Energy from Nuclei

In analogy with combustion,
  
"Heat generated by molecule burning to more stable molecules.
  
and with Einstein's insight --E = mc2 --
  
Energy generated by less stable nucleus "burning" into more stable ones.

What then is "burning" and

In chemistry, O2 sets zero for stability.
In nuclear physics, the atomic mass unit u is set by 168O = 16 u,

Weighting the nucleus -- nuclear chemistry

Definitions -- what is

Nuclear Landscape -- Isotope Zoo

Stable nucleons more neutrons than protons.
Last stable isotope is Bi-209 (see 208Pb).
U half lives.234U: ~½ 106 y; 235U: ~109 y; 238U: 4 109 y.
Decay routes:
  Above stability line: Z -> → Z-1 + β + + &gamma
  Below stability line: Z -> → Z+1 + β - + &gamma
  Large Z elements Z -> → Z-2 + α + γ
Closed shells. Chemistry: coulomb potential produces closed shells at A=2, 10, 18, 36, 54, 86. Nuclear physics: nuclear potential produces closed shells at at Z or N = 2, 8, 20, 28, 50, 82.

Binding Energy of Nuclei

Difference between nucleus mass & constituent nucleons
5626Fe: most bound
⇒ merge: fusion
⇐ split: fission
Leftmost dip:
  4He = α + 2 e-

Units: Standard: 168O ≡ 16 u.
u (amu or atomic mass unit) = 1.6606 10-27 kg
Convert: u = 930 MeV = 0.149 nJ;   neutron = 1.0087 u, proton = 1.0073 u. Ref: Nuclei Mass Table

Fission: Energy & Decay Products

92-U-235 +n = 56-Ba-141 +
36-Kr-92 + 2 n ElementEnergyunit
U-235235.0439u
Ba-141-140.9144u
Kr-92-91.9262u
2 n (1.0078)-2.0156u
 excess 0.1877u
174.4 MeV
28pJ
72TJ/kg

On a mass basis (235 1.66 10-27 = 0.39 10-24 kg); 235U fission produces 71.7 TJ/kg.

Methane: 50 MJ/kg. (TNT: 4 MJ/kg). Fission is million times more efficient!
23592U plus slow neutron has other fission reactions; see text.

Chain Reactions and Fission Products

Average number of neutron released is 2.4. That there is more than one neutron permits a chair reaction .

U-235 + n chain reaction

1. Stable energy requires neutrons cooled & most absorbed.
2. Reaction products in excited states that decay by emitting α (He-4 nucleus), β (both + & - electrons) and γ (MeV xrays).

Most neutrons are prompt; 0.7% delayed ms→min. This will be important for controlling nuclear fission reactors for safety.

Fission products distribution and radioactive

U-235 fission product
distribution penetration of
alpha, beta, gamma Relative penetration of α, β & γ.
Distribution 235U fission fragments. Most radioactive; some long lifetimes.


To cite this page:
Energy from Nuclei
<http://www.physics.ohio-state.edu/~wilkins.5/energy/Resources/Lectures/nucenergy-land.html>
[Saturday, 16-Dec-2017 23:11:10 EST]
Edited by: wilkins@mps.ohio-state.edu on Monday, 19-Oct-2009 17:32:45 EDT