L 38 Modern Physics [4]L 38 Modern Physics [4]

Nuclear physics Nuclear physics what’s inside the nucleus and what what’s inside the nucleus and what

holds it together holds it together what is radioactivity, halflife what is radioactivity, halflife carbon dating carbon dating

Nuclear energyNuclear energynuclear fissionnuclear fissionnuclear fusionnuclear fusionnuclear reactorsnuclear reactorsnuclear weaponsnuclear weapons

Nuclear Reactions and E = mcNuclear Reactions and E = mc22

Energy is often released in nuclear Energy is often released in nuclear reactions, for examplereactions, for example

If you add up the masses on the left side If you add up the masses on the left side we find that it is slightly greater than the we find that it is slightly greater than the masses of the products on the right sidemasses of the products on the right side

The extra mass is converted to energy, The extra mass is converted to energy, mostly as kinetic energy of the neutronmostly as kinetic energy of the neutron

4 9 12 1

2 4 6 0He Be C n 13 13

Biological effects of nuclear Biological effects of nuclear radiationradiation

Nuclear reactions can produce alphas, betas, Nuclear reactions can produce alphas, betas, neutrons and gamma radiation (particles or neutrons and gamma radiation (particles or photons)photons)

Nuclear radiation is ionizing radiation, i.e., Nuclear radiation is ionizing radiation, i.e., energetic enough to knock electrons out of energetic enough to knock electrons out of atoms or moleculesatoms or molecules

Ionizing radiation is potentially harmful to Ionizing radiation is potentially harmful to humans because the ionization it produces can humans because the ionization it produces can alter significantly the structure of molecules alter significantly the structure of molecules within a living cell which can lead to alterations within a living cell which can lead to alterations of the cell (make them cancerous) or to the of the cell (make them cancerous) or to the death of the celldeath of the cell

Hazards of radiationHazards of radiationThe hazards of radiation can be minimized The hazards of radiation can be minimized

by limiting overall exposureby limiting overall exposureThe effects of absorbed doses or ionizing The effects of absorbed doses or ionizing

radiation is measured in a unit called the radiation is measured in a unit called the rem.rem.

The effects of radiation exposure areThe effects of radiation exposure areShort term or acute effects appearing within a Short term or acute effects appearing within a

matter of minutes of exposurematter of minutes of exposureLong-term effects that may appear in years, Long-term effects that may appear in years,

decades or even in future generationsdecades or even in future generations

Average radiation doses received by a US resident

Source of radiation dose in mrem/yr*

Natural Background radiation Cosmic rays 28 Earth and air 28 Internal radioactive nuclei 39 Inhaled radon 200

Man-made radiation Medical / dental x-yars 39 Nuclear medicine 14

*Current federal standards limit exposure to 500 mrem/yr

Radiation sicknessRadiation sicknessThis is the general term applied to the This is the general term applied to the

acute effects of radiationacute effects of radiationA dose less than 50 rem causes no short A dose less than 50 rem causes no short

term ill effectsterm ill effectsA dose of 50 – 300 rem A dose of 50 – 300 rem at one timeat one time brings brings

on radiation sicknesson radiation sicknessA whole body dose of 400 – 500 rem is A whole body dose of 400 – 500 rem is

lethal for about 50% of people exposedlethal for about 50% of people exposedWhole body doses greater than 600 rem Whole body doses greater than 600 rem

result in death for almost all individualsresult in death for almost all individuals

Energy from the nucleusEnergy from the nucleus

Huge amounts of energy are given off in two Huge amounts of energy are given off in two nuclear processesnuclear processesNuclear fissionNuclear fission: splitting a: splitting a

heavy nucleus in twoheavy nucleus in two

Nuclear fusionNuclear fusion: fusing two: fusing two

light nuclei into onelight nuclei into one

A lot of energy from a little massA lot of energy from a little mass The energies released when a large nucleus The energies released when a large nucleus

undergoes fission or small nuclei undergo fusion undergoes fission or small nuclei undergo fusion are enormous compared to chemical energies are enormous compared to chemical energies (e.g. burning fossil fuel)(e.g. burning fossil fuel)

When Uranium splits apart about 0.1% of its When Uranium splits apart about 0.1% of its mass is converted into energymass is converted into energy

Pound for pound, nuclear reactions release Pound for pound, nuclear reactions release about 10 million times more energy than about 10 million times more energy than chemical reactionschemical reactions

1 pound Uranium 1 pound Uranium 1 million gallons of gasoline 1 million gallons of gasoline

FissionFission

To split a uranium To split a uranium nucleus apart takes nucleus apart takes energyenergy

A neutron hitting a A neutron hitting a uranium nucleus can uranium nucleus can cause it to splitcause it to split

A neutron can split U-A neutron can split U-235 into Cs-143 and 235 into Cs-143 and Rd-90 plus a Rd-90 plus a few few extra neutronsextra neutrons

Nuclear PhysicistsNuclear Physicists

Otto Hahn and Lise MeitnerDiscovered fission

Enrico Fermi“Father of the Atomic bomb”

Edward Teller“Father of the

Hydrogen bomb”

The nuclear chain reactionThe nuclear chain reaction When U-235 splits, on When U-235 splits, on

average 2.5 neutrons are average 2.5 neutrons are releasedreleased

These neutrons can then These neutrons can then go on to cause other U-go on to cause other U-235’s to split, this resulting 235’s to split, this resulting in a in a chain reactionchain reaction

This can result in a This can result in a catastrophic process with catastrophic process with enormous energy enormous energy released.released.

Reactor vs BombReactor vs Bomb If the energy released in a nuclear chain If the energy released in a nuclear chain

reaction is allowed to proceed in a reaction is allowed to proceed in a controlled way, then this can be used as controlled way, then this can be used as an energy source an energy source nuclear reactornuclear reactor

If the chain reaction occurs in an If the chain reaction occurs in an uncontrolled manner then you have uncontrolled manner then you have

atomic bombatomic bombFermi produced the first nuclear reactor Fermi produced the first nuclear reactor

under the west stands of Stagg Field at under the west stands of Stagg Field at the University of Chicago in 1942the University of Chicago in 1942

Nuclear reactorsNuclear reactors

The fuel elements contain the The fuel elements contain the fissile fuel in the form of rods offissile fuel in the form of rods of 1 cm diameter. There may be 1 cm diameter. There may be thousands of such rods stacked together thousands of such rods stacked together

in the reactor corein the reactor coreThe most common fuel is enriched U-235The most common fuel is enriched U-235Some type of moderator material is also Some type of moderator material is also

used to slow down the neutrons to make used to slow down the neutrons to make their capture more efficienttheir capture more efficient

Inside a nuclear reactorInside a nuclear reactor

Control rods

Nuclear Power Plant

Steel and Concrete Containment vessel

Reactor operationReactor operationThe reactor is usually operated in the so-The reactor is usually operated in the so-

called called criticalcritical state in which each fission state in which each fission leads to only one additional fission.leads to only one additional fission.

In the critical state the reactor produces a In the critical state the reactor produces a steady output of electrical energysteady output of electrical energy

The reactor is designed not to go into the The reactor is designed not to go into the supercriticalsupercritical state – in this state the state – in this state the reactor produces an uncontrolled and reactor produces an uncontrolled and increasing amount of energy which can increasing amount of energy which can cause it overheat and lead to meltdown.cause it overheat and lead to meltdown.

Controlling the nuclear reactorControlling the nuclear reactor To keep the reactor in the critical state the To keep the reactor in the critical state the

operators adjust the operators adjust the control rodscontrol rods The control rods can be moved into or out of the The control rods can be moved into or out of the

reactor core. They contain an element, such as reactor core. They contain an element, such as cadmium or boron which absorbs neutrons.cadmium or boron which absorbs neutrons.

If the reactor is getting too hot, the control rods If the reactor is getting too hot, the control rods are pushed into the core to slow down the chain are pushed into the core to slow down the chain reactionreaction

The heat generated within the fuel rods is The heat generated within the fuel rods is carried away by water surrounding the rodscarried away by water surrounding the rods

Fuel rods

To start the reactorthe control rods arepulled out of the core

To stop the reactorthe control rodsare pushed intothe core

France

Japan

USA

Nuclear Power Generation

Pros and Cons of Nuclear energyPros and Cons of Nuclear energy

Plentiful fuelPlentiful fuel no greenhouse gasesno greenhouse gases no poisonous no poisonous

emissionsemissions non-pollutingnon-polluting efficient power efficient power

productionproduction

must deal with must deal with nuclear wastenuclear waste

possibility of possibility of catastrophic accidentcatastrophic accident

expensive to buildexpensive to build

Advantage Disadvantage

The atomic (fission) bombThe atomic (fission) bombThe key is to achieve a critical mass of The key is to achieve a critical mass of

fissionable materialfissionable material if a critical mass can be achieved than an if a critical mass can be achieved than an

self-sustained uncontrolled reaction occursself-sustained uncontrolled reaction occursTo achieve critical mass (60 kg), 2 lumps To achieve critical mass (60 kg), 2 lumps

of a non-critical mass of U-235 are brought of a non-critical mass of U-235 are brought together quickly using a cannontogether quickly using a cannon

When the U-235 becomes supercritical, a When the U-235 becomes supercritical, a catastrophic fission will quickly turn into a catastrophic fission will quickly turn into a fireballfireball

Little BoyLittle Boy

FAT MAN

Nuclear FusionNuclear FusionTwo light nuclei (likehydrogen) are combined into one, with a large releaseof energy

If this is done with aa large number of nucleithe energy is releasedcatastrophically asa Hydrogen Bomb

ITER- Demonstrate FusionITER- Demonstrate Fusion

The Hydrogen Bomb A fusion bomb releases A fusion bomb releases

energy by fusing deuterium energy by fusing deuterium with tritium nuclei to form with tritium nuclei to form helium and neutronshelium and neutrons

To achieve this, the To achieve this, the hydrogen must be heated to hydrogen must be heated to 100 million C using a fission 100 million C using a fission bomb bomb thermonuclear thermonuclear

Thermonuclear fusion is Thermonuclear fusion is what powers the starswhat powers the stars

Effects of a nuclear explosionEffects of a nuclear explosion

The released neutrons produce the fireball The released neutrons produce the fireball by heating everything around themby heating everything around them

The ultra hot fireball produces an intense The ultra hot fireball produces an intense flash of light, x-rays and gamma raysflash of light, x-rays and gamma rays

The explosion creates a huge pressure The explosion creates a huge pressure surge surge blast wave blast wave

Long after the blast there is the fallout Long after the blast there is the fallout the creation and release of radioactive the creation and release of radioactive nuclei that are carried away in the airnuclei that are carried away in the air

The EndReview on Friday

this is a cartoon of a chain reactionthis is a cartoon of a chain reaction

http://www.nanopolis.net/subject.php?id=1335

http://www.pbs.org/wgbh/amex/bomb/sfeature/videos.htmlIn Andrei Sakharov's Layer Cake design, several layers of light and heavy elements were alternated.High explosives surrounding the Layer Cakewould be used to implode and ignite the atomic bomb at the center of the device. The atomic explosion would then set off a fusion reaction in the deuterium.

http://www.pbs.org/wgbh/amex/bomb/sfeature/effects.html