Earth as an Ecosystem

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Define nuclear fission
Reactions in which an atoms nucleus splits into smaller parts (and 2/3 neutrons) releasing a large amount of energy in the process. It can be spontaneous, or occur when the nucleus of a heavy atom captures a neutron
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Define nuclear reaction
when neutrons released from fission produce an additional fission in at least on other nucleus, which in turn produces neutrons etc.
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Give two ways that n's can be lost?
1) leaving into other surroundings 2) neutron capture by other nuclides e.g. 238U
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What kind of neutrons does U235 absorb? What does this lead to?
thermal- 236U and subsequent nuclear fission
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What kind of neutrons does U238 absorb? What does this lead to?
Much faster neutrons- 239U and further neutron absorption (does not emit neutrons as cannot overcome energy barrier)
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Therefore what problems does U238 pose in nuclear reactor? How do they overcome this
It would mean the reaction was no longer self sustaining. Therefore they slow down neutrons to thermal speed so can only react with 235U
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In an atomic bomb, what would they do?
Use of highly enriched 235U or 239Pu, both of which are fissionable
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What is the critical mass of 235U and 239Pu and how are they made?
U- 50kg (separation from 238U) Pu-10kg (exposure of 239U to neutrons)
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What is the fuel used in nuclear reactors? what is it encased in?
uranium oxide pellets (12cm long, 1cm diameter) encased in zirconium alloy (Which is strong and low neutron absorbing)
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Name three ways of controlling nuclear reactors and explain how
coolants (absorb heat from fission reaction and transport from core to turbine), moderators (slows neutrons by causing to collide) and control rods (shuts down nuclear fission process when required by absorbing ALL neutrons)
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what are control rods made out of?
borium carbide+silver+cadmium +hafmium +indium
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Name two methods of uranium enrichment
1) centrifuge (separates isotopes by mass difference- spun at 2000 revs so walls enriched by heavier 238U) 2) gas diffusion (UF6 pumped through filter barriers- smaller 235UF6 passes through more easily- diffuses 0.4% faster)
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What reaction must precede both of these types of enrichment?
Conversion of Uranium into a volatile compound- UF4 + F2 --> UF6
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After enrichment?
UF6 +2H2O + H2 --> UO2 + 6HF
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What three groups can nuclear waste be divided into?
1) highly radioactive, short-lived isotopes 2) isotopes of intermediates half-lives (100yrs)
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Describe nuclear reprocessing
A series of chemical reactions that separate Pu and U from other nuclear waste, which canteen be re-used as fuels (but doesn't eliminate long term components)
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What are the requirements for a permanent nuclear waste storage site?
must fulfil stable geolocial as well as social-political requirements and safe for over 100 000 yrs
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What is transmutation?
Exposure to neutron to produce shorter lived isotopes which can then be stored
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Describe liquid flouride thorium reactor
232Th bred to fissile 233U, hence only producing type 2 waste (also requires neutron flow so no risk of runaway accident)
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how are fission reactions contained safely?
In a large steel building with a hemispherical dome- capable of withstanding high pressures.
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How is the internal pressure of fission reactors controlled?
spray system, fan cooler system, ice condenser system
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Define nuclear fusion
The reaction in which two or more atomic nuclei collide at very high energy and join to form a new nucleus
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Why is nuclear fusion so much more difficult to achieve than nuclear fission?
fusion can only occur by the collision of positively charged nuclei. Therefore, to overcome electrostatic repulsion, extremely high energy particles are required (100 million K). Contrast to fission which absorbs neutral neutrons= much easier
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Give the chemical equation for the fusion reaction that requires the lowest ignition temperature
Li+D --> 2He + 22.4MeV
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Where does deuterium come from? What % of naturally occurring H does this make up?
Water. 0.02%
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Where does tritium come from?
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What are two ways of making D+T fusion more efficient?
1) myon catalysis 2) cold fusion
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What is myon catalysis? At how many cycles is it energy neutral?
The use of high energy myon particle (207x heavier than e-) pulls DT nuclei closer- each particle fuses 150DT at 1000K . Energy neutral at 250 cycles
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Describe how cold fusion was supposedly achieved
electrolysis of D20 on palladium electrodes
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Define water pollution
Any physical/chemical change in H20 that adversely affects the health of humans/other organisms
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Define a dead zone
a section of the ocean or a sea in which oxygen has been depleted to the point that most animals and bacteria cannot survive; often caused by runoff of chemical fertilizers or plant and animal wastes
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Define biogeochemical cycle
The complete path a chemical takes through the four components of the Earth's system: the hydrosphere, the biosphere, the geosphere and the atmosphere
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Define BOD
Biological oxygen demand- amount of oxygen required by bacteria to decompose biological waste into CO2, water and minerals
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Define virtual water
hidden flow of water if food or other commodities are traded from one place to another. For instance, it takes 1,600 cubic meters of water on average to produce one metric tonne of wheat.
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Other cards in this set

Card 2


Define nuclear reaction


when neutrons released from fission produce an additional fission in at least on other nucleus, which in turn produces neutrons etc.

Card 3


Give two ways that n's can be lost?


Preview of the front of card 3

Card 4


What kind of neutrons does U235 absorb? What does this lead to?


Preview of the front of card 4

Card 5


What kind of neutrons does U238 absorb? What does this lead to?


Preview of the front of card 5
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