OCR Specification focus:
‘Discuss nuclear waste impacts; describe fusion and high temperatures; balance nuclear transformation equations.’
Understanding nuclear fusion and its associated environmental impacts is essential for appreciating modern energy challenges, including how high-temperature conditions enable fusion reactions and how nuclear transformations are accurately balanced.
Impacts of Nuclear Waste and Nuclear Processes
Nuclear technologies offer significant benefits, yet they also lead to environmental and societal consequences that must be responsibly managed. The term nuclear waste refers to radioactive by-products generated from nuclear reactions or industrial and medical sources.
Nuclear waste: Radioactive materials remaining after nuclear reactions that continue to emit ionising radiation and require controlled long-term management.
Nuclear waste impacts are important to consider because they influence energy policy, waste-handling strategies, and public attitudes. These impacts arise mainly from high-level waste produced in fission reactors, although fusion itself generates far less long-lived waste.
Nuclear waste poses challenges because:
Long half-lives of certain isotopes can require containment for thousands of years.
Ionising radiation may harm living tissue or contaminate ecosystems if containment fails.
Storage and disposal demand engineered stability, often in geological repositories.
Public concern increases the regulatory and financial burden of nuclear projects.
Mitigation strategies prioritise secure containment and reduction of materials with long-term radiotoxicity.
Conditions Required for Nuclear Fusion
Fusion is the process in which two light nuclei combine to form a heavier nucleus, releasing energy due to an increase in binding energy per nucleon. Achieving fusion on Earth requires conditions similar to those found in stellar cores.
Nuclear fusion: A reaction in which two light nuclei combine to form a heavier nucleus, releasing energy because the product nucleus has higher binding energy per nucleon.
Fusion requires extremely high temperatures so that nuclei possess enough kinetic energy to overcome the electrostatic repulsion between positively charged protons. At lower energies, this Coulomb barrier prevents nuclei from approaching closely enough for the strong nuclear force to act.
Diagram showing repulsive Coulomb forces at larger separations and the attractive strong nuclear force at close range, emphasising why extremely high temperatures are needed for fusion. Source.
Key Fusion Conditions
Students should understand why fusion in terrestrial reactors demands extreme engineering measures:
High temperatures, typically over 100 million kelvin, provide the kinetic energy for nuclei to collide with sufficient velocity.
High particle density increases the frequency of collisions, improving the likelihood of fusion events.
Confinement, either magnetic (as in tokamaks) or inertial (as in laser-driven fusion), maintains these conditions long enough for net energy release.
Fuel purity and isotopic selection, commonly using deuterium and tritium, ensure optimal reaction rates.
These conditions require advanced technologies such as superconducting magnets, lasers, and vacuum chambers capable of containing ionised gases. Despite these challenges, fusion is pursued because it offers potentially abundant energy and avoids the long-term waste issues of fission.
A notable consequence of fusion reactions is the production of high-energy neutrons, which may activate surrounding materials, creating short- to medium-lived radioactive isotopes. This effect contributes to reactor component wear and disposal considerations but is significantly less severe than the waste from fission reactors.
Balancing Nuclear Transformation Equations
Balancing nuclear equations is essential for analysing fusion, fission, and radioactive processes. A nuclear transformation equation shows the reactants and products in a nuclear reaction and must obey two conservation laws.
Nuclear transformation equation: A symbolic representation of a nuclear reaction showing reactants and products, subject to conservation of charge and nucleon number.
Students must apply conservation rules systematically:
Conservation of nucleon number (A) ensures that the total number of protons and neutrons on each side of the equation is the same.
Conservation of charge (Z) ensures that total proton number remains constant throughout the reaction.
These principles are applied to all fusion reactions. For example, when deuterium and tritium fuse, the combined nucleon numbers and charges on the reactant side must equal the sum on the product side.
Diagram of the D–T fusion process, with clear labelling showing conservation of nucleon number and charge, directly supporting analysis of balanced nuclear equations. Source.
Understanding these rules is vital for analysing energy changes, determining missing particles, and checking reaction feasibility.
Balancing equations also helps identify any accompanying emission such as neutrons or gamma photons. Gamma emission often occurs when a nucleus transitions to a lower energy state, and the balancing process ensures this is indicated when appropriate.
Steps for Balancing Nuclear Equations
Identify the nucleon number and charge of each reactant.
Add these values to obtain the total A and Z for the reactant side.
Ensure the sum of A and Z on the product side matches exactly.
Insert any particles (e.g., neutrons, gamma photons, beta particles) required to maintain these equalities.
Check that all resulting particles are physically plausible within the reaction context.
Balancing nuclear equations supports deeper understanding of nuclear processes, enabling students to follow how energy release corresponds to mass change according to mass–energy equivalence.
FAQ
The deuterium–tritium (D–T) reaction has the highest fusion cross-section at the lowest achievable temperatures, making it the most practical option for current experimental reactors.
Other reactions such as deuterium–deuterium or proton–boron require far higher temperatures and are therefore less feasible with present technology.
• D–T fuel is also comparatively accessible: deuterium is abundant in seawater, and tritium can be bred from lithium inside the reactor.
The neutron carries most of the reaction energy and escapes the magnetic confinement because it is uncharged.
It strikes the surrounding reactor blanket, transferring energy as heat, which can be used to produce electricity.
• This neutron bombardment can also breed tritium by interacting with lithium-bearing materials.
• However, it gradually makes the reactor structure radioactive through neutron activation.
Magnetic confinement allows continuous or long-duration plasma operation, which is more compatible with steady electrical output.
Inertial confinement involves short, intense bursts of laser-driven compression, making it better suited to experimental studies than routine power generation.
• Magnetic systems like tokamaks can enclose larger plasma volumes, improving potential energy gain.
Reactor components are made from specialised alloys designed to resist neutron-induced swelling, embrittlement, and transmutation.
Key strategies include:
• Using low-activation steels that limit long-term radioactivity
• Designing replaceable modules for areas exposed to the highest flux
• Incorporating cooling systems to remove heat before structural damage accumulates
Even with these measures, materials must be regularly inspected and replaced to maintain safety.
Fusion does not involve splitting heavy nuclei, so it avoids generating long-lived fission products with extremely long half-lives.
The isotopes produced by neutron activation in fusion reactors typically have much shorter lifetimes, often decaying to safe levels within decades rather than millennia.
• This significantly reduces long-term storage requirements.
• However, careful handling is still needed for components exposed directly to neutron bombardment.
Practice Questions
Question 1 (2 marks)
Explain why very high temperatures are required for nuclear fusion to occur in a reactor.
Mark scheme:
• High temperatures give nuclei enough kinetic energy to overcome electrostatic (Coulomb) repulsion between positively charged nuclei. (1)
• Allows nuclei to get close enough for the strong nuclear force to act and fusion to occur. (1)
Question 2 (5 marks)
A fusion reactor uses deuterium and tritium as fuel.
(a) Write a balanced nuclear equation for the fusion of deuterium and tritium.
(b) Describe two engineering conditions required to sustain fusion in a reactor, and explain why each condition is necessary.
(c) State one advantage and one challenge of using fusion as an energy source.
Mark scheme:
(a) Balanced equation:
• 2 1 H + 3 1 H → 4 2 He + 1 0 n (1)
(Allow equivalent notation as long as A and Z are conserved.)
(b) Engineering conditions (any two, with explanation):
• Very high temperatures required to give nuclei enough kinetic energy to overcome Coulomb repulsion. (1)
• Confinement (magnetic or inertial) needed to keep the hot plasma in place long enough for fusion to occur. (1)
• Sufficient fuel density required to increase collision frequency and fusion probability. (1)
(c) Advantage and challenge:
• Advantage: produces no long-lived high-level radioactive waste / produces large amounts of energy from abundant fuels / no risk of runaway chain reaction. (1)
• Challenge: extremely difficult to maintain required temperatures and confinement / neutron activation of reactor materials / current reactors have not yet achieved net energy gain. (1)
