Energy & Sustainability

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Lily Elliott
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1 Energy & Sustainability Lecture 20: Nuclear Power April 9, 2009

2 Radioactive Decay Each radioactive isotope has a characteristic lifetime and decays pathway Each isotope has a given probability of decay in a unit of time, INDEPENDENT of past history dn = number of atoms decaying in time interval dt dn = C N dt, C: Constant characteristic of isotope

3 Exponential Decay N(t) = N(0) e Ct = N(0) e t/τ, τ: lifetime Specific half-life T 1/2 = ln2τ = 0.639τ

4 Radioacitivity Amount of radioactivity depends on: Amount of material Lifetime Example 1 g of 235 U half life y = s Mole: amount of substance of a system which contains as many elementary entities as there are atoms in kilogram of carbon atoms/mol (Avogadro)

5 Radiocitivity i

6 Important Radioactive Elements 3 H (tritium) 12 y half life used in nuclear weapons/ fusion power 60 Co (cobalt 60) used for cancer therapy, food sterilization 90 Sr (strontium 90) nuclear fallout, settles in bones 137 Cs (cesium 137) nuclear fallout 235 U (uranium 235) nuclear power, nuclear weapons 238 U (uranium 238) most abundant, used in artillery shells (and particle detectors) 239 Pu (plutonium 239) nuclear power, nuclear weapons

Polonium 210 210 Po half life 138 d (alpha decay) Lethal Dose 1 microgram Alexander Litvinenko Former KGB agent living in

7 Polonium Po half life 138 d (alpha decay) Lethal Dose 1 microgram Alexander Litvinenko Former KGB agent living in London Very critical of Putin and Russian govt. Nov 1, 2006 dinner with ex KGB agent Poisoned (tea) with 210 Po Dead three weeks later

8 Energy from Nuclear Physics

9 Naturally Occurring Uranium Consists of three radioactive isotopes: Main: U 238 (t 1/2 = y ) 1/140 th : U 235 ( t 1/2 = y ) >>> : U 234 (t 1/2 = y ) As we saw: decay of any of these uranium nuclei produces new nucleus ( daughter ) and so on > long series of isotopes (Decay series/paths/chains) Until number of protons and nucleons is small enough to form stable nucleus (in each case final product is an isotope of lead (Pb 206 or Pb 207) Uranium ore contains all these isotopes (e.g. Radon) > total radioactivity many times that of uranium alone

10 Nuclear Fission A little history: : Rutherford s gold foil experiment 1932: J. Chadwick first identified free neutrons (proved to be the by far most effective projectiles) Fermi among the first to use neutrons to study matter, found β emission, led him to believe that elements one place higher in the periodic table were produced Fermi finally reached the ultimate target material: uranium, led him to believe that he produced elements with atomic numbers higher than 92: transuranic elements or actinides

11 A little history: Nuclear Fission : Hahn, Meitner, Strassmann repeated Fermi s experiments (others as well) 1938: F. and I. Joliot Curie reported that they found a lanthanium(a.n.57) like element in the products of irradiated uranium On hearing this, Hahn and Strassmann worked on a chemical analysis establishing that one product definitely was barium (a.n.56) At first they could not bring themselves to to take such a drastic step which goes against all previous experiences in nuclear physics (Hahn & Strassmann, 1938)

12 Nuclear Fission A little history: Hahn wrote to Meitner in Stockholm: Perhaps you can suggest some fantastic explanation 1939:

13 A little history: Nuclear Fission 1939 Theoretical physics conference in Washington: Bohr and Fermi describe the results Fermi made the startling suggestion that free neutrons would appear

14 Nuclear Fission Neutron + heavy atom

15 Chain reaction

16 Exponential Growth: Chain reaction Controlled nuclear power Uncontrolled nuclear bomb 1939 Washington conference: This [Fermi s] suggestion threw the meeting into an uproar while physicists who had facilities initiated phone calls to their laboratories to start the search for fission neutrons. Manley, 1962

17 Critical Mass Bohr established that only U 235 readily underwent fission Many of the produced high energy fast neutrons are absorbed by U 238, on average less than 1 neutron causes another fission (and no chain reaction occurs) Uranium needs to be enriched: increased U 235 proportion Even with enriched Uranium there would be losses in the process because neutrons escape though the surface, volume dependent: CRITICAL MASS

18 Critical Mass Work on fission continued despite reservations and doubts 1939: group of scientist approached FDR for funding Feb 1940: $6000 (!) for fission research About the same time in England: Otto Frisch and Rudolf Peierls concluded (based on available data and some inspired guesses) that the critical mass pure uranium would be 1 kg => Frisch Peierls memorandum, included suggestion that an explosion could be achieved by rapidly bringing together two subcritical masses

19 Critical Mass If there is enough material (critical mass), get an exponential increase and explosion, if less, it peters out U 235 critical mass = 52 kg (17 cm sphere) Use centrifuges for enrichment (on going dispute with Iran) For 3 5% enrichment (used for power) cannot explode 20% enrichment critical mass = 400 kg >20% highly enriched uranium Pu 239 critical mass = 10 kg (10 cm sphere)

20 The first reactor 1941: groups in several countries had projects for the construction of a controllable nuclear reactor (called atomic pile in the UK, uranium pile in the US, and uranbrenner in Germany) Fermi had shown in the 1930s that the chances of neutron absorption by U 235 greatly increased if fast neutrons were slowed down by colliding with relatively light nuclei

21 Moderators Best moderators > similar mass (compare to moving ball bearing colliding with a stationary ball bearing or with a stationary cannonball) Slow neutrons > thermal neutrons Why is hydrogen not used? Used: deuterium possible Fermi s choice for first reactor: carbon as graphite (German project: heavy water)

22 The first reator 1942, Dec 2 Fermi team achieved first controlled chain reaction (piling up lumps of U metal and graphite with strips of cadmium, a neutron absorber)

23 New elements Neptunium (93) and plutonium (94) also produced when bombarding uranium with neutrons (so Fermi was right)

24 Plutonium 239 Discovery report for Pu 239 was submitted to PRL immediately, but publication withheld until after the war Pu 239 fissile (U 238 fertile) Pu 239 production requires fast neutrons Lead to fast breeder reactors designed to produce more fissile material than they consume, maintaining the chain reaction with fast neutrons (fuel in these however is not natural uranium)

25 First atomic bomb With the discovery of Pu, two fissile materials were available and decision was taken to proceed on both routes 1945, 16 July: first bomb (Pu) was exploded in Alamogordo, NM

27 Building a bomb

Nuclear 2. Fission and Fusion

Nuclear 2. Fission and Fusion Nuclear 2 Fission and Fusion History 1896: Becquerel discovers radioactivity 1898: Marie & Pierre Curie discover radium 1911: Rutherford discovers nucleus 1932: Chadwick discovers neutrons 1933: Hitler