3.8 Nuclear Physics Flashcards
Define Rutherford’s Experiment
Fired alpha particles at a very thin gold sheet.
They mostly passed through but some were deflected at large angles.
This disproved the ‘plum pudding’ model.
The deflections were caused by the electrostatic repulsion between the positive nucleus and positive alpha particles.
Most passed through proving atoms are mostly free space.
What are alpha particles?
Two protons and two neutrons (the equivalent of a helium nucleus).
Weakly penetrating.
Highly ionising.
Easily absorbed.
Positively charged.
What are the applications of alpha radiation?
used in fire/smoke alarms:
alpha particles cannot penetrate through smoke, this is detected and sets off the alarm.
What are beta particles?
electrons or positrons
moderately penetrative (cannot penetrate through about 5mm of aluminium/ 30cm of air)
fairly ionising
either positively or negatively charged
What are the applications of beta particles?
used for gauging the thickness of paper in a paper mill
a beta plus emitter is used in medical PET scanning
When are beta particles emitted?
beta-minus = by neutron-rich nuclei
beta-plus = by proton-rich nuclei
What are some radioactivity protective measures?
wear protective goggles and gloves
point sources away
keep source at arm’s length
reduce exposure time
keep source in a lead-lined box
What is gamma radiation?
part of the EM spectrum
highly penetrative
not very ionising
What are some application of gamma radiation?
used in medical imaging, treating cancer by destroying tumorous cells, tracing, sterilising medical equipment
used to irradiate food to stop it from going bad
What is background radiation?
radiation which is always present, even when there is no radioactive source
What are some sources of background radiation?
radon gas
cosmic rays
medical procedures
food and drink
What is radioactive decay?
when an object is radioactive it releases radioactive particles, but as time passes, the rate of particles decreases
it is random
What is half-life?
the time taken:
for the number of radioactive nuclei in a sample, or for the activity (the number of decays per second) to halve
What are the uses of half-life?
used in the dating of a substance.
-by knowing the original count rate, the current count rate and the half-life of the material, we can calculate the time passed:
What are atomic excited states?
in an atom, an electron can gain energy and be promoted to a higher energy level
when the electron returns to the original level, the energy is released as EM waves
What are nuclear excited states?
when the protons and neutrons rearrange to a lower energy state, the excess energy is released as gamma radiation
What is binding energy?
A nucleus is made up of protons and neutrons
The mass of a nucleus is less than the mass of the individual protons and neutrons
This is because when the protons and neutrons combine, they release energy, which is the biding energy
What is fission?
Nuclear fission is a reaction in which the nucleus is split:
Energy is released when heavy nucleus split apart
The new nuclei (daughter nuclei) have a larger binding energy per nucleon than the parent
This means that a large amount of energy is released
What is a chain reaction?
Fission occurs when an unstable atoms splits, by absorbing a neutron
The reaction produces daughter nuclei, and releases energy and some neutrons
These neutrons will go on to cause more fission reactions, producing more neutrons
What is fusion?
Nuclear fusion is a reaction in which two nuclei are combines to form a larger nucleus:
When an atom is formed, it releases binding energy
Where in a nuclear reactor do all fission reactions take place?
Reactor core
What ‘fuels’ the reactor?
Uranium made into pellets and contained in the fuel rods
What do the control rods do?
Absorb free neutrons - the deeper they are in the core, the slower the fission reactions happen.
Why do neutrons need to be slower in a nuclear reactor?
Safer
Increases the chance of collision, so increases the chance of fission reactions occurring
What surround the fuel rods and why?
2 moderators, Granite, High pressure water.
Neutrons collide with the moderator atoms and slow down.
The water acts as a coolant.
What are thermal neutrons?
Neutrons once they have been slowed to speeds similar to particles at room temperature.
What type of uranium do fuel rods contain?
‘Enriched uranium’
What is ‘enriched uranium’?
97% U-238 [non-fissionable]
3% U-235 [fissionable]
What is natural uranium?
99% U-238 [non-fissionable]
1% U-235 [fissionable]
What is critical mass?
Not all neutrons hit each other, some escape.
Needs to be a minimum amount of U-235 present to ensure a chain reaction happens = critical mass.
What are safety considerations for a nuclear reactor?
Reactor core has thick steel walls
Core surrounded by thick concrete walls
Emergency shut down
All materials are handled remotely.
What do the thick steel walls in the reactor core do?
Withstands pressure (from the heat).
Also blocks beta and gamma radiation from leaving the core.
What does the thick concrete wall surrounding the reactor do?
Absorbs any neutrons and gamma radiation that escapes.
What does the general emergency shut down for a nuclear reactor do?
Lowering the control rods completely will stop the chain reaction.
What are some examples of low level waste?
Laboratory equipment
protective clothing
What is the handling and storage for low level waste?
Sealed in metal drums and buried in large trenches (geology provides a barrier against radioactivity escaping)
What are some examples of intermediate level waste?
Radioactive material with low activity
Reactor components
Used filters
What is the handling and storage for intermediate level waste?
Sealed in drums, enclosed in concrete and stored in specially constructed buildings with reinforced concrete walls.
What are some examples of high level waste?
Spent fuel rods containing fission fragments
What is the handling and storage for high level waste?
‘Ventrification’ - waste mixed with glass to create molten product. then poured into steel containers, then placed in an air cooled store (until there is available disposal route) - allows it to cool and decay.
Stored in underwater cooling ponds.
