The Sun generates its energy by nuclear fusion. In the core of the Sun, hydrogen nuclei (protons) are forced together at extremely high temperatures and pre...
Question 1 Report
The Sun generates its energy by nuclear fusion. In the core of the Sun, hydrogen nuclei (protons) are forced together at extremely high temperatures and pressures to form helium nuclei, releasing enormous amounts of energy. Fig. 64.1 shows a simplified cross-section of the Sun with its core temperature labelled. The core temperature is approximately 1.5 × 10⁷ K. Scientists on Earth are attempting to reproduce fusion in experimental reactors, but achieving and maintaining the necessary conditions is extremely challenging. A fusion reactor would have significant advantages over a fission reactor as a source of energy. The fuel for fusion (hydrogen isotopes deuterium and tritium) is abundant, and the reaction produces helium, which is not radioactive. However, no fusion power station is yet in commercial operation.
(a) Define nuclear fusion. [2]
(b) Explain why extremely high temperatures are required for fusion to occur. [2]
(c) State two advantages of fusion over fission as a source of energy. [2]
(d) Suggest why scientists have not yet been able to build a commercial fusion power station. [2]
Answer Details
(a) Define nuclear fusion [2]
Nuclear fusion is the process in which two light nuclei join together (combine) to form a single heavier nucleus, releasing energy in the process. [2]
Both conditions matter for the definition: the nuclei must be light (hydrogen, helium range), and energy is released because the product nucleus has a lower mass per nucleon than the reactants. The "missing" mass is converted to energy via \(E = mc^2\).
(b) Why extremely high temperatures are required [2]
Both hydrogen nuclei carry a positive charge, so they experience strong electrostatic (Coulomb) repulsion. [1] At extremely high temperatures (of order 107 K), the nuclei gain enough kinetic energy to overcome this repulsion and approach close enough for the strong nuclear force to bind them together. [1]
Temperature is a measure of the average kinetic energy of particles. Only when \(\frac{3}{2}kT\) is large enough can the nuclei get within the very short range (~10-15 m) at which the strong force dominates over the Coulomb barrier.
(c) Two advantages of fusion over fission [2]
- The fuel for fusion (deuterium and tritium, isotopes of hydrogen) is abundant and readily available; deuterium can be extracted from seawater. [1]
- The product of fusion (helium) is not radioactive, so fusion produces far less radioactive waste than fission, which generates highly radioactive fission fragments and spent fuel. [1]
(d) Why commercial fusion power stations do not yet exist [2]
The plasma must be heated to temperatures exceeding 107 K, and no known solid material can withstand direct contact with such a plasma. [1] Magnetic confinement (tokamaks) or inertial confinement (laser-driven) techniques are used instead, but so far none has sustained a net energy gain: the energy input needed to maintain the plasma exceeds the fusion energy output. [1]
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