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

• 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.

• Mouse Trap Chain Reaction

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 heats.

• 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

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

• BWR: P=1000 psi T=545F

• PWR P=2250 psi T=600F

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

Reactor is inside a large containment building

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

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?

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.

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.

US Nuclear Power Plants

World Nuclear Power