Know that research is being carried out to investigate how energy released by nuclear fusion can be used to produce electrical energy on a large scale

5.1 The nuclear model of the atom

5.1.1 Atomic structure and experimental evidence

The atom consists of a tiny, dense nucleus surrounded by a cloud of electrons. The existence of the nucleus was proved by the Rutherford scattering experiment (1911): α‑particles from a radioactive source were directed at a thin gold foil. Most passed straight through, but a few were deflected at large angles, showing that the positive charge and most of the mass are concentrated in a very small region.

5.1.2 The nucleus

• Protons (p) – positively charged particles; each carries a charge of +1 e.
• Neutrons (n) – electrically neutral particles; mass ≈ 1 u.
• Atomic number (Z) – number of protons; determines the element.
• Mass number (A) – total number of nucleons (protons + neutrons).
• Neutron number (N) – N = A − Z.

Example: For the nuclide \(\,^{14}_{6}\text{C}\) the number of neutrons is \(N = 14 - 6 = 8\).

5.1.3 Isotopes

Atoms of the same element (same Z) that have different A are called isotopes. They have identical chemical behaviour but different nuclear properties (e.g., stability, radioactivity).

5.2 Radioactivity

5.2.1 Detection of radiation and background correction

• Geiger‑Müller (GM) tube – a gas‑filled detector that produces a pulse each time ionising radiation passes through it.
• Read‑out is usually given as counts per second (cps) or counts per minute (cpm).
• Because the environment always contains some radiation, a background count must be measured and subtracted from the sample reading.

Example of background subtraction:

1. Measure background for 60 s → 120 cpm.
2. Measure sample for 60 s → 560 cpm.
3. Net activity = 560 − 120 = 440 cpm.

5.2.2 Three types of emission

5.2.3 Radioactive decay

Radioactive nuclei spontaneously transform into more stable nuclei, emitting radiation. The parent nucleus changes into a different element (or a different isotope of the same element).

Typical decay equations (in nuclide notation):

• α‑decay: \(\displaystyle ^{A}{Z}\text{X} \;\rightarrow\; ^{A-4}{Z-2}\text{Y} + ^{4}_{2}\text{He}\)
• β⁻‑decay: \(\displaystyle ^{A}{Z}\text{X} \;\rightarrow\; ^{A}{Z+1}\text{Y} + ^{0}_{-1}e\)
• β⁺‑decay (positron emission): \(\displaystyle ^{A}{Z}\text{X} \;\rightarrow\; ^{A}{Z-1}\text{Y} + ^{0}_{+1}e\)
• γ‑decay: \(\displaystyle ^{A}{Z}\text{X}^{*} \;\rightarrow\; ^{A}{Z}\text{X} + \gamma\) (no change in A or Z, just loss of excess energy)

Example: α‑decay of uranium‑238: \(\displaystyle ^{238}{92}\text{U} \;\rightarrow\; ^{234}{90}\text{Th} + ^{4}_{2}\text{He}\).

5.2.4 Half‑life

The half‑life (t½) is the time required for half of a radioactive sample to decay.

Worked example: Cobalt‑60 has a half‑life of 5 years. A 8 g sample is present now. How much remains after 15 years?

1. Number of half‑lives in 15 yr: \(15 \text{yr} / 5 \text{yr} = 3\).
2. Remaining fraction = \((\frac{1}{2})^{3} = \frac{1}{8}\).
3. Mass remaining = \(8 \text{g} \times \frac{1}{8} = 1 \text{g}\).

5.2.5 Safety precautions

• Time – minimise exposure duration.
• Distance – increase distance from the source (inverse‑square law).
• Shielding – use appropriate materials (e.g., paper for α, aluminium for β, lead or concrete for γ).

Example: When handling a sealed \(\,^{137}\text{Cs}\) source (γ‑emitter), a technician works behind a 5 cm lead shield and stays at least 2 m away, reducing the dose to a few µSv h⁻¹.

1.7.3 Energy Resources – Nuclear Fusion

Learning objective

Understand that research is being carried out to investigate how the energy released by nuclear fusion can be used to produce electrical energy on a large scale.

What is nuclear fusion?

Fusion is the process in which two light atomic nuclei combine to form a heavier nucleus. The combined mass is slightly less than the mass of the reactants; the missing mass is released as energy according to Einstein’s equation \(E=mc^{2}\).

The reaction most widely studied for power generation is the deuterium‑tritium (D‑T) reaction:

\[

^{2}{1}\text{H} + ^{3}{1}\text{H} \;\rightarrow\; ^{4}_{2}\text{He} + n + 17.6\ \text{MeV}

\]

• Deuterium (D) – abundant in seawater (≈0.015 % of hydrogen).
• Tritium (T) – scarce, but can be bred inside the reactor from lithium‑6:

\[

^{6}{3}\text{Li} + n \;\rightarrow\; ^{4}{2}\text{He} + ^{3}_{1}\text{H}

\]

Why fusion is attractive for large‑scale electricity generation

• Fuel abundance – deuterium from seawater and lithium for tritium breeding are virtually limitless.
• Very high energy density – ≈\(3\times10^{14}\ \text{J kg}^{-1}\), orders of magnitude above chemical fuels.
• Low‑level radioactive waste – only short‑lived activation products; no long‑lived high‑level waste.
• Inherent safety – the reaction stops automatically if plasma confinement fails; there is no risk of a runaway chain reaction.
• Zero greenhouse‑gas emissions during operation.

Major international research projects (status 2025)

Key technical challenges

• Achieving the required temperature – ≈100 million °C so that nuclei have enough kinetic energy to overcome Coulomb repulsion.
• Plasma confinement – keeping the ultra‑hot plasma away from material walls:

  • Magnetic confinement (tokamaks, stellarators, spherical tokamaks).
  • Inertial confinement (laser or particle‑beam compression).

• Plasma stability and control – avoiding disruptions, controlling turbulence, and maintaining the required density and confinement time (Lawson criterion).
• Materials durability – first‑wall and blanket must survive intense 14 MeV neutron flux and high heat loads.
• Net energy gain (Q‑factor) – the reactor must produce more fusion power than the total power used for heating, current drive and auxiliaries (Q > 1; commercial aim Q ≥ 10).
• Tritium breeding and fuel cycle – a lithium blanket must breed enough tritium to sustain operation.

How fusion energy is converted to electricity

1. Heat extraction – high‑energy neutrons escape the plasma and are absorbed in a lithium‑containing blanket, heating a coolant (water, helium, or liquid metal).
2. Steam generation – the hot coolant transfers heat to a secondary water circuit, producing high‑pressure steam.
3. Power conversion – the steam drives a turbine connected to an electrical generator (the same principle as conventional thermal power stations).
4. Direct‑conversion concepts (research stage) – e.g., using the kinetic energy of charged fusion products to drive magnetohydrodynamic (MHD) generators or electrostatic converters.

Comparison of major energy sources

Future outlook and milestones

• ITER – first plasma (2025); full‑power D‑T operation (late 2020s); aim to demonstrate Q≈10.
• DEMO – design completion early 2030s; construction mid‑2030s; target net‑electricity delivery to the grid by the 2040s.
• Commercial fusion power plants – envisaged for the 2050s if DEMO succeeds and engineering challenges are solved.
• Technology spin‑offs – advances in superconducting magnets, high‑heat‑flux materials and plasma diagnostics benefit medicine, industry and space exploration.

Key points to remember

1. Fusion joins light nuclei; the mass loss is released as energy via \(E=mc^{2}\).
2. The most promising fuel pair is deuterium‑tritium, releasing 17.6 MeV per reaction.
3. Research programmes such as ITER, DEMO, NIF, JET, EAST and KSTAR aim to achieve net‑positive energy gain (Q > 1) and eventually commercial electricity generation.
4. Principal challenges: reaching ≈100 million °C, confining the plasma long enough, handling neutron‑induced material damage, and establishing a self‑sustaining tritium‑breeding fuel cycle.
5. If successful, fusion could provide large‑scale, low‑carbon electricity with abundant fuel and minimal long‑lived waste.