Splitting The Atom Nuclear Fission Fission Large mass

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 Ø~0. 1% less • “Missing mass” converted into energy ØE = mc 2 • 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 Me. V 1 Me. V (million electron volts) = 1. 609 x 10 -13 j • This corresponds to 3. 2 x 10 -11 j • Production of one molecule of CO 2 in fossil fuel combustion only generates 4 ev or 6. 5 x 10 -19 j of energy • This is 50, 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% (3 x 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 UF 6 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 D 2 O – 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 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)