Nuclear energy and radioactive decay is an interactive 3D simulation of the atomic nucleus: why some nuclei are unstable, what they emit, what that radiation does, and where nuclear energy comes from. You build a nucleus from protons and neutrons, watch alpha, beta and gamma radiation leave it, shield a detector with paper, aluminium and lead, follow a decaying population through its half-life, and weigh a nucleus before and after it decays. The same binding-energy curve then explains both fission in a reactor and fusion in the Sun. It covers the radioactivity and nuclear energy chapter of Danish upper-secondary physics.
A nucleus is held together by the strong nuclear force, which acts only between neighbouring nucleons, while the electric repulsion between its protons reaches across the whole nucleus. A nucleus with too many neutrons or too many protons for its size moves toward stability by beta decay (β⁻ or β⁺), which turns a neutron into a proton or the other way round. A very heavy nucleus, where the repulsion between its many protons has grown too strong, can shed an alpha particle. Gamma radiation is not a way to fix the balance: it is the extra energy a nucleus gives off when it is left in an excited state, usually right after an alpha or beta decay.
Alpha radiation is a helium-4 nucleus with charge +2; it ionizes very densely and is stopped by a sheet of paper. Beta radiation is a fast electron (β⁻) or positron (β⁺); it ionizes less and is stopped by a few millimetres of aluminium. Gamma radiation is a high-energy photon with no charge and no mass; it is never fully stopped, only weakened, which is why thick lead is used as shielding.
The half-life T½ is the time it takes for half of the nuclei in a sample to decay. Each nucleus decays at random, but a large number follows the decay law N = N₀·(½)^(t/T½). The activity is A = k·N, where the decay constant is k = ln 2 / T½ and activity is measured in becquerel (decays per second). Carbon-14, with a half-life of about 5,730 years, is used to date organic material.
Gray (Gy) measures absorbed dose, the radiation energy absorbed per kilogram: D = E/m. Sievert (Sv) measures equivalent dose, H = D·Q, where the quality factor Q accounts for how harmful the type of radiation is — 20 for alpha and 1 for beta and gamma. The same absorbed dose of alpha radiation therefore gives a much larger equivalent dose than gamma.
The binding energy per nucleon is highest for nuclei around iron and nickel. Splitting a heavy nucleus such as uranium-235 and fusing light nuclei such as hydrogen both produce more tightly bound nuclei, so the products have less mass than the starting nuclei. The missing mass is released as energy according to E = mc² — about 200 MeV per uranium fission, and 26.7 MeV each time the Sun turns four hydrogen nuclei into one helium nucleus, counting the energy from the two positrons annihilating with electrons (a small part escapes with neutrinos).
Ionizing radiation carries enough energy to knock electrons out of atoms, turning them into ions. In living tissue this breaks molecules, and when a hit reaches DNA it can break the strands. Cells repair most of this damage, but a large dose can kill cells, and a lasting error in the DNA can lead to cancer. The same effect is used on purpose in radiotherapy to destroy tumour cells.
The nuclide chart is a grid with neutron number N on the x-axis and proton number Z on the y-axis, where every known nucleus has its own square. Stable nuclei form a narrow band. Nuclei with too many neutrons lie below the band and undergo β⁻ decay, nuclei with too many protons lie above the band and undergo β⁺ decay or electron capture, and the heaviest nuclei undergo α decay. Each decay moves the nucleus to a new square, so the chart shows which daughter nucleus forms.
A nuclear power plant works like a coal-fired plant, except that the boiler is replaced by a reactor. In the reactor, uranium-235 nuclei split when they absorb a slow neutron, releasing energy and 2–3 new neutrons. The new neutrons are fast, so a moderator — usually the water in the reactor — slows them down so they can cause new fissions. Some neutrons escape or are absorbed by other materials, and control rods adjust the absorption so that on average exactly one neutron from each fission causes a new fission, which keeps the power constant. The heat boils water into steam, the steam drives a turbine connected to a generator, and a condenser turns the steam back into water. Only about a third of the heat becomes electricity.
Cosmic radiation constantly produces carbon-14 (C-14) from nitrogen-14 in the upper atmosphere, and living organisms keep taking up carbon, so they contain the same small share of radioactive carbon-14 as the atmosphere. When the organism dies, the uptake stops and carbon-14 decays with a half-life of about 5,730 years. Measuring how much carbon-14 is left compared with a living sample gives the number of half-lives that have passed since the organism died, and therefore the age.
Background radiation is the radiation everyone constantly receives from natural sources: radon gas seeping from the ground into houses, radioactive substances in soil and building materials, cosmic radiation from space, and radioactive potassium-40 and carbon-14 inside our own bodies. In Denmark radon indoors is the largest single contribution. Medical examinations such as CT scans add to the dose.