Induced fission (AQA A-Level Physics): Revision Notes

Induced fission

What is induced fission?

While fission can occur spontaneously in some heavy nuclei, this is an extremely rare event. However, fission can be deliberately triggered through a process called induced fission. This involves directing neutrons at heavy nuclei such as uranium-235, which absorb the neutrons and subsequently split apart.

This process releases a substantial amount of energy, typically between 150 and 200 MeV per fission event.

The fission process

Each fission event produces:

• Two mid-sized nuclei called fission fragments (also known as daughter nuclei or fission products)
• Several free neutrons (typically 2 to 3 neutrons, though this varies)
• Gamma photons
• Large amounts of kinetic energy

The fission fragments themselves are often unstable and may undergo further radioactive decay. The energy released appears primarily as the kinetic energy of the fission fragments and neutrons, which can be calculated by determining the mass loss in the fission reaction.

Self-sustaining chain reaction

The neutrons produced during fission are the key to obtaining large amounts of energy from nuclear reactions. These free neutrons can potentially go on to trigger additional fission events in other uranium-235 nuclei, creating what is known as a self-sustaining chain reaction.

The number of fission fragments produced in induced fission varies, averaging between two and three neutrons per event. If just one fission event produces three neutrons, and each of these causes another fission, the process rapidly multiplies: three neutrons produce nine, then nine produce twenty-seven, and so on. This multiplication effect is what makes nuclear chain reactions so energetically productive.

Controlled fission in nuclear reactors

The basic principle of operating a nuclear reactor is to establish and maintain a controlled self-sustaining chain reaction in nuclear fuel. The first self-sustaining chain reaction was achieved in the 1940s, and nuclear power stations have been generating electricity since the 1950s.

For a reactor to operate efficiently and generate the required amount of heat energy, several technical requirements must be met. The probability of fission occurring must be high enough to ensure that sufficient fission events take place every second. Additionally, enough fission neutrons must go on to create more fission events so that a self-sustaining chain can be established, and the heat energy generated in the reactor must be extracted efficiently.

Fast neutrons and thermal neutrons

The neutrons produced directly from fission events are called fast neutrons. A fast neutron has a kinetic energy of approximately $2$ MeV, which means it is travelling at about $2 \times 10^7$ m $\cdot$ s$^{-1}$.

A neutron that is in thermal equilibrium with its surroundings is called a thermal neutron. A thermal neutron has a kinetic energy much less than 1 eV (written as $\ll 1$ eV). For a nuclear reactor to function effectively, it must contain a material that can slow down the fast fission neutrons to thermal speeds. The reactor must also contain materials that allow the fast neutrons to collide with them, resulting in the neutrons' speed being reduced very significantly at each collision.

The moderator

The material used to slow down fast fission neutrons is known as the moderator. Understanding how the moderator works requires considering elastic collisions between particles.

How moderation works

In practice, a neutron will undergo a series of elastic collisions before losing enough kinetic energy to become thermal. The material chosen as the moderator must have particles with which the fast neutrons can collide elastically. For maximum energy transfer, these particles should have a mass similar to that of a neutron.

Choice of moderator material

A commonly used moderator is water, as it contains protons (hydrogen nuclei), which have almost the same mass as neutrons. When fast neutrons collide with water molecules, they interact with the protons, progressively losing their kinetic energy until they reach thermal speeds.

To maintain a self-sustaining chain reaction, it is important that materials in the reactor core, other than the uranium-235 fuel, do not readily absorb neutrons. The moderator must therefore have a low probability of absorbing neutrons. Early reactors used carbon in the form of graphite as a moderator. Although carbon nuclei are about twelve times more massive than neutrons, they are still sufficiently small to act as effective moderators. Graphite also has a very low probability of neutron absorption. Some neutron collisions with moderator nuclei can be inelastic, which may result in the moderator nuclei becoming excited and subsequently emitting gamma photons.

Nuclear fuel and fuel rods

The nuclear fuel used in most reactors is natural uranium, though the fuel composition can vary. Natural uranium consists primarily of uranium-238 (about 99%) with only a small fraction being uranium-235 (less than 1%). It is specifically the uranium-235 nuclei that undergo induced fission and are described as fissile material.

A fuel rod is constructed from a column of uranium oxide pellets sealed into zirconium alloy tubes. These tubes are typically several metres in length. Zirconium is chosen as the cladding material because it has a very low probability of absorbing neutrons. The fuel rods are assembled in bundles, with some rod positions left vacant to allow for the insertion of movable control rods, neutron source rods, and measuring instruments.

Control rods

The operation of a nuclear reactor requires a critical chain reaction, in which, on average, exactly one of the neutrons produced in a fission event goes on to produce another fission event. To maintain this critical condition, control rods are used.

Control rods are made from materials with a high probability of absorbing neutrons, such as cadmium or boron. They can be raised from or lowered into the fuel assemblies. By controlling the neutron density within the reactor core, the control rods regulate the rate at which fission occurs and therefore control the power output of the reactor.

When the control rods are lowered into the fuel assemblies, they absorb more neutrons, reducing the number available to cause fission. This decreases the reaction rate. When they are raised, fewer neutrons are absorbed, and the reaction rate increases. Additional control rods are positioned so they can be lowered very quickly into the core should an unsafe condition arise, providing an emergency shut-down mechanism for the reactor.

Neutron source rods provide the neutrons required for the initial start-up of the reactor, initiating the chain reaction before the fission process becomes self-sustaining.

The coolant

The kinetic energy of the fission fragments and neutrons generates a substantial amount of heat in the reactor core. This heat energy must be transferred efficiently to convert water into high-pressure steam, which then drives the turbines that generate electricity.

The material that passes through the reactor core and absorbs the heat energy generated by fission is called the coolant. The coolant must have several important characteristics: either a high specific heat capacity or the ability to be pumped very quickly around the system. Examples of suitable coolants include liquid water and carbon dioxide gas.

In the United Kingdom, some reactors use carbon dioxide as the coolant. However, over 60% of reactors worldwide are pressurised water reactors (PWRs), in which water acts as both coolant and moderator, enabling a more compact reactor design.

The pressure vessel containing the reactor core is made of steel, designed to withstand these high temperatures and pressures.

Water used as a moderator and coolant is sometimes called light water in this context. While it has excellent moderating and heat transfer properties, ordinary water is over 100 times more likely than carbon dioxide to absorb neutrons. Therefore, all light water reactors require fuel with an enriched uranium content rather than natural uranium.

Critical mass of fuel

Understanding what could happen to a fission neutron produced by the fission of a uranium-235 nucleus within a fuel rod in the core of an operating nuclear reactor helps explain the concept of critical mass.

Several of these possible outcomes mean that the neutron would not contribute to maintaining the chain reaction. The choice of materials for the cladding, moderator, and coolant is made to reduce the likelihood of neutrons being absorbed by these components rather than causing further fission.

However, some neutron absorption by these materials is inevitable. Neutron escape from the reactor is also more difficult to control. The larger the mass of fuel present, the smaller the surface area to volume ratio becomes, which means a smaller percentage of neutrons produced are likely to escape from the reactor core.

As the reactor operates over time, the uranium-235 content of the fuel rods in an assembly gradually decreases, while increasing amounts of fission fragments and plutonium decay products accumulate. These fission fragments and their decay products can absorb neutrons. After approximately four to five years of operation, the fissile content of the fuel rods in an assembly becomes much reduced. Given that fission fragments and their decay products absorb neutrons, the efficiency of the fuel assembly becomes too low. At this point, it must be removed from the reactor core during a shut-down period.

Safety features

The workforce at a nuclear power station must be protected from radiation emitted by the reactor. Outer concrete shielding absorbs any escaping neutrons and gamma photons, preventing exposure to these forms of ionising radiation.

The reactor design includes multiple safety systems, including the emergency shut-down capability provided by additional control rods that can be inserted very quickly if necessary.

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