P.3.8.1.7 Induced fission

What effect are the control rods intended to have on the average kinetic energy and number of fission neutrons in a thermal nuclear reactor?

• A

• B

• C

• D

Which of the following substances can be used as a moderator in a nuclear reactor?

• Boron

• Concrete

• Uranium-238

• Water

The moderator of some nuclear reactors is made from graphite.

What is the principal purpose of the graphite?

• to absorb all the heat produced

• to decrease the speed of neutrons

• to absorb α and β radiation

• to prevent the reactor from going critical

For a nuclear reactor in which the fission rate is constant, which one of the following statements is correct?

• There is a critical mass of fuel in the reactor.

• For every fission event, there is, on average, one further fission event.

• A single neutron is released in every fission event.

• No neutrons escape from the reactor.

Which line, A to D, in the table gives a combination of materials that is commonly used for moderating, controlling and shielding respectively in a nuclear reactor?

• A

• B

• C

• D

Which one of the following statements is not true about the control rods used in a nuclear reactor?

• They must absorb neutrons.

• They must slow down neutrons to thermal speeds.

• They must retain their shape at high temperatures.

• The length of rod in the reactor must be variable.

Why is a moderator required in a thermal nuclear reactor?

• to prevent overheating of the nuclear core

• to absorb surplus uranium nuclei

• to shield the surroundings from gamma radiation

• to reduce the kinetic energy of fission neutrons

A thermal nuclear reactor is shut down by inserting the control rods fully into the core. Which line, A to D, shows correctly the effect of this action on the fission neutrons in the reactor?

• A

• B

• C

• D

In a thermal reactor, induced fission is caused by the ²³⁵₉₂U nucleus capturing a neutron, undergoing fission and producing more neutrons. Which one of the following statements is true?

• To sustain the reaction a large number of neutrons is required per fission.

• The purpose of the moderator is to absorb all the heat produced.

• The neutrons required for induced fission of ²³⁵₉₂U should be slow neutrons.

• The purpose of the control rods is to slow down neutrons to thermal speeds.

Artificial radioactive nuclides are manufactured by placing naturally-occurring nuclides in a nuclear reactor. They are made radioactive in the reactor as a consequence of bombardment by

• α particles.

• β particles.

• protons

• neutrons