Topic 5.4: Fission Questions

State what is meant by induced nuclear fission.

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

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 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?

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

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.

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.

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

Identify one suitable moderator material from this list.

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

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 nuclear engineer is designing a thermal-fission reactor and must choose a material for the component that surrounds the fuel rods. The purpose of this component is to slow down the fast neutrons released by fission so that they are more likely to be captured and sustain a chain reaction.

Which property makes a material a suitable choice for this component, and which material below has that property?

In a hypothetical nuclear reactor, each fission event releases exactly 4 free neutrons.

The reactor is operating in a steady state, so that the rate of fission events stays constant from one generation to the next.

On average, how many of the 4 neutrons released in each fission must be lost (by escaping the core or being absorbed without causing fission) for this steady state to be maintained?

In a fusion reactor, 5.0 g of hydrogen fuel is consumed and a mass of 4.0 × 10⁻⁵ kg is converted into energy. In a separate fission reactor, 8.0 g of uranium fuel is consumed and a mass of 6.4 × 10⁻⁶ kg is converted into energy. What is the ratio

(energy released per unit mass of hydrogen fuel) : (energy released per unit mass of uranium fuel)?

In one fission event the total mass of the products is 0.215 u less than the mass of the original nucleus plus the absorbed neutron.

Using the conversion 1 u ≡ 931.5 MeV, determine the energy released in this fission, in MeV.

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.

Both nuclear fission and nuclear fusion can release energy.

Outline, in terms of the nuclei involved, the difference between the two processes.

A nitrogen-14 nucleus contains 7 protons and 7 neutrons.

Proton mass = 1.007276 u, neutron mass = 1.008665 u, and the nitrogen-14 nucleus has mass 13.999234 u.

Determine the binding energy per nucleon of nitrogen-14, in MeV.

(1 u = 931.5 MeV c⁻².)

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.

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