nuclear Flashcards
advantages of fusion compared to fission
- fuel is readily available (hydrogen)
- releases more energy per nucleon (only need a small mass of fuel)
- no radioactive products from fusion
disadvantages of fusion compared to fission
high temps and pressures are needed - currently, it requires more energy input to provide this than we get out
what is fission
the splitting of a nucleus into two (approx equal) fragments. Induced fission caused by an incoming neutron colliding with a nucleus
what are fission neutrons
neutrons released when a nucleus undergoes fission and which may collide with nuclei to cause further fission
what is fusion
the fusing together of light nuclei to form a heavier nucleus
what fuel is normally used in nuclear reactors and why
Enriched Uranium - U - 235 because induced fission is more probable with it
when a U-235 nucleus absorbs a thermal neutron, this induces fission to occur - what is produced
the nucleus splits into two smaller nuclei (fragments) and several (2 or 3) fast moving electrons
the fission fragments are neutron rich
how can a chain reaction occur
the neutrons released in 1 fission can go on and cause fission of another nucleus
if more than 1 neutron from each fission causes another fission then the rate of fission will increase
what are thermal neutrons and why are they needed to induce fission
- thermal neutrons - slow moving neutrons (in thermal equib in the moderator)
- thermal neutrons are much more likely to be absorbed by the nucleus than a fast moving electron
what is the moderator used for in a nuclear reactor
fast moving neutrons produced in fission collide with the moderator molecules and lose EK in the collision so they are moving slower after
example of a suitable material for a moderator
water or graphite
what is meant by the critical mass of uranium
- the critical mass is the smallest mass of fissile isotope (uranium) required for a nuclear chain reaction to be maintained
- when the mass is less than the critical mass, the rate of fission will decrease because less than 1 neutron from each fission will cause another fission
- number of neutrons escaping is proportional to the surface area
if the rate of fission is too high, what should we do with the control rods and what do the control rods do
-push them further into the reactor
-control rods absorb some of the neutrons that have not escaped, so fewer neutrons are available to induce fission
why are cadmium and boron good materials to use for control rods
-they have a high cross-section for neutrons - easily absorb neutrons
-they retain their shape at high temps
why is water or CO2 chosen as a coolant in a nuclear reactor
- it has a high specific heat capacity, so it can gain a lot of energy without the temp getting too high and hence the pressure form getting too high
-the energy is then used to heat water to turn turbines in the generator
why do we use multiple fuel rods
- neutrons need to pass through a moderator to slow them down and the neutrons leaving the fuel rod are unlikely to re-enter the same fuel rod
- makes it easier to replace the fuel in stages
what are the safety features used in a nuclear reactor
- remote handling of fuel (fuel rods inserted and removed from the reactor)
- shielding (steel reactor vessel to absorb neutrons, some beta and some gamma, very thick concrete walls to absorb gamma)
- emergency shut down (inserts control rods fully into the reactor to stop fission completely)
sources of radioactive waste and their levels from a nuclear reactor
High level - fission fragments (spent fuel rods)
Intermediate level - radioactive materials with low activity are sealed in drums that are enclosed in concrete and stored in specially constructed buildings with walls of reinforced concrete
Low level - lab equipment and protective clothing is sealed in metal drums and buried in large trenches
Advantages of nuclear power compared to fossil fuels
-less mass of fuel used because more energy produced per kg
-less harm to the environment because nuclear power does not generate greenhouse gases
Disadvantages of nuclear power compared to fossil fuels
-hazardous waste because fission products are radioactive
-long term responsibility because waste needs to be stored for many years
benefits of slowing neutrons
- slow neutrons are less damaging
-slow neutrons increase the chance of causing fission
-slow neutrons transfer heat energy to the moderator
what is meant by the decay constant
the probability of an individual nucleus decaying per second and nuclear decay constant is the constant of proportionality
why is it difficult to obtain relatable age using carbon dating
- the object may have been made with the wood some time after the tree was cut down
- the background activity is high compared to the observed count rates
- possible contamination
- the count rates are low or sample size / mass is small or there is statistical variation in the recorded results
-uncertainty in the ratio of carbon-14 thousands of years ago
two advantages of using high energy electrons rather than alpha particles to estimate nuclear radii
- free electrons are easier to accelerate
- electrons can get closer to the nuclei as there is not electrostatic repulsion
- electrons are easier to produce
what was observed in the rutherford scattering experiment
- most of the alpha particles passed straight through the foil with little or no deflection ( 1 in 2000 were deflected)
- a small percentage of alpha particles ( 1 in 10000) were deflected through angles more than 90 degrees
what were the conclusions of the rutherford scattering experiment
- most of the atom’s mass is concentrated in a small region, the nucleus, at the centre of the atom
- the nucleus is positively charged because it repels alpha particles (which carry positive charge) at the centre of the atom
what is alpha radiation
occurs when the nucleus of an atom becomes unstable and alpha particles are emitted to restore balance
what is beta radiaiton
when the ratio of neutrons to protons in the neutrons is too high
what is gamma radiation
photons that are often emitted along with alpha particles and beta particles
what is the most ionising radiation
alpha
what is the most penetrating radiation
gamma
where does beta decay come from
from naturally occurring radioactive substances consists of fast moving
what is the most dangerous radiation inside the body
alpha because it is highly ionising and easily absorbed by cells
which type of radiation would leave the most easily visible tracks in a cloud chamber and why
- alpha radiation
- alpha particles produce straight tracks that radiate from the source and are easily visible
- the tracks from the isotope are all the same length, indicating that the alpha particles have the same range
what is the most dangerous radiation outside the body
gamma because gamma rays are very penetrating and can pass through a person and damage cells
what stops alpha radiation
a piece of paper
what stops beta radiation
thin sheet of aluminium foil
what stops gamma radiation
piece of lead
how should radioactive sources be stored
- materials should be in lead-lined containers
- containers should be kept under ‘lock and key’ and a record of the sources is kept
how should radioactive sources by handled
- no source should be allowed to come into contact with the skin
- solid sources should be transferred using handling tools, such as tongs or a glove-box or using robots
- radioactive sources should not be used for longer than is necessary
- liquid and gasses should be stored in sealed containers so you don’t breathe it in
list some sources of background radiation
- air
- medical
- ground and buildings
- food and drink
- cosmic rays
- nuclear weapons
- nuclear weapons
- air travel
- nuclear power
- radon gas in the atmosphere
- rocks
what does the alpha particle do as it approaches the nucleus
loses ke and gains epe
which gives a more accurate value for the diameter of a nucleus
high energy diffraction
what must the de broglie wavelength of the electrons must be for electron diffraction to work and what pd should the electrons be accelerated through to achieve this wave length
- order of 10^-15 because it is around the same as the diameter of the nucleus
- order of a hundred million volts
definition of radioactivity
the emission of energy from an unstable nucleus
can radiation cause something to be radioactive
no because the radiation only affects the electrons and not the nucleus
what does it mean that radioactive decay is exponential decay
mass decreases exponentially - the mass drops by a constant factor in equal intervals of time
what is half life
- the half life of a radioactive decay isotope is the time taken for the number of parent nuclei to decrease to half the initial number of parent nuclei
define activity
- the activity of a radioactive isotope is the number of nuclei of the isotope that disintegrates per second
- the rate of change of the number nuclei of the isotope
what is the unit of activity
Bq (becquerel)
how can you work out the power of of the source
the energy transferred per second from a radioactive source (power) = AE
define the decay constant
- the probability of an individual nucleus decaying per second
how are decay constant and half life linked
the longer the half life, the smaller the decay constant because the probability of decay per second is smaller
what is a radioactive tracer
used to follow the path of a substance through a system
what characteristics should a tracer have
- have a half life which is stable enough for the necessary measurements to be made short enough to decay quickly after use
- emit beta or gamma radiation so it can be detected outside the flow path
why is technetium-99m widely used as a tracer in medicine
the gamma radiation it emits allows the medical practitioner to image internal organs causing hardly any radiation damage to the patient
describe some uses of radioactivity
- detecting underground pipe leaks
- modelling oil reservoirs mathematically to improve oil recovery
- investigating the uptake of fertilisers by plants
- monitoring the uptake of iodine
explain gamma emission in terms of nuclear energy levels
- after an unstable nucleus emits an alpha or beta particle, or undergoes electron capture it might emit a gamma photon
- gamma photon does not change the number of protons or neutrons in the nucleus but it does allow the nucleus to lose energy
- this happens if the ‘daughter’ nucleus is formed in an excited state after it emits an alpha or beta particle - this excited state is short lived and the nucleus moves to its ground state
what does metastable mean and give an example
- when the atom stays in the excited state long enough to be separated from the parent isotope
- example: technetium-99m is metastable
why is metastable (technetium-99m) useful in medicine
it traps and holds the excitation state
examples of radioactivity in medicine
- tracers
- gamma treating for cancer
- gamma sterilisation
alpha decay - energy
energy is shared between the alpha particle and the nucleus
the excess energy is lost by emitting a gamma ray
beta decay - energy
energy released is shared in variable proportions between the beta particle and the neutrino
the excess energy is lost by emitting a gamma ray
electron capture - energy
the nucleus emits a neutrino which carries away the energy released in the decay
- causes a proton to change into a neutron and this makes the nucleus unstable and it emits gamma radiation
in radioactive decay or reactions, the products are more stable than the parent nucleus/reactants, this means that energy has been released in the decay, what else does this mean?
this loss of energy results in a loss of mass
compare the range of the strong nuclear force and the electrostatic force and what is the consequence of this on the stability of large nuclei
- snf = 3 to 4 x 10^-15 m
- elcf = infinity
- the snf between 2 nucleons must become repulsive at separations of about 0.5 fm or less, otherwise nucleons would pull each other closer and closer together and the nucleus would be much smaller than it is
define binding energy
the binding energy of the nucleus is the work that must be done to speparate a nucleus into its constituent neutrons and protons
define mass defect
the difference between the mass of the separated nucleons and the mass of the nucleus
if a nucleus has a greater binding energy per nucleon, is it more or less stable
more stable
which has a higher change in binding energy per nucleon - fusion or fission
fusion is 10x bigger
describe the process of induced fission
- bombarding different elements with neutrons produces radioactive isotopes
- lighter elements were present after bombardment
- nuclei were split into two approx equal fragment nuclei as a result of neutron bombardment
- fission neutrons ( neutrons released in a fission event), are capable of causing a further fission event as a result of a collision with another uranium nucleus - causes a chain reaction
- energy is released because the fragments repel each other to overcome the snf therefore, the nuclei and fission neutron gain kinetic energy
give examples of of possible fusion reactions in the sun
nuclear fusion of hydrogen to helium
why should the foil be thin in alpha scattering experiment
less interaction with atoms in the foil and so the foil doesnt absorb the alpha particles
what is the force responsible for the rutherford scattering
electrostatic force
why do alpha particles need to have the same kinetic energy and why should the detector be at constant distance from the metal foil
- need the same angle of deflection
- same angle and all the particles hit the detector
advantages of nuclear power
- less fossil fuel used so cleaner air
- small amount of fuel used to get the same/ large amounts of power
- can be produced continuously (does not depend on wind or sun like renewables)
- some nuclear power stations can adjust their output quickly
- benefit of producing medical isotopes
