Splitting The Atom

Nuclear Fission

Fission

• Large mass nuclei split into two or more smaller mass nuclei– Preferably mass numbers closer to 56

• Neutrons also emitted

• Fission can occur when – unstable large mass atom captures a neutron– can happen spontaneously

Fission is Exothermic

• Masses resulting nuclei < original mass~0.1% less

• “Missing mass” converted into energy E = mc2

• Large quantities of energy are released because the products are higher up the Binding Energy Curve

The Fission Process

Energy Released By A Fission

• U + n --> fission + 2 or 3 n + 200 MeV1MeV (million electron volts) = 1.609 x 10-13 j

• This corresponds to 3.2 x 10-11 j

• Production of one molecule of CO2 in fossil fuel combustion only generates 4 ev or 6.5 x 10-19 j of energy

• This is 50,000,000 times more energy

Fissile Nuclei

• Not all nuclei are capable of absorbing a neutron and then undergoing a fission reaction

• U-235 and Pu-239 do undergo induced fission reactions

• U-238 does not directly undergo an induced fission reaction

The Fission of U-235

Nuclear Chain Reactions

• The process in which neutrons released in one fission reaction causes at least one additional nucleus to undergo fission.

• The number of fissions doubles generation if each neutron releases 2 more neutrons

• In 10 generations there will be 1024 fissions and in 80 generations about 6 x 1023 (a mole)

Nuclear Chain Reaction

Critical Mass

• If the amount of fissile material is small, many of the neutrons will not strike another nucleus and the chain reaction will stop

• The critical mass is the amount of fissile material necessary for a chain reaction to become self-sustaining.

Nuclear Chain Reactions

• An uncontrolled chain reaction is used in nuclear weapons

• A controlled chain reaction can be used for nuclear power generation

Uncontrolled Chain Reactions

The Atomic Bomb

Little Boy Bomb

• The atomic bomb dropped on Hiroshima on August 6, 1945

• Little boy was a U-235 gun-type bomb

• Between 80,000 and 140,000 people were killed instantly

The Gun-Type Bomb

• The two subcritical masses of U-235 are brought together with an explosive charge creating a sample that exceeds the critical mass.

• The initiator introduces a burst of neutrons causing the chain reaction to begin

Fat Man

• A plutonium implosion-type bomb

• Dropped on Nagasaki on August 9, 1945

• 74,000 were killed and 75,000 severely injured

Plutonium Implosion-Type Bomb

• Explosive charges compress a sphere of plutonium quickly to a density sufficient to exceed the critical mass

Controlled Nuclear Fission

• Maintaining a sustained, controlled reaction requires that only one of the neutrons produced in the fission be allowed to strike another uranium nucleus

Controlled Nuclear Fission

• If the ratio of produced neutrons to used neutrons is less than one, the reaction will not be sustained.

• If this ratio is greater than one, the reaction will become uncontrolled resulting in an explosion.

• A neutron-absorbing material such as graphite can be used to control the chain reaction.

From Steam To Electricity

• Different fuels can be used to generate the heat energy needed to produce the steam– Combustion of fossil fuels

– Nuclear fission

– Nuclear fusion

Types of Fission Reactors

• Light Water Reactors (LWR) – Pressurized-light water reactors (PWR)– Boiling water reactors (BWR)

• Breeder reactors

Light Water Reactors

• Most popular reactors in U.S.

• Use normal water as a coolant and moderator

Pressurized Water Reactor

• The PWR has 3 separate cooling systems.• Only 1 should have radioactivity

– the Reactor Coolant System

Inside Containment Structure

• Fuel Rods contain U (3-5% enriched in U-235) or Pu in alloy or oxide form

• Control rods (Cd or graphite) absorb neutrons. Rods can be raised or lowered to change rate of reaction

U-235 Enrichment

• Naturally occurring uranium is 99.3% non fissile U-238 & 0.7% fissile U-235

• In light water reactors, this amount of U-235 is not sufficient to sustain the chain reaction

• The uranium is processed to increase the amount of U-235 in the mixture

Graham’s Law of Diffusion & Effusion

• Diffusion- rate at which two gases mix

• Effusion- rate at which a gas escapes through a pinhole into a vacuum

• In both cases the rate is inversely proportional to the square root of the MW of the gas

• Rate 1/(MW)0.5

• For 2 gases the rate of effusion of the lighter is:Rate = (MWheavy/MWlight)0.5

U-235 Enrichment• Rate U-235/238= (352/349)0.5= 1.004• The enrichment after n diffusion barriers is

(1.004)n

• Minimum amount of U-235 needed in LWR is 2.1% (3x more concentrated than in natural form)

• (1.004)263=3 or 263 diffusion stages needed • Enrichment is an expensive process• Large amount of energy is needed to push the

UF6 through so many barriers

Inside Containment Structure

• Coolant performs 2 functions– keeps reactor core from getting too hot

– transfers heat which drives turbines

Water as Coolant

• Light Water Reactor (LWR)– uses ordinary water– needs enriched uranium fuel– common in U.S.– 80% of world’s reactors

• Heavy Water Reactor (HWR)– uses D2O

– can use natural uranium– common in Canada and Great Britain– 10% of world’s reactors

Water As Coolant

• Pressurized Water Reactors – uses a heat exchanger– keeps water that passes the reactor core in a

closed loop– steam in turbines never touches fuel rods

• Boiling Water Reactors – no heat exchanger– water from reactor core goes to turbines– simpler design/greater contamination risk

PWR vs. BWR

The Moderator

• The moderator is necessary to slow down neutrons (probability of causing a fission is increased with slow moving neutrons)

• Light water will capture some neutrons so enriched enriched fuel is needed

• Heavy water captures far fewer neutrons so don’t need enriched fuel

Breeder Reactors

• Generate more fissionable material than they consume

• Fuel U-238, U-235 & P-239• No moderator is used

– Fast neutrons are captured by non fissionable U-238 to produce U-239

– U-239 decays to fissile Pu-239

• Coolant is liquid sodium metal• None in U.S.

– France, Great Britain, Russia

Breeder Reactor Processes

Breeder Reactors

• Advantages– creates fissionable material by transforming U-

238 into Pu-239– Fuel less costly

Breeder Reactors• Disadvantages

– no moderator (if something goes wrong, it will happen quicker)

– liquid sodium coolant is extremely corrosive and dangerous

– Plutonium has a critical mass 50% less than uranium

• more widely used for weapons

• more actively sought by terrorists

– Fuel rods require periodic reprocessing to remove contaminants resulting from nuclear reactions (cost consideration)