INTRODUCTION to

FUSION AND FUSION AND FISSIONFISSION

There are 1200 students in a hall. A student starts talking.

If each talking student starts 3 others to talk each second, ...

… what is the minimum time required for all students to be talking?

Warm-up

Sec. No. of talking students

0 1

1 1 + 1 3 = 4

2 4 + ____ 3 = ____

3 ____ + ____ 3 = ____

4 ____ + ____ 3 = ____

5 ____ + ____ 3 = ____

6 ____ + ____ 3 = ____

Warm-up The time required = 6 s

How to stop it How to stop it from being from being out of out of control? control? 4 16

16 16 64

64 64 256256 256 10241024 1024 4096

Ask teachers responsible for discipline to patrol around the hall.

Nuclear Nuclear ReactionsReactions

When a heavy nucleus splits upsplits up,,

When 2 light nuclei join togetherjoin together to form a heavy nucleus,

huge amount of energy is released.

E.g. atomic bomb

huge amount of energy is released.

E.g. the Sunthe Sun

Nuclear FissionNuclear Fission Fission is the process of splitting a nucleus

into two nuclei with smaller masses. Large amount of energy is releases A chain reaction is an on-going series of

fission reactions. Critical massCritical mass is the amount of fissionable

material required to continue a reaction at a constant rate

Exothermic Exothermic ReactionReaction

Nuclear fissionNuclear fission

neutron

U-235 nucleus

Ba-144 nucleus

Kr-90 nucleus

neutron

uncharged not repelled on

approaching nuclei

U23592

14456

Ba + Kr9036

+ n10

n10+ n1

0+ energy+

neutron

Nuclear fissionNuclear fission

Naturally-occurring uranium consists of 2 isotopes, U-238 (99.3%) & U-235 (0.7 %).

It is the rare isotope (U-235)(U-235) which undergoes fission.

The total mass of the fission products is found lessless than the total mass before fission takes place.

U23592

14456

Ba + Kr9036

+ n10

n10+ n1

0+ energy+

Chain reactionChain reactionWhen a uranium nucleusuranium nucleus splits, 2 or 3 neutrons are emitted.

neutronU-235

nucleus splits

If these neutrons carry on splitting other uranium nucleus...

Chain reactionChain reaction

neutron

U-235 nucleus splits

self-sustaining chain reaction

escapes

Chain reactionChain reaction

the fraction of neutrons escaping .

If a small piecesmall piece of uranium is used,

the chain will break very quickly.

If the mass of uranium is below a critical mass,

NuclearNuclear fissionfission

Neutrons may:1 - Cause another fission by colliding with a U235 nucleus

2 - Be absorbed in other material3 - Lost in the system

If sufficient neutrons are present, we may achieve a chain reaction

Each split (fission) is accompanied by a large quantity of E-N-E-R-G-Y

• Creates two smaller nuclides and free neutrons• The free neutrons potentially collide with nearby U235 nuclei• May cause the nuclide to split as well

Nuclear FUSION

DEUTERIUM

TRITIUM

HELIUM

NEUTRON

http://fusioned.gat.com

Nuclear FusionNuclear Fusion

Fusion occurs when two nuclei with low masses are combined to form one nucleus of larger mass.

Can only happen when the nuclei are moving fast enough to get “close” to each other

Temperature in stars are high enough for fusion to occur (millions of degrees Celsius)

FUSION video clip

Nuclear fusionNuclear fusion

A HUGE AMOUNTHUGE AMOUNT of energy is also released when two light nucleilight nuclei join together to form a heavy nucleus.

This process is called nuclear fusion.

Nuclear fusion

hydrogen-3 nucleus

hydrogen-2 nucleus

neutronhelium-4

He42

+H21

n10+ +energyH3

1

fusion

Nuclear fusionNuclear fusion

Since nuclei carry “+” charges, they repel each other.

For fusion to occur, the 2 hydrogen2 hydrogen nuclei must approach each other with very high very high speedspeed..

He42

+H21

n10+ +energyH3

1

hydrogen gas of high temperature (108 C)!

Nuclear fusion

Nuclear fusion occurs in the core of the SunSun, giving out heat and light. The reaction takes place continuously for billions of years.

Q1 Which of the following…

A U-235

B U-238

C Hydrogen

D Helium

Which of the following elements is used in nuclear fission?

Q2 According to Einstein’s… According to Einstein’s mass-energy equation E = mc2, what is the amount of energy produced if 1 kg of a certain element completely changes into energy?A 3 108 J

B 3 1016 J

C 6 1016 J

D 9 1016 J

Q3 In nuclear… (U)

In nuclear ___________, nuclei of uranium

are split into smaller pieces and large amount

of energy will be released. In order to

maintain the ___________ reaction, mass of

uranium used should exceed the __________

mass.

critical

fission

chain

Q4 In nuclear… (H)

In nuclear __________, nuclei of

hydrogen are joined together and

____________ nucleus is formed.helium

fusion

Every second, the sun converts Every second, the sun converts 500 million metric tons of 500 million metric tons of hydrogen to helium. Due to the hydrogen to helium. Due to the process of fusion, 5 million process of fusion, 5 million metric tons of excess material is metric tons of excess material is converted into energy in each converted into energy in each second. This means that every second. This means that every year, 157,680,000,000,000 metric year, 157,680,000,000,000 metric tons are converted into energy. tons are converted into energy.

THE SUN

Complete the Reaction

1H

4Be

1H

8O

4Be

2He

2He

1H

2He

2He

2He

6C

1H element atomic number (protons)

6C

Nuclear Fission

Neutron induced in U235

Fission is Exothermic

The sum of the masses of the resulting nuclei is less than the original mass (about 0.1% less)

The “missing mass” is converted to energy according to E=mc2

U.S. Electrical Power Production by Source

Source: EIA

(2004)

Nuclear Fuel Costs

• Nuclear Fuel Costs Include– Uranium – Enrichment– Manufacturing– Waste Disposal

• Total Nuclear Fuel Cost is Only About 0.5 cents per kilowatt-hour– Uranium accounts for only about 20% of this cost or

0.1 cents per kilowatt-hour– Increasing Uranium Cost has Minimal Impact

Review

Nuclear fission:

A large nucleus splits into several small nuclei when impacted by a neutron, and energy is released in this process

Nuclear fusion:

Several small nuclei fuse together and release energy.