Nuclear fission

Question 1

What does the fission of uranium-235 generate?

Answer 1

large quantities of energy, which is used to heat water in power stations

Question 2

nuclear fission

Answer 2

splitting up a larger unstable nucleus into two smaller ones

e.g.
a uranium-235 nucleus spits up when it absorbs a neutron
the fission produces two daughter nuclei, two or more neutrons, and energy
the daughter nuclei are also radioactive

Question 3

What does a fission reaction produce?

Answer 3

• daughter nuclei
• neutrons
• energy

Question 4

controlled/stable chain reactions

Answer 4

if only one neutron released from a reaction is absorbed by another atom then the reaction is stable and will not increase in scale/speed

only one neutron from a fission can cause another fission

Question 5

runaway chain reactions

Answer 5

if more than 1 (or all 3) neutrons are absorbed by other atoms, each of those nuclei then undergoes fission and produces even more neutrons, leading the reaction to increase in scale

if the chain reaction is not controlled there will be a nuclear explosion as the amount of energy keeps increasing

Question 6

Describe how a chain reaction can be controlled [3]

Answer 6

a chain reaction can be controlled by using a different material to absorb some of the neutrons

this slows the reaction down because there are fewer neutrons to cause more nuclei to undergo fission

Question 7

Describe what happens in the fission of uranium-235 [3]

Answer 7

slow moving neutrons are absorbed by a nucleus which becomes unstable and splits into two smaller nuclei, as well as releasing at least 2 neutrons and energy

Question 8

Explain the difference between a runaway chain reaction and a controlled chain reaction [4]

Answer 8

in a runaway chain reaction each of the neutrons produced by fission can cause another nucleus to undergo fission
so the number of atoms undergoing fission increases very rapidly and can cause an explosion

in a controlled chain reaction some of the neutrons are absorbed by a different material so only one neutron from each fission can cause another atom to fission, keeping the number of atoms undergoing fission constant

Question 9

Explain why:

a) neutrons in a nuclear reaction need to be slow moving [2]
b) only one neutron should be absorbed by a uranium-235 nucleus [2]

Answer 9

a) so that they are easily absorbed by the nuclei/do not miss the nucleus causing the nucleus to become unstable

so that the nucleus will become unstable and split, releasing energy

b) so that the extra neutrons are absorbed as only one neutron per fission needs to go on to produce further fission as otherwise the reaction would go out of control, leading to an explosion