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Fundamentals of Nuclear Power
The Nucleus
Protons – 1.672 × 1027 kg
Neutrons – 1.675 × 1027 kg
X
NZA
AZ
Strong Nuclear Force
• Holds the protons and neutrons together.• Third Fundamental Force (after gravity and the
electromagnetic forces)• operates on distances like 1015 m• the more protons you have, the more neutrons
needed for stability
Binding Energy
• What does it take to break a nucleus into its constituents?
binding energy = (mass defect)c2
Example: Helium Nucleus
• two protons + two neutrons
• mass of intact helium nucleus
• difference is converted to binding energy
27 27 272(1.672 10 kg) 2(1.675 10 kg) 6.695 10 kg)
kg1064776 27.
MeV328J10534kg)1005030 12227 ...( c
Curve of Binding Energy
Nuclei with the largest binding energyper nucleon are the most stable.-------------------------------
The largest binding energy per nucleon is 8.7 MeV, for mass number A = 56 (Fe).
Beyond bismuth,A = 209, nucleiare unstable.
Nuclear Fission
• We convert mass into energy by breaking large atoms (usually Uranium) into smaller atoms. Note the increases in binding energy per nucleon.
A slow moving neutron induces fission in Uranium 235
Fission products
• The fission products shown are just examples, there are a lot of different possibilities with varying probabilities
Expanding Chain Reaction
• The fission reaction produces more neutrons which can then induce fission in other Uranium atoms.
Linear Chain Reaction• Obviously, an expanding chain reaction cannot
be sustained for long (bomb). For controlled nuclear power, once we reach our desired power level we want each fission to produce exactly one additional fission
Tricks of the trade
• Slow moving (thermal) neutrons are more effective at inducing fission, but, fissions produce fast moving electron. We need to slow neutrons down.
• Fissions typically produce several neutrons, but a linear chain reaction only needs one. We need to get rid of a good fraction of our neutrons.
Moderator• Neutrons are slowed
down by having them collide with light atoms (Water in US reactors).
• Highest level of energy transfer occurs when the masses of the colliding particles are equal (ex: neutron and hydrogen)
Control Rods
• Control rods are made of a material that absorbs excess neutrons (usually Boron or Cadmium).
• By controlling the number of neutrons, we can control the rate of fissions
Basic Ideas
• The Uranium is both the fuel and the source of neutrons.
• The neutrons induce the fissions• The Water acts as both the moderator and
a heat transfer medium.• Control rods regulate the energy output by
“sucking up” excess neutrons
Practicalities
• Processing of Uranium• Each ton of Uranium ore
produces 3-5 lbs of Uranium compounds
• Uranium ore is processed near the mine to produce “yellow cake”, a material rich in U3O8.
• Only 0.7% of U in yellow cake is 235U. Most of the rest is 238U which does not work for fission power.
US Uranium Deposits
World Distribution of Uranium
Enrichment• To be used in US
reactors, fuel must be 3-5% 235U.
• Yellow cake is converted into UF6 and this compound is enriched using gaseous diffusion and/or centrifuges.
• There are some reactor designs that run on pure yellow cake.
• NOTE: A nuclear bomb requires nearly 100% pure 235U or 239Pu. The 3% found in reactor grade Uranium CANNOT create a nuclear explosion!
Fuel Pellets• The enriched UF6 is
converted into UO2 which is then made into fuel pellets.
• The fuel pellets are collected into long tubes. (~12ft).
• The fuel rods are collected into bundles (~200 rods per bundle
• ~175 bundles in the core
Cladding
• The material that the fuel rods are made out of is called cladding.
• It must be permeable to neutrons and be able to withstand high temperatures.
• Typically cladding is made of stainless steel or zircaloy.
Controlling the chain reaction depends on
• Arrangement of the fuel/control rods• Quality of the moderator• Quality of the Uranium fuel• Neutron energy required for high
probability of fission
Other Options
• Other countries use different reactor designs.• Some use heavy water (D2O) as a moderator.
Some use Graphite as a moderator.• Some are designed to use pure yellow cake
without further enrichment• Liquid metal such as sodium or gasses such as
Helium are possibilities to use for coolants
Nuclear Power in the US
• We currently generate approximately 21 % of our electricity using nuclear power.
• No new nuclear power plants have been “ordered” since the late 1970’s. The last one to come online was in 1996 in Tennessee.
• Even the newest plants are nearing 30 years old and will start to need replacing.
US Nuclear Power Plants
World Nuclear Power
Jan 2011
Would you have a miniature nuclear plant in your neighborhood?
Yes
Mayb
e N
o
33% 33%33%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
1) Yes
2) Maybe
3) No
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