Nuclear Energy

The Periodic Table

• Dates from around 1880, invented by the Russian Gregor Mendeleev.

• Organizes the elements into groups (columns) and periods (rows).

• Groups have similar chemical properties.

• Periods are arranged by how the atoms’ electron shells fill.

The Numbers

• Each element has two critical numbers: its atomic mass and its atomic number.

• The atomic mass is the average mass of a particular large number of its atoms. Rounded off it represents the number of nucleons (protons + neutrons) it has.

• The atomic number is its number of protons.

Nucleons

• The number of protons in an atom determines its name: 6 for Carbon, 26 for Iron, etc.

• The number of neutrons determines the isotope: 14C, 12C, etc.

• The difference between the atomic mass and number is the number of neutrons.

Stability

• As you look at the Periodic Table, you notice that the heavier element have disproportionately more neutrons than protons.

• This causes them to be unstable, meaning that nuclear decay is imminent.

• Instability is caused by a weakening of the force that holds the protons together, despite their positives charges.

• Too many neutrons separate the protons from each other, and the binding force is a inverse distance proportion.

Decay

• A natural occurrence;

• Three kinds of decay: alpha, beta, and gamma.

• Alpha: fairly low energy; a Helium nucleus

• Beta: higher energy; an electron from the nucleus

• Gamma: high energy photon.

E=mc2

• The famous law says mass can be converted into energy and back again.

• When a nucleus decays, its mass changes up or down, due to the equation above.

• When, say, 238U decays in an alpha emission, the sum of the masses after weighs more than the initial nucleus.

• The extra mass comes from some of the energy released.

Half Life

• After a period of time, enough atoms in a lump of a radioactive element have decayed into other elements so that only half the original element remains.

• Called a Half Life.

• The rates of many isotopes are well-known.

Radiometric Dating

• No such thing as a radiometric blind date.• If a material with a known quantity of a

radioactive element is found to half the amount expected, one half-life has passed for that material.

• 14C is very effective in dating carbon-based artifacts.

• Potassium-Argon is useful for geologically long periods of time.

How much remains?

• A = Ao(1/2)n where n is the number of half lives.

• Po is the amount in the beginning, P is the amount left after some many n’s.

• Non-integer values for n are allowed.

So…

• In nuclear reaction, elements transmute into other elements: called nucleosynthesis.

• AND the amount of material before a reaction is not the same as after. The difference is called the mass defect.

• Definitely not chemistry!

Human instigated nuclear processes.

• Fission: breaking apart of heavy atoms.

• Fusion: a “welding” of light atoms.

Fission

• Uses heavy elements (Uranium/Plutonium)

• “Splitting the atom”

• The splitter is a neutron with just the speed:

• Too fast and it bounces off, too slow and it gets absorbed into the nucleus.

Reactions

• In a controlled reaction, only one neutron survives out of the first split to split more atoms.

• The controlling factors are called, ah, control rods, usually cadmium, which absorbs neutrons.

• But……

Uncontrolled Reaction

• Without control rods, more neutrons are liberated with every split, causing a chain reaction.

• Also known as a BOMB!

• These early atomic bombs had the explosive power of 20,000 tons of TNT.

Reactors

• Consists of the core, where the fission process takes place, giving off enormous heat,

• A moderator, water in US plants, graphite elsewhere, which adjusts the speed of the neutrons,

• Control rods, and• A closed system heat exchanger to move the heat

outside the reactor to another heat exchanger.

Electricity

• The heat which has been moved outside the reactor is used to make steam out of local water (river, ocean) to power a turbine electric generator.

• 1kg of 234U makes as much heat as 3300 tons of coal.

• No “greenhouse gases”, but the waste material and the reactor itself are highly radioactive for many many years.

• Cannot be turned into an atomic bomb: wrong material.

Fusion

• Light elements (Hydrogen)

• “Welding” together

• The energy that powers the sun. (E=mc2)

Lawson Criteria

• High temperature (~15 million degrees);

• High pressures;

• Time for reaction to occur.

• Currently these conditions can only be reproduced consistently in a thermonuclear (Hydrogen) bomb.

• Yield: 1 million tons of TNT

Tokamack

• The name given to the most promising container for a controlled fusion reaction.

• Looks like an octopus on a bad day,• The convoluted loops of a tokamack form a

magnetic bottled to contain the super hot Hydrogen (picture in text).

• So far more energy to start the reaction than is withdrawn from it.

• But…

Clean Energy

• If such a process is achieved, it will be very clean energy.

• The fuel is water, the waste product is Helium.