Nuclear Energy lessons

Inside the nucleus

A nucleus is made of protons and neutrons, together called nucleons. The number of protons is the atomic number Z, and it decides the element.

The total number of nucleons is the mass number A. We write A on top and Z below, beside the element's symbol.

Nuclide notation: ^{A}_{Z}\text{X}

Isotopes of an element have the same number of protons but different numbers of neutrons. The number of neutrons is N = A - Z.

Pick an element and change its neutrons; the nucleus, its notation and its stability update.

Practice: Read nuclide notation

Type the number of protons, neutrons or nucleons in a nuclide.

The missing mass

A nucleus has slightly less mass than the protons and neutrons it is made from. The missing mass is called the mass defect.

That mass was released as energy when the nucleus formed. Pulling the nucleus apart takes the same energy back, called the binding energy.

Mass-energy equivalence: E = mc^2

Here m is mass in kilograms and c is the speed of light, 3.00 \times 10^8\ \text{m/s}. Because c^2 is so large, a tiny mass is a lot of energy.

A balance weighs loose nucleons against the nucleus they make, and shows the mass defect as energy.

Practice: Turn mass into energy

Use E = mc² to find the energy from a few grams of mass, in joules.

The binding energy curve

Dividing a nucleus's binding energy by its mass number gives the binding energy per nucleon. The higher it is, the more tightly each nucleon is held.

The curve peaks near iron. Any change that moves nuclei up the curve releases energy.

Light nuclei climb it by joining, which is fusion. Heavy nuclei climb it by splitting, which is fission.

The binding energy per nucleon curve, stepping through fusion, fission and the peak near iron.

Practice: Read the curve

Tap the nucleus on the binding energy curve that answers each question.

Splitting uranium

In fission, a heavy nucleus absorbs a slow neutron and splits in two. Uranium-235 is the usual fuel.

The fragments vary from split to split. But the mass numbers on each side always add to the same total, and so do the atomic numbers.

Each split releases about 200 MeV and 2 or 3 new neutrons. Most of that energy is in the fast fragments, which heat the fuel.

One fission played in stages, with its equation and the mass and atomic number totals.

Practice: Balance a fission equation

Type the mass number and atomic number of the missing fission fragment.

Chain reactions

The neutrons from one fission can split more nuclei. This is a chain reaction.

The average number of new fissions each fission causes is called k. When k = 1 the reaction is critical and runs steadily, as in a power station.

Below 1 it is subcritical and dies away. Above 1 it is supercritical and grows each step.

Fissions per generation for k below, at and above 1.

Practice: Critical or not?

Decide whether a reactor scenario is subcritical, critical or supercritical.

Inside a reactor

The fuel rods hold the uranium, where fission makes heat. A moderator, usually water, slows the neutrons, because slow neutrons split uranium-235 far more easily.

Control rods absorb neutrons. Lowering them slows the reaction, and dropping them fully in shuts it down.

A coolant carries the heat away to make steam. The steam spins a turbine, which turns a generator.

A pressurised water reactor, highlighting each part in turn with what it does.

Practice: Match the reactor parts

Match each reactor part to its job.

Radioactive decay

Some nuclei are unstable. They settle by giving out radiation, which is called radioactive decay.

In alpha decay the nucleus throws out 2 protons and 2 neutrons, a helium nucleus. In beta decay a neutron turns into a proton, and a fast electron leaves.

In gamma decay the nucleus gives out energy as an electromagnetic wave. Its A and Z stay the same.

Alpha, beta and gamma decay of real nuclei, with what happens to A and Z.

Practice: Find the new nucleus

Pick the nucleus left after an alpha or beta decay.

Stopping radiation

Radiation is ionising: it knocks electrons off the atoms it passes. In living cells, this can damage DNA.

Alpha is stopped by paper or the dead outer layer of skin. It is the most ionising, so it does most harm if breathed in or swallowed.

Beta is stopped by a few millimetres of aluminium. Gamma is the least ionising but the most penetrating, and thick lead or concrete cuts it down.

Alpha, beta and gamma fired at paper, aluminium and lead.

Practice: Alpha, beta or gamma?

Sort facts about radiation into alpha, beta and gamma.

Half-life

No one can predict when a single unstable nucleus will decay. But in a large sample, half of the nuclei decay in a fixed time called the half-life.

After one half-life, half the sample remains. After two, a quarter remains, and so on.

After n half-lives: N = N_0 \times \left(\frac{1}{2}\right)^{n}

Here N_0 is the starting amount and n is the number of half-lives. Half-lives range from fractions of a second to billions of years.

A sample of iodine-131 halving every 8 days, beside its decay curve.

Practice: Half-life sums

Find the amount left, the time taken or the half-life of a real isotope.

Nuclear waste and safety

Used fuel is very radioactive and hot, because it is full of fission fragments. It cools under water for years, then moves into sealed steel and concrete casks.

Most of its radioactivity fades within a few hundred years, but some lasts for many thousands. That part is meant for deep stores in stable rock, and Finland is the first country to build one.

Reactors keep several barriers around the fuel, and the control rods go in automatically if something goes wrong. Serious accidents like Chernobyl and Fukushima are rare, and each one led to stricter rules.

How much of three isotopes in used fuel is left after 1 to 10,000 years.

Practice: Put the steps in order

Order the steps from fission to electricity, then the journey of used fuel.

Fusion

In fusion, two light nuclei join to make a heavier one. It powers the Sun, which turns hydrogen into helium.

Nuclei are positive, so they push each other away. They only fuse if they move fast enough to touch, which on Earth takes about 150 million °C.

Deuterium-tritium fusion: ^{2}_{1}\text{H} + ^{3}_{1}\text{H} \to ^{4}_{2}\text{He} + ^{1}_{0}\text{n} + 17.6\ \text{MeV}

In 2022, a laser experiment in the USA got more energy out of fusion than its lasers put in. No power station runs on fusion yet, but its fuel is plentiful and its waste is short-lived.

Deuterium and tritium bounce apart when cool and fuse when hot enough.

Practice: Fission, fusion or both?

Sort statements into fission, fusion or both.