The Lab · Nuclear energy
The Modern Alchemy
Every atom has a tiny core. We call it the nucleus. There are two ways to get energy from it.
In both cases, the pieces at the end weigh a little less. The lost mass becomes energy. Einstein's rule tells us how much. The c is the speed of light, a huge number. So one gram of mass holds about 90 trillion joules. A big power station makes that much in about a day. Play with E = mc² →
01 · Fission
Split a heavy nucleus
A neutron is a tiny particle with no charge. It hits a uranium-235 nucleus. The number counts the particles in the nucleus.
One split starts more splits
Each split frees about 3 neutrons. Each one can split another uranium nucleus. This is a chain reaction.
Control rods
Control rods are bars that soak up neutrons. They keep it at one new split per split. So the power stays steady.
Heat makes electricity
The heat boils water. The steam spins a turbine. The turbine turns a generator.
About 200 MeV per split
MeV is a tiny unit of energy for single atoms. Most of it is the speed of the two pieces. It ends up as heat.
02 · Fusion
Join two light nuclei
Deuterium and tritium are heavy forms of hydrogen. They fuse into helium-4.
Nuclei push apart
Both nuclei have a plus charge. Like charges repel. Only extreme heat makes them crash together. A fusion machine needs about 150 million °C.
Not a power plant yet
In 2022, a US lab got more energy out of a fuel pellet than its lasers put in. No fusion power plant exists yet.
The Sun: a fusion reactor you can see
The Sun's core is about 15 million °C. Gravity squeezes the core very hard. So the Sun needs no machine. Four hydrogen nuclei join, step by step, into one helium-4. On the way, two protons turn into neutrons. The lost mass leaves as energy. It reaches us as sunlight.
The Sun turns this much mass into energy each second.
Sunlight takes this long to reach Earth.
The Sun has been doing this for this long.
Sun vs. reactor. The Sun fuses ordinary hydrogen. It does this slowly. Reactors on Earth use deuterium and tritium. They fuse much more easily.
03 · A new idea
Fusion could also make gold
Fast fusion neutrons hit a thick layer around the hot fuel. We call it the blanket. One idea puts mercury in the blanket.
STEP 1 OF 6Deuterium and tritium fuse. A fast neutron flies out.
The diagram follows each step. Swipe it to look around.
- Deuterium and tritium fuse. A fast neutron flies out.
- The neutron hits mercury-198.
- Mercury-198 takes in 1 neutron. It lets out 2. Now it is mercury-197.
- The 2 spare neutrons hit lithium-6. Each lithium-6 splits into tritium and helium-4. Tritium is new fuel.
- Mercury-197 is unstable. One proton turns into a neutron. In a few days, it becomes gold-197. See the close-up ↓
- The blanket still gets hot. That heat still makes electricity.
Do not misread it. Mercury does not turn into lithium. The lithium-6 is already in the blanket. Mercury only gives it spare neutrons.
Close-up: how mercury-197 becomes gold
An atom has electrons around its nucleus. The closest ones sit in the inner shell. This change is called electron capture.
STEP 1 OF 5Mercury has 80 protons.
This is not fission. Nothing splits. One proton just changes into a neutron. The total stays 197. 80 protons is mercury. 79 protons is gold.
This plan comes from a 2025 study by the company Marathon Fusion. It is based on computer models. Other scientists still need to check it. Mercury-197 has a half-life of about 64 hours. Half-life is the time for half of it to change.
Read the study (arXiv 2507.13461) · See the full story: Mercury Into Gold →
04 · Side by side
Fission vs fusion
Energy figures assume pure uranium-235, fully split (about 82 TJ per kg), and a deuterium–tritium mix, fully fused (about 340 TJ per kg). Coal gives about 24 MJ per kg. Real reactor fuel uses only part of its uranium.