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: 14 C, 12 C, 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=mc 2 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, 238 U 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. 14 C is very effective in dating carbon-based artifacts. Potassium-Argon is useful for geologically long periods of time.

How much remains? A = A o ( 1 / 2 ) n where n is the number of half lives. P o 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 234 U 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=mc 2 )

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.