The Atomic Incumbent: Why Uranium Rules the Reactor

And why better alternatives remain on the sidelines

In the periodic table of elements, few hold as much geopolitical and scientific weight as element 92. Uranium is the undisputed king of nuclear energy. It powers our submarines, lights up 10% of the world’s electricity grids, and defines modern energy security.

But here is the paradox: from a purely physics standpoint, uranium is not necessarily the "best" fuel. We have Thorium (element 90), which is more abundant and safer. We have Plutonium (element 94), which is more energy-dense.

So, why are we locked into a uranium economy? The answer is a complex tapestry woven from the unique physics of the atom, the urgent pressures of World War II, and the massive inertia of industrial infrastructure.

1. The Physics: The Magic of U-235

To understand uranium’s dominance, you must understand the concept of "fissile" versus "fertile."

The vast majority of natural uranium (99.3%) is the isotope Uranium-238. This isotope is heavy and stable; if you hit it with a neutron, it usually just absorbs it. It doesn't split easily.

However, the remaining 0.7% is Uranium-235. This specific isotope is the only naturally occurring fissile material on Earth.

The "Fissile" Advantage

Being "fissile" means that if a slow-moving (thermal) neutron hits the nucleus of U-235, it becomes unstable and splits apart (fissions), releasing

1. Massive Energy: In the form of heat.
2. More Neutrons: usually 2 or 3 extra neutrons.

These extra neutrons go on to hit other U-235 atoms, creating a chain reaction.

Why this matters: No other naturally occurring element can sustain a chain reaction on its own.

• Thorium-232 is fertile, not fissile. You can't just put thorium in a reactor and start it up. You have to bombard it with neutrons first to turn it into Uranium-233.
• Plutonium doesn't exist in nature in significant quantities; it must be manufactured inside a reactor.

Uranium-235 is nature's only "turnkey" nuclear fuel. You mine it, enrich it slightly, and the fire starts.

2. The History: The Shadow of the Bomb

If physics provided the spark, history poured the gasoline. The reason we use uranium—and specifically the type of uranium reactors we use today—is largely due to the Manhattan Project and the Cold War.

The Dual-Use Dilemma

During World War II, the goal was not electricity; it was a weapon. The US government pursued two paths to the atomic bomb:

1. The Uranium Path: Enriched U-235 (used in the "Little Boy" bomb).
2. The Plutonium Path: Irradiating U-238 to create Plutonium-239 (used in the "Fat Man" bomb).

To get Plutonium, you need a reactor. The US developed reactors specifically optimized to turn uranium into plutonium.

The Rickover Legacy

After the war, Admiral Hyman Rickover was charged with building a nuclear navy. He needed a reactor that was compact, power-dense, and reliable enough to fit inside a submarine. He chose the Pressurized Water Reactor (PWR) using enriched uranium.

It worked brilliantly. When the US started building civilian power plants (like Shippingport in 1957), they didn't reinvent the wheel. They simply scaled up the submarine design. Because the government had already spent billions developing the supply chain for uranium enrichment and water-cooled reactors, it became the cheapest option for commercial companies.

We use uranium today because we built a massive industrial complex to use it 80 years ago.

3. The Challengers: Why Not Thorium or Plutonium?

If uranium has historical baggage and isn't the most abundant option, why haven't we switched?

The Case of Thorium (Th-232)

Thorium enthusiasts often point out its benefits:

• Abundance: It is roughly three times more common than uranium.
• Safety: It is harder to weaponize.
• Waste: It produces less long-lived transuranic waste.

The Barrier: Thorium reactors (specifically Molten Salt Reactors) require liquid fuel chemistry that is incredibly corrosive. We need materials that can withstand hot, radioactive salts for decades. Furthermore, because thorium isn't fissile on its own, you still need uranium or plutonium to "kickstart" the reaction. Switching to thorium requires building an entirely new infrastructure from scratch—a cost in the trillions.

The Case of Plutonium (Pu-239)

We actually do use plutonium. In "Breeder Reactors," we can turn useless U-238 into fuel (Plutonium). This theoretically allows us to extract 60x more energy from uranium ore.

The Barrier:

1. Proliferation Risk: Plutonium is the easiest material for making nuclear weapons. A global economy based on plutonium increases the risk of theft or misuse.
2. Complexity: Liquid metal fast breeder reactors (which use sodium as a coolant) are notoriously finicky and expensive to maintain compared to standard uranium water reactors.

4. The Inertia of Infrastructure

The final reason is economic gravity.

• The Supply Chain: We have massive mines in Kazakhstan, Canada, and Australia dedicated to uranium.
• Enrichment Plants: We have centrifuges spinning globally specifically tuned for U-235.
• Regulation: The Nuclear Regulatory Commission (NRC) and global bodies have 70 years of safety codes written specifically for solid-fuel, light-water uranium reactors. Licensing a new type of reactor (like a Thorium liquid salt reactor) takes decades and billions of dollars in paperwork alone.

Summary: The Perfect Storm

Uranium’s dominance isn't just about it being the "best" fuel; it is the result of a convergence of three powerful factors:

1. The Scientific Spark (Physics) Uranium-235 is unique in nature. It is the only isotope found in the ground that is "fissile," meaning it can start and sustain a nuclear chain reaction on its own. While other elements like Thorium can eventually become fuel, they need a jump start. Uranium is nature's only "turnkey" solution—you just add neutrons, and it works.

2. The Shadow of History (Momentum) The nuclear industry was born out of World War II and the Cold War. The US government spent massive amounts of money developing uranium enrichment and water-cooled reactors for atomic bombs and nuclear submarines. By the time civilian power plants were being built, the uranium technology path was already paved, tested, and paid for by the military.