Fundamentals of Nuclear Power

The Nucleus

Protons – 1.672 × 1027 kg

Neutrons – 1.675 × 1027 kg

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NZA

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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 No

33% 33%33%

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1) Yes2) Maybe3) No