Lecture 6 Flashcards
What is problem of using 238U
- Absorbs neutrons - stops chain as absorbs neutron but doesn’t emit
- 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
- 239Pu produced instead
What fission of U is exothermic
- Only 235U is exothermic - fissile isotope
- Has very small activation energy
Describe neutron absorption by 235U
- 235U + n –> 236U + binding energy
- –> 236U* - Excited state is accessible as lower activation energy than binding energy - exothermic
- 236U* leads to fission
Why does 238U neutron absorption require more energy
- Contains 92 protons
- Even number of protons + neutrons
- So more stable
- Adding neutrons would make odd number - high energy reaction
What U is more abundant
- 238U is more abundant but only absorbs neutron
Describe problem of difficulties of using 235U and 239Pu
- 235U and 239Pu are fissionable materials
- 235 is rare and must be separated from 238U
- 239Pu is made by exposing 238U to neutrons
What is solution to problem of 235U low abundance and high activation energy of 238U
- Use of highly enriched 235U or 239Pu
- Or use of moderators- nuclear reactor
- Can used purified 233U and 239Pu
What do moderators do
- Slow down neutrons to 0.025eV by collision with suitable atoms (most of 238U is unreactive)
- This is below speed needed to be absorbed by 238U so only absorbed by 235U
How did they make an atomic bomb
- 2 bits of 235U which is below critical mass of 50Kg and are far enough apart to not lead to fission
- When fired, bits to join to make super critical sphere which triggers fission reaction
What are the 5 components a nuclear reactor requires to produce power via the fission of uranium or plutonium by bombardment with neutrons
- Nuclear fuel consisting of fissile material
- A nuclear moderator- slows down neutrons so doesn’t matter if some 238U is present
- A reactor coolant - transfers heat generated
- Control rods
- A shield and containment system - dome of concrete
How does a reactor work
- Heat produced from reactor
- Heat utilised- heat a material e.g. water
- This generates pressure to drive a turbine and generate power
What does a nuclear fuel require
- Comprises a fissile isotope such as 235U which must be present in sufficient quantity to provide a self-sustaining chain reaction
- Generally needs to be enriched more than natural percent present in the ore of 0.7% 235U and 99.3% U238
- Uranium ores contain from 0.05% to 0.3% of the uranium oxide
- Normally enriched to have about 5% mass
What is a thermal neutron
- One which is slowed down to normal thermal temperature
- Ensures
Does 239Pu need to be enriched for use in nuclear reactors
- Doesn’t have to be enriched
- Once made just needs to be purified
What is a fast neutron
- High energy electrons emitted in fission
Describe how 235U acts as a nuclear fuel with thermal neutrons
- Much larger amounts lead to fission than captured
- Some are captured though- lead to 236U ground state - not react further
- Some scattering but not very prominent
Describe how 235U acts as a nuclear fuel with fast neutrons
- Similar to thermal neutrons
- More scattering compared to capture and fission
- Still low amounts of capture
- Still fairly large amount of fission
Describe how 238U acts as a nuclear fuel with thermal neutrons
- Large amount of scattering
- Absorb a small amount
- Virtually no fission
Describe how 238U acts as a nuclear fuel with fast neutrons
- Large amount of scattering
- Significant amount of capture
- Some fission if neutron can carry enough energy to get to excited state 238U
Describe how 239Pu acts as a nuclear fuel with thermal neutrons
- Capture some but majority leads to fission
Describe how 239Pu acts as a nuclear fuel with fast neutrons
- Similar to thermal neutrons
- More scattering compared to capture and fission
- Still low amounts of capture
- Still fairly large amount of fission
Give example of natural nuclear reactor
- In Gabon
- Nuclear reactor zones - deposits of U in Uranium ore layer- content of 235U was higher in past
- Water slips into system- good moderator so slows down neutrons - leads to nuclear fission reactor - self sustained
- When heated- water evaporates- slows reaction as no moderator- reaction slows down
- Then water goes in when cold
- Depleted U
- Can find decay products
How is uranium enrichment carried out
- Requires a volatile U compound as gases easier to separate: UF6
- UF4 + F2–> UF6
- A unique compound of U that is in gas phase- only under reduced pressure
- Isotope separation by centrifuge or gas diffusion
- After enrichment: UF6 + 2H2O + H2 –> UO2 + 6HF
What are problems with uranium enrichment using UF6 and solutions
- Difficult process because UF6 is very corrosive and is a powerful fluorinating agent
- Centrifuges made of stainless steel of Al alloy- coated in an impervious layer of metal fluorides
- Also UF6 is very easily hydrolysed to involatile UO2F2 so must be used under vacuum conditions
Describe gas centrifuge separation used in Uranium enrichment
- Separate 235U and 238U by mass difference
- 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
- Heavier particles driven to edge
- Requires a cascade of many centrifuges
- Achieve an enrichment of 2-3%
- Thousands of cycles
Describe how gas diffusion is used for uranium enrichment
- UF6 is pumped through thousands of filter barriers
- The slightly smaller 235UF6 passes through the pores marginally easier- slightly faster diffusion
- Use of many cycles
How are fuel rods made
- 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
- The pellets are then inserted into thin tubes to form fuel rods
- These fuel rods are then grouped together to form fuel assemblies which are several metres long
- Made of zirconium alloy- selected because of strength and low neutron absorption
What are control rods used for
- 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
What are control rods typically made of
- Boron, cadmium or hafnium
- Able to absorb neutrons
- Boron-10 absorbs neutrons by a reaction that produces Li-7 and alpha particles
- Need to be good at capturing neutrons
What happened in Chernobyl
- 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
- Reaction became supercritical- melted fuel rods- couldn’t insert control rods
- The pressure generator blew off the concrete reactor lid and hot pieces of the fuel core landed on the reactor building causing fires
What are the 3 control elements needed for a nuclear reactor
- Coolants
- Moderators
- Cool rods
- All used to ensure reaction stays as critical point
What are most abundant radioisotopes from Chernobyl and problems
- 131I and 137 Cs
- Thyroid cancer due to 131I has significantly increased in adolescents and children
