Write out and balance the following reactions involved in fission: Uranium 235 absorbs a neutron to give uranium 236 Uranium 236 fissions to give barium 140, krypton 93 and other particles Use the equations to explain why fission can rapidly run out of control.

  12. Fission questions Q1. Explain the meaning of the term chain reaction. What is the purpose of the control rods in a reactor? What is the purpose of the moderator? Q2. Calculate the energy released per fission event (in J and MeV) by doing Q1-5 from Qs 250S. Q3. What rate of fission is needed to give a reactor thermal power of 1200 MW? (see P239) Q4. How much U 235 would be consumed in running the reactor for a year? Q5. If the fuel contains 4 % U 235, how much would be needed each year?

  13. Fission questions Q1. Explain the meaning of the term chain reaction. What is the purpose of the control rods in a reactor? What is the purpose of the moderator? Q2. Calculate the energy released per fission event (in J and MeV) by doing Q1-5 from Qs 250S. Answers to Q2-5 (numerical) 2. 236.052 588 u 3. 235.859 799 u 4. –0.192 789 u; energy lost. 5. 179.49 MeV Q3. What rate of fission is needed to give a reactor thermal power of 1200 MW? (see P239) Answer: 4.2 x 1019 fissions s-1 Q4. How much U 235 would be consumed in running the reactor for a year? Answer: 516 kg Q5. If the fuel contains 4 % U 235, how much would be needed each year? Answer: 12 917 kg

  14. Critical mass A fission reaction cannot become self-sustaining unless the rate at which neutrons are lost from the surface of the mass of fissile material is less than the rate at which they are produced within it. Consider the fissile mass to be spherical. Q1. Write down an expression for the rate of production of neutrons, if the rate of production per unit volume is P. Q2. The rate at which neutrons are lost from the fissile mass is proportional to its surface area. Write down an expression for the rate of loss of neutrons from the mass, if the loss rate per unit area is L. Q3. Using your answers to Q1 and Q2, explain why there is a minimum critical size (critical mass) for self-sustaining fission.

  15. Fusion • Estimate temperature needed for fusion in stars • Explain nuclear reaction schemes taking place in the Sun • Explain why the Sun’s lifetime is billions, not millions of years Starter: Q1. Why are high temperatures required in order for protons to fuse together ? Q2. Write down an expression for the potential energy of a system of 2 charges q1 and q2, separated by a distance r. (Ch.16) Q3. Write down an expression for the average energy of a particle at a temperature T. (Ch.14) Q4. Write down an expression for the radius of a nucleus in terms of its nucleon number A. (Ch.17)

  16. If two protons are to fuse, they must get close enough for the attractive nuclear strong force to dominate electrostatic repulsion. As a first approximation, we may assume that they need to touch for this to happen. [Review: PE-KE exchanges in nuclear collisions; nuclear radii from electron diffraction; average energy per particle] • What is the potential energy of two protons touching? • How much kinetic energy will two colliding protons need to have if they are going to get close enough to touch? • How is the average kinetic energy of a system of particles related to the temperature? Use the relevant equation and your answer to the previous question to estimate the temperature needed for fusion. • In fact, the core of the Sun is at a temperature of only 10 million K. Which assumption made in your model does this observation suggest is at fault? • The energy ε that protons need to fuse is about 8.4 x 10-15 J. What is the Boltzmann factor for proton fusion in the core of the Sun? Comment on the feasibility of the fusion process in light of the numerical value of the Boltzmann factor you have just calculated. • The Boltzmann factor may be interpreted as either the fraction of particles with energies in excess of ε at one instant, or as the fraction of time that an individual particle spends with energy in excess of ε. If a proton in the core of the Sun makes on average 109 collisions s-1, show that it takes an average of 3 billion years before it gains enough energy to fuse with another proton. (1 year = 3.2 x 107 seconds) 7. What are the implications for this period of time for life on Earth?

  17. Fusion plenary Q1. What is the kinetic energy of a proton when it has reached its distance of closest approach to another proton prior to fusion? What is the potential energy of the system at this point? (give equation) Q2. Why are higher temperatures needed to initiate the fusion of heavier nuclei? Q3. Giant stars with high mass are generally much shorter-lived than low-mass stars like the Sun. Explain this observation carefully Homework: Write out the nuclear fusion reactions on p240 as balanced equations, paying attention to conservation. Revise for Ch.18 test next lesson

More Related