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Chapter 7:The Fires of Nuclear Fission

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A nuclear fuel pellet contains about 4 grams offuel

It produces the same amount of energy as a tonof coal or 150 gallons of gasoline

Its fairly cheap - $3 per pellet (compare to 150gallons of gasoline!)

It produces no greenhouse gases, nor VOCs,nor NO, nor SO2

It does not rely on petroleum

So why isnt it the primary fuel used in the U.S.?

Nuclear Fuel

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The answer: radioactivity

Spent fuel pellets emit radioactive particles(well learn what this means later)

Radioactivity carries with it images ofNagasaki, Hiroshima, Chernobyl

But these catastrophes are NOT typical ofnuclear fuel

The leftover residue is toxic ...

... but is that so different than gasoline and

coal?

Nuclear Fuel

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Consider This 7.1:

A) Given a choice between your town building anuclear power plant and a coal-burningpower plant, which would you choose?

B) Under what circumstances, if any, would yoube willing to change your mind?

Nuclear Power

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In the U.S., 20% of electricity is generated bynuclear plants

But nonew nuclear plants have beenconstructed since 1978

In 1979, Three Mile Island (Harrisburg, PA)experienced a partial meltdown

Since then, 9 nuclear plants have closed:

Couldnt compete with natural gas

Recent demands for energy have slowed the

decommissioningwill new plants be built?

Nuclear Power

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What isnuclear power?

Nuclear power plants run on the principle ofnuclear fission:

The process of splitting a large nucleusintosmaller ones, usually by bombarding thetarget nucleus with neutrons

Why does this produce energy?

The products of this reaction actually possess

slightlyless massthan the reactants

Nuclear Power

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The products of this reaction actually possess

slightlyless massthan the reactants

But wait, you say...

Nuclear Fission

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The products of this reaction actually possessslightlyless massthan the reactants

But wait, you say...

In earlier chapters, we said that mass was

conserved (Ch. 1) and that energy wasconserved (Ch. 4).

This is, strictly speaking, not true.

Neither property is conserved independently ofthe other

In ALL normal reactions, the assumption thatthey are independently conserved is valid

Nuclear Fission

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Recall that we said that the total energy of asystem must be conserved, but that energycould be transformed from one type ofenergy to another

This is what takes place in nuclear reactionsthe mass of the nucleus itself is convertedinto energy

Nuclear reactions are NOT normal theyinvolve tremendousamounts of energy

Nuclear Fission

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In fact, we will see that noneof theseconserved properties must be conserved in

a nuclear (abnormal?) reaction

Nuclear Fission

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If we accept that mass can be converted into

energywhich is to say that mass is justanother form of energythere must be a wayto express that conversion

The Einstein Equat ion:E = mc2

Energy = mass x (speed of light)2

Note the units here... c is a large number!

(3.0 x 108 m/s)2= 9.0 x 1016m2/s2

1 Joule = 1 kg m2/s2

Small changes in mass make for HUGE changes in

energy

Nuclear Fission

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Lets take a specific example:

The fission of Uranium-235

Recall from Chapter 2 that atoms can exist asdifferent isotopes, each of which must contain

the same number of protons as each other, butwhich contain a different number of neutrons

Protons = atomic number, this definestheelement

Neutrons, together with protons, make up themass of the nucleus

Nuclear Fission

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Nuclear FissionWe write the symbol for these isotopes differently, to

reflect the different number of neutrons and thus thedifferent atomic masses

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Nuclear Fission

Similarly, Uranium (element number 92) hasseveral isotopes, and U-235 has a massnumber of 235, written:

In order for Uranium-235 to undergo fission, itmust be struck by a neutron

A bare neutron has no protons, but a massnumber of one, and can thus be written:

U235

92

n1

0

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Nuclear Fission

So, if we want to write down our reaction, wewould begin by writing our reactants:

nU 1

0

235

92

But what are our products? It is true that wecouldproduce a different isotope of Uraniumhere by adding a neutron:

UnU 236

92

1

0

235

92

But thats not what happens. What happens is

more complicated.

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Nuclear Fission

Rather than simply adding a new neutron, theUranium nucleus undergoes fission, breakingapart into smaller nuclei. One such reaction is:

nKrBanU

1

0

92

36

141

56

1

0

235

92 3Note that rules of balancing DO still apply tonuclear equations:

Protons on the left = 92Protons on the right = 56 + 36 =92Mass numbers on the left = 235 + 1 = 236

Mass numbers on the right = 141+ 92+ 3 = 236

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Nuclear Fission

nKrBanU

1

0

92

36

141

56

1

0

235

92 3But two things should worry you about thisanalysis:

- Why do neutrons appear on both sides?Shouldnt they cancel?- If the mass numbers on both sides are equal,

where is the energy coming from?

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A chain reaction is one inwhich the productsof

an initial step undergofurther reaction

Here, the three neutronsemitted by the fission

process can strike othernearby U-235 atoms,and induce fission inthem

... Producing moreneutrons, which can goon to strike morenearbyU-235 atoms...

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An important concept withregard to chain reactionsis that of critical mass:

The amount of fissionablematerial which isnecessary to sustain thechain reaction

For U-235, this is 15 kg:

If 15 kg of U-235 iscontained in the same

place, it will undergospontaneousfission

This is the principlebehind nuclear bombs

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Nuclear Fission

nKrBanU 1

0

92

36

141

56

1

0

235

92

3

If the mass numbers on both sides are equal,where is the energy coming from?

Recall that the actualmass of a nucleus is notsimply the mass number:In order to define the mass number, wedeclared that protons and neutrons weighexactly the same amount, and that electronsdont weigh anythingNeither of these statements is true, although

both are good approximations

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Nuclear Fission

nKrBanU

1

0

92

36

141

56

1

0

235

92 3In fact, an atom of U-235 weighs 235.043924amu

An atom of Kr-92 weighs 91.926156 amuAn atom of Ba-141 weighs 140.914412 amuA neutron weighs 1.00866 amu

So, the reactants weigh 236.052584 amuThe products weigh 235.866548 amuOver the course of this reaction, 0.186036 amuof matter is converted into energy

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Nuclear Fission

nKrBanU 1

0

92

36

141

56

1

0

235

92

3

Over the course of this reaction, 0.186036 amuof matter is converted into energy

Thats about 1/1000thof the total massHow much energy is produced from the fissionof 1 kg of U-235?

E = mc2= [(1/1000)(1 kg)](9.0x1016m2/s2)= 9.0 x 1013Joules!

This is the same amount of energy as from

33,000 tons of TNT, or 3300 tons of coal

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Change of Pace:Your Letters Assignment

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Letter: The Letter is worth 20%of your finalgrade.

It is due on April 26ththere will be no quiz and nohomework that week to ensure that you have thetime to complete the assignment

As a large part of the job of an environmentalscientist is that of communicating to the masses, Iwould like you to show your proficiency in

describing the science weve covered in class to alay-person.

You will write a letter to your fictional great aunt,

who is 85 years old and lives in Nebraska.

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Your aunt has found out you were taking this course.She is so thrilled because she watches CNN andhears all about environmental issues, but since she

hasnt taken a science class in 70 years and hasspent her whole life working on her farm, she doesnt

quite understand what the problems and solutionsare.

Could you please explain to her (in no m ore thantwo pages)what is the big deal about ONE of the

following issues, andwhat can she doto make adifference:Global WarmingThe Ozone Hole

Alternative fuels to replace petroleum

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Write this assignment as a personal letter

Do not attempt to persuade your aunt with math and

chemical equationsits all gobbledygook to her.

Use everyday language to explain the issues and thepossible solutions

You cant explain everything in two pages youllhave to decide what is important enough to include

If your aunt is really confused by your letter, she maywrite back and ask for some clarification. If youre

going to write her a second letter, it will be due on the

last day of classes