Buxton & District U3A Science Discussion Group

“Nuclear Fission: Nuclear Power & The Atom Bomb”

John Estruch17 May 2013

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Science DiscussionWhat are we going to talk about

• The basic physics• Uranium fission• Nuclear power generation• Atom bomb• Nuclear waste• Alternative fuels

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Science DiscussionQuiz Time

What are the following:Atom

Element

Nucleus

Isotope

Neutron

Proton

Electron

Proton

Neutron

Electron

Nucleus

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Science DiscussionElements and Isotopes

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number (number of protons).

Protium Deuterium TritiumH1

1 H21 H3

1

1H 3H2Hhydrogen-1 hydrogen-2 hydrogen-3

Isotopes are variants of a particular element. All isotopes of a given element have the same number of protons, each isotope differs from the others in its number of neutrons.They have the same chemical properties but different physical properties.

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Science DiscussionNuclear Fission

• The protons and neutrons in a nucleus are held together by the Strong Nuclear Force which has a short range.

• Without the Strong Nuclear Force the positively charged protons would be pushed apart by electrostatic repulsion.

• If a nucleus has enough excess energy to deform its shape then the protons/neutrons may move far enough apart for the electrostatic repulsion to overcome the Strong Nuclear Force – the nucleus breaks into 2 or more pieces

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Science DiscussionEnergy stored in nuclei

• The binding energy of a nucleus is the amount of energy needed to pull it apart.

• If you rearrange the same number of protons and neutrons (nucleons) from nuclei with lower to higher binding energy the difference in energy is released from the nuclei

• If a nucleus with about 240 nucleons fissions into 2 nuclei of about 120 nucleons each then:• The binding

energy changes by about 1 MeV per nucleon

• Therefore a total of 200-300MeV is released.

MeV = million electron-volts 1eV = 1.6 x 10-19

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Science DiscussionNuclear energy v. chemical energy

• Fission 1 atom of Uranium approx 200 MeV• Burn 1 atom of Carbon approx 1 eV• Uranium about 20x heavier than Carbon

Fission 1 ton Uranium = Burn 10,000,000

ton coal

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Science DiscussionWhat are we going to talk about

• The basic physics• Uranium fission• Nuclear power generation• Atom bomb• Nuclear waste• Alternative fuels

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Science DiscussionUranium

• Atomic number 92 (92 protons)• Naturally occurring Uranium is a mixture of isotopes:• 0.7% 235U (92 protons + 143 neutrons)• 99.3% 238U (92 protons + 146 neutrons)• 0.0055% 234U

• 235U is the only “fissile” isotope occurring naturally in useable quantity on earth

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Science DiscussionUranium fission

• Uranium can undergo “spontaneous” fission (only rarely – half life 7 x 10 8 years)

• Also undergoes “induced” fission

235Uneutron

“Fission product” e.g. 90Rb

“Fission product” e.g. 143Cs

Neutrons (2.5 on ave.)

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Science DiscussionChain Reaction

235Uneutron

235U

235U

235U

235U

235USome

neutrons escape

If at least 1 neutron from each fission goes on to cause another fission then we have a “chain reaction”

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Science DiscussionCritical Mass

• If you have a small piece of 235U then lots of neutrons escape – no sustained chain reaction

• If you get bigger piece then more neutrons will cause fission before they escape

• When piece is just big enough so on average 1 neutron from each fission goes on to cause another fission - just get chain reaction – this is “critical mass”

The nominal critical mass for a sphere of pure 235U is 52kg (17cm diameter)

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Science DiscussionIf only it were that simple!!

• Natural Uranium is 0.7% 235U and 99.3% 238U• 238U is not fissile, it absorbs neutrons so tends to

prevents chain reaction.• Fission produces “fast” neutrons• Fast neutrons more likely to be absorbed by 238U• Slow neutrons more likely to cause fission in 235U

Any suggestions?

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Science DiscussionNeutron Moderators

235USlow (or “thermal”) neutron

235U

235U238U

Mod

erat

or

Fast neutrons

238U

Thermal neutrons

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Science DiscussionModerators

Moderator Advantages Disadvantages

Hydrogen - Light Water (H2O)

• Very efficient moderator• Cheap

• Neutron absorber

Deuterium - Heavy Water (D2O)

• Efficient moderator • Expensive

Carbon – Graphite • Cheap• Not neutron absorber

• Only moderately efficient

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Science Discussion

The Heroes of Telemark(a small historical aside)

• The 1965 film is a dramatisation based on a number of real Norwegian/British commando raids on the Vermork Norsk Hydro plant.

• The plant was producing heavy water (D2O) which the Germans could use as a moderator in a Uranium reactor as part of a nuclear weapons programme.

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Science DiscussionEnrichment

Another way to increase number of neutrons causing 235U fission is to increase percentage of 235U i.e. “enrichment”

Grade % 235U Use

Natural 0.7% CANDU, Magnox

Reactor 3-4% PWR, BWR, AGR…

Weapons 85%+ Bombs

Can’t be separated chemically so use complex /expensive technology such as gas centrifuge cascade.

IAEA / UN get very concerned about export of enrichment technology as a nuclear weapons proliferation issue.

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Science DiscussionWhat are we going to talk about

• The basic physics• Uranium fission• Nuclear power generation• Atom bomb• Nuclear waste• Alternative fuels

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Science DiscussionNuclear reactor

Fuel

Moderator

ContainmentCoolant

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Science DiscussionControlling the reactor

Control Rods

• Control rods are made of a neutron absorbing material (e.g. cadmium )• Pushing them in or out of reactor controls the flow of neutrons

When the rods are inserted more neutrons are absorbed – power decreases

When the rods are withdrawn fewer neutrons are absorbed – power increases

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Science DiscussionChicago Pile 1 (CP-1)– The 1st Reactor

• Team led by Enrico Fermi built CP-1 in rackets court under the stand of Chicago University football field.

• Uranium pellets separated by graphite bricks “a pile of black blocks and wooden timbers”

• Controls were rods coated in cadmium.• On 2 December 1942 the first sustained

chain reaction was achieved. • No radiation shield, no coolant!

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Science DiscussionTypes of reactor

Reactor type Fuel Moderator Coolant Countries

Pressurised Water Reactor (PWR)

Enriched UO2 Light water (H2O) Light water (H2O) USA, France, Russia, Japan, China

Boiling water Reactor (PWR) Enriched UO2 Light water (H2O) Light water (H2O) USA, Japan, Sweden

Magnox Natural U metal Graphite CO2 UK

Advanced Gas Cooled Reactor (AGR)

Enriched UO2 Graphite CO2 UK

Pressurised Heavy Water Reactor (CANDU)

Natural UO2 Heavy water (D2O)

Heavy water (D2O) Canada

Light Water Graphite Reactor (RMBK)

Enriched UO2 Graphite Light water (H2O) Russia

Fast Neutron Reactor PuO2 & highly enriched UO2

None Liquid sodium Russia

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Science DiscussionNuclear Power Plant

PWR electricity generation plant

Pressurised water in reactor:• Acts as moderator• Is primary cooling circuit• Takes heat out of reactor

and uses it to boil water in secondary circuit

Secondary circuit is just like coal, gas or oil power station

Steam turbine and generator is good old 19th century technology

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Science DiscussionWhat are we going to talk about

• The basic physics• Uranium fission• Nuclear power generation• Atom bomb• Nuclear waste• Alternative fuels

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Science DiscussionFission bomb (a.k.a. Atom bomb)

• A nuclear reactor without the controls• Want to release as much energy as possible as

quickly as possible• Moderator cannot be used (too slow)• Requires highly enriched Uranium (>85% 235U)• The number of neutrons/fissions can double

every 10-8sec• In theory several hundred tons of 235U could

fission in 1/1,000,000 sec.• In practice there are only a few Kg and bomb

blows itself apart before it all fissions• Hiroshima bomb only fissioned 1.3% of

available 235U,

Trigger mechanisms

“Gun method” – a sub-critical mass fired into another

“Implosion method” – lots of sub-critical masses surrounded by high explosive

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Science DiscussionHistory of nuclear bombs

“Little Boy”• Uranium bomb• Gun type mechanism• Detonated Hiroshima

6 August 1945• 16 kilotons TNT

equivalent

“Fat Man”• Plutonium bomb• Implosion mechanism• Detonated Nagasaki

9 August 1945• 21 kilotons

Nuclear states• USA• USSR/Russia• UK• France • China• Apartheid era

South Africa.• India• Pakistan• North Korea• Israel?

USA & USSR built bombs up to 50 megatons

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Science DiscussionWhat are we going to talk about

• The basic physics• Uranium fission• Nuclear power generation• Atom bomb• Nuclear waste• Alternative fuels

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Science DiscussionSource of nuclear waste

235Uneutron

Many fission products are highly radioactive

Some neutrons do not cause fission but are absorbed by 238U and 235U to produce heavy (actinide) radioactive isotopes e.g. 234U, 237Np, 238Pu, 239Pu, 241Am

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Science DiscussionRadioactive isotopes

• When unstable nuclei “decay” by giving off , or radiation

• and change nucleus to a different element/isotope

• puts the isotope into a more stable state

• So the higher the rate an isotope emits radiation the quicker it stops emitting the radiation.

• This is measured by the “half-life” (how long it takes for half the nuclei of a particular isotope to decay)

Short Lived isotopes• When reactor shuts down radioactive

isotopes continue to decay• Short lived isotopes decay quickly• High rate of decay generates lots of

heat (5%-10% of reactor power)• The rate reduces rapidly reaching “cold

shutdown” in a few days• Until cold shutdown, the reactor must

be actively cooled.• Failure of cooling after shutdown

caused all the problems at Fukushima

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Science Discussion

Example isotopes in nuclear waste

Iso-tope

Half-life (million yrs)

99Tc 0.211126Sn 0.230

79Se 0.32793Zr 1.53

135Cs 2.3 107Pd 6.5

129I 15.7

Iso-tope

Half-life (yrs)

155Eu 4.7685Kr 10.76

113Cd 14.190Sr 28.9

137Cs 30.23121Sn 43.9

151Sm 90

Long-lived fission products

Medium-lived fission products

Iso-tope

Half-life (thousand yrs)

229Th 7.3243Am 7.3

239Pu* 24.1236Np 154233U* 159 242Pu 373

236U 2,348

Iso-tope

Half-life (yrs)

252Cf 2.6241Pu* 14

227Ac 21244Cm 18243Cm 29232U* 69238Pu 88

Long-lived actinides

Medium-lived actinides

* Fissile

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Science DiscussionHow nasty (or useful) is the waste?

It depends on the isotope:Property Impact

Decay rate Short half-life isotopes give off radiation at faster rate but disappear more quickly

Type of radiation , ,

Energy of the radiation

For same type of radiation different energies will cause different effects

Does it accumulate in the body

Some substances are kept in the body (e.g. 131I accumulates in the thyroid gland) others pass through (137Cs is water soluble and is excreted in urine)

Is it fissile? Fissile isotopes could be useful as fuel but are a proliferation risk.

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Science DiscussionWhat to do with nuclear waste?

1. Leave it alone, cool it, keep it safe

When first removed it is very radioactive and generates lots of heat, very hard to handle. Usually kept in ponds full of water (often at reactor site)

2. Leave it alone some more, cool it, keep it safe

After a year or so it has become a little less radioactive and may be moved to ponds at a larger storage facility (e.g. Sellafield)

3. Reprocess it (optional) After a few tens of years in is easier to handle with care (robots, remote handling etc). We can remove the useful fissile isotopes for fuel and separate the nasty from the not so nasty.

4. Keep it safe for a very long time

The remaining nasty stuff may not need to be kept in water but it needs to be kept from entering the environment or getting into hands of naughty people for a long time (100,000 years?). Big holes in the ground have their uses!

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Science DiscussionWhat are we going to talk about

• The basic physics• Uranium fission• Nuclear power generation• Atom bomb• Nuclear waste• Alternative fuels

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Science DiscussionPlutonium

• 239Pu and 241Pu are fissile• Does not occur naturally (except in minute amounts)• Pu is created in Uranium reactors

(1n + 238U => 239U ==> 239Pu)

• Not a neutron absorber• Can be used with fast neutrons – no need for a moderator –

“fast reactor”

2 decays

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Science DiscussionThorium

• Predominant isotope (232Th) is not fissile• 232Th is “fertile” (1n + 232Th => 233Th ==> 233U)

• Lots of excitement in recent years that thorium will provide abundant, cheaper, cleaner power.

2 decays

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Science DiscussionThorium

(Claimed) Advantages Disadvantages

More abundant than Uranium Need to convert 232Th into 233U

Not radioactive Current solid fuel technology requires U or Pu reactor then reprocessing

Produces less Pu and other heavy actinides

Proposed liquid fuel reactors could avoid reprocessing but technology is unproven

Produces 232U as well as 233U. Mixture is more difficult to weaponise.

232U production can be eliminated in operation of reactor. Not easy for terrorists but could be possible for states to combine power/weapons production.

Has some favourable physical/chemical properties

Very high cost to develop, test, prove safety and licence thorium reactors.

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Science DiscussionQuestions?