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aa Radiochemistry > Lecture 6 > Flashcards

Lecture 6 Flashcards

1
Q

What is problem of using 238U

A
  1. Absorbs neutrons - stops chain as absorbs neutron but doesn’t emit
  2. Much higher activation energy required to absorb neutron- and then binding energy produced isn’t enough to produce 239* excited state which would lead to fission
  3. 239Pu produced instead
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2
Q

What fission of U is exothermic

A
  1. Only 235U is exothermic - fissile isotope
  2. Has very small activation energy
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3
Q

Describe neutron absorption by 235U

A
  1. 235U + n –> 236U + binding energy
  2. –> 236U* - Excited state is accessible as lower activation energy than binding energy - exothermic
  3. 236U* leads to fission
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4
Q

Why does 238U neutron absorption require more energy

A
  1. Contains 92 protons
  2. Even number of protons + neutrons
  3. So more stable
  4. Adding neutrons would make odd number - high energy reaction
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5
Q

What U is more abundant

A
  1. 238U is more abundant but only absorbs neutron
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6
Q

Describe problem of difficulties of using 235U and 239Pu

A
  1. 235U and 239Pu are fissionable materials
  2. 235 is rare and must be separated from 238U
  3. 239Pu is made by exposing 238U to neutrons
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7
Q

What is solution to problem of 235U low abundance and high activation energy of 238U

A
  1. Use of highly enriched 235U or 239Pu
  2. Or use of moderators- nuclear reactor
  3. Can used purified 233U and 239Pu
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8
Q

What do moderators do

A
  1. Slow down neutrons to 0.025eV by collision with suitable atoms (most of 238U is unreactive)
  2. This is below speed needed to be absorbed by 238U so only absorbed by 235U
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9
Q

How did they make an atomic bomb

A
  1. 2 bits of 235U which is below critical mass of 50Kg and are far enough apart to not lead to fission
  2. When fired, bits to join to make super critical sphere which triggers fission reaction
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10
Q

What are the 5 components a nuclear reactor requires to produce power via the fission of uranium or plutonium by bombardment with neutrons

A
  1. Nuclear fuel consisting of fissile material
  2. A nuclear moderator- slows down neutrons so doesn’t matter if some 238U is present
  3. A reactor coolant - transfers heat generated
  4. Control rods
  5. A shield and containment system - dome of concrete
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11
Q

How does a reactor work

A
  1. Heat produced from reactor
  2. Heat utilised- heat a material e.g. water
  3. This generates pressure to drive a turbine and generate power
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12
Q

What does a nuclear fuel require

A
  1. Comprises a fissile isotope such as 235U which must be present in sufficient quantity to provide a self-sustaining chain reaction
  2. Generally needs to be enriched more than natural percent present in the ore of 0.7% 235U and 99.3% U238
  3. Uranium ores contain from 0.05% to 0.3% of the uranium oxide
  4. Normally enriched to have about 5% mass
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13
Q

What is a thermal neutron

A
  1. One which is slowed down to normal thermal temperature
  2. Ensures
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14
Q

Does 239Pu need to be enriched for use in nuclear reactors

A
  1. Doesn’t have to be enriched
  2. Once made just needs to be purified
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15
Q

What is a fast neutron

A
  1. High energy electrons emitted in fission
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16
Q

Describe how 235U acts as a nuclear fuel with thermal neutrons

A
  1. Much larger amounts lead to fission than captured
  2. Some are captured though- lead to 236U ground state - not react further
  3. Some scattering but not very prominent
17
Q

Describe how 235U acts as a nuclear fuel with fast neutrons

A
  1. Similar to thermal neutrons
  2. More scattering compared to capture and fission
  3. Still low amounts of capture
  4. Still fairly large amount of fission
18
Q

Describe how 238U acts as a nuclear fuel with thermal neutrons

A
  1. Large amount of scattering
  2. Absorb a small amount
  3. Virtually no fission
19
Q

Describe how 238U acts as a nuclear fuel with fast neutrons

A
  1. Large amount of scattering
  2. Significant amount of capture
  3. Some fission if neutron can carry enough energy to get to excited state 238U
20
Q

Describe how 239Pu acts as a nuclear fuel with thermal neutrons

A
  1. Capture some but majority leads to fission
21
Q

Describe how 239Pu acts as a nuclear fuel with fast neutrons

A
  1. Similar to thermal neutrons
  2. More scattering compared to capture and fission
  3. Still low amounts of capture
  4. Still fairly large amount of fission
22
Q

Give example of natural nuclear reactor

A
  1. In Gabon
  2. Nuclear reactor zones - deposits of U in Uranium ore layer- content of 235U was higher in past
  3. Water slips into system- good moderator so slows down neutrons - leads to nuclear fission reactor - self sustained
  4. When heated- water evaporates- slows reaction as no moderator- reaction slows down
  5. Then water goes in when cold
  6. Depleted U
  7. Can find decay products
23
Q

How is uranium enrichment carried out

A
  1. Requires a volatile U compound as gases easier to separate: UF6
  2. UF4 + F2–> UF6
  3. A unique compound of U that is in gas phase- only under reduced pressure
  4. Isotope separation by centrifuge or gas diffusion
  5. After enrichment: UF6 + 2H2O + H2 –> UO2 + 6HF
24
Q

What are problems with uranium enrichment using UF6 and solutions

A
  1. Difficult process because UF6 is very corrosive and is a powerful fluorinating agent
  2. Centrifuges made of stainless steel of Al alloy- coated in an impervious layer of metal fluorides
  3. Also UF6 is very easily hydrolysed to involatile UO2F2 so must be used under vacuum conditions
25
Q

Describe gas centrifuge separation used in Uranium enrichment

A
  1. Separate 235U and 238U by mass difference
  2. Centrifuge is spun at around 2000 Revs per second and the gas at the walls become enriched in the heavier 238UF6 and 235UF6 in the centre
  3. Heavier particles driven to edge
  4. Requires a cascade of many centrifuges
  5. Achieve an enrichment of 2-3%
  6. Thousands of cycles
26
Q

Describe how gas diffusion is used for uranium enrichment

A
  1. UF6 is pumped through thousands of filter barriers
  2. The slightly smaller 235UF6 passes through the pores marginally easier- slightly faster diffusion
  3. Use of many cycles
27
Q

How are fuel rods made

A
  1. 235U enriched UF6 is converted to UO2 powder which is pressed to form small fuel pellets before being heated to make a hard ceramic material
  2. The pellets are then inserted into thin tubes to form fuel rods
  3. These fuel rods are then grouped together to form fuel assemblies which are several metres long
  4. Made of zirconium alloy- selected because of strength and low neutron absorption
28
Q

What are control rods used for

A
  1. Control the fission rate of the nuclear fuel by adjusting the number of slow neutrons present to keep the rate of the chain reaction at a safe level
29
Q

What are control rods typically made of

A
  1. Boron, cadmium or hafnium
  2. Able to absorb neutrons
  3. Boron-10 absorbs neutrons by a reaction that produces Li-7 and alpha particles
  4. Need to be good at capturing neutrons
30
Q

What happened in Chernobyl

A
  1. UO2 fuel overheated as a result of raising the boron carbide control rods causing the fuel rods to rupture and come in contact with the water coolant
  2. Reaction became supercritical- melted fuel rods- couldn’t insert control rods
  3. The pressure generator blew off the concrete reactor lid and hot pieces of the fuel core landed on the reactor building causing fires
31
Q

What are the 3 control elements needed for a nuclear reactor

A
  1. Coolants
  2. Moderators
  3. Cool rods
  4. All used to ensure reaction stays as critical point
32
Q

What are most abundant radioisotopes from Chernobyl and problems

A
  1. 131I and 137 Cs
  2. Thyroid cancer due to 131I has significantly increased in adolescents and children

aa Radiochemistry (11 decks)

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