P2 topic 5 Flashcards
fission reactions
take place in a nuclear reactor to generate electricity on a large scale
what happens in a fission reaction?
a slow-moving neutron is absorbed by a uranium-235 nucleus, making it unstable; the new nucleus splits into two smaller daughter nuclei and 2 or more fast moving neutrons; energy is released as the kinetic energy of the daughter nuclei and the neutrons
how can fission reactions be triggered?
by bombarding uranium-235 nuclei with neutrons
chain reaction
a process in which an enormous amount if energy is produced when neutrons from previous fission reactions go on to produce further uncontrolled fission reactions
how do nuclear power stations create energy?
most nuclear power stations use uranium or plutonium as fuel for the reactions; a large amount of energy, in the form of KE of the neutrons and the daughter nuclei is released in fission reactions; KE turned into heat and used to boil water to mane steam; steam powers turbines
why is disposal of nuclear waste a major concern?
for daughter nuclei produced in fission reactions can remain active for thousands of years
what are the four main components of a reactor?
fuel rods, coolant, moderator and control rods
fuel rods
contain pellets of nuclear fuel in the form of uranium dioxide
coolant
removed the thermal energy produced in the fission reactions in the reactor core, so it can be used to heat water to create steam to power generator turbines (in a water-cooled reactor)
moderator
surrounds the nuclear fuel rods and slows down the fast-moving neutrons-slow-moving neutrons have a greater chance of reacting with uranium nuclei than fast-moving neutrons
control rods
can be lowered into the reactor to absorb the neutrons and so slow down the fission reactions and control the chain reactions
fusion reactions
cause the energy generated by the Sun and the stars
critical mass
a chain reaction can only be sustained by a large amount of uranium-235- in small amounts, too many neutrons will escape and not take part in fission reactions; the minimum mass of a fissile material required to sustain a chain reaction
what is the energy released in fission reactions used by?
nuclear reactors to produce energy
nuclear fusion
a nuclear reaction in which two smaller, lighter nuclei join or fuse together to produce one larger nucleus, creeping vast amounts of energy- the fusing together of hydrogen nuclei to produce helium nuclei
what is needed for nuclear fusion to take place?
extremely high temperatures
what is the energy source that keeps our Sun and other stars burning?
the fusion of hydrogen and other lighter nuclei
isotopes of hydrogen
deuterium and tritium
who were the scientists who claimed to have carried out cold fusion?
Stanley Pons and Martin Fleischmann in 1989
cold fusion
an invalidated theory that proposed nuclear fusion occurring at room temperature- the scientists claimed that their experiment had produce vast amounts of thermal energy
why were Pons and Fleischmann criticised by other scientists?
their announcement gained worldwide publicity- but they had not published enough technical details of their experiment for other scientists to reproduce their results
what do the majority of scientists think about cold fusion?
they reject the theory, as it couldn’t be validated by reproducing the experiment
why are fusion reactions more difficult to trigger than fission reactions?
hydrogen nuclei (protons) are positively charged and therefore repel one another
how can you improve the changes of a fusion reaction taking place?
You can increase the speed at which the nuclei move
what happens at temperatures around 10 million degrees Celsius?
hydrogen nuclei move rapidly enough to overcome the electrostatic repulsive forces and join together in fusion reactions
when can nuclear fusion not take place?
at low temperatures and pressures
what do you need to do in order to create fusion?
hydrogen nuclei must be heated to temperatures of about 100 million degrees Celsius and contained by very strong magnetic fields produced by super cooled electromagnets- it’s Beth difficult to create these conditions on earth
nucleons
a term used to refer to either protons or neutrons
ion
a charged atom that had lost or gained electrons
how can positive ions be created?
rubbing insulators together (the friction removes electrons grin the atoms of one insulator) or by heating a gas
how does heating a gas create positive ions?
thermal energy ionises the gas atoms; electrons of the atoms gain energy and fly off
isotopes
nuclei of atoms with the same number of protons but a different number of neutrons
why do isotopes of an element have the same chemical properties?
they all have the same number of electrons
radioactive decay
some isotopes are unstable- over time, the nucleus breaks up and emits a particle or wave in an attempt to become more stable
alpha particles
each alpha particle is identical to a helium nucleus, with two protons and two neutrons
beta particles
each beta particle is an electron emitted from inside the nucleus
gamma rays
EM waves of very short wavelength
ionisation
a process in which radiation transfers some or all of its energy to liberate an electron from an atom
what does ionisation leave behind?
positive ions
what is the charge of alpha particles?
2+
what’s the charge of gamma rays?
0
what is the typical speed of alpha particles?
10 million m/s
what is the typical speed of beta particles?
100 million m/s
what is the typical speed of gamma rays?
3 x 10 to the 8 m/s
what is the mass of an alpha particle?
4
what is the mass of a beta particle?
0.00055
what is the mass of a gamma ray?
0
how good is the ionising effect of alpha?
strong
how good is beta at ionising?
weak
how good is gamma at ionising?
very weak
what does it take to stop alpha radiation?
paper, skin or about 6cm of air
what does it take to stop beta radiation?
few mm of aluminium
what does it take to stop gamma radiation?
never completely stopped, but reduced significantly by thick lead or concrete
what is the nucleus known as before it decays?
the parent
what is the nucleus left behind after decay called?
the daughter
why can’t radioactive decay be predicted or affected by external conditions?
it’s random and spontaneous
half life
the average time it takes for half of the undecayed nuclei in a sample to decay- it can be micro-seconds or thousands of years
activity
the rate of decay of a source’s nuclei
what is activity measured in?
becquerel (Bq)- 100 Bq means that 100 nuclei decay per second and that 100 alpha or beta particles are emitted per second
what is the relationship between activity and the number of undecayed nuclei in a source?
it is directly proportional
what is activity inversely proportional to?
half-life of an isotope
why does the activity of a source decrease over time?
as radioactive nuclei decay, there are fewer undecayed nuclei
what will a source with a short half life have?
a large activity
how can radioactive decay be useful?
the kinetic energy of the alpha or beta particles emitted from the nuclei can be used to generate electricity on a small scale
why is the chain reaction of the uranium nuclei in a nuclear power station controlled?
to maintain a steady output of power
what does a neutral atom have?
the same number of electrons and protons
nucleon number
mass number
proton number
atomic number
what is ionisation the process of?
removing electrons from atoms, leaving behind positive ions
what do the numbers decrease by in alpha decay? e
proton number decreases by 2 and the mass number decreases by 4
is the daughter nuclei of the same element after radioactive decay?
no
