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Chapter 5 Physics POWERPOINT
Terms in this set (27)
Exponential growth is basically the opposite of exponential decay. Starts off slow and gets faster and faster.
exponential growth examples
population growth, compound interest, human growth, cell divisions, epidemics, computer viruses, nuclear bombs.
In fission, a large nucleus breaks up into two smaller nuclei, some free neutrons, and some gamma radiation.
Spontaneous fission —extremely rare
Induced fission —a nucleus fissions after absorbing a neutron. These nuclei are called fissionable.
Fissionable: Fissions after absorbing a neutron (induced fission)
•U233, U235, U238, Pu239, Pu240, Pu241
•not that many nuclei are fissionable
Capable of sustaining a chain reaction. One fission leads to more fissions, and more energy. (They generally can fission with low energy neutrons.)
Only naturally occurring fissile isotope is U235. It is only 0.7% of all Uranium. Most of the rest is U238.
Others are U233 and Pu239 (and Pu241). Where do we get them?
We get Pu239 from U238
We get U233 from Th232
Decay into a fissile isotope after absorbing a neutron. U239 and Th232 are fertile.
We have three options for nuclear power
•U235—all US reactors
•Pu239 (from U238)—Breeder reactors
•U233 (from Th232)
•Experimental plant 1965‐1969
•Research and development are underway.
We have two basic options for nuclear bombs
•(Th232—Presence of Th233 creates, and thus is hard to handle)
•Multistage devices use fusion
For a chain reaction to occur, the free neutrons must be absorbed by another U235. They are "fast", and can be absorbed, but not well. There are two solutions
1. We can slow them down, after which they are absorbed more easily—the neutrons undergo collisions with a "moderator". This is the option used in power plants.
2. We can have lots and lots of U235, so that each neutron is more likely to be absorbed— "enrichment". This is the option used in bombs.
Uranium reactors (U235) use moderators; some also use enrichment. (Reminder, natural uranium is 0.7% U235.)
Water + enrichment to 3%: All US reactors.
Heavy water + no enrichment: CANDU reactors
•CANadian DeUterium reactors
Uranium reactors safety features
The moderator— overheating evaporates the moderator which stops the reaction
Control rods— absorb neutrons and stop the reaction. The rate is controlled by inserting or removing the control rods
Containment vessel— Nuclear explosions, i.e., exponential growth of reactions, are impossible, but pressure explosions ARE possible.
-The plutonium initially comes from a Uranium reactor, because some U238 absorbs neutrons and becomes U239, then Pu239
-Subsequently, the Plutonium reactor can "breed" its own fuel from U238
-No enrichment needed, though the Pu and U must be separated from other waste products. Separating different elements is easier than separating different isotopes
-No moderator needed
-All US reactors are Uranium, most worldwide reactors are Uranium
-Require less fuel and produce a smaller volume of waste
Breeder reactors: Two types
•238 Uranium breeds 239 Plutonium
•Uses fast neutrons, therefore called a "fast breeder"
•No moderator needed
•232 Thorium to breed 233 Uranium
•Uses thermal neutrons, therefore called a "thermal breeder"
Both must be "seeded" with fuel derived from a
Uranium reactor (initially)
Breeder Reactors: Pros and Cons
•They burn more Transuranics (sometimes called Actinides). This is a big advantage for waste storage. See half‐life chart.
•Initially, we did not know how much Uranium we really have, so breeders were very attractive
•Fast breeders (Plutonium) do not have the safety advantage of a moderator.
•Thermal breeders (Thorium) have no real advantage over Uranium reactors. (India, however, has a lot of Th and not so much U.)
Nuclear Accidents -Chernobyl
Chernobyl, Ukraine (USSR) (1986), during an electrical test.
•Safety procedures were not followed due to lack of coordination and safety "culture"
•Power surge >> explosions >> destruction of plant
•Design flaws: Graphite moderator (caught fire, leading to a pressure explosion); no containment vessel (led to release of radioactive isotopes into upper atmosphere)
•Lethality: 30 deaths within the first few months; 14 among exposed workers within 10 years; 15 childhood thyroid cancer deaths.
•About 24,000 cancer deaths, based on LNTH
Nuclear Accidents - Fukushima, Japan
Fukushima, Japan (2011), resulted from a tsunami after an
•Operator did not meet basic safety requirements.
•Reactors shut down immediately after the earthquake.
• Tsunami >> disabled the generators that powered pumps that cooled the reactor >> fuel meltdowns in 3 reactors >> pressure explosions >> release of radioactivity. (It had a containment structure, though.)
•Design flaws: Pumps were protected from the tsunami, but not the switching station that sent power to the pumps, despite previous warnings.
•No direct deaths; 1 related cancer death in 2018; ~1600 secondary deaths (e.g., evacuation of elderly patients from nursing homes.
Tanks holding contaminated water.
Fission Products—refers to the daughter elements. These are present for both reactors and bombs Transuranics—refers to isotopes heavier than uranium that are created in a reactor. Also called Actinides. Spent fuel per year:
•33 tons of Uranium (0.9%)
•1.2 tons of fission products
• 360 kg of Plutonium
•27 kg of minor actinides
T 1/2 Fission Products
a = years (anno)
Ma = million year
Notes: Short‐lived are not listed because they do not pose long‐term concerns. There are no fission products listed with half‐lives between 90 years and 211,000 years.
Transuranics (Actinides) in waste
Some neutron capture is not followed by fission, leading to "transuranic" isotopes in the waste. (heavier than uranium)
•These are also radioactive
•Reminder: 27 kg per year
•Half lives are all ranges
•After about 200 years, the radioactivity from the actinides is greater than from the fission products
Radioactivity over time from reactor waste
Goes down a lot ~ 100 years
Nuclear waste storage (high level)
~ 5 years in spent fuel pools. This is when the waste is most radioactive. The water keeps the spent fuel rods cool and acts as a moderator. Racks are made of neutron‐absorbing metal such as cadmium.
•Above ground storage in casks. This is a "60‐year" solution, not a long‐term solution. This is the only current US storage!!
•Deep geological storage
•Mined repositories: 250‐1000m
•Yucca Mountain—mired in politics
•WIPP (Waste Isolation Pilot Plant)—military waste only.
•Straight bore holes
•Horizontal drilling technology allows more storage per hole.
(Reminder, natural uranium is 0.7% U235.)
A water moderator is not very practical for a bomb. Why not? Instead of a moderator, a bomb has to rely on enrichment.
•Uranium bombs are enriched to about 90% U235. This is the main impediment to building a uranium bomb (even for a country).
•Critical mass—above a certain mass, the chain reaction will detonate before you are ready from spontaneous fission.
•Fairly high for U235. Hence, U235 bombs use a gun design.
•Requires about 33 pounds of U235 (using a neutron reflector)
Uranium bomb: Gun design
Little Boy - Hiroshima (1.38% efficiency. i.e., 1.38% of the uranium fissioned)
In essence, the Little Boy design consisted of a gun that fired one mass of uranium 235 at another mass of uranium 235, thus creating a supercritical mass. A crucial requirement was that the pieces be brought together in a time shorter than the time between spontaneous fissions. Once the two pieces of uranium are brought together, the initiator introduces a burst of neutrons and the chain reaction begins, growing exponentially until the energy released becomes so great that the bomb simply blows itself apart
Get the plutonium from a reactor. No need to enrich.
•This is one impediment to building a plutonium bomb (even for a country).
•Lower critical mass than U235, due to contamination from Pu240 (rapid spontaneous fission)
•Design is much more difficult.
•This is a second impediment to building a plutonium bomb (even for a country).
Plutonium bomb: Implosion design
Fat Man - Nagasaki (13% efficiency. i.e., 13% of the
Seth Neddermeyer, a scientist at Los Alamos, developed the idea of using explosive charges to compress a sphere of plutonium very rapidly to a density sufficient to make it go critical and produce a nuclear explosion.
Design- starts on edges works its way to center
Deuterium and Tritium and
Lithium 6 are added.
• The heat from the initial fission causes some fusion
• Fusion releases neutrons
• The extra neutrons cause extra fission events
• More of the plutonium fuel fissions.
Note: Less than 2% of the energy is from the fusion, but over 30% of the plutonium undergoes fission, compared to 13% for Fat Boy
Multi stage devices
40‐ 50% efficient. 25‐50% of energy is from fusion
1. Chemical explosion compresses fission fuel to initiate fission
2. X-rays from primary are reflected by casing and heat foam
3. Foam, now a plasma, compresses secondary; fissionable "spark plug" ignites
4. Fusion fuel ignites
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