Topic 5.4: Fission Questions

State what is meant by the mass defect of a nucleus.

State the function of the fuel in a nuclear fission reactor, and name one material commonly used as the fuel.

A nuclear engineer is designing a thermal fission reactor that uses uranium-235 as its fuel. Fast neutrons released during fission are too energetic to be efficiently captured by the fuel nuclei, so one component is included specifically to reduce the kinetic energy of these neutrons through repeated collisions, increasing the probability that they cause further fission events.

Which reactor component performs this function?

A pressurised-water fission reactor uses uranium-235 as its fuel.

Surrounding the fuel rods is a material chosen specifically to act as the moderator.

What is the function of the moderator in this reactor?

A nuclear reactor design lists ordinary water, graphite and heavy water as candidate moderator materials.

Identify one suitable moderator material from this list.

Four nuclei have the following binding energies per nucleon: nucleus W, 1.1 MeV; nucleus X, 8.8 MeV; nucleus Y, 7.6 MeV; nucleus Z, 7.4 MeV.

Identify which nucleus is the most stable.

Each fission in a reactor releases on average 4 neutrons.

A student lists four possible average values for the number of neutrons per fission that are lost or absorbed: 1, 2, 3 and 4.

(a) Identify which value keeps the chain reaction steady.

(b) State, for one of the other values, whether that reaction would grow or die out.

State what is meant by induced nuclear fission.

Sketch a graph of binding energy per nucleon (vertical axis) against nucleon number (horizontal axis) for naturally occurring nuclei.

On your sketch, mark the approximate position of the most stable nuclei.

A carbon-12 nucleus contains 6 protons and 6 neutrons.

Proton mass = 1.007276 u, neutron mass = 1.008665 u, and the carbon-12 nucleus has mass 11.996709 u.

(a) Determine the mass defect of the nucleus.

(b) Determine the binding energy of the nucleus in MeV.

(c) Determine the binding energy per nucleon. (1 u = 931.5 MeV c⁻².)

A student says: "Only the splitting of heavy nuclei such as uranium can release nuclear energy." Using the shape of the binding-energy-per-nucleon curve, explain why this statement is incorrect.

Outline the role of the moderator and the role of the heat exchanger in a nuclear fission reactor, and explain why a moderator is needed for the chain reaction to be sustained.

Explain why fast neutrons produced in fission must be slowed down, and identify the component that does this.

Outline the role of each of the four main components of a fission reactor: the fuel, the moderator, the control rods and the heat exchanger.

Describe how a nuclear reactor transfers the energy released by fission to a turbine that generates electricity.

Outline why energy must be supplied to separate a stable nucleus into its individual protons and neutrons.

During a controlled shutdown the operators of a reactor lower all of the control rods fully into the core.

Describe and explain the effect this has on the chain reaction.

In a particular nuclear fission reaction, a uranium-235 nucleus absorbs a slow neutron and splits, releasing on average 2.4 neutrons.

(a) State what is meant by a self-sustaining chain reaction.

(b) Determine the average number of neutrons per fission that must be lost or absorbed for the chain reaction to remain steady.

(c) State and explain what would happen to the reaction rate if fewer neutrons than this were lost.

A pressurised-water reactor uses ordinary water both to slow the neutrons and to carry heat away from the core.

The reactor also contains rods made of boron.

(a) State the function of the boron rods.

(b) State what happens to the rate of the chain reaction when these rods are raised, and explain why.

A nuclear reactor generates thermal power at a rate of 6.0 × 10⁸ W.

Each fission of a fuel nucleus releases 3.2 × 10⁻¹¹ J of energy.

Calculate the number of fission events that occur in the reactor core each second.