Nuclear Fission Flashcards
Peak maxima for fission products of 235U.
A = 95, A = 138
On average, how many neutrons do 6 fission reactions produce?
15
6 => chain reaction
5 => absorbed by 238U
4 => lost by leakage/control rods
What is the function of using a moderator to slow neutrons down to 0.025 eV?
Allows for selective activation of 235U (not 238U) to increase the likelihood of fission
Fuel rods
= small UO2 pellets placed inside rods
12 ft long
1 cm diameter
made of Zr alloy (Zr is strong, stable, has low neutron absorption)
arranged into an array (= “assembly”) - the core of a nuclear reaction usually contains 180 assemblies
Coolants
Absorb heat from the fission reaction and transport the heat from the core to the turbine
Also play in a minor role in slowing neutrons down to 0.025 eV (by absorbing their energy)
The same as the moderator in many plants
Fission of 235U releases 202 MeV of energy in the form of…
Hot fission products (170 MeV)
(Delayed) beta radiation from fission products (20 MeV)
Gamma radiation (7 MeV)
Fast (hot) neutrons (5 MeV)
How many neutrons are produced per fission reaction?
2.5
Moderators
Slow neutrons down by causing them to collide
The same as the coolant in many plants
** Higher levels of moderation mean lower fuel needs because more uranium nuclei undergo fission **
Properties of effective moderators
Of similar mass to neutron for efficient energy transfer - if the atom is too big, the neutron will just bounce off it as fast as it collided with it
Examples of moderators
2H (in D2O) (but also water itself) - deuterium has a smaller likelihood of absorbing the neutron than 1H because tritium is less stable than deuterium
12C (as pure graphite of CO2)
12C and 16O are very stable
Cheaper than D2O
Control rods
= “the brakes!”
Regulate the distribution of / control the power in the reactor during operation by absorbing all neutrons
Most important function = STOP nuclear fission process when required
Can be inserted into assembly in 2 secs
Examples of control rods
Boron carbide Silver Indium Cadmium Hafnium
Gas diffusion method of uranium enrichment
Utilises the different diffusion coefficients of 235- and 238UF6
UF6 is pumped through 1000s of filter barriers (‘pinholes’) - 235UF6 passes through 0.4 % faster due to its slightly smaller size
What are the 3 reactor types?
- Gas-cooled
- Pressurised water
- Boiling water
Gas-cooled reactors
Coolant = CO2
Moderator = graphite (allows use of natural U)
Hot CO2 causes water to boil into stream that drives the turbine
Pressurised-water reactors
Water = coolant and moderator
There is a second cycle of water away from the core of the reactor that is not involved in producing steam - this increases safety
Most common type of reactor
Boiling water reactors
Water = coolant and moderator
Water boils in the reactor and the stream drives the turbine
Bulk boiling of water in the reactor
Controls rods inserted from the bottom
What does THORP stand for?
Thermal Oxide Reprocessing Plant
Why are the products of nuclear fission radioactive?
They have a high n/p ratio
Critical mass of 235U
50kg of pure 235U
Critical mass of 239Pu
10kg of pure 239Pu
Disadvantages of/problems associated with nuclear energy
Environmental pollution
Decommissioning of old reactors
Reprocessing/storage of spent nuclear fuel
UF6 isotope separation equipment
Made of stainless steel/an Al alloy coated in an impervious layer of metal fluorides
(Because UF6 is corrosive and very reactive)
Safety systems in nuclear power plants
Contained within large steel buildings approx. 3ft thick to sustain the high internal pressures
The containment pressure is controlled by containment spray systems/fan cooler systems/ice condensers
Emergency feed water will continue to supply water to the steam generator to continue heat removal from the core after shutdown
