Reactor Types

Why Different Designs?

Different countries developed different reactor types based on what was available to them — their resources, their expertise, and their priorities at the time.

The UK's reactor evolution tells a story of adapting to constraints: from the 1950s when we couldn't enrich uranium, to today when we're building the world's most advanced designs.

Gas-cooled reactors use carbon dioxide (CO₂) to carry heat away from the fuel, and graphite to slow neutrons. This combination was the UK's answer to a 1950s problem: how to build reactors without enriched uranium.

The key thing to understand: in these designs, the moderator (graphite) and coolant (CO₂) are separate materials. This has important safety implications we'll explore below.

Magnox: Britain's Pioneering Design

Magnox reactors were the world's first commercial nuclear power programme. The name comes from the magnesium non-oxidising alloy used to clad the fuel.

Designed when the UK lacked enrichment capability, Magnox used natural uranium with graphite moderator and CO₂ coolant.

AGR: The Advanced Design

The Advanced Gas-cooled Reactor evolved from Magnox with one key improvement: enriched uranium with stainless steel cladding allowed operation at much higher temperatures.

AGRs are uniquely British — no other country has built this design.

The Graphite Challenge

AGR operating life is limited by the graphite core. Over decades, neutron bombardment causes graphite bricks to crack and lose weight.

This is why AGRs cannot run indefinitely — the graphite eventually reaches its safe operating limits. Heysham 2 and Torness will close by March 2030.

Here's the fundamental shift from gas-cooled reactors: in a PWR, water does both jobs — it cools the fuel AND slows neutrons. This isn't just simpler engineering; it creates a crucial safety feature called thermal feedback.

When we compare AGR and PWR below, pay attention to how this single difference — separate vs combined moderator/coolant — affects the reactor's behaviour when things heat up.

One Material, Two Jobs

The PWR's defining characteristic: water serves as both coolant AND moderator. This dual-role approach dominates globally because it provides inherent thermal feedback — a safety feature gas-cooled reactors cannot match.

The EPR is a PWR at its core — it has the same thermal feedback safety feature. But "Generation III+" means it's been designed with extreme scenarios in mind: what if multiple safety systems fail simultaneously? What if the core melts?

The answer isn't to prevent every failure (impossible), but to ensure that even worst-case accidents stay contained. That's the EPR philosophy.

Building on PWR Technology

The EPR is fundamentally a PWR — it benefits from the same thermal feedback. But it incorporates lessons from Three Mile Island, Chernobyl, and Fukushima into a design that can contain even the most severe accidents.

What Makes It "Generation III+"?

Every reactor we've discussed so far — Magnox, AGR, PWR, EPR — is a bespoke megaproject. Each one built on-site over a decade. SMRs flip this model: instead of bringing workers to the site, you bring the reactor to the site.

The Rolls-Royce SMR is still a PWR with thermal feedback. The revolution is in how it's built, not what it is.

A Different Way to Build

Instead of building massive power stations entirely on site, SMRs are factory-manufactured in standardised modules and transported for assembly.

The UK has selected Rolls-Royce SMR for deployment, with the first site at Wylfa on Anglesey.

Why "Small" Can Mean "Safer"

A smaller core produces less decay heat after shutdown. This has profound implications:

Build Your Own SMR

Click each module to assemble the plant. Unlike conventional reactors built on-site, SMR modules arrive factory-complete and are assembled like building blocks.

Looking across all five reactor types, two clear trends emerge. First: moderator and coolant converged — from separate materials (graphite + CO₂) to a single material doing both jobs (water). Second: passive safety improved — from relying on operators and pumps to designs that cool themselves using physics alone.

The table below highlights these differences. Pay particular attention to the thermal feedback row — it's the single most important safety distinction.

Key Trends

Notice the pattern: almost every UK nuclear site is coastal. That's not coincidence — reactors need vast amounts of cooling water. The sea provides an unlimited, reliable heat sink.

Use the filters below to explore where different reactor types are located, from decommissioned Magnox sites to the EPR under construction at Hinkley Point C.