How energy is released in fission
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How energy is released in fission. How nuclear bombs work. How nuclear power works. Nuclear Binding Energy. The mass of nucleons: mass of protons: 1.673 E-27 mass of neutron: 1.675 E-27

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How energy is released in fission

How energy is released in fission

How nuclear bombs work.How nuclear power works.

Nuclear binding energy
Nuclear Binding Energy

  • The mass of nucleons:

    • mass of protons: 1.673 E-27

    • mass of neutron: 1.675 E-27

  • Sum of the masses of protons and neutrons in an atom is greater than the mass of the nucleus

    • Missing mass was converted to energy used to overcome repulsion: binding energy.

  • The binding energy is a function of the specific nucleus; calculated as binding energy/nucleon

Relative binding energy per nucleon

The nucleon gives up some mass as energy as it becomes part of the nucleus. The more energy it gives up, the less mass each nucleon has.

Rope and knot analogy
Rope and knot analogy of the nucleus. The more energy it gives up, the less mass each nucleon has.

Part of the rope is used to tie the knot. The bigger the knot, the less rope available for tying something up.

The knot-tying is a one time expenditure of energy w/ lasting effect.

Nuclear fission
Nuclear fission of the nucleus. The more energy it gives up, the less mass each nucleon has.

  • Large elements are intrinsically unstable and will split when they absorb a neutron: Fission

  • Example: U-238

    • Average Binding energy is 7.6 MeV/nucleon

  • Suppose it splits into two atoms of 119 each

    • An atom of atomic mass 119 normally has a binding energy of 8.5 MeV/nucleon.

    • The A.M. 119 atom produced by fission of U-238 has not given up enough of its mass as energy.

      • So it does.

Relative binding energy per nucleon

Energy release in fission
Energy release in fission

  • 8.5 (normal) – 7.6 (fission product) = 0.9 MeV

  • An additional 0.9 MeV of energy (mass TO energy) must be given up PER NUCLEON.

    • 238 x 0.9 MeV = 214 MeV per atom of uranium split.

  • In what form is this energy?

    • Fission products (atoms) moving away

    • Gamma rays

    • Subsequent radioactive decays

    • neutrons


  • When other U-238 atoms are close enough, neutrons released from fission are absorbed, causing another atom to undergo fission.

    • The amount of U needed for this to occur is the critical mass; the situation: criticality.

    • Result, an exponentially increasing number of fission reactions with release of binding energy

  • Plutonium (Pu-242) even more readily undergoes fission, making it more “useful”

The atomic bomb
The Atomic Bomb

  • Einstein discovers e = mc2

    • Scientists recognize that purified uranium can be used to make a bomb, and WW II Germany starts enriching uranium.

    • Einstein alerts US Government, and the Manhattan Project begins

  • Bomb: chain reaction, an exponentially increasing number of fission reactions

    • Requires purified uranium (or plutonium) brought to together rapidly to create a critical mass


  • Subcritical quantities of U or Pu brought together rapidly by conventional explosives

  • Massive chain reaction perpetuated by neutrons releases nuclear binding energy

    • Matter transformed into energy e = mc2

    • Energy released in the form of: heat, light, gamma rays, and lots of neutrons (which make other atoms radioactive)

Result of boom
Result of Boom

  • Fallout: tons of soil and debris into atmosphere by heat and updraft

    • Incl. fission daughters and atoms made into radioisotopes from neutrons

    • Principle components: C-14, Na-24, Sr-89, Pu-239, I-131, Cs-137, and Sr-90

    • I-131 falls on fields, grazed by cattle, appears in milk, ingested by children, concentrated in thyroid.

      • High incidence or thyroid cancer

The hydrogen fusion bomb
The Hydrogen (fusion) bomb

  • Fusion: 2 atoms of H combine to make He

    • Avg. binding energy per nucleon much higher for helium than hydrogen, so lots of energy released.

Fusion continued
Fusion continued

  • To get 2 atoms of H to fuse requires energy

    • Feature of the sun, a fusion reactor

    • Thus the search for “cold fusion”; cold being less than thousands of degrees

    • Limitless source of energy without radioactive waste

  • Hydrogen bomb

    • Heat from conventional A-bomb drives fusion reaction.


Nuclear terrorist threats
Nuclear Terrorist threats

  • Dirty Bomb

    • Conventional explosive packed with radioisotope such as Cs-137 or other “hot” industrial isotope.

    • Relatively cheap and easy to make

    • Relatively little radiological damage, but high fear factor, good terrorist weapon

  • Suitcase nuke

    • Miniaturized A bomb

    • Plans on internet

    • Need U or Pu

Nuclear power
Nuclear Power

  • Radiation from radioactive decay gives up its energy ultimately as heat

  • Fission reactions are controlled (moderated) to prevent an exponential increase in the fission reaction.

  • Result is a steady liberation of heat that can be used to generate steam to drive turbines to generate electricity

Nuclear power1
Nuclear Power

More details on nuclear power
More details on nuclear power

  • Moderators

    • Various options, but plain water most common

    • Slow down the neutrons to the energy level of “thermal neutrons”; these are readily absorbed by nuclei to promote fission reactions.

  • Control rods

    • Absorbing material that blocks neutrons from hitting fissile material, slows down chain reactions.

  • Waste: consists of daughter isotopes

    • Neutrons can be made to create radioisotopes