So how does splitting a rock boil enough water to power a city?
This is the part that completely fascinated me.
Uranium isn’t like coal.
Nothing is burning.
No oxygen is involved.
No flames.
No smoke.
Instead, the energy comes from the nucleus of the atom itself.
Most commercial reactors use Uranium-235, an isotope that makes up less than one percent of naturally occurring uranium.
When a neutron collides with a Uranium-235 atom, something remarkable can happen.
The nucleus becomes unstable.
Instead of simply absorbing the neutron, the atom splits into two smaller atoms known as fission products.
That split releases three incredibly important things.
- Heat.
- Two or three more neutrons.
- Gamma radiation.
The heat is the part we’re interested in.
It isn’t chemical energy like burning coal.
It’s nuclear binding energy.
A tiny amount of mass literally disappears during the split and is converted directly into energy according to Einstein’s famous equation:
E = mc²
The amount of mass involved is unbelievably small.
The amount of energy released is enormous.
That’s why a fuel pellet about the size of the tip of your thumb contains roughly the same usable energy as around a tonne of coal.
The chain reaction
The really clever bit isn’t splitting one atom.
It’s controlling billions upon billions of them.
Remember those extra neutrons released during fission?
They don’t simply disappear.
Many go on to strike neighbouring Uranium-235 atoms.
Those atoms split too.
Which releases more neutrons.
Which split more atoms.
Left completely uncontrolled, the reaction would increase extremely rapidly.
A power station doesn’t want that.
It wants something wonderfully boring.
A reaction that stays almost exactly the same every second of every day.
Engineers achieve this using several systems working together.
The moderator
Fresh neutrons are travelling incredibly fast.
Too fast to efficiently split more uranium atoms.
The reactor contains a moderator, usually water or graphite depending on the reactor design.
The moderator slows the neutrons down.
Oddly enough, slower neutrons are actually better at causing further fission.
Control rods
Control rods are made from materials that absorb neutrons extremely effectively.
They’re essentially neutron sponges.
Push the rods further into the reactor…
They absorb more neutrons.
The chain reaction slows down.
Pull them out…
Fewer neutrons are absorbed.
The reaction speeds up.
This allows operators to carefully control exactly how much heat the reactor produces.
Cooling
Water performs another important job.
It continuously removes heat from the fuel.
That heat is then used to generate steam.
The steam drives turbines.
The turbines drive generators.
From this point onwards…
The engineering starts looking remarkably similar to every other thermal power station we’ve looked at in this series.
Coal burns to make heat.
Gas burns to make heat.
Biomass burns to make heat.
Concentrated solar focuses sunlight to make heat.
Nuclear simply gets its heat from somewhere entirely different.
Why the pressure vessel is such an engineering marvel
One thing I hadn’t appreciated before researching nuclear was just how extraordinary the reactor vessel itself is.
It has to safely contain:
- High temperatures.
- High pressures.
- Intense radiation.
- Fast-moving neutrons.
- Highly purified water.
- Fuel assemblies operating continuously for years.
The steel walls can be more than 20 centimetres thick.
The welds undergo exhaustive inspection.
Every pipe.
Every valve.
Every pump.
Every sensor.
Everything is designed with redundancy.
Everything is monitored.
Because unlike many industries, nuclear engineering assumes components will eventually fail.
The objective is ensuring that when they do…
The reactor remains safe.
Gareth Winterman