Flashcards · 9630 · A Level Topic 12

Nuclear energy (A Level) flashcards

25 cards on A Level Topic 12, subtopics 3.12.1 to 3.12.5. Pick a subtopic or study them all.

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Distance of closest approach for an alpha particle fired at a nucleus: All the kinetic energy becomes electric potential energy: Ek = (2e × Ze)/(4πε₀r). It gives only an upper limit for the nuclear radius.
Why are high-energy electrons used to measure nuclear radius by diffraction?: Electrons are not affected by the strong interaction, only the nuclear charge. At hundreds of MeV their de Broglie wavelength is comparable with nuclear size. First minimum: sin θ ≈ 1.22λ/D.
Nuclear radius and nucleon number: R = R₀A^(1/3), with R₀ ≈ 1.2 fm (1 fm = 10⁻¹⁵ m).
Why is nuclear density the same for all nuclei?: Volume ∝ R³ ∝ A and mass ∝ A, so mass ÷ volume is independent of A: about 2.3 × 10¹⁷ kg m⁻³.
How does lg R against lg A test R = R₀A^(1/3)?: It gives a straight line of gradient 1/3, with intercept lg R₀.

Mass defect: The mass of the separate nucleons minus the mass of the nucleus: Δm = Zm_p + (A − Z)m_n − m_nucleus.
Binding energy: The minimum energy needed to separate a nucleus into its individual protons and neutrons. It equals Δmc².
Atomic mass unit and its energy equivalent: 1 u = 1/12 of the mass of a carbon-12 atom = 1.66 × 10⁻²⁷ kg, equivalent to 931.5 MeV.
Binding energy per nucleon graph: where is the peak, and what does it mean for fission and fusion?: Peak near A = 56 (iron), the most tightly bound nuclei. Fusion of light nuclei (left of the peak) and fission of heavy nuclei (right of it) both increase binding energy per nucleon, so release energy.
How do you find the energy released in a nuclear reaction from masses?: Total mass before − total mass after, in u, × 931.5 gives MeV. Use nuclear masses throughout (or atomic masses when the electron numbers balance).

Induced fission: A heavy nucleus (e.g. uranium-235) absorbs a neutron and splits into two fragments, releasing two or three neutrons and energy.
Thermal neutron: A neutron slowed until its kinetic energy is of the order of the thermal energy of its surroundings (about 0.04 eV at room temperature).
Chain reaction and critical mass: Chain reaction: neutrons from each fission go on to cause further fissions. Critical mass: the least mass of fissile material that can sustain a chain reaction.
Why is there a critical mass?: Neutrons are produced throughout the volume but escape through the surface. The surface-to-volume ratio falls as size increases, so below a certain mass too many neutrons escape.
Roles of the moderator, control rods and coolant: Moderator: slows neutrons by elastic collisions (water, graphite). Control rods: absorb neutrons to control the rate (boron, cadmium). Coolant: removes energy from the core (pressurised water, carbon dioxide).

How is a reactor shut down in an emergency?: Control rods drop fully into the core (under gravity or on an alarm); absorber can be added to the coolant. Cooling must continue because of decay heat from the fission products.
Why is spent fuel handled remotely?: It is intensely radioactive and hot. Machines move it, often under water, which both shields and cools it.
Shielding around a reactor core: A steel pressure vessel inside thick concrete. Water absorbs neutrons; dense materials absorb gamma rays.
Treatment of low-, intermediate- and high-level waste: Low: sealed and buried at shallow depth. Intermediate: shielded and set in concrete. High (spent fuel): cooled, sealed in glass and steel, stored deep underground.
Benefits and risks of nuclear power: Benefits: high energy per kilogram, no carbon dioxide from the reactor in normal use, steady output. Risks: rare but severe accidents, long-lived waste, decommissioning cost, links to weapons.

Nuclear fusion: Two light nuclei combine to form a heavier nucleus, releasing energy because the mass of the products is less than the mass of the reactants.
Why does fusion need a very high temperature?: The nuclei are both positive and repel. They need enough kinetic energy (mean Ek = (3/2)kT) to get close enough for the strong interaction to act, so temperatures of about 10⁸ K are needed.
Which nuclei are used in fusion reactors?: Deuterium (²₁H) and tritium (³₁H). Tritium can be made from lithium-6 by neutron absorption in a lithium blanket.
Practical problems with fusion power: Reaching about 10⁸ K; confining the plasma with magnetic fields; achieving enough density and confinement time for net energy gain; neutron damage to the structure; tritium supply.
Plasma: An ionised gas of nuclei and free electrons.