Uses and Dangers of Radiation, Fission and Fusion
Uses of Radioactivity
Ionising radiation, though hazardous, has well-established uses in medicine, food safety and industry. Each application exploits a specific property of a particular radiation type.
Household smoke alarms use a small americium-241 source emitting alpha particles. Alpha particles ionise air between two electrodes, maintaining a small current. Smoke particles absorb the alpha particles, reducing the current and triggering the alarm. Alpha is chosen because it is strongly ionising (effective at ionising air) and does not penetrate the housing, so it poses minimal risk outside the device.
Irradiating food: gamma rays from a radioactive source are used to kill bacteria and other pathogens in food, extending shelf life without significantly affecting taste or nutrition. The food itself does not become radioactive.
Sterilisation of medical equipment uses gamma rays to kill micro-organisms on surgical instruments. This is preferred for heat-sensitive items that cannot withstand autoclaving.
Thickness gauging: beta sources are used in manufacturing to control the thickness of paper, metal sheet or plastic film. Beta radiation partially penetrates the material; a detector on the other side monitors the intensity. If the material is too thick, less radiation reaches the detector; the machinery adjusts accordingly. Beta is used (not alpha, which would be blocked entirely, or gamma, which would pass through without being affected by small thickness changes).
Tracing: radioactive tracers are injected or ingested and their progress through a system monitored externally. In medicine, tracers help diagnose blockages in blood vessels or thyroid disorders. In industry, tracers locate leaks in underground pipes.
Diagnosis and Treatment of Cancer
Gamma radiation is used in radiotherapy to destroy tumour cells. An external beam is targeted at the tumour from multiple angles to maximise the dose at the tumour while minimising damage to surrounding healthy tissue. For internal tumours, radioactive implants (seeds) can be placed directly inside or near the tumour.
A PET scanner uses a tracer that emits positrons (beta-plus particles). The positron immediately annihilates with a nearby electron, producing two gamma rays that travel in exactly opposite directions. Detectors surrounding the patient record these paired gamma rays and reconstruct a 3D image of metabolic activity, helping identify tumours and brain conditions.
The tracer used in PET scanners must have a short half-life to limit the patient’s radiation dose, and must also decay by positron emission. Fluorine-18 is widely used, with a half-life of approximately 110 minutes. Because of this short half-life, the tracer must be produced in a nearby cyclotron and used quickly — it cannot be transported over long distances or stored for later use.
Contamination vs Irradiation
It is essential to distinguish between two types of radiation hazard:
Irradiation is exposure to radiation from an outside source. The exposed object or person does not become radioactive. Irradiation from a gamma source (e.g. during a medical scan) stops as soon as the source is moved away.
Contamination occurs when radioactive material gets onto or inside the body. This is potentially more dangerous because the radioactive source remains close to cells and continues to irradiate them. Alpha contamination is particularly dangerous: outside the body, alpha is stopped by skin, but if inhaled or ingested, alpha particles are absorbed by internal organs at very short range.
Irradiation: external, exposure stops when source moves. Contamination: internal (on or in the body), continuous exposure, harder to control. Alpha contamination is especially dangerous inside the body despite low external penetrating power.
Safety Precautions
Precautions to reduce radiation dose include: keeping radioactive sources in lead-lined containers, using tongs or remote handling to avoid direct contact, minimising the time spent near sources, maintaining maximum possible distance from sources, wearing dosimetry badges to monitor cumulative dose, and ensuring patients receive the minimum dose necessary for diagnosis or treatment.
Nuclear Fission
Nuclear fission occurs when a large, unstable nucleus absorbs a neutron and splits into two smaller daughter nuclei, releasing two or three more neutrons and a large amount of energy. The most commonly used fissile material is uranium-235 (U-235).
Each fission event releases neutrons that can trigger further fissions in neighbouring U-235 nuclei. If each event causes on average more than one further fission, a chain reaction develops. In an uncontrolled chain reaction, energy release escalates exponentially (as in a nuclear weapon). In a nuclear reactor, the chain reaction is kept controlled so that on average exactly one new fission is triggered per event.
Control is achieved using: control rods (made of boron or hafnium) that absorb excess neutrons — inserting them deeper into the reactor slows or stops the chain reaction; a moderator (commonly graphite or water) that slows the neutrons down to a speed at which they are most effective at causing fission. The thermal energy released is used to heat water, producing steam to drive turbines that generate electricity. The products of fission (daughter nuclei) are radioactive and must be stored safely.
Moderator: slows neutrons to a speed where they are effective at causing fission. Control rods: absorb neutrons to regulate the rate of the chain reaction. Inserting rods further slows or stops the reaction; withdrawing them speeds it up.
Nuclear Fusion
Nuclear fusion is the process by which two light nuclei combine to form a larger nucleus. The total mass after fusion is slightly less than the total mass before; this ‘missing’ mass is released as energy. Fusion releases much more energy per kilogram of fuel than fission and its reactants (such as deuterium and tritium, both isotopes of hydrogen) are relatively abundant.
Fusion is the energy source of stars, including our Sun. In stellar cores, extreme gravitational pressure and temperatures many millions of degrees Celsius provide the conditions for sustained fusion. On Earth, reproducing these conditions is enormously challenging: the positively charged nuclei repel each other strongly (electrostatic repulsion), so they must be given enough kinetic energy to overcome this barrier, requiring temperatures of tens of millions of degrees.
At the extreme temperatures needed for fusion, matter exists as a plasma — electrons are stripped from atoms, leaving a gas of free nuclei and electrons. This plasma cannot be contained by physical materials (it would instantly vaporise any wall) and must be held in place by powerful magnetic fields (as in a tokamak design) or compressed by powerful lasers.
The main engineering challenges are: reaching and sustaining the required temperature (tens of millions of kelvin); confining the plasma without it touching the reactor walls; and achieving net energy gain (the energy output from fusion exceeds the energy input to heat and confine the plasma). International research projects are working towards making fusion a practical, economic energy source, but as of the mid-2020s no fusion reactor has yet achieved sustained net energy gain at a power-plant scale.
Fission: large nucleus splits, releases neutrons and energy, fuel = U-235. Fusion: light nuclei combine, loss of mass releases energy, fuel = hydrogen isotopes (deuterium, tritium). Fusion powers stars. Both are sources of nuclear energy; neither involves combustion.