Nuclear
A fleet retiring, a PWR commissioning, and the small-modular debate.
Nuclear supplied 14 percent of GB electricity in 2024. Four Advanced Gas-cooled Reactor plants retire by 2028 (Heysham 1, Hartlepool, Heysham 2, Torness). Sizewell B runs to at least 2035. Hinkley Point C commissions in 2029-30. Sizewell C was approved and received a final investment decision in 2025. Great British Nuclear is running the SMR competition. This route maps the fleet transition that determines 2030s nuclear output.
07Route 7 of 12 · Energy vectorsAfter this route you will be able to
- Name the operational GB nuclear fleet, the retirement schedule, and Hinkley and Sizewell plans.
- Distinguish an AGR from a PWR, and explain why the AGR technology is unique to GB.
- Describe the role of the Office for Nuclear Regulation (ONR) and the NDA.
- Identify the Great British Nuclear SMR competition and its candidate vendors.
- Make a reasoned call on nuclear capacity in the 2030s mix.
22 July 2025Sizewell C Final Investment Decision · £38 bn build cost, 3.2 GW
For the first time since Hinkley in 2016, the government committed to a new large nuclear station.
Under the Nuclear Regulated Asset Base (RAB) funding model,
The RAB model allows consumers to pay a small levy on electricity bills during construction, spreading the capital cost across the build period. That is a direct contrast to the 2016 Hinkley Point C financing, which used a 35-year CfD at £92.50/MWh (2012 prices, now £126/MWh indexed), fully private during construction.
The Sizewell FID was not the end of the debate about GB nuclear capacity. It started a new one. With Hinkley Point C still in commissioning, Sizewell C in early construction, four AGRs retiring by 2028, and only Sizewell B as a long-running PWR, GB nuclear output falls between 2028 and 2030 before Hinkley rebuilds it. The 2030s mid-point requires either Small Modular Reactors or a decision to build more large plants.
Britain just committed to one new large nuclear station. Is that enough for a zero-carbon 2030s system, or does the fleet need SMRs as well, and if so, by when?
The answer starts with understanding the fleet today. Five nuclear stations operating. Retirement dates confirmed. Replacements committed but not yet delivering.
Section 01 · The current fleet
Five stations, one PWR, four AGRs retiring by 2028.
The current fleet was built between 1976 and 1995. Every AGR is more than 35 years old. Sizewell B, the only PWR, is under a life-extension study. The table below is the operational picture.
Source: ONR reactor status; EDF Energy operational updates. AGR = Advanced Gas-cooled Reactor, a GB-unique design using CO₂ coolant and graphite moderator. PWR = Pressurised Water Reactor, the international standard design.
Section 02 · Why AGRs are different
Gas-cooled, graphite-moderated, uniquely British.
The Advanced Gas-cooled Reactor is a British design from the 1960s. No new AGR has been built since the 1980s. The four remaining plants are the end of the line.
AGRs use CO₂ gas as the primary coolant (rather than water in a PWR) and a graphite moderator to slow neutrons. This allows higher outlet temperatures (~650°C vs ~330°C for a PWR), meaning AGRs can drive high-temperature steam turbines efficiently but also demand more of their graphite cores over time.
Graphite ageing is the core life-limiting factor. Graphite bricks in the reactor core develop cracks under neutron irradiation. Each AGR's extension has been driven by inspection of graphite condition and ONR approval. Once the ONR judges the core unsafe for further operation, the plant must close.
All four retiring AGRs have had multiple life extensions already. None of the remaining plant has a practical route to further extension. The technology cannot be replicated: there is no international supply chain, no new designs, no continuing expertise at the scale a new build would need.
The Office for Nuclear Regulation may issue a notice requiring the licensee to cease operation where it is not satisfied that operation can be safely continued. A nuclear site licence carries 36 conditions, of which Condition 4 (restriction on nuclear matter) and Condition 23 (operating rules) govern core integrity.
Nuclear Installations Act 1965 as amended; ONR nuclear site licence conditions
Section 03 · The SMR competition
Great British Nuclear narrowed to two preferred vendors in 2024.
Small Modular Reactors (SMRs) are nuclear plants below roughly 300 MW per unit, designed for factory fabrication and site assembly. The SMR programme is GB policy's answer to the 2030s capacity gap.
Great British Nuclear, a public body established under the Energy Act 2023, ran a 2024 competition to select SMR vendors. Six competitors entered: Rolls-Royce SMR (UK), GE Hitachi (US), Holtec (US), Westinghouse (US), NuScale (US), and EDF (France). After technical evaluation,
Rolls-Royce SMR is a 470 MW PWR intended for deployment at decommissioned nuclear sites and industrial clusters. UK nuclear regulator (ONR) Generic Design Assessment Phase 3 is underway, with completion targeted for 2026.
GE Hitachi BWRX-300 is a 300 MW Boiling Water Reactor design derived from a larger operating fleet internationally. Licensing in GB would start from scratch, adding time.
The earliest realistic first-of-a-kind SMR in GB is late 2030s, possibly 2040. This is consistent with vendor and ONR communications; it is not consistent with some political framing that suggests SMRs before 2030.
Common misconception
SMRs will be delivering electricity to the GB grid before 2030.
No SMR vendor has operational units supplying commercial grids anywhere in 2026. GDA Phase 3 for Rolls-Royce SMR completes in 2026 (best case); site licensing, Environment Agency permits, construction and commissioning then follow. Realistic first-of-a-kind deployment in GB is late 2030s. Any 2030 claim confuses announcement dates with operational dates.
Section 04 · The 2030s call
Large, small, or pause. Pick a fleet-build strategy.
The question is how much nuclear capacity GB should target for the mid-2030s. Three defensible answers. Each has different risk and different cost shape.
Check your understanding
Three questions on what you have just read.
Key takeaways
- 14 percent of GB generation in 2024. Five stations, four AGRs retiring by 2028, Sizewell B running to at least 2035.
- Hinkley Point C 3.2 GW commissions 2029-30. Sizewell C FID in July 2025 at £38 bn.
- AGR technology is uniquely British and cannot be replicated. Graphite ageing is the retirement driver.
- GBN selected Rolls-Royce SMR and GE Hitachi BWRX-300 as preferred SMR vendors. First deployment late 2030s.
- The 2030s fleet gap runs from 2028 (AGR retirement) to Hinkley commissioning. Sizewell C alone does not close it.
References
- Office for Nuclear Regulation
Licensed sites, operational status, GDA progress.
Primary regulatory source.
- EDF Energy: UK power stations
Operational AGR fleet and Sizewell B status.
Operator reference for current fleet.
- Great British Nuclear
SMR competition progress, preferred vendors, deployment plan.
Primary source for SMR programme.
- DESNZ: Civil Nuclear Roadmap to 2050
Policy direction for nuclear capacity across the 2030s and 2040s.
Strategic policy reference.
- Nuclear Decommissioning Authority
Decommissioning of retired plants, long-term waste management.
Retirement and waste authority.
The next route covers oil. The UK's declining Continental Shelf, the refining footprint, and what security of supply looks like for an import-dependent oil system.