Nuclear Fuel Cycle Lecture 2: Basic Nuclear Chemistry, Part 1

Transcription

1 Nuclear Fuel Cycle 2011 Lecture 2: Basic Nuclear Chemistry, Part 1

2 Home page of the course: KTH Social

3 What is Nuclear Chemistry? Chemistry related to nuclear technology Chemistry of radionuclides Studies of chemical processes by using radionuclides as tracers: Radiochemistry Radiation induced chemical reactions: Radiation Chemistry

4 The Nucleus Building blocks: Protons and neutrons Forces: Electromagnetic forces and the strong nuclear force Proton Neutron Mass g g Charge +1 0 Spin s = ½ s = ½

5 Repulsive Attractive The Strong Nuclear Force Exchange of mesons keep the nucleons together Nucleons π Meson m Distance The force acts at very short distances

6 Nuclear density and radius Measured charge and nuclear density for 40 Ca and 209 Bi as a function of nuclear radius Density: 0.2 nucleons/fm 3 => g/cm 3!

7 The Nucleus A ZR Z+N = A Z = Number of protons N = Number of neutrons A = Mass number (number of nucleons) Nuclide Specific nuclear species Examples Isotopes Isotones Isobars Isomers Identical Z Identical N Identical A Identical Z and A S Cl U 92U 92U Ar K 20 Ca Sr Y 40 Zr 99 43Tc 43Tc 99m 99m 43 t½ Tc 6. 01h 99 Tc 43

8 Chart of Nuclides Note! Colors may change between different charts More than isotopes 80 elements have together 246 stable isotopes Lightest element with no stable isotopes: Tc Heaviest element with (almost) stable isotope: 209 Bi (t ½ = y) (else Pb) Heaviest naturally occurring element: U Most stable isotopes: Sn; 10 stable isotopes 26 monoisotopic elements α β + β - sf p n Stable Alpha decay Positron decay Electron capture Negatron decay Spontaneous fission Proton decay Neutron decay

9 Number of Nucleons on the Stability of the Nuclei Even Z Odd Z Odd N Even N Magic numbers of protons or neutrons: 2, 8, 20, 82 and Xe σn barns N 81 Xe 6 σ barns N n

10 Nuclear Stability: Nucleon Orbitals

11 Mass Defect (ΔM) and Mass Excess (δ A ) ΔM A = M A Z M p N M n M A = Mass of atom M H = Mass of Proton (hydrogen) M n = Mass of Neutron Deuterium, 2 1H M p + M n = = u M A = u => ΔM A = u All stable isotopes have negative mass defect, ΔM A Mass excess: δ A = M A A (sometimes used in tables, no practical use) M = atomic mass unit, measured in u.

12 Binding energy ΔE = Δmc 2 Nuclear Heat of formation Δm = M A (Z m p + N m n ) c = m/s Nucleus: 5-10 MeV / nucleon ( J/mol) Covalent bond: J/mol

13 Isotope effects Due to the difference in nucleons there are very small differences between two isotope s Freezing point Boiling point Density Heat of vaporization Viscosity Surface tension Optical emission spectra

14 Isotope separation i. Equilibrium processes (light elements) ii. Rate processes Multi-stage processes (for instance distillation) Chemical exchange Electrolysis Gaseous diffusion Electromagnetic separation Gas centrifugation

15 Gaseous diffusion Lighter isotopes diffuse faster than heavy isotopes 235 UF 6 / 238 UF 6

16 Other methods of isotope separation Distillation Extraction Ion-exchange Photoionization Photoexcitation

17 Radioactive decay α-decay (He-nucleus) β-decay (electron/positron) γ-decay Unusual modes of decay (proton, neutron, heavy particles) Spontaneous fission

18 Chart of Nuclides Note! Colors may change between different charts α β + β - sf p Stable Alpha decay Positron decay Electron capture Negatron decay Spontaneous fission Proton decay

19 Nuclide chart EC, β + β - α

20 Decay chain of 238 U

21 Decay chain of 238 U 238 U 234 Th + α 234 Th 234 Pa + β Pa 234 U + β U 230 Th + α 230 Th 226 Ra + α Or simplified 238 U(α) 234 Th(β - ) 234 Pa(β - ) 234 U(α) 230 Th(α) 226 Ra

22 Decay schemes

23 Conservation laws Nuclear Reaction: X 1 + X 2 X 3 + X 4 Energy: E 1 + E 2 = E 3 + E 4 Linear momentum: p=mv p 1 + p 2 = p 3 + p 4 Charge: Z 1 + Z 2 = Z 3 + Z 4 Mass number: A 1 + A 2 = A 3 + A 4

24 α-decay A Z X A 4 Z2 Y 4 2 He 238 U 234 Th + 4 He

25 Decay energy (Q-value) E=mc 2 1 u = 1/ = g c 2 = m 2 /s 2 1 J = MeV E = * * = MeV/u Q(MeV) = ΔM (u) Q α = (M Z-2 + M He M Z ) Q α > 0 if (M Z-2 + M He M Z ) < 0 Q α > 0 => Spontaneous decay For α-particles the Q-value is 2 10 MeV

26 Alpha spectrum

27 β-decay A Z X A Z 1 Y β - A negatron (electron) is emitted A proton in the nucleus is coverted to a neutron A Z X A Z1 Y β A positron (anti-partivle to the electron) is emitted. A neutron in the nucleus is converted to a proton

28 β-decay 137 Cs 137m Ba + β - Why not one peak? (also the nuclear spin quantum number of the reaction is not an integer, which is a forbidden transition)

29 β-decay continued Another particle is emitted: a neutrino (ν) The neutrino has no charge and very small or no mass and does not interact readily with matter 137 Cs 137m Ba + β - + ν ν is an anti-neutrino, emitted in a β - -decay ν is a neutrino, emitted in a β + -decay

30 Energy of β - decay A Z X A Z1 Y β The formed Y has Z orbit electrons and must capture one electron from the surroundings. => The mass of the β - -particle needs not to be included when calculating the energy of the decay. Q β - = (M Z+1 M Z ) Example: n H + β - Q β - = ( ) = 0,782 MeV

31 Energy of β + decay A Z X A Z1 Y β A Z1 Y e β - The formed Y has now one extra orbit electron which it must loose. => Both emitting a β + -particle and loosing an electron must be included when calculating the energy of the decay. Q β - = (M Z M e M Z ) Example: N C β Q β - = ( ) + 2* 0.511) = 1,2 MeV

32 Electron capture A Z X EC A Z 1 Y An inner shell electron is captured by the nucleus. Energy similar to β - decay. Q β - = (M Z-1 M Z )

33 γ-emisson Most α and β-decays do not go all the way to the daughter s ground state. The remaining energy is released as γ-rays. Isomeric transition When the meta-stable state is more long-lived

34 Spontaneous fission Some heavy radionuclides are so unstable that they undergo spontaneous fission Rare modes of decay Proton emission Neutron emission

35 Gamma spectrum

36 Radioactive decay A dn dt λn dn N λdt N N 0 1 dn N t 0 λdt lnn-lnn 0 λt N N 0 e t

37 Half-life N N 0 1 dn N t 0 λdt lnn-lnn 0 λt N0 N 2 t ½ lnn 0 N ln 2 λ 0 ln2 λ

38 Units SI unit: 1 Becquerel (Bq) = 1 decay / s Older unit: 1 Curie (Ci) = Bq (1 Ci is approximately the actvity of 1 gram 226 Ra)