Nuclear fission

Which is not a use of water in the nuclear industry?

The purpose of a moderator in a thermal nuclear reactor is to:

One purpose of the coolant in a thermal nuclear reactor is to maintain a safe working temperature within the core.

State the other purpose.

State two properties that engineers consider when choosing a liquid to use as a coolant in a thermal nuclear reactor.

Explain how the power output of a thermal nuclear reactor is decreased.

The equation represents a typical fission reaction.

235 1 87 146 1

Which statement about this reaction is not true?

Carbon is used as the moderator in some thermal nuclear reactors.

Identify one other material commonly used as a moderator.

State two benefits of slowing down the neutrons released during fission.

The collision of a neutron with the nucleus of a moderator atom is modelled using two gliders on a horizontal frictionless air track.

In Figures 13 and 14 the glider N of mass m_N represents the neutron and the glider M of mass m_M represents the moderator nucleus.

Figure 13 shows glider N travelling with initial speed u towards the stationary glider M.

The gliders collide. N rebounds with speed v as shown in Figure 14.

Figure 15 shows the variation of the ratio with the ratio^M.

Show that when is 12, N loses about 30% of its initial kinetic energy in the collision.

In a reactor, the speed of a fast-moving neutron is reduced by a series of y random collisions with carbon-12 nuclei.

The final kinetic energy E_f of the neutron is

where E₀ is the initial kinetic energy of the neutron and b = 0.73

A thermal neutron has kinetic energy equivalent to that of the average particle of an ideal gas with a temperature of 350 K.

One neutron has an initial kinetic energy of 1.0 MeV.

Calculate the minimum value of y required so that this neutron becomes a thermal neutron.

Explain, with reference to Figure 15, why elements with a small nucleon number are preferred as moderator materials.

Section B

Each of Questions 08 to 32 is followed by four responses, A, B, C and D.

For each question select the best response.

235 In a thermal reactor, induced fission occurs when a 92 U nucleus captures a neutron. Which statement is true?

What is the main purpose of a moderator in a thermal nuclear reactor?

A thermal nuclear reactor uses enriched uranium as its fuel. This is fuel in which the ratio of U-235 to U-238 has been artificially increased from that found in naturally-occurring ore.

Describe what happens when neutrons interact with U-235 and U-238 nuclei in a thermal nuclear reactor.

The amounts of U-235 and U-238 in the ore decrease due to radioactive decay at different rates.

A sample of uranium ore today contains 993 g of U-238 The mass of U-238 in this sample was greater 2.00 × 10⁹ years ago.

Show that the mass of U-238 in this sample at that time was about 1.4 kg.

decay constant of U-238 = 1.54 × 10⁻¹⁰ year⁻¹

A thermal nuclear reactor requires a minimum of 3.0% of its uranium mass to be U-235

The ratio of U-235 to U-238 in the ore has changed over time. 2.00 × 10⁹ years ago, the sample in Question 06.2 contained 52 g of U-235

Deduce whether the sample had a high enough U-235 content to be used in a reactor

A thermal nuclear reactor uses a moderator to lower the kinetic energy of fast-moving neutrons.

Explain why the kinetic energy of neutrons must be reduced in a thermal nuclear reactor.

As a result of a collision with an atom of a particular moderator, a neutron loses 63% of its kinetic energy.

A neutron has an initial kinetic energy of 2.0 MeV.

Calculate the kinetic energy of the neutron after five collisions.

The kinetic energy of a neutron in a thermal nuclear reactor is reduced from about 2 MeV to about 1 eV.

Explain why the number of collisions needed to do this depends on the nucleon number of the moderator atoms.

One fission process which can occur in a thermal nuclear reactor is represented by the equation

235 1 142 90 1 U + n = Xe + Sr + 4 n 92 0 54 38 0

Calculate in MeV the energy released in this fission process.

Many magazine and newspaper articles focus on the risks of using nuclear power.

State three benefits of using nuclear power.

Section B

Each of Questions 07 to 31 is followed by four responses, A, B, C and D.

For each question select the best response.

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

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

State what happens to a neutron that is incident on the moderator.

State what happens to a neutron that is incident on a control rod.

A slow-moving neutron is in collision with a nucleus of an atom of the fuel which causes fission.

Describe what happens in the process.

A thermal nuclear reactor produces radioactive waste.

State the source of this waste and discuss some of the problems faced in dealing with the waste at various stages of its treatment.

Your answer should include:

the main source of the most dangerous waste

a brief outline of how waste is treated

problems faced in dealing with the waste, with suggestions for overcoming these problems.

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