Nuclear fission, fusion and half life

Question 1

What does nuclear fission mean?

Answer 1

The splitting of a nucleus.

Question 2

What happens when a uranium-235 or plutonium-239 nucleus is hit by a neutron?

Answer 2

1. The neutron splits the nucleus into two smaller nuclei which are radioactive
2. two or three more neutrons are released
3. energy is released
4. The additional neutrons may hit other nuclei and cause them to split which causes a chain reaction
5. The chain reaction in nuclear reactors is controlled to stop it going too fast

Question 3

What is nuclear fusion?

Answer 3

Nuclear fusion is where two nuclei joining to make a large nucleus. A lot of energy is released

Question 4

What nuclear fusion reactions takes place in a star?

Answer 4

Hydrogen nuclei with helium nuclei

Question 5

What are the disadvantages of nuclear fission?

Answer 5

1. Products left over after nuclear fission are radioactive making them difficult and expensive to dispose of safely
2. Overall cost of nuclear power is high due to cost of powerplant and decomissioning
3. Risk of radiation leaks

Question 6

What are the disadvantages of nuclear fusion?

Answer 6

1. More energy is needed to create the conditions for fusion than what we can get out of it, it can only happen at really high temperatures
2. You can’t hold the hydrogen at high temperatures and pressures required for fusion

Question 7

What are the advantages of nuclear fusion?

Answer 7

1. It doesn’t leave behind radioactive waste like fission

2. There is plenty of hydrogen to use for fuel

Question 8

What are the advantages of nuclear fission?

Answer 8

1. It releases a lot more energy than chemical processes

Question 9

How is a star formed?

Answer 9

1. Stars form from clouds of dust and gas into a protostar
2. Gravitational energy transfers into heat energy which raises the temperature. When the temperature is high enough hydrogen nuclei undergo nuclear fusion to form helium nuclei and give out mass amounts of heat and light. A star is born. Dust and gas around the star form planets that orbit the star.

Question 10

What happens when a star has formed?

Answer 10

1. The star enters a stable period where heat from nuclear fusion provides an outward pressure to balance the force of gravity pulling everything inwards. The star maintains this energy for millions of years due to massive amounts of hydrogen it consumes. Its stable period its called the MAIN SEQUENCE STAR and lasts several billion years.

Question 11

What happens when the MAIN SEQUENCE STAR runs out of hydrogen?

Answer 11

The star runs out of hydrogen. Heavier elements like iron are made by nuclear fusion of helium and the star swells into a RED GIANT, if its small, or a RED SUPER GIANT if its a big star.

Question 12

What is a supernova?

Answer 12

When a red super giant starts to glow brighter as it undergoes more fusion. It expands and contracts forming heavy elements like iron in nuclear reactions. It explodes in a SUPERNOVA forming heavier elements ejecting into the universe and creating new planets and stars. The supernova ejects the dust and gas into space leaving a black hole.

Question 13

Define half life.

Answer 13

Half life is the average time it takes for the number of nuclei in a radioactive isotope sample to halve.

Question 14

What does a short half life indicate?

Answer 14

Activity falls quicker because nuclei decay quicker

Question 15

What does a long half life indicate?

Answer 15

Activity falls slower because the nuclei doesn’t decay very quickly

Question 16

The activity of a radioscope is 640cpm. Two hours later it is 80cpm. Find the half life of the sample.

Answer 16

640/2 = 320 320/2= 160 160/2=80 after three half lives

It takes three half lives to get to 80cpm so two hours is three lives. So half life = 120 mins / 3 = 40 minutes