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Fundamentals of Nuclear Power. 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.

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Fundamentals of Nuclear Power

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Fundamentals of nuclear power l.jpg

Fundamentals of Nuclear Power


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Nuclear Fission

  • We convert mass into energy by breaking large atoms (usually Uranium) into smaller atoms. Note the increases in binding energy per nucleon.


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A slow moving neutron induces fission in Uranium 235


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Fission products

  • The fission products shown are just examples, there are a lot of different possibilities with varying probabilities


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Expanding Chain Reaction

  • The fission reaction produces more neutrons which can then induce fission in other Uranium atoms.

  • Mouse Trap Chain Reaction


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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


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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.


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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)


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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


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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


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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.


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US Uranium Deposits


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World Distribution of Uranium


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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.


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  • NOTE: A nuclear bomb requires nearly 100% pure 235U or 239Pu. The 3% found in reactor grade Uranium CANNOT create a nuclear explosion!


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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


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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 heats.

  • Typically cladding is made of stainless steel or zircaloy.


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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


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  • Two common US reactor types: Boiling Water Reactor and Pressurized Water Reactor.

  • BWR: P=1000 psi T=545F

  • PWRP=2250 psi T=600F

  • PWR is most common and is basis of marine nuclear power.


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Reactor is inside a large containment building


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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


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Breeder Reactors

  • A big problem with nuclear power is the creation of Plutonium in the reactor core.

  • This is a long lived radioactive element that is difficult to store.

  • Q: Why not use it as a fuel too?


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Basic Idea

  • Process that creates the Pu.

  • During fission use one of the extra neutrons to create a Pu atom


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  • Somewhat difficult in that we want fast neutrons to “breed” the 239Pu out of the 238U, but we want slow neutrons to induce the fission of 235U.

  • Requires a different design of reactor.

  • Doubling time: Time required to produce twice as many 239Pu atoms as 235U destroyed. A good design will have a 6-10 doubling time.

  • There are no currently operating breeder reactors in the US.


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Nuclear Power in the US

  • We currently generate approximately 20% of our electricity using nuclear power.

  • No new nuclear power plants have been “ordered” since the late 1970’s.

  • Even “new” plants are nearing 20 years old and will start to need replacing.


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US Nuclear Power Plants


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World Nuclear Power


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