Nuclear reactions

Micro-world Macro-world

Lecture 17

Using the strong nuclear force to produce useful energy

Strong Nuclear Force

• It is very strong– It overcomes the electrical

repulsion between positively charged protons that are only 10-

15m apart.

• It acts over a very short range– It is not felt by nucleons when they

are more than 10-15m apart.

• It is selective– It is felt by neutrons & protons, but

not by electrons

Nuclear “bullets”Protons are repelled by electrical the repulsion force of the positively nucleus. Only protons with KE ofa few MeV or more can get within the range of thestrong nuclear force & produce “nuclear reactions”

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Producing nuclear reactionswith protons (or any other

charged nuclei) is a challenge

Neutron induced nuclear reactions

Neutrons don’t feel the electrical force so even very slow, low-energy neutrons can strike the nucleus & produce “nuclear reactions”

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Low energy neutrons areeffective nuclear “bullets”

Nuclear fission

n + 92U 56Ba + 36Kr + 2n235 142

92

Energy balance in a fission reaction

141Ba +92Kr + 2n

200 MeV KE heat 235U + n

Chain reaction

Use the neutrons produced by onefission to initiate another fissionEnrico Fermi

Requirements for A-bomb

• Fissionable material: 235U or 239Pu

• Critical mass

• Mechanism

Critical Mass

Enriched 235U 50kg

239Pu 10kg

Mcrit

Fissionable Material

Fortunately, only certain nuclear isotopes undergo the fission process:

235U only 0.7% of naturally occurring U(99.3% is 238U, which doesn’t fission)

239Pu doesn’t occur naturally, but is produced in nuclear reactors

…. There are other fissionable isotopes, e.g. 233U &232Th, but they are very rare

Little boy (235U)

(doughnut-like)

Fat man (239Pu)

Devastation

Hiroshima Aug 6 1945 8:15AM

80,000 people killed immediately;

~100,000 people were exposed to lethal radiation & died painful slow deaths

Hiroshima aftermath

Devastation

Nagasaki Aug 9 1945 10:45AM

39,000 people killed immediately;

~70,000 people were exposed to lethal radiation & died painful slow deaths

Nagasaki aftermath

Nuclear fusion

2H + 3H 4He + n

Two light nuclei fuse togetherto form a heavier one

Here the nuclei have to start outwith large energy in order to

overcome the electrical repulsion

Energy balance in a fusion reaction

4He+n

12.3 MeV KE heat 2H +3H

Need to overcome electric repulsion

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Protons need ~2MeVenergy to get within10-15 m of each other(where strong nuclear force can be felt)

This requires super-hightemperatures (severalMillion degrees). Such hightemperatures exist in thecore of the Sun or in an Atomic-Bomb explosion

H-bomb: powered by nuclear fusion

Nuclear fission bomb “detonator”produces the high temperature

required to initiate fusion processes

Nuclear fusion bomb

Brighter than 1000 suns

1000 times the power of an A-bomb!!

Dangers of teaching nuclear physics

Oh, and I suppose it

was me who said ‘what

harm could it be to give the

chickens a book on nuclear

physics?’

Fusion in the Sun

The core temperatureis ~14 million degrees

Here a tiny fractionof the protons haveenough thermal energyto undergo fusion

Solar fusion

processes

+ 5.5 MeV

+ 1.4 MeV

+ 12.9 MeV

pp-cycle

6 protons 4He + 2 protons + 2 “positrons” + 2neutrinos

Energy balance in the pp-cycle

4He

25 MeV KE heat 4 protons

+ 2 neutrinos

How do we know what goes oninside the Sun?

Superkamiokande

Superkamiokande

Direction of neutrinosdetected in Superkamiokande

Sun as seen by a neutrino detector

Neutrinos come directly from solar core

Neutrinos are everywhere

T esttest