4.4.4.1 Nuclear Fission

Question 1

Nuclear fission:

Answer 1

the splitting of a large and unstable nucleus into 2 smaller nuclei (e.g. uranium or plutonium)

Question 2

How common is spontaneous fission?

Answer 2

rare

Question 3

What usually needs to take place for fission to occur?

Answer 3

• for fission to occur the unstable nucleus must first asorb a neutron
• the nucleus undergoing fission splits into 2 smaller nuclei, roughly equal in size, and emits 2 or 3 neutrons plus gamma rays
• goes on to start a chain reaction

Question 4

How does a chain reaction occur?

Answer 4

• Neutron may collide with another radioactive nucleus
• This nucleus absorbs the neutron and becomes unstable
• This nucleus splits, releasing another neutron and produces more energy
• This is a chain reaction, as energy is being released and one ‘split’ causes another to occur
• A neutron needs to be absorbed initially before it splits, because it needs a little extra energy before it can split and release lots of energy
• After the nucleus splits, it leaves two smaller nuclei roughly equal in size
• And they both (along with the neutrons) have kinetic energy
• If the chain reaction is not controlled, it will increase at an exponential rate – which is what
  happens in a nuclear weapon
• This process is used in nuclear fission, with uranium nuclei

Question 5

Why is the chain reaction controlled?

Answer 5

chain reaction is controlled in a nuclear reactor to control the energy released

Question 6

How is a nuclear explosion caused?

Answer 6

the explosion caused by a nuclear weapon is caused by an uncontrolled chain reaction

Question 7

What do nuclear reactors use?

Answer 7

• nuclear reactors use rods of uranium that are rich in 235U as fuel for fission reactions
• these fission reactions produce more neutrons which induce other nuclei to fission - chain reaction

Question 8

When will neutrons cause a chain reaction?

Answer 8

neutrons will only cause a chain rection if they are slowed down - allows them to be captured the uranium nuclei

Question 9

What is sent around the reactor and why?

Answer 9

• coolant is sent around the reactor to remove heat produced by the fission
• often, same water that is being used in the reactor as a moderator is used

Question 10

How does nuclear fission generate electricity?

Answer 10

• heat from reactor can be used to make steam for powering turbines
• turbines generate electricity

Question 11

What are fuel rods used for?

Answer 11

• fuel rods need to be placed in a moderator to slow down and/or absor neutrons

Question 12

What are control rods used for?

Answer 12

• control rods absorb neutrons - lowering control rods increases the number of neutrons absorbed - decreases energy released
• chain reaction needs to continue at a steady rate
• control rods control the chain reactio by limiting the number of neutrons in the reactor
• made of a material that absorobs neutron (e.g. graphite, boron, water etc.) can be inserted by varying amounts to control the chain reaction

Question 13

What is the nuclear reactor surrounded by and why?

Answer 13

• nuclear reactor is surrounded by a thick cement case, which acts as shielding
• prevents radiation escaping and reaching people

Question 14

What happens in an emergency in a reactor?

Answer 14

• rods can be shut down completely by release of control rods into the reactor
• the control rods are lowered fully into the reactor, which slows down the reaction as quickly as possible