Nuclear Fission, Uncontrolled and Controlled Chain Reactions

What is Nuclear Fission?

Nuclear fission is a type of transmutation where a large nuclide splits to form two smaller nuclides.

In nuclear fission, the binding energy of the products is greater than that of the parent nuclide. The total mass of products is also smaller than the mass of reactants. By Einstein's mass-energy equivalence principle, this mass difference is transformed into energy. As a result, energy is released during nuclear fission, typically in the form of kinetic energy and heat. 

Calculations involving nuclear fission can be found here.

In addition to small daughter nuclides, neutrons are sometimes formed from fission. The emitted neutrons are important as they allow for chain reactions to occur. 

Diagram shows nuclear fission of uranium-235 to form two smaller nuclide fragments: krypton-92 and barium-141. This process involves the formation of an intermediate (uranium-236) and emission of 3 neutrons.

 

While a parent nuclide can undergo fission spontaneously, nuclear fission is sometimes induced by firing a neutron with sufficient kinetic energy into a heavy isotope (e.g. uranium-235). When the neutron is captured by the parent nucleus, an unstable nuclide is temporarily formed (due to changes in neutron to proton ratio). Consequently, fission occurs to produce smaller nuclides that are more stable.

What is a Chain Reaction? 

A chain reaction in nuclear physics refers to the process during which nuclear fission reactions become self-sustaining. In other words, one fission can induce another without any further intervention.

 

 

Critical reaction - each fission reaction produces exactly one further fission reaction and maintains constant reaction rate. This kind of perfect reaction is called a critical reaction and the amount of fissionable material used to facilitate this one-for-one sustainability is called the critical mass (smallest amount of fissile material that will facilitate sustained chain reaction).

Uncontrolled Chain Reaction

 

Uncontrolled chain reaction occurs when more than one further fission reaction is induced by neutrons emitted from fission. The amount of fissionable material required to facilitate this an uncontrolled chain reaction must exceed the critical mass.

The rate of fission and power of an uncontrolled chain reaction are exponentially increasing. As a result, uncontrolled chain reactions are seen in the use of nuclear weapons. 

While moderators are used in uncontrolled chain reactions to increase the likelihood of successful fission, control rods are not used.

Controlled Chain Reaction

A controlled chain reaction refers to a chain reaction whose rate of fission is controlled and thus the generated energy is also controlled. A controlled chain reaction allows for fission to occur in a self-regenerating manner while limiting the amount of energy produced to a sufficient but safe level.

The rate of fission and power of a controlled chain reaction is constant, and can be controlled with the use of moderators and control rods. As a result, controlled chain reactions are used in nuclear power plants (nuclear generators).

A controlled chain reaction requires:

• fissionable material that meets the critical mass
• control rods
• moderator

Control Rods

Control rods are materials that capture free neutrons emitted from fission. Since absorbed neutrons cannot induced further fission reactions, the use of control rods reduces the rate of fission and hence is essential to facilitate a controlled chain reaction.

While all neutrons in an uncontrolled chain reaction are allowed to induced fission reactions, the number of neutrons allowed to do this in a controlled chain reaction is determined by the number of control rods used. 

Control rods are commonly made of cadnium or boron. 

Moderator

For a neutron to induce fission, it must first be captured in the parent nuclide to form a temporary unstable nuclide. This unstable nuclide will then undergo fission. A neutron can only be captured if its kinetic energy (speed) is not too low nor too high. The kinetic energies of neutrons emitted from fission are typically too high to successfully induced further fission and cause a chain reaction to occur.

 

 

Diagram illustrates a simple set-up of moderated and controlled chain reaction of Uranium-235. Moderators are positioned after each 'row' of fissile material (Uranium-235) to slow down emitted neutrons. Controlled rods are dispersed throughout the reaction chamber to reduce reaction rate.

 

Moderators slow down free neutrons produced in the fission reactions. The slowing down of neutrons increases the likelihood of successful fission reactions and hence increases the rate of fission. Moderators are required for both controlled and uncontrolled chain reactions.

Examples of moderators include heavy water, graphite. 

Nuclear Generators

A nuclear generator harnesses the energy produced from controlled chain fission reactions. This energy is initially in the form of heat which is used to transform water into steam (at high pressure). The steam is used to power a turbine which in turn powers an electric generator. 

 

 

It is important for controlled chain reactions to occur as it would otherwise be unsafe.

 

Fuel Rods

Fuel rods are hollow rods made of metal, typically alloy steel, and contain a sub-critical mass of fuel or fissionable material (usually enriched uranium oxide or plutonium).

They are inserted into the reactor in a grid pattern and their combined fuel provides enough fissionable material for critical mass.

Moderator & control rods

Moderators and control rods are essential to cause a controlled chain reaction to occur. The number of moderators and control rods in a reaction chamber can be adjusted to change the rate of fission.

Moderators increase the rate of fission. Removal of moderators therefore decreases the rate of fission.

Control rods decreases the rate of fission. Removal of control rods therefore increases the rate of fission.

Coolant

The heat produced by the fission reaction is transferred to the coolant. The coolant in a nuclear reactor prevents the fuel rods from melting and transfers the heat to the water. The water that is heated at the heat exchange will turn into steam which drives a turbine and produces electricity via an electric generator.

Shielding

Multiple layers of shielding surround the reactor for safety and efficiency reasons.

A graphite shield reflected neutrons back into the core, followed by a thermal shield to prevent unwanted heat loss, a pressure vessel to isolate and contain everything inside the core and a biological shield of 3 metres of concrete mixed with lead pellets to absorb gamma rays and neutrons.

   

Previous section: Modelling Radioactive Decay (Calculation)

Next section: Mass Defect and Binding Energy

 

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