Buxton & District U3A
Science Discussion Group
“Nuclear Fission:
Nuclear Power & The Atom Bomb”
John Estruch
17 May 2013
Buxton & District
What are we going to talk about
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The basic physics
Uranium fission
Nuclear power generation
Atom bomb
Nuclear waste
Alternative fuels
Science Discussion
Buxton & District
Quiz Time
What are the following:
Science Discussion
Electron
Atom
Nucleus
Electron
Element
Proton
Proton
Neutron
Isotope
Neutron
Nucleus
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Elements and Isotopes
A chemical element is a pure
chemical substance consisting of
one type of atom distinguished
by its atomic number (number of
protons).
Science Discussion
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.
Protium
Deuterium
1
1H
2
1H
1H
2H
hydrogen-1
Tritium
3
1H
3H
hydrogen-2 hydrogen-3
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Nuclear 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
Science Discussion
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Energy 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
MeV = million electron-volts
Science Discussion
• 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.
1eV = 1.6 x 10-19
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Nuclear energy v. chemical energy
Science Discussion
• 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
Buxton & District
What are we going to talk about
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The basic physics
Uranium fission
Nuclear power generation
Atom bomb
Nuclear waste
Alternative fuels
Science Discussion
Buxton & District
Uranium
• 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
Science Discussion
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Uranium fission
• Uranium can undergo
“spontaneous” fission
(only rarely – half life
7 x 10 8 years)
• Also undergoes
“induced” fission
Science Discussion
“Fission product”
e.g. 90Rb
neutron
235U
Neutrons
(2.5 on ave.)
“Fission product”
e.g. 143Cs
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Chain Reaction
Science Discussion
235U
235U
neutron
235U
If at least 1 neutron
from each fission
goes on to cause
another fission then
we have a “chain
reaction”
235U
235U
Some
neutrons
escape
235U
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Critical Mass
Science Discussion
• 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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If only it were that simple!!
Science Discussion
• 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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Neutron Moderators
Science Discussion
238U
Slow (or
“thermal”)
neutron
235U
Fast
neutrons
Moderator
235U
Thermal
neutrons
235U
238U
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Moderators
Moderator
Advantages
Hydrogen - Light Water • Very efficient moderator
(H2O)
• Cheap
Science Discussion
Disadvantages
• Neutron absorber
Deuterium - Heavy
Water (D2O)
• Efficient moderator
• Expensive
Carbon – Graphite
• Cheap
• Not neutron absorber
• Only moderately
efficient
The Heroes of Telemark
(a small historical aside)
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Science Discussion
• 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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Enrichment
Science Discussion
Another way to increase number of neutrons causing 235U fission
is to increase percentage of 235U i.e. “enrichment”
Can’t be separated chemically so use
complex /expensive technology such
as gas centrifuge cascade.
Grade
% 235U Use
Natural
0.7%
CANDU, Magnox
Reactor
3-4%
PWR, BWR, AGR…
Weapons
85%+
Bombs
IAEA / UN get very concerned about
export of enrichment technology as a
nuclear weapons proliferation issue.
Buxton & District
What are we going to talk about
•
•
•
•
•
•
The basic physics
Uranium fission
Nuclear power generation
Atom bomb
Nuclear waste
Alternative fuels
Science Discussion
Buxton & District
Nuclear reactor
Science Discussion
Containment
Coolant
Fuel
Moderator
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Controlling the reactor
Science Discussion
Control Rods
When the rods
are inserted
more neutrons
are absorbed –
power
decreases
When the rods
are withdrawn
fewer neutrons
are absorbed –
power increases
• Control rods are made of a neutron absorbing material (e.g. cadmium )
• Pushing them in or out of reactor controls the flow of neutrons
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Chicago Pile 1 (CP-1)– The 1st Reactor
Science Discussion
• 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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Types of reactor
Science Discussion
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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Nuclear Power Plant
Science Discussion
 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
PWR electricity generation plant
 Steam turbine and generator
is good old 19th century
technology
Buxton & District
What are we going to talk about
•
•
•
•
•
•
The basic physics
Uranium fission
Nuclear power generation
Atom bomb
Nuclear waste
Alternative fuels
Science Discussion
Buxton & District
Fission 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,
Science Discussion
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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History of nuclear bombs
Science Discussion
Nuclear states
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“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
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USA
USSR/Russia
UK
France
China
Apartheid era
South Africa.
India
Pakistan
North Korea
Israel?
USA & USSR built
bombs up to
50 megatons
Buxton & District
What are we going to talk about
•
•
•
•
•
•
The basic physics
Uranium fission
Nuclear power generation
Atom bomb
Nuclear waste
Alternative fuels
Science Discussion
Buxton & District
Source of nuclear waste
Science Discussion
Many fission products
are highly radioactive
neutron
235U
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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Radioactive 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)
Science Discussion
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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Example isotopes in nuclear waste
Science Discussion
Long-lived
fission products
Isotope
Half-life
(million yrs)
Medium-lived
fission products
Isotope
Half-life
(yrs)
Long-lived
actinides
Isotope
Medium-lived
actinides
Half-life
(thousand yrs)
Isotope
Half-life
(yrs)
99Tc
0.211
155Eu
4.76
229Th
7.3
252Cf
2.6
126Sn
0.230
85Kr
10.76
243Am
7.3
241Pu*
14
79Se
0.327
113Cd
14.1
239Pu*
24.1
227Ac
21
93Zr
1.53
90Sr
28.9
236Np
154
244Cm
18
135Cs
2.3
137Cs
30.23
233U*
159
243Cm
29
107Pd
6.5
121Sn
43.9
242Pu
373
232U*
69
129I
15.7
151Sm
90
236U
238Pu
88
2,348
* Fissile
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How nasty (or useful) is the waste?
Science Discussion
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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What to do with nuclear waste?
Science Discussion
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!
Buxton & District
What are we going to talk about
•
•
•
•
•
•
The basic physics
Uranium fission
Nuclear power generation
Atom bomb
Nuclear waste
Alternative fuels
Science Discussion
Buxton & District
Plutonium
Science Discussion
• 239Pu and 241Pu are fissile
• Does not occur naturally (except in minute amounts)
• Pu is created in Uranium reactors
239Pu)
(1n + 238U => 239U 2==>
 decays
• Not a neutron absorber
• Can be used with fast neutrons – no need for a moderator –
“fast reactor”
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Thorium
Science Discussion
• Predominant isotope (232Th) is not fissile
• 232Th is “fertile” (1n + 232Th => 233Th ==> 233U)
2  decays
• Lots of excitement in recent years that
thorium will provide abundant, cheaper,
cleaner power.
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Thorium
Science Discussion
(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
Has some favourable
physical/chemical properties
Very high cost to develop, test, prove safety and
licence thorium reactors.
production can be eliminated in operation of
reactor. Not easy for terrorists but could be possible
for states to combine power/weapons production.
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Questions?
Science Discussion