P2.6 Nuclear fission and nuclear fussion Flashcards
definition of nuclear fission
the splitting of an atomic nucleus
What 2 fissionable substances are commonly used in nuclear reactors?
uranium-235
plutonium-239
Describe how nuclear power stations generate electricity
NUCLEAR FISSION
a controlled reaction takes place in which atomic nuclei split up and release energy in the form of heat
Heat is used to heat water to make steam, which is used to drive a turbine which is connected to a generator.
Describe nuclear fission
for fission to occur, the uranium-235 or plutonium-239 nucleus must first ABSORB A NEUTRON
The addition of a neutron makes the nucleus unstable, causing it to split into 2 smaller nuclei and 2 or 3 neutrons are released
energy is also released
The neutrons may hit another nucleus- starting a CHAIN REACTION
Definition of nuclear fusion
The joining of 2 atomic nuclei to form a larger one
Nuclear fusion is the process by which energy is….
released in stars
What are some advantages of using nuclear FUSION to generate electricity?
fusion releases more energy than fission does for a given mass
fusion does not leave behind a lot of radioactive waste like fission
What are some disadvantages of using nuclear FUSION to generate electricity?
fusion can only happen at really high temperatures (10,000,000 degrees)- it takes more power to get up to the temperature than the reactor can produce.
you can not hold nuclei (eg hydrogen) at the high temperature and pressure required for fusion
When do stars form?
when enough dust and gas from space is pulled together by gravitational attraction
When do planets form?
when smaller masses of dust and gas from space are pulled together by gravitational attraction and are attracted by a larger mass
Why during the ‘main sequence’ period of a star’s life cycle is it stable?
because the forces within the star are balanced
heat created by nuclear fusion provides an outward pressure to balance the force of gravity pulling everything inwards
Describe the life cycle of a star which is about the same size as the Sun
1) PROTOSTAR -dust and gas is is pulled together by gravitational forces
2) MAIN SEQUENCE STAR- gravitational energy converted to heat energy, temp rises. hydrogen nuclei undergo nuclear fusion to form helium nuclei. Give out large amounts of heat and light. Is a long stable period, forces within it are balanced
3) RED GIANT- hydrogen begins to run out so nuclear fusion of helium instead, forming heavier element, up to iron. Star swells into red giant. Becomes red and surface cools
4) WHITE DWARF- becomes unstable, ejects outer layer of dust and gas. leaves behind a hot white core
5) BLACK DWARF- white dwarf cools to form black dwarf. - eventually disappears
Describe the life cycle of a star which is much bigger then the Sun
1) PROTOSTAR -dust and gas is is pulled together by gravitational forces
2) MAIN SEQUENCE STAR- gravitational energy converted to heat energy, temp rises. hydrogen nuclei undergo nuclear fusion to form helium nuclei. Give out large amounts of heat and light. Is a long stable period, forces within it are balanced
3) RED SUPER GIANT- hydrogen begins to run out so nuclear fusion of helium instead, forming heavier element up to iron. Star swells into red super giant. It becomes red and surface cools
4) SUPERNOVA- starts to glow brightly again as undergo more fusion and core shrinks. Explode, forming elements heavier than iron and ejecting them into the universe to form new planets and stars.
5) NEUTRON STAR or BLACKHOLE- exploding supernova throws outer layers of dust and gas into space leaving a dense core (neutron star). If mass is large enough, the core collapses to form a black hole.
What does the fusion process in stars produce?
all the naturally occurring elements
large amounts of energy
What eventually happens to the elements produced by fusion in stars?
they may be distributed throughout the universe by the explosion of a supernova at the end of its life
What elements are formed in stars during the stable period?
elements up to iron
When are elements heavier than iron formed in stars?
in a supernova
Explain how nuclear fission could lead to a chain reaction
released neutrons are absorbed by other nuclei
so more neutrons are released when the nuclei splits
In nuclear fission, how does the mass number of an atom change when the nucleus absorbs a neutron?
it will increase by 1
In a nuclear reactor at a power station, what do the control rods do?
they absorb neutrons which slows down the chain reaction
this means less energy is released during fission
Explain the difference between a protostar and a main sequence star
protostar is at a lower temperature
protostar does not emit radiation/energy
This is because nuclear fusion reactions have not started
Explain how the different elements now contained in the universe were formed
by nuclear fusion
of hydrogen to helium
elements heavier than iron are formed in a supernova
Why are stars stable during the main sequence period of the life cycle?
as forces within the star are balanced
Explain why scientists can not give the exact number of years a star will be in the main sequence period
insufficient evidence
do not know exact amount of hydrogen in the star
time too long to measure
The rate of fusion in a large star is …… than in a small star
faster
Explain why the rate of nuclear fusion in a large star is faster than in a small star
as larger stars have a shorter main sequence period so they must have a faster rate of nuclear fusion
end of main sequence happens as the hydrogen in the star is used up
(they use up hydrogen at a faster rate)
Describe what happens to a star much bigger than the sun once the star reaches the end of the main sequence period of its life cycle
- star starts to run out of hydrogen , star expands and cools, helium starts to fuse to form elements up to iron
- Red super giant is formed, outer layers explode
- forms supernova, elements heavier than iron are formed, core shrinks
- becomes a neutron star, if mass is large enough the core collapses to form a black hole
Why does lowering control rods reduce the amount of energy released per second from a nuclear fuel in a nuclear reactor?
neutrons are absorbed by the control rods
so their are fewer neutrons
which causes the chain reaction to slow down
Why do some stars not turn into black dwarfs?
as only stars about the same size as our sun become black dwarfs
What will happen to a star at the end of the red super giant stage?
explode
become a supernova
suggest why scientists can only estimate and only calculate the mass of some stars
do not have the technology
suggest why in the future it will be possible to calculate the mass of most stars
advances in technology/knowledge
Describe what happens to a star after the main sequence period, for the star to eventually become a black dwarf
star expands to become a red giant
heavier elements are formed by fusion
stars shrink to become a white dwarf
star cools and stops emitting radiation to form a black dwarf
