Physics (9630) | Induced fission

Welcome to Induced Fission: The Power of Splitting Atoms!

Hello future physicists! This chapter dives into one of the most powerful processes we've harnessed: nuclear fission. This is the heart of how nuclear power plants generate electricity, making it a crucial topic in the "Nuclear Energy" section.

Don't worry if the reactor components sound complicated—we will break down the function of the moderator, control rods, and coolant using simple analogies. By the end of these notes, you'll understand exactly how we manage to safely split an atom and turn that energy into reliable power.

1. The Mechanism of Induced Fission

What is Fission?

Fission literally means "splitting." Nuclear fission is the process where a large, unstable nucleus splits into two smaller nuclei, releasing a huge amount of energy and several neutrons.

We are focusing specifically on induced fission—meaning we force the split to happen.

The Key Particle: The Thermal Neutron

To induce fission in common reactor fuel like Uranium-235, we need to use a neutron. Crucially, this neutron must be a thermal neutron.

• Thermal Neutron: A slow-moving neutron, possessing kinetic energy comparable to the thermal energy of the atoms in its surroundings.
• Did you know? Uranium-235 is much more likely to absorb a slow neutron than a fast one. Think of it like a slow-moving, heavy object being easier to catch than one moving at high speed!

The Fission Process (Step-by-Step)

The standard reaction uses the fissile isotope Uranium-235 (\(^{235}_{92}\text{U}\)):

1. A thermal neutron (\(^{1}_{0}\text{n}\)) is absorbed by the Uranium-235 nucleus.
2. This creates a highly unstable intermediate nucleus, Uranium-236 (\(^{236}_{92}\text{U}\)).
3. The unstable nucleus immediately splits into two smaller nuclei (fission fragments, or daughter nuclei), typically releasing 2 or 3 additional neutrons.
4. A massive amount of energy (mostly kinetic energy of the fragments and neutrons) is released.

A general reaction equation might look like this (though the fission products vary greatly):
\(^{1}_{0}\text{n} + ^{235}_{92}\text{U} \longrightarrow ^{236}_{92}\text{U} \text{ (unstable)} \longrightarrow ^{141}_{56}\text{Ba} + ^{92}_{36}\text{Kr} + 3\times ^{1}_{0}\text{n} + \text{Energy}\)

Key Takeaway: Induced fission requires a thermal neutron to strike a large nucleus (like U-235), resulting in two smaller nuclei, energy, and *more* neutrons.

2. Chain Reactions and Criticality

The Fission Domino Effect

Because the fission process releases 2 or 3 new neutrons, these newly produced neutrons can go on to strike other U-235 nuclei, causing them to split too. This process is called a chain reaction.

In a nuclear reactor, the chain reaction must be controlled so that, on average, exactly one neutron from each fission event causes a subsequent fission.

Critical Mass

If you have too little fissile material, too many neutrons will escape the surface before they can cause further fission, and the reaction will stop.

The critical mass is the minimum mass of fissile material required for a self-sustaining chain reaction.

• Sub-critical: Too few fissions occur; the reaction dies out.
• Critical: The reaction is sustained at a constant, steady rate (ideal for a nuclear power station).
• Super-critical: The reaction rate increases rapidly (leads to an uncontrolled energy release, like in a nuclear weapon).

Key Takeaway: We must maintain a critical state where the chain reaction produces energy at a constant, controlled rate.

3. Components of a Thermal Nuclear Reactor

A modern thermal nuclear reactor uses the heat generated by controlled induced fission to boil water, which then drives turbines to generate electricity. This control is achieved through three essential components: the moderator, control rods, and coolant.

Function of the Moderator

When U-235 fissions, it releases fast neutrons (high kinetic energy). As we learned, these fast neutrons are terrible at causing new fission in U-235. They must be slowed down to become thermal neutrons.

• Function: To slow down the fast neutrons released during fission.
• Mechanism: The moderator works via elastic collisions. The fast neutron collides repeatedly with the nuclei of the moderator material. In an ideal elastic collision, the maximum kinetic energy is transferred when the colliding particles have similar masses.
• Analogy: Imagine a fast-moving billiard ball (the neutron) hitting a heavy bowling ball (a uranium nucleus)—it barely slows down. Now, imagine it hitting another billiard ball (a light moderator nucleus)—it slows right down.
• Material Choice: Materials are chosen for their small mass number and low tendency to absorb neutrons. Common examples include graphite or water (light water or heavy water).

Function of the Control Rods

If the reaction were left unchecked, the number of neutrons would quickly multiply, leading to a meltdown or explosion. We need a brake!

• Function: To absorb excess neutrons, thereby controlling the rate of the chain reaction.
• Mechanism: Control rods are inserted into the reactor core. By adjusting their depth, operators can fine-tune the number of available neutrons, ensuring the reactor stays exactly at the critical state.
• Material Choice: Materials chosen for their high neutron-absorption cross-section. Common examples are Cadmium or Boron.

Function of the Coolant

Fission generates extreme heat—that's the whole point! This heat needs to be removed from the core to prevent the fuel rods from melting, and to be transferred to the boiler system.

• Function: To circulate through the core and transfer thermal energy (heat) away from the nuclear fuel.
• Material Choice: Liquids or gases with high specific heat capacity and good flow characteristics. Examples include water or carbon dioxide gas.

Key Takeaway: The moderator slows neutrons (via elastic collisions), control rods absorb neutrons (controlling the rate), and coolant removes heat.

4. Safety Aspects and Waste Management (Risk vs. Benefit)

Generating nuclear power carries inherent risks due to the immense radioactivity involved. Safety is paramount in reactor design and operation.

Safety Features and Procedures

Shielding

The core, which produces intense levels of dangerous alpha, beta, gamma, and neutron radiation, must be surrounded by heavy shielding.

• Purpose: To absorb harmful radiation, protecting personnel and the environment.
• Materials: Thick concrete walls, often reinforced with steel and lead.

Fuel Handling and Emergency Shutdown

• Fuel Handling: Fresh fuel is radioactive, but spent fuel is intensely so. Both insertion and removal must be done via remote handling, often using automated machinery, to minimise human exposure.
• Emergency Shut-down (SCRAM): In an emergency, control rods are fully and rapidly inserted into the core. This absorbs almost all neutrons, halting the chain reaction instantly.

Radioactive Waste

The by-products of fission (fission fragments) are highly radioactive and have very long half-lives. Handling and disposal pose a significant long-term challenge.

• Production and Handling: Spent fuel is handled remotely and stored temporarily (often underwater) to allow short-lived isotopes to decay and the material to cool.
• Storage: Permanent disposal requires isolating the waste from the environment for thousands of years. This usually involves complex deep geological storage—sealing the waste in stable rock formations deep underground.

The Balance: Risk vs. Benefit

The decision to use nuclear power requires balancing the risks and benefits.

• Benefits:

  Nuclear power plants produce vast amounts of reliable energy without burning fossil fuels, meaning they release almost zero greenhouse gases (low carbon emissions).

• Risks:

  The risk of a catastrophic accident (though extremely rare) and the ongoing challenge of managing radioactive waste that must be stored safely for millennia.

As physicists, understanding this balance is key to making informed judgments about future energy policy.

Key Takeaway: Reactor safety relies on heavy shielding, remote handling, and emergency shutdown mechanisms. The greatest challenge remains the safe, long-term storage of highly radioactive waste.