Nuclear energy Flashcards
Binding energy of the nucleus
The work that must be done to separate a nucleus into its constituent neutrons and protons
Mass defect
The difference between the mass of the separated nucleons and the mass of the nucleus
Binding energy per nucleon
The average work done per nucleon to remove all the nucleons from a nucleus
Nuclear fission
The process in which a large unstable nuclear splits into two fragments which are more stable than the original nucleus
Binding energy per nucleon increases in this process
Induced fission is fission caused by a neutron colliding with a 235-U or a 235-Pu nucleus
Nuclear fusion
The process of making small nuclei fuse together to form a larger nucleus
The product nucleus has more binding energy per nucleon than the small nuclei that formed it
Lots of energy is released
Chain reaction
A series of reactions in which each reaction causes a further reaction
A steady chain reaction occurs when one fission neutron on average from each fission event produces a further fission event
Plasma
A state of matter achieved during fusion reactions and other high temperature areas
At such high temperatures atoms are stripped of their electrons
Control rods
Rods made of neutron-absorbing substance
Usually cadmium or boron
Moved in or out of the core of a nuclear reactor to control the rate of fission events in the reactor
Coolant
A fluid that is used to prevent a machine or device from becoming dangerously hot
The coolant of a nuclear reactor is pumped through the core of the reactor to transfer thermal energy from the core to a heat exchanger
Must be efficient at transferring heat from the reactor
Needs to be a gas or liquid at room temperature to be pumped around the reactor
Water - high specific heat capacity
CO2
Moderator
Substance in a thermal nuclear reactor that slows the fission neutrons down so they can go on to produce further fission
Thermal speeds mean that neutrons are the same temperature as the reactor and therefore the uranium
Slows them down via elastic collisions - takes approx 50 collisions to reach thermal speeds
Neutrons won’t disable the nucleus if they are too fast
Slow down to ~ 200ms^-1 (thermal neutrons)
Graphite, water
Heat exchanger
A steel vessel containing pipes through which hot coolant in a sealed circuit is pumped, causing water passing through the steel vessel in separate pipes to turn to steam which is used to drive turbines
Reactor core
The fuel rods and the control rods together with the moderator substance are in a steel vessel through which the coolant (which is also the moderator in ‘pressurised water reactor’) is pumped
Thermal nuclear reactor
A nuclear reactor which has a moderator in its core as opposed to the coolant being the moderator
Critical mass
The minimum mass of a fissile isotope in a nuclear reactor necessary to produce a chain reaction
If the mass of the fissile isotope is less than the critical mass then a chain reaction won’t occur because too many fission neutrons escape from the reactor or are absorbed without inducing fission
Safety features of a nuclear reactor
Reactor core is a thick steel vessel designed to withstand high temperatures and pressures
Core is in a building with very thick concrete walls which absorb the neutrons and radiation that example from the reactor
Every reactor has an emergency shut-down system designed to fully insert the fuel rods into the core to stop fission completely
Sealed fuel rods are inserted and removed from the reactor by means of remote handling devices
Rods are more radioactive after removal than before due to shifting from being alpha emitters to gamma and beta emitters
This is due to due to the increased number of neutron rich fission products formed
High level radioactive waste
Examples include spent fuel rods
Has high activity
Firstly, this waste is placed in cooling ponds
In Britain the rods are transferred to Sellafield in steel casks by rail
Unused uranium and plutonium is removed
Waste is vitrified mixed with molten pyrex glass and then solidifies, then placed in steel/lead/concrete cylinders - then stored deep underground
In other countries, it is stored in underground caverns - can be mixed with molten glass and stored as glass blocks in the caverns
Intermediate-level waste
Examples include materials with low activity and used containers of radioactive materials
Sealed in drums that are encased in concrete and stored in specially constructed buildings with walls of reinforced concrete
Low-level waste
Examples include laboratory equipment and protective clothing
Sealed in metal drums and buried in large trenches
Uses of fission
Generating energy
Making new isotopes (medical, military)
Making new elements for different types of fission reactor
Fuel rods
Composed of predominantly U-235
The build up of daughter products slows down the reaction - results in the need to replace the fuel rods
Heating the plasma during fusion
Inducing a current in the plasma
Electrical heating
Fire lasers at the plasma - EM radiation
Alpha particles leaving the nucleus and energy changes
When an alpha particle is emitted the nucleus recoils
So the energy released is shared between the alpha and the nucleus
Energy released is shared in inverse proportion to their masses - conservation of momentum
Beta decay and energy changes
Energy released is shared in variable proportions between the beta particle and neutrino and antineutrino
Electron capture energy changes
Nucleus emits a neutrino which carries away the energy
The atom also emits a photon (usually X-Ray) when the inner-shell vacancy is filled
Curve of stability
Graph of N against Z
Where N is number of neutrons
Z is number of protons
Starts curving upwards when N = 20
Alpha emitters to the top right of the curve
Beta minus emitters to the middle left of the curve
Beta plus emitters to the middle right of the curve
Graph of binding energy per nucleon against nucleon number
Curve which steeply rises until its peak - 8.7 Mev per nucleon for iron (56)
Then gradually decreases
Elements before iron undergo fusion and after iron undergo fission
Advantages of Nuclear power
Nuclear power doesn’t release much greenhouse gases
Nuclear power an extremely large amount of energy per kilogram of fuel
Power can be produced almost continuously
Nuclear power produces medical tracer isotope
Plants can sometimes change power output quickly
What type of energy changes does E =mc^2 apply to?
All energy changes
Calculating mass defect
mass of nucleons - mass of nucleus
e.g. for mass of nucleons, mass of neutron = 1.00728 u
so multiply this by the number of nucleons
Similarities between fission and fusion
Both have a mass defect
Both transform mass into energy
Products formed have higher binding energy
Both can involve the strong nuclear force
Differences between fission and fusion
Fusion isn’t yet available in nuclear power plants
Fusion requires high temperatures and pressures
Fission splitting whereas fusion joining
Fission forms radioactive products whereas fusion forms mostly stable products
What could happen to a neutron released as a result of a fission event?
Could be absorbed by U-235 and induce fission
Could be absorbed by U-235 and not induce fission
Could be absorbed by U-238
Could be scattered by the uranium nuclei
Could leave the reactor without being absorbed
Could be absorbed by the control rods
Problems and solutions for dealing with radioactive waste
Waste is initially very hot - placed in cooling ponds to remove heat
Waste is initially highly radioactive - it is handled using remote handling devices
In liquid form the waste may leak - the waste is vitrified
Difficult to transport waste - waste is transported in thick steel casks by rail
Waste will be radioactive for a long time - so it is stored deep underground to increase distance to humans and because it is geologically stable
