1

How energy is released in

fission

How nuclear bombs work.

How nuclear power works.

http://www.gandhiserve.org/streams/images/einstein.jpg

2Nuclear 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

3

http://library.thinkquest.org/17940/texts/binding_energy/binding_energy.html

Relative binding energy per nucleon

4

A B

energy

Mass

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 analogy5

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.

6Nuclear fission

• 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.

7

http://library.thinkquest.org/17940/texts/binding_energy/binding_energy.html

Relative binding energy per nucleon

8Energy 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

9Criticality

• 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”

10The 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

11Boom

• 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)

www.immediart.com/catalog/product_info.php?cP...

12Result 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

13The 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

• 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. – www.willy-brandt.org/bwbs_biografie/Soviet_Un...

14

15Nuclear 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

http://www.warinformation.com/images/Dirty-bomb.gif

16Nuclear 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

17Nuclear Power

http://www.chm.bris.ac.uk/motm/uf6/fission.gif

18More 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