nuclear energy. the periodic table dates from around 1880, invented by the russian gregor mendeleev....
TRANSCRIPT
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.
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…