P2.6 Nuclear fission and nuclear fussion

Question 1

definition of nuclear fission

Answer 1

the splitting of an atomic nucleus

Question 2

What 2 fissionable substances are commonly used in nuclear reactors?

Answer 2

uranium-235

plutonium-239

Question 3

Describe how nuclear power stations generate electricity

Answer 3

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.

Question 4

Describe nuclear fission

Answer 4

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

Question 5

Definition of nuclear fusion

Answer 5

The joining of 2 atomic nuclei to form a larger one

Question 6

Nuclear fusion is the process by which energy is….

Answer 6

released in stars

Question 7

What are some advantages of using nuclear FUSION to generate electricity?

Answer 7

fusion releases more energy than fission does for a given mass

fusion does not leave behind a lot of radioactive waste like fission

Question 8

What are some disadvantages of using nuclear FUSION to generate electricity?

Answer 8

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

Question 9

When do stars form?

Answer 9

when enough dust and gas from space is pulled together by gravitational attraction

Question 10

When do planets form?

Answer 10

when smaller masses of dust and gas from space are pulled together by gravitational attraction and are attracted by a larger mass

Question 11

Why during the ‘main sequence’ period of a star’s life cycle is it stable?

Answer 11

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

Question 12

Describe the life cycle of a star which is about the same size as the Sun

Answer 12

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

Question 13

Describe the life cycle of a star which is much bigger then the Sun

Answer 13

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.

Question 14

What does the fusion process in stars produce?

Answer 14

all the naturally occurring elements

large amounts of energy

Question 15

What eventually happens to the elements produced by fusion in stars?

Answer 15

they may be distributed throughout the universe by the explosion of a supernova at the end of its life

Question 16

What elements are formed in stars during the stable period?

Answer 16

elements up to iron

Question 17

When are elements heavier than iron formed in stars?

Answer 17

in a supernova

Question 18

Explain how nuclear fission could lead to a chain reaction

Answer 18

released neutrons are absorbed by other nuclei

so more neutrons are released when the nuclei splits

Question 19

In nuclear fission, how does the mass number of an atom change when the nucleus absorbs a neutron?

Answer 19

it will increase by 1

Question 20

In a nuclear reactor at a power station, what do the control rods do?

Answer 20

they absorb neutrons which slows down the chain reaction

this means less energy is released during fission

Question 21

Explain the difference between a protostar and a main sequence star

Answer 21

protostar is at a lower temperature

protostar does not emit radiation/energy

This is because nuclear fusion reactions have not started

Question 22

Explain how the different elements now contained in the universe were formed

Answer 22

by nuclear fusion

of hydrogen to helium

elements heavier than iron are formed in a supernova

Question 23

Why are stars stable during the main sequence period of the life cycle?

Answer 23

as forces within the star are balanced

Question 24

Explain why scientists can not give the exact number of years a star will be in the main sequence period

Answer 24

insufficient evidence

do not know exact amount of hydrogen in the star

time too long to measure

Question 25

The rate of fusion in a large star is …… than in a small star

Answer 25

faster

Question 26

Explain why the rate of nuclear fusion in a large star is faster than in a small star

Answer 26

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)

Question 27

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

Answer 27

• 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

Question 28

Why does lowering control rods reduce the amount of energy released per second from a nuclear fuel in a nuclear reactor?

Answer 28

neutrons are absorbed by the control rods

so their are fewer neutrons

which causes the chain reaction to slow down

Question 29

Why do some stars not turn into black dwarfs?

Answer 29

as only stars about the same size as our sun become black dwarfs

Question 30

What will happen to a star at the end of the red super giant stage?

Answer 30

explode

become a supernova

Question 31

suggest why scientists can only estimate and only calculate the mass of some stars

Answer 31

do not have the technology

Question 32

suggest why in the future it will be possible to calculate the mass of most stars

Answer 32

advances in technology/knowledge

Question 33

Describe what happens to a star after the main sequence period, for the star to eventually become a black dwarf

Answer 33

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