NUCLEAR FISSION AND FUSION

Question 1

Induced fission

Answer 1

When a large heavy nucleus absorbs a slow moving neutron and becomes unstable, splitting into two smaller nuclei and releasing more neutrons. Energy is released as the binding energy per nucleon of the products is greater than the initial nuclei.

Question 2

Moderator

Answer 2

Material (water or graphite) used to slow down high energy neutron to speeds at which further controlled fission reactions can happen.

Question 3

Control rod

Answer 3

Typically cadmium or boron rods which control the reaction in a nuclear reactor by absorbing excess neutrons.

Question 4

Thermal neutron

Answer 4

A slow moving (low energy) neutron

Question 5

Fuel rod

Answer 5

Uranium or Plutonium rods in a nuclear reactor, providing the fuel for the reaction.

Question 6

Chain reaction

Answer 6

When the product of one nuclear reaction leads to one or more subsequent reactions, a chain reaction has occurred.

Question 7

Fusion

Answer 7

When two small light nuclei come close enough together to join (and for the strong nuclear force to act), to form one heavier nucleus. Releasing energy as the binding energy per nucleon of the product is higher than the initial nuclei.

Question 8

Conditions for fusion

Answer 8

Very high densities to ensure high collision rate of nuclei, very high temperatures to provide nuclei with enough kinetic energy to overcome/do work against the electrostatic force of repulsion and come close enough together for the strong force to act.

Question 9

Practicalities of fusion on earth:

Answer 9

Fusion has happened but sustained fusion has not because as very high temperatures are required, the plasma must be carefully controlled as if it were to touch the container walls it would transfer energy to the wall and its kinetic energy would be reduced, reducing its temperature. Strong magnetic fields are required to do this (and to maintain high densities), but they have not been successful for long periods of time yet.