1. Nuclear Energy
Chapter 25
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2. Nuclear binding forces
Nucleus: protons+neutrons
electrostatic repulsion forces between protons
WHAT HOLDS THE NUCLEONS TOGETHER?
Nuclear forces: or strong interaction: much stronger than the electrostatic repulsion force between the protons.
Energy associated to the strong interaction: Nuclear energy
Nuclear energy: a new possible source of energy
- rearrangement of the nucleons in the nuclei can liberate this energy
studying nuclear forces and energy:
- more complicated than studying the electronic shells
(smaller, and higher energy required)
- radioactive decay
- bombarding nucleons with heavy particles: , p, n
- in order to reach and penetrate the nucleus high energy required --> achieved with particle accelerators 2
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3. Accelerators
Obtaining high energy electrically charged nuclear “projectiles”
Basic principle: using electric field to accelerate charges
Two main types of accelerators:
- linear accelerators: passing the charged particles through several consecutive accelerating regions (ex. Stanford Linear Accelerator, 3km long, electrons with energies up to: 50 GeV)
- circular accelerators (sincrotron) : allows the particles to pass through a single accelerating region many times; particles kept on circular trajectories by strong magnetic fields; maximum
energy limited by the overall size; important radiation losses for light particles--> appropriate for heavier charged particles
(Fermilab near Chicago, up to 1 TeV!) 3
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4. The Nuclear Glue
Forces between nucleons are revealed by accelerator experiments
proton - proton collisions; proton-neutron collisions reveal the nature of the interactions between them
n-n interaction obtained indirectly
Results:
- nuclear forces have a very short range! (3 x m)
- strongly attractive; 100 times stronger than electrical repulsion
- at very-very close distance is repulsive like the electrostatic one
- force is a complicated one: depends on relative orientation and some purely quantum mechanical effects
- is independent of charge: same: n-n, n-p, p-p
- an new additional force in the nucleus, responsible for the  decay process: n--> p + e; weak force (one-billionth the strength of the strong nuclear force) 4
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5. Nuclear Binding Energy
Due to the strong nuclear forces: force is needed to pull them apart--> it takes work to take a stable nucleus apart
energy is released when we allow nucleons to combine to form a stable nucleus
average binding energy per nucleon: the total amount of energy required to completely disassemble a nucleus divided by the number of nucleons.
Total energy of the nucleon related with the total mass: E=mc2
- total energy contains the total nuclear binding energy also
- measuring the total mass of a given nucleus and comparing it with the mass of the free nucleons--> information to the binding energy
average binding energy per nucleon as a function of nucleon nr. 5
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6. 6 Stability of a nucleus Not all nuclei are stable
line of stability: relates the number
of protons and neutrons
heavier nuclei has more neutrons than protons
- explanation: electrostatic interaction between
protons (long range forces)
- quantum mechanical energy diagrams with or without electrostatic force between the nucleons
filling up the available energy levels respecting the minimal energy principle and Pauli’s exclusion principle
if the nucleus is above the line of stability: extra neutrons
(n-->p+e; decays through  radiation)
if the nucleus is below the stability line: extra protons
( or + decay or electron capture) 6
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7. Nuclear Fission
nuclear fission: Heavy elements nuclei (with large number of nucleons) when interacting with projectiles (n, p or  particles) can split in several lighter nuclei --> minimizing the binding energy/nucleon
a typical fission
fission releases a large amount of energy from the nuclear binding energy; ( ex. above around: 210 MeV)
typical energies from chemical reactions: a few eV !!! 7
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8. Chain Reactions
The key to release nuclear energy: a single reaction can trigger additional reactions (neutrons released during the fission of U, can start additional reactions)
a single reaction can cause a chain of events--> chain reaction
conditions for getting chain reaction:
- the produced neutrons should not
leave the material before triggering
a new reaction (critical mass)
- the neutrons should not be
captured by other atoms ( ex.
isotopes of U-235: U-238…)
the reaction can be:
- subcritical (less than one new n
produces new reaction)
- critical (one new n produces in
average a new reaction)
- supercritical (more than one n from
each reaction initiates another
reaction)--> nuclear bomb
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9. Nuclear Reactors
To use the nuclear energy--> to control the chain reaction
the reaction should not die-out, neither become subcritical!
We should work under critical condition--> energy released at steady state
controlling and driving the reaction in reactors:
- fuel: mixture of U-235 and U-238
(can be enhanced in U-235)
bigger amount than the critical mass!
- U-238 captures the fast neutrons,
and produces no useful reaction
- U-235 fission mainly due to slow
neutrons
- moderator: materials that slow
down the neutrons (heavy water,
graphite ….)
- neutron absorbent rod (boron,…):
control mechanism for fne-tuning the criticality
- heat-exchanger: remove the heat from the core to
drive electric generators (water, steam, etc…)
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10. Summary
nuclei stay together in spite of the electromagnetic repulsion between protons because of a nuclear force.
the strong interaction between nucleons has a very short range and is independent of charge
accelerated particles from radioactive decay are used as probes to study the structure of stable nuclei
the average binding energy per nucleon varies for the stable nuclei.
(this can be used to produce nuclear energy). Combining light nuclei or splitting heavier nuclei releases energy.
bombarding uranium with neutrons splits the uranium nuclei, releasing large amount of energy and extra neutrons. A single reaction emits two or three neutrons which can trigger additional fission reactions.
Nuclear reactors are using fission chain reactions under critical
condition
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11. Home-work assignment 664/ 1-6, 8-18, 22-24;
665/ 25-28, 31, 33-34, 37-40, 43-46
666/ 1-6, 8-14, 17-20
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