The Fuel Cycle & Waste

Every kilowatt of nuclear electricity starts with a rock in the ground — and ends, eventually, hundreds of metres below it. Between mining and disposal lies a complex global journey spanning decades and multiple continents.

Explore the interactive diagram below. Toggle between the UK's once-through approach (use fuel once, dispose of it) and the closed cycle used by France (recycle the fuel, minimise waste).

🔑 Why This Matters

The fuel cycle isn't just an abstract process — it shapes energy security, waste management, and nuclear economics. Here's what you'll discover in this module:

The "front end" of the fuel cycle covers everything that happens before uranium enters a reactor. It's a global journey — uranium mined in Kazakhstan might be converted in Canada, enriched in the UK, and fabricated into fuel for a French reactor.

Let's follow the complete journey, step by step.

Inside Springfields: The Fabrication Process

✓ Front End Complete

The fuel assemblies are now ready to be transported to a reactor, where they'll generate electricity for 3–5 years. This marks the end of the front end of the fuel cycle.

Before we explore what happens after the reactor (the back end), let's take a closer look at the most sensitive step: enrichment.

🔬 A Closer Look at the Critical Step

You've just seen enrichment as step 4 in the fuel cycle journey. But this single step deserves special attention — it's technically challenging, internationally regulated, and central to understanding how nuclear fuel is prepared.

Let's explore how it actually works, and why it's such a sensitive technology.

🎯 The Problem: Natural Uranium Can't Power Most Reactors

Natural uranium is 99.3% U-238 and only 0.7% U-235. But it's U-235 that can sustain a chain reaction — U-238 mostly just absorbs neutrons without fissioning.

For most reactor designs, we need to increase that U-235 concentration to 3–5%. This process is called enrichment.

📊 What Does "Enrichment" Actually Mean?

The enrichment percentage tells you how many U-235 atoms are present compared to U-238. Use the interactive below to visualise what different enrichment levels look like at the atomic scale.

⚠️ The Challenge: They're Chemically Identical

U-235 and U-238 are the same element. They have the same number of protons, the same electrons, the same chemical behaviour. You cannot separate them using chemistry.

The only difference: U-238 has 3 extra neutrons, making it about 1.3% heavier.

The solution: Convert uranium to a gas (UF₆), then exploit that tiny mass difference using physical processes — specifically, spinning at extreme speeds.

⚙️ The Solution: Gas Centrifuges

UF₆ gas is fed into a cylindrical rotor spinning at up to 50,000–70,000 RPM — faster than a jet engine turbine. The slight mass difference between U-235 and U-238 molecules causes them to separate under these extreme forces. The enriched stream (higher U-235 concentration) is extracted from one output, whilst the depleted stream (higher U-238) exits from another.

🔗 Why Thousands of Centrifuges?

Here's the catch: a single centrifuge barely changes the concentration — each pass might increase the U-235 proportion by a fraction of a percent. To go from 0.7% to 3–5%, the gas must flow through hundreds or thousands of centrifuges arranged in a cascade — each stage feeding slightly more concentrated gas into the next, with the enrichment compounding gradually.

🎮 Challenge: Can You Sustain a Chain Reaction?

Now you understand what enrichment means — but can you feel the difference it makes? In the simulator below, red nuclei are U-235 (they fission and release more neutrons) and blue nuclei are U-238 (they absorb neutrons without fissioning).

🤯 What Just Happened? You Discovered the Nuclear Weapons Problem

If you pushed the simulator to high enrichment, you just experienced one of the most concerning aspects of nuclear technology: the same process that makes fuel for peaceful power generation can make material for weapons.

You started with an innocent goal — sustain a chain reaction for electricity. But by simply running the enrichment process longer, you discovered you could create something much more destructive. This is the fundamental dilemma of nuclear technology.

🧠 Wait — So Could a Power Plant Explode Like a Bomb?

You might now be thinking: "If nuclear plants use the same physics as bombs, couldn't a reactor explode like Hiroshima?"

Absolutely not. It is physically impossible.

You just experienced exactly why. In the simulator, what happened at 5% enrichment versus 90%? At 5%, you struggled to keep the chain going. At 90%, it went explosive almost instantly.

This is one of the most common misconceptions about nuclear power. Surveys show that nearly 40% of people believe a nuclear plant could explode like an atomic bomb. Now you know the physics that makes this impossible:

• Wrong enrichment: Bombs need 90%+, reactors use 3–5%
• Wrong geometry: Bombs need precise compression, reactors are spread out
• Wrong design: Reactors have control rods, coolant, and passive safety systems

What can happen? The worst-case scenario is a meltdown (like Fukushima) — where decay heat melts the fuel. When fuel overheats, the zirconium cladding reacts with steam to produce hydrogen gas. If this hydrogen accumulates and ignites, it can cause a hydrogen explosion — this is what blew the roofs off the Fukushima reactor buildings. These explosions are chemical, not nuclear, but they can damage containment structures and release radioactive material. Even robust containment may not withstand such events. This is serious, but it's fundamentally different from a nuclear bomb — there's no mushroom cloud or city-levelling blast.

🛡️ But Here's Some Good News...

Remember we said enriching uranium requires thousands of centrifuges in massive cascades? This inefficiency is actually a built-in safeguard. The same difficulty that makes power plant fuel expensive also makes it extremely difficult for rogue actors to secretly produce bomb material.

You can't just build a weapons program in your basement — it requires massive industrial infrastructure, huge amounts of electricity, sophisticated technology, and years of operation. All of which are hard to hide from satellites and international inspectors.

🌍 That's Why These International Controls Exist

Now you understand why there are strict international controls around uranium enrichment. Because the line between "peaceful" and "weapons" enrichment is just a matter of degree — as you just discovered:

🕵️ "Iran Has Enriched to 60%" — But How Do We Know?

You've probably seen headlines like "Iran enriches uranium to 60%" or "North Korea expanding enrichment capacity." Now you understand what these percentages mean and why they matter. But how does the world actually know what's happening inside secret facilities?

🤔 Curious About Higher Enrichment Levels?

If you've been cautious with the simulator above and stuck around 5% enrichment — good instincts! But if you're curious about what happens when you push the enrichment slider much higher...

Go back up and experiment. Try 50%, 70%, even 90%+. See what happens when you really test the limits. Don't worry — it's just a simulation, and you might learn something surprising about why uranium enrichment is such a sensitive topic worldwide.

Come back here after you've experimented...

What's in Spent Fuel?

After years of fission, the fuel's composition has changed dramatically:

Only about 4% is actual waste. Countries like France reprocess spent fuel to recover the uranium and plutonium for reuse.

The Cooling Challenge

When fuel leaves the reactor, the chain reaction stops — but the heat doesn't. Radioactive decay of fission products continues generating heat for years. This decay heat is why spent fuel management is so critical.

This is why Fukushima was a disaster — when the tsunami knocked out cooling systems, decay heat had nowhere to go. The fuel overheated even though the reactors had already shut down.

What is Reprocessing?

Spent fuel still contains 96% usable material — uranium and plutonium that could theoretically be recycled into new fuel. Reprocessing is the chemical process to separate these from the waste.

Wait — Where Does Plutonium Come From?

Plutonium doesn't exist in nature in useful quantities. It's created inside the reactor while uranium is fissioning. Here's how:

Remember that natural uranium is 99.3% U-238 (the "unfissile" isotope) and only 0.7% U-235. While U-235 atoms are busy fissioning and releasing energy, some U-238 atoms do something different — they absorb neutrons without fissioning.

💡 Key Insight: Protons Define the Element

You might wonder: how does uranium become plutonium? The answer is beta decay. When a neutron converts to a proton (emitting an electron), the element changes because the number of protons changes:

• 92 protons = Uranium
• 93 protons = Neptunium
• 94 protons = Plutonium

This is how all elements heavier than uranium are made — neutron absorption followed by beta decay. Plutonium doesn't exist naturally because it has to be manufactured this way.

Why Does This Matter?

This explains several things:

• Why spent fuel contains plutonium — it's created during normal reactor operation
• Why reprocessing exists — to extract this valuable fissile material
• Why the UK has 140 tonnes of plutonium — decades of reprocessing extracted it, but we never used it all as fuel
• Why plutonium is a proliferation concern — it can be used for weapons

But What Happens After You Burn Plutonium?

If France recycles plutonium into MOX fuel and burns it in a reactor, what's left? Can they recycle again?

When plutonium fissions, it generates energy — but not all of it fissions. Some absorbs neutrons and becomes heavier plutonium isotopes (Pu-240, Pu-241, Pu-242). After one cycle:

• ~95% uranium remains (can be recycled)
• Plutonium remains, but it's now "degraded" — a mix of isotopes, harder to use
• More fission products (the actual waste)

Can you recycle spent MOX?

In current thermal reactors: 1-2 times maximum. Each cycle degrades the plutonium quality further. France currently recycles once — spent MOX is then stored, not recycled again.

However, fast reactors could change this equation entirely — potentially recycling fuel many times and extracting 60-100x more energy from uranium.

First: What Counts as "Radioactive Waste"?

Not everything from a nuclear site is dangerously radioactive. In fact, most of it isn't. The UK classifies radioactive waste into four categories based on how radioactive it is — not where it came from.

The Paradox: Less Volume = More Danger

Here's something that surprises people: the most dangerous waste takes up the least space. Why?

The takeaway: Most radioactive waste by volume is barely radioactive at all. The truly dangerous material fits in a relatively small space — which makes it much easier to manage and contain.

Wait — Where Does Spent Fuel Fit?

Good question. Spent fuel isn't classified as waste — at least not yet. It still contains 96% usable uranium and plutonium, so it's considered a "material" with potential value, not waste.

In the UK, spent fuel from operating reactors is stored at reactor sites or at Sellafield, waiting for eventual disposal in the GDF. If it were ever reprocessed, the fission products would become HLW and the cladding would become ILW. Since the UK has stopped reprocessing, most spent fuel will eventually go directly to the GDF as "spent fuel" — its own category.

What Does Treated Waste Look Like?

HLW

Vitrified HLW

Glass block inside a stainless steel canister. About 1.3m tall, 0.4m diameter. Contains ~400kg of glass.

ILW

Encapsulated ILW

Waste set in cement inside a steel drum or larger box. Solid, stable, stackable.

The Challenge

HLW and ILW will remain hazardous for tens of thousands of years. No human institution has ever lasted that long. So we can't rely on future generations to keep maintaining storage facilities indefinitely.

The solution? Put the waste somewhere it will be safe passively — no ongoing human intervention required. That place is deep underground, in stable rock that hasn't moved for hundreds of millions of years.

What is a GDF?

A Geological Disposal Facility is a highly engineered underground repository built 200m to 1km below the surface. It combines human engineering with natural geological barriers to isolate waste from the living environment — permanently.

The Multiple Barrier Concept

A GDF doesn't rely on any single barrier. If one fails, others remain. It's defence in depth — applied to waste disposal.

International Progress — It's Happening

Geological disposal isn't theoretical. Countries are building these facilities right now.

These countries have shown it can be done — technically, politically, and socially. The UK is learning from their experience.

UK GDF Programme — Where We Are

The UK's approach is consent-based. No community will have a GDF forced upon it. The process requires both suitable geology and a willing host community.

What Communities Receive

Engaging with the process brings investment to local areas — regardless of whether a GDF is eventually built there.

Timeline — A Long-Term Project

Geological disposal is measured in decades, not years. This is deliberate — rushing would be irresponsible.

Common Questions

UK Fuel Cycle Facilities

The UK has a complete nuclear fuel cycle infrastructure, though reprocessing has now ended. These sites represent decades of nuclear expertise.

The Nuclear Decommissioning Authority

Established in 2005, the NDA is the government body responsible for cleaning up the UK's nuclear legacy. It oversees 17 sites across the country.

Decommissioning: A Multi-Generational Commitment

"We're not just cleaning up the past — we're building an industry for the future."