Nuclear Energy

The Fuel: Uranium

Present nuclear power plants consume U-235 as fuel

Uranium has 92 protonsTwo isotopes are important. U-235 has an

atomic mass of 235 and U-238 has an atomic mass of 238.

Usable Fuel: 5% U-235

Separating isotopes is expensivePlants exist for separating U-235 from the

U-238 in natural uraniumFor nuclear reactors, enriched uranium (4

to 5% U-235) is used instead of the 0.7% U-235 of natural uranium

Bombs need over 90 percent U-235.

Fission

When an atom of U-235 absorbs a neutron it fissions: breaks apart into two atoms of smaller elements and some neutrons

When a reactor is operating, fission of an atom of U-235 generates on the average a bit more than two neutrons per fission

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Meltdown

If each of two neutrons produced by a fission was absorbed by an atom of U-235 the number of fissions would double again and again generating enough power to melt the reactor

Chain Reaction-simplified

One U splits yielding 2 n which can go on to hit 2 U which will split to yield 4 n

Controlling Fission

When the reactor is turned on, the multiplication of fissions is allowed to continue until the reactor is generating power at the desired rate

Control rods are inserted to absorb neutrons until exactly one neutron from each fission causes another fission.

Types of Nuclear Plants

moderator: control rods/ coolant: heat absorber

Safety Precautions

Some of the neutrons caused by a fission are emitted only after a delay of a minute

It is not difficult to control the power level of the reactor

Redundant safety systems are in place that will stop the reactor if the power level gets too high or if the cooling water stops flowing

Power Generation

The power to produce electricity comes from high speed products of fission

They quickly hit something and kinetic energy is changed to heat energy

If the heat weren't removed, the reactor would meltdown

Power Generation

The heat from fission is taken up by water or steam pumped through the reactor

The hot steam goes through turbines connected to electric generators

About 2/3 of the heat energy is lost, and is emitted to the atmosphere or to a body of water

Post Power Production

U-235 fuel rods last for about 2 years leaving mainly fission products which still produce heat and are radioactive

Fuel rods cool in large pools of water and become less radioactive with time and can be chemically reprocessed to remove and then reuse remaining U and newly produced plutonium as reactor fuel