Radiation safety and nuclear energy

Nuclear radiation is a powerful tool in modern medicine and energy production, but it carries significant risks. In this topic, we will explore how to protect ourselves from these hazards and examine the physics behind nuclear reactors and the stars.

What you'll learn

• The crucial differences between irradiation and contamination and how we protect people from both.
• How doctors use internal and external radiation to diagnose and treat diseases like cancer. (Separate Physics Only)
• The physics of nuclear fission and how chain reactions are controlled in a nuclear power station. (Separate Physics Only)
• How nuclear fusion powers the stars and why it is so difficult to replicate on Earth. (Separate Physics Only)

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1. Protecting Against Radiation: Irradiation vs Contamination

To understand radiation safety, we must first distinguish between two terms that are frequently confused: irradiation and contamination.

The diagram below illustrates this fundamental difference:

Comparing the Hazards

The hazard level of irradiation versus contamination depends heavily on the type of radiation emitted:

• Alpha radiation is highly ionizing but has a very short range in air and is blocked by skin. Therefore, alpha is extremely dangerous as an internal contaminant (if swallowed or inhaled) because it concentrates all its ionizing damage on delicate internal organs. However, it is a relatively low hazard for external irradiation.
• Beta and Gamma radiation are highly penetrating. They are highly hazardous during external irradiation because they can pass through skin and damage internal tissues from a distance.

Radiation Precautions

To protect people, we must limit their radiation dose. Medical staff and nuclear workers take strict precautions to minimize their risks:

• Shielding: Placing absorbing materials between the source and the worker. Lead screens or lead aprons are used to block beta and gamma rays.
• Distance: Using long-handled tongs to handle sources, as the intensity of radiation drops rapidly with distance.
• Time: Minimizing the time spent near radioactive materials.
• Personal Monitoring: Workers wear photographic film badges. The film inside darkens as it is exposed to radiation, providing a clear record of the total dose the worker has received over time.

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2. Medical Uses of Radioactive Substances (Separate Physics Only)

Medical physics utilizes radioactive isotopes for both diagnosing and treating diseases.

Internal vs External Radiotherapy

Cancerous tumours can be destroyed using ionizing radiation. This is called radiotherapy, and it can be administered in two main ways:

• External Radiotherapy: High-energy gamma rays are directed at the tumour from outside the body. The source is rotated around the patient with the tumour at the centre. This maximizes the dose delivered to the cancer cells while minimizing the damage to surrounding healthy tissues.
• Internal Radiotherapy: A radioactive beta-emitter or alpha-emitter is placed directly inside or next to the tumour (e.g., using radioactive implants or targeted chemical carriers). Because alpha and beta have short ranges, they deposit their energy precisely in the tumour, sparing healthy tissue further away.

Medical Tracers and PET Scanners

A medical tracer is a radioactive isotope injected into or swallowed by a patient to track how a substance moves through the body.

• The isotope must emit gamma radiation so that the rays can easily penetrate out of the body to be detected by external cameras.
• It must have a short half-life (typically a few hours) so that it quickly decays to a safe level, minimizing the long-term radiation dose to the patient.

Positron Emission Tomography (PET) scanners are an advanced diagnostic tool. They inject a tracer that emits positrons (beta-plus decay). When a positron meets an electron in the body, they annihilate each other, producing two gamma rays traveling in opposite directions. The PET scanner detects these gamma rays to build a highly detailed 3D image of active metabolic areas (such as active tumours).

Our understanding of radioactive decay also allows us to calculate how rapidly a tracer's activity decreases over time.

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3. Nuclear Fission (Separate Physics Only)

Nuclear reactions—including decay, fission, and fusion—are massive sources of energy. Nuclear fission is the splitting of a large, unstable nucleus into smaller, more stable nuclei.

The Fission of Uranium-235

In a nuclear reactor, the isotope uranium-235 is commonly used. Fission does not occur spontaneously in a controllable way; it must be induced:

1. A slow-moving neutron (often called a thermal neutron) is absorbed by a uranium-235 nucleus.
2. This forms an extremely unstable uranium-236 nucleus.
3. The unstable nucleus splits into two smaller daughter nuclei (such as barium and krypton) and releases two or three fast-moving neutrons, along with a massive amount of energy (carried away as kinetic energy of the products and gamma radiation).

$$neutron+{}_{92}^{235}U\to {}_{92}^{236}U\to {Daughter}_1+{Daughter}_2+3neutrons+Energy$$

Controlled Chain Reactions

Because each fission event releases multiple neutrons, those neutrons can go on to strike other uranium-235 nuclei, triggering further fission events. This self-sustaining sequence is called a chain reaction.

In a nuclear power station, we must control this chain reaction so that exactly one neutron from each fission event goes on to cause another fission. If too many neutrons cause fission, the reaction will escalate exponentially and release heat too fast, leading to a meltdown.

Inside a Nuclear Reactor

A nuclear reactor uses specific components to maintain and control this chain reaction, and to harness its thermal energy:

• Moderator (often water or graphite): Fission is only triggered by slow-moving neutrons. The neutrons released during fission are moving far too fast to be easily absorbed. The moderator slows down these fast neutrons through collisions, turning them into "thermal" neutrons.
• Control Rods (made of boron or cadmium): These materials are excellent neutron absorbers. They can be lowered into or raised out of the reactor core. Lowering the control rods absorbs more neutrons, slowing down or stopping the chain reaction. Raising them increases the reaction rate.
• Coolant (usually water or carbon dioxide gas): The coolant absorbs the thermal energy generated in the reactor core and carries it to a heat exchanger.
• Generating Electricity: In the heat exchanger, the hot coolant transfers its thermal energy to boil water, producing high-pressure steam. This steam drives a turbine, which turns a generator to produce electricity.

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4. Nuclear Fusion (Separate Physics Only)

Nuclear fusion is the process where two light, smaller nuclei collide and join together to create a larger, heavier nucleus.

Fission vs Fusion

It is crucial to be able to compare these two nuclear processes:

Why Fusion is Extremely Difficult on Earth

Unlike fission, we have not yet built practical, economic nuclear fusion power stations. Fusion requires two positive nuclei (such as hydrogen nuclei, which are single protons) to fuse.

• Because both nuclei have a positive charge, they experience a massive electrostatic repulsion (like charges repel) as they get close.
• To overcome this electrostatic force and get close enough for the strong nuclear force to pull them together, the nuclei must be moving incredibly fast.
• This requires extremely high temperatures (millions of degrees Celsius) and extremely high pressures (to squeeze the nuclei close together, increasing collision rates).

Creating and containing plasma at these extreme temperatures and pressures on Earth requires more energy than the fusion reaction currently yields back, making it a major engineering hurdle.

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5. Pros and Cons of Nuclear Power (Separate Physics Only)

Nuclear fission power stations provide a large percentage of the world's electricity, but they are a topic of intense public debate.

Advantages

• No Carbon Dioxide Emissions: Unlike burning fossil fuels, operating nuclear power stations does not release greenhouse gases ($C{O}_2$ or $S{O}_2$), helping to combat climate change.
• High Energy Density: A tiny amount of nuclear fuel produces millions of times more energy than an equivalent mass of coal or oil.
• Reliability: Unlike solar or wind power, nuclear power is not dependent on the weather and provides a steady "base-load" of electricity.

Disadvantages

• Radioactive Waste: Safe storage of highly radioactive fission products is required for thousands of years, which is expensive and politically difficult.
• Accident Risks: While rare, accidents (like Chernobyl or Fukushima) can release large amounts of radioactive material into the atmosphere, contaminating vast areas.
• High Setup and Decommissioning Costs: Building nuclear reactors is incredibly expensive, and safely dismantling (decommissioning) them at the end of their lifetime takes decades and billions of pounds.

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