Transcript Nuclear Energy

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 wellknown.
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
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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.