The basics and origins of nuclear energy including descriptions of the fission reaction.

1. URANIUM AND THORIUM
New (and Old) Energy Resources
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This presentation will, for the sake of brevity, deal only with uranium.
Thorium is more abundant than uranium in the earth’s crust, but it has two
disadvantages. First, in source rock, thorium is not very soluble in water,
so it loses the benefit of Redox processes that help to concentrate uranium
and thereby lead to commercially economic ore bodies.
Second, thorium is not associated with an appreciable amount of fissile material,
as is uranium. To begin using thorium, one must first produce Uranium-233
in a uranium-fuelled reactor.

2. Where and When
it All Began
A star exploded as a super-nova
near our location in this galaxy,
~ 6.5 billion years ago.
2http://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-a-supernova.html

3. • Heavy “natural” elements were formed in that blast
• Fragments eventually became part of our earth
Uranium (235 and 238) and Thorium 232 remain from that
event; U235 and U238 have decayed significantly with time
– They are still heating the interior of the earth
Uranium accumulated in earth’s crust via lava flows
– early life forms produced oxygen in atmosphere
– redox processes in rivers concentrated uranium in ore bodies
At Oklo, West Africa, at least 17 natural reactors started up
– they operated at low power for thousands of years
Uranium underwent radioactive decay; eventually to stable
lead 207
– less than 1 percent of uranium is fissile today
– Humans came along just in time & “invented” fission
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4. Uranium in
Earth’s Crust
http://physics.stackexchange.c
om/questions/144758/in-the-
earths-crust-why-is-there-far-
more-uranium-than-gold
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5. ENGR 4780 2016 Week 1 5
Fragments from these Super-Nova explosions accumulated in orbit
around other stars, and new planets – including earth – were formed.
Fortunately, an unusually large amount of the heaviest
natural elements are present in today’s earth. They help to keep us
warm.
Our atmosphere (and the greenhouse gasses contained in it)
moderates our day-to-night temperature swings.
Different and various processes have led to concentration of uranium
in the continental crust of earth.
Increasing oxidation of the atmosphere increased the mobility of
uranium due to the relatively high solubility of uranium oxides in water.
The inset above shows the roughly 100-fold increase relative to
primitive mantle material in meteorites.
Uranium was originally produced as a byproduct of radium mining.
It was used as a yellow pigment in glass and for luminous luminous
instrument dials.

6. Elements Present After a Super-Nova Explosion
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Isotope Half Life (Years)
Thorium 232 1.4 x 1010
Uranium 233 1.6 x 105
Uranium 235 7.0 x 108
Uranium 238 4.5 x 109
https://en.wikipedia.org/wiki/Binding_energy

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All earth’s heavier elements were created in these supernovae, by progressive
absorption of neutrons to form heavier nuclei. These elements all were formed
during the massive explosion that we call a supernova.
The explosion itself was caused by compression, heating, and then fusion of light
elements after the precursor star collapsed under its own gravitational force.
With the exception of Thorium, Uranium 235 and Uranium 238, the heaviest
nuclei have all undergone radioactive decay and have vanished. The lighter
elements in the Periodic Table all include stable isotopes – these never decay.
A large amount of potential energy is stored in these heavy nuclei. It is our
objective to draw on this energy storehouse.
Nuclear binding energy is the amount of energy that would be required to
“take apart” the nucleus into its component parts. This graph shows binding
energy divided by the number of nucleons in each nucleus. When uranium
undergoes “fission”, a small fraction of its original energy is released – and
appears mainly in the kinetic energy of the fission fragments.

8. 8
Nature’s Reactors
• The remains of seventeen natural reactors were found in Oklo, Gabon, West Africa in 1972
(see American Scientist, November 2005)
• As oxygen built up in earth’s atmosphere, uranium from banks of a river in East Africa was
dissolved in flowing water and carried to the river delta
• Uranium precipitated at high concentration in the river silt and sand
• The uranium-sand-silt mixture was soaked with water. Natural fission reactors were
created. They operated for thousands of years at low power.
Uranium
Deposit
http://www.iaea. /programmes/inis/index.html

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It is likely that the power level of these deposits was cyclic – water seeped into
the deposit, increasing the slow neutron population. The reactor power increased.
As power increased, steam forced liquid water out of the deposit and decreased
the neutron slowing down, so the reactor shut down. After shutdown the reactor
cooled down, water returned and the cycle repeated.
Presto! Automatic control.
All fission product decay isotopes can be found in the deposit, except for the
noble gases. This finding is important for long-term waste disposal.
There could be other deposits of this type in the world, but they have not been
found as yet.
If the earth’s uranium actually arose from two or more supernovae that occurred
at times later than 6.5 billion years, we could expect, of course find natural
uranium with higher percentages of uranium 235.

10. https://wordlesstech.com/oklo-natural-nuclear-reactor/
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11. Cross section of OKLO uranium mine
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The Gabon reactors were found in a long, narrow strip of uranium ore between
sandstone and the shale overburden. Seventeen “critical” reactor core locations
were eventually identified. All fission products except for the noble gasses were
found in or immediately around the reactor sites.
The anomaly of low U235 content in the ore, which was intended for further
enrichment in France, led to discovery of these reactors. The IAEA published
a book on this topic, showing the details of the discovery and the scientific
analysis of the findings.
Canadian physicist George Laurence built a “pile” including uranium and
graphite, but did not have enough of the right materials (his graphite
contained impurities), so could not achieve a sustained chain reaction.
The Italian Enrico Fermi achieved the first sustained chain reaction in Chicago
with the help of British, French, and Canadian experts.

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The second critical reactor in the world was constructed at Oak Ridge.
The X-10 reactor (uranium and graphite) reached first critical in late 1943 and
4 MW thermal power in 1944. It was operated as a plutonium producer
until 1945, after which it was reassigned to research purposes.
The liquid fueled “LOPO” reactor first started up in Los Alamos in 1943.
It was fueled with uranium in a uranyl nitrate solution. The critical mass was
only 565 grams of uranium 235. The upgraded SUPO and HYPO units were
used at Los Alamos for several years, mostly for measurement of cross sections.
The first nuclear reactor operated outside the USA was the ZEEP machine
at Chalk River Lab. ZEEP was firs started up on September 5, 1945.

14. Our First Reactors
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Ottawa 1939-42
Chicago December 1942
Los Alamos late 1943
LOPOCP1
George Laurence
Enrico Fermi
Wigner, Weinberg, Fermi
Fuel Vessel
(1-foot dia)
LOPO
1 metre
FILLING (COKE, WITH OR WITHOUT
SPACED SACKS OF URANIUM DIOXIDE)

15. Fundamentals of Power Generation
COAL-FIRED POWER PLANT URANIUM-FIRED POWER PLANT
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FLY ASH
CARBON DIOXIDE
BOTTOM ASHCOAL
AIR
HEAT ENERGY
CONTROL
URANIUM
HEAT ENERGY
USED FUEL
NEUTRONS
CONTROL
ABSORBERS
Burning of coal releases energy through increased electron binding energy of the hydrocarbon
molecules when they are oxidized.
Fission of uranium releases energy through increased per-nucleon binding energy
of the heavy nuclei, when they break apart.
The second (nuclear) effect is much larger than the first (chemical) effect.

16. Comparison of Coal vs Uranium Stations
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COAL-FIRED POWER STATION
Requires continuous coal and air input
The end objective is to boil water.
Useful output is heat energy.
Waste streams exist in CO2, fly ash, bottom ash
There is a small amount of potential energy in
the firebox at any one time - continuous
fuelling
Temperature is limited to less than the “flame
temperature” of the fuel-air mixture
Large amount of fuel per year (2000 train-cars
full of coal) to produce 1000 MWe
Large amount of poisonous waste that lasts
forever. CO2 is difficult to capture
Control by adjusting amount of fuel and air --
shutdown takes several minutes.
Reliable safety systems necessary to limit
damage to equipment after malfunctions.
High pressure steam is the main hazard.
URANIUM-FIRED POWER STATION
Requires only batch-fed uranium -- no air
The end objective is to boil water.
Useful output is heat energy.
No waste streams. Used fuel is replaced
periodically
There is a large amount of potential energy in the
reactor at any one time. Fuel is added in batches.
There is no intrinsic limit to the “burning”
temperature of the fuel.
Small amount of fuel per year (4 truck loads of
uranium) to produce 1000 MWe
Radioactive waste is confined when used fuel is
taken out. Waste decays to low level in ~1000
years.
Control by adjusting the population of neutrons –
shutdown takes a few seconds.
Reliable safety systems are essential to be certain
that neutron population does not “run away”.
Fuel temperature must be kept below melting.

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Burning of coal releases energy through
increasing electron binding energy of the
hydrocarbon molecules.
Fission of uranium releases energy through
increasing nucleon binding energy of the
heavy nuclei.
The second effect is much larger, so much
less fuel is required for a given electrical output.

18. The Reactor Designer’s Charge
• Controlled fission of heavy elements
– Re-emphasize the word Controlled
• Release the correct amount of heat energy as required
by the turbine-generator
• Allow for highly reliable heat removal design
• Economic reactor production & control systems
• Provide highly reliable protective systems
• Practical refueling & inspection/maintenance
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Reactor design:
 multi-dimensional –several dependent and independent variables.
 experimental to some degree - testing and verification are essential contributors
to achieve long-term reliability and good economics.

19. Time Scales in Reactor Design
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 Add one neutron to fissile nucleus, either radiative capture or fission occurs.
Fission is very fast – about 10 E-12 seconds.
Chain reaction time scales range from 10 E-8 to 10 E-3 seconds.
 Various tasks of the reactor designer involve phenomena on time scales from
about 10 E-7 seconds to 10 E+10 seconds.
 The reactor designer is responsible for all aspects of design from this time scale
all the way out to the end of power plant life, at up to more than 100 years.

20. It’s All About Neutrons & Nuclei!
The fission chain reaction process
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Some fission fragments decay
by neutron emission –
Delayed Neutrons – 0.2 to 55 sec
Prompt Fission Neutrons
Fission – 10-12 sec Neutron slowing
down by
scattering
2-3 Fission
Neutrons
 Time between fissions in a single chain varies between about 1 millisecond in CANDU
to 10 nanoseconds in a very fast reactor.
 Fission rate determines reactor power. 3.1 E+10 fissions produce one watt-second
of thermal energy in the fuel.

21. What’s Fast about this reactor?
– It’s the Neutrons!
Neutron Energy Distributions
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FNR
LWR
Prompt neutron
fission spectrum
PHWR

22. ENGR 4780 2016 Week 1 22
The prompt fission spectrum (red line) is degraded by scattering;
mostly inelastic scattering in uranium steel, and sodium.
The thermal spectrum of PHWR is much more pronounced
than that of the PWR, because of its well-thermalized,
low-absorption characteristic.
The least possible amount of inelastic scattering is preferred
in an FNR because it reduces Eta, the ratio of neutrons
produced to neutrons absorbed..

23. Eta - Neutrons
produced per
neutron
absorbed
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233U
235U
     239Pu
241Pu
1.0
2.0
3.0
4.0
Eta(E)
10010-2 10+2 10+4 10+6
Neutron Energy (eV)
233U
233U
239Pu
239Pu
235U
241Pu

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At very low neutron energy, 233U has the best value of “Eta”
– the number of neutrons produced per neutron absorbed.
At high neutron energies, 239Pu is best. No fuel performs
particularly well at intermediate energy because of the
complicated structure of resonance absorption in this energy range.

25. Fission Neutron Yield
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235U
Plutonium 239 shows the best value of the number of prompt
neutrons emitted during fission, at all energies.

26. Fission
Cross
Section
vs Neutron
Energy
ENGR 4780 2016 Week 1
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233U
235U
239Pu
1.0eV
1.0keV
1.0MeV
1 barn
1 barn
1 barn
1000 barns
Resonance Absorption Energy Range
1000 barns
1000 barns

27. ENGR 4780 2016 Week 1 27
This graph Illustrates the reason that fast reactors require
such high fissile fuel concentrations – the microscopic c
ross section for fission is much smaller at high neutron energy.
It also shows the main reason for designing heterogeneous reactors
– slowing down the neutrons outside the fuel, past the resonance
absorption energy range “saves” neutrons that would otherwise be
captured in those resonances.

28. The Neutron Chain Reaction
Conversion of Internal Energy to Kinetic Energy - in a reactor, it appears as heat
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constant, the system is critical.
• When Number of Slow Neutrons is
• Delayed Neutrons appear after
about 10 seconds.
• Fast Neutrons slow down in
1 millisecond or less.
Neutrons Slowing
Down
Leaked Neutrons
Delayed Neutrons
from Fission
Neutrons
Diffusing Leaked Neutrons
CONTROL THIS TO
CONTROL HEAT
HEAT
Slow NeutronsU235
Fission
Prompt
Neutrons
from
Fission
PRODUCTION
Captured
Neutrons
(Some neutrons are
captured in U238 and so
produce useful fuel - Pu239)
"ASHES”
(Fission
Products)
KINETIC
ENERGY

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This is what Leo Szilard foresaw – a steady state neutron chain
reaction similar to the process in a chemical autocatalytic reactor.
The main difference is that, in a chemical reactor, thermal heat from
one stage of the reaction initiates the next stage of reaction– a steady
state thermal energy chain reaction.
This reaction would be uncontrollable, if not for the fact that a
few neutrons – the so-called delayed neutrons – are emitted
from fission products long after the fission event.
(This uncertainty is what caused the fear present under
the Chicago University bleachers when CP1 was first started
up. That fear has since spread all around the world.)

30. The Big Ideas
• Physicist Ernest Rutherford 1908 – internal structure of nuclei (Nobel Prize)
• Chemist Ida Noddack 1934 – the fission idea; her idea was ignored
• Physicists Meitner, Frisch & Hahn 1938 – experiment & discovery of fission
• Otto Frisch Jan 1939 – saw fission products, named process “fission”
• Leo Szilard Jan 1939 – chain reaction possible, large energy expected
• Albert Einstein Aug 1939 – letter to President Roosevelt
• Otto Frisch, Rudolph Peierls Feb 1940 – first design of U bomb
• UK MAUD committee Sep 1941 -- informed US of bomb concept
• President Roosevelt Oct 09, 1941 -- US decision to build bomb
• Enrico Fermi Dec 2, 1942 -- first sustained reaction in “reactor” or “pile” by
University of Chicago “Metallurgical Lab” staff in the CP-1 reactor at Chicago
• Trinity bomb tested -- Jul 16, 1945
• Hiroshima Japan bombed -- Aug 06, 1945
• Nagasaki bombed -- Aug 09, 1945
• Japan surrendered -- Sep 02, 1945
• Argonne National Lab was established in 1946, and took over most civilian nuclear
energy development tasks.
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31. Now, Shrink Yourself!
• The diameter of a uranium nucleus is about 2 x 10-14 metres. This is about
one ten-thousandth of the diameter of an atom itself, since atoms range
from 1 × 10-10 to 5 × 10-10 metres in diameter.
• The distance between atoms in solid metal uranium is about two
angstroms, or ≈ 2 x 10-10 metres.
• So, in a square crystal lattice of uranium metal, the diameter of a uranium
nucleus is about 1/10,000 of the distance to its nearest neighbour.
• A lot of electrons are flying around in between two uranium nuclei, but
most of “solid” matter is just empty space.
• We could call this technology femto-power.
• Many parallel and sequential discoveries have combined during the past
century to make this technological achievement possible.
• The ancient Greeks spoke about atoms, Ernest Rutherford spoke of the
components from which atoms are built, and here we are today planning
the supply of inexhaustible energy in the service of our descendants.
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