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Nuclear Reactions andReactor Safety
DO NOT LICK
We haven't entirely nailed down what element it is yet, but I'll tell you this: it's a lively one and it does not like the human skeleton.
-Cave Johnson
The Model of the Atom
Plum Pudding idea
Rutherford's Gold Foil Experiment
Why do we care?
The necessity of neutrons
The discovery of the neutron, 1932 -Needed to account for mass of nucleus -How do you keep protons from pushing each other away? -Observed uncharged particles penetrating matter
First nuclear chain reaction, 1938
Nuclear Chain ReactionsDo nuclear chain reactions lead to runaway explosions?
or ?
-Controlled nuclear chain reactions possible: control energy release/sec -> More on this later!
--->
-Crucial concept on whether a chain reaction will lead to useful power or destruction:
Critical Mass
[Not this: ]-Subcritical mass: reaction dies out! -Critical mass: one reaction yields exactly one other reaction-Supercritical mass: increasing reaction rate!-Can occur without human input!-Critical mass depends on :
-Temperature -Density -Amount of TNT used (somewhat kidding) -Shape(!) - Use of neutron reflectorsInteresting story about that last one...will come back to it in a second
Critical mass of some elements
Typical values for:
(what am I leaving out??)
Do humans have access to this amount of material?
Yes, but not readily.
U-235 Pu-239 Ne-236
52 kg 10 kg 7 kg
The Manhattan Project
Chain reaction sustained: 1938Einstein-Szilard letter: 1939Beginning of World War II: 1939First successful nuclear power "plant" : 1942
Famous names:J Robert Oppenheimer, Richard Feynman, Enrico Fermi, Albert Einstein, Julian Schwinger, Isidore Rabi, Eugene Wigner, Harold Urey, Niels Bohr, Hans Bethe, Luis Alvarez....(and more) Famous places: Hanford Site, Oak Ridge National Lab, Los Alamos National Lab
German competition?
"Tickling the Dragon's Tail"
Most severe criticality accident to occur -Louis Slotin, Harry Daghlian and the demon core-Ionizing radiation released, killing Slotin/Daghlian after few days
-Why was there no explosion? -What was difference between two "criticality" incidents? Were they the same mechanism? -->Remember what determines the critical mass
There's something odd about higher Z elements...
Nuclear FissionThese say it all:
+
Neutrons released/reaction > 1!!!!Energy released/mass, some comparison
Questions/details:Why Uranium (235)?Neutron radiation crucial!So is neutron "capture"
Remember N vs. Z?Another type of "radiation"Occurs in nature rarely! Half-life: who cares?
Typical Fission Coal Gasoline
8.8 x 1013 J/kg 3.5 x 107 J/kg 4.4 x 107 J/kg
Critical mass and nuclear fission, the bad version (Chris: the good kind)
05:29:45 MWT 16 July, 1945, 6.2 kg Pu-239, 21 kT TNT yield
J Robert Oppenheimer: "Now I am become Death, the destroyer of worlds."
Humans have never done anything so terrifying as this, in all history.
Weapons aren't the only thing we can do
Recap: from our demo earlier that chain reactions occur in various conditions
-We can control chain reactions, or not control them[99.3% of world's reactors] vs. [Chernobyl, Fukushima, Three Mile Island, Hiroshima, Nagasaki, etc.]
-Explosions only occur when the critical mass is reached and exceeded -Being exactly at the critical mass or a bit above- and being able to keep it from running off- can yield a controlled chain reaction!
Oh, and one last question...Based upon this plot, what is the purpose of the Universe?
Hint: you may need to ask me some questions about the plot!
Nuclear Chain Reactions to Nuclear Power Plants
And now, we change gears to talk about how we can use the aforementioned nuclear chain reactions and harness their power.
---->
Much better!
Reactor Core
Pressurized Water Reactor
Boiling Water Reactor
Meltdown
Corium 'lava' flow in basement of Chernobyl
Three Mile Island:
-Secondary pumps shut down
-Loss of heat exchange causes primary to lose some cooling capacity
-Automatic SCRAM occurs
-Valve opens to release excess pressure in containment vessel
-Valve gets stuck open, all coolant escapes
Chernobyl
-Build up of Xenon-135 slows down reaction.
-Control rods are removed to compensate
-Coolant valves are closed as part of a test
-Heating reactor causes coolant to partially boil, increasing reactivity
-Deploying control rods briefly flushes moderating water coolant, further increasing reactivity
-Reactor heats to 30 GW (10x maximum operating power) in about 1 second, flash boiling all water
-Lack of containment structure results in destruction of reactor and spread of radioactive materials through much of Eastern Europe and Scandanavia
Fukushima
-Earthquake shuts down power grid in northern Japan
-Back up generators do not work well underwater
-No electricity means no water can be pumped in to cool the reactor
Benefits of Nuclear Power
-No carbon emissions*
-No mercury or other heavy metals
-Fuel comes primarily from US and Canada
-Small foot print
*cement production
Dangers of Nuclear Power
-Accidents have very long term consequences
-Spent fuel disposal is problematic
-waste heat damages river ecosystems
No
