China’s Tianwan Unit 7 Reaches First Criticality in Nuclear Milestone

On or around 28 September 2026, the seventh reactor at China’s Tianwan Nuclear Power Plant initiated and sustained a controlled nuclear chain reaction for the first time, reaching what engineers call first criticality. It is the most significant commissioning milestone the unit has cleared to date.

The achievement follows a long build sequence. Intergovernmental agreements for the two new VVER-1200 reactors were signed in June 2018, and construction of Unit 7 began on 19 May 2021. Fuel loading ran from 12 to 18 August 2026, and hot functional tests, which replicate the temperature and pressure of normal operation, were completed on 30 December 2025.

First criticality does not mean the reactor is generating power for the grid. It means the unit has moved out of construction and into an active power-generating commissioning phase.

Here is what that shift actually involves, why this particular reactor sits at the centre of the Russia-China strategic partnership, and what Tianwan’s eventual full capacity signals for the trajectory of global nuclear energy.

What first criticality means, and what comes next for Unit 7

First criticality is a precise technical threshold. It is the moment a reactor sustains a controlled nuclear chain reaction at its minimum controllable power, the lowest stable output at which the reaction holds steady. It is not power generation, and it is not commercial operation. It is the point at which the reactor becomes an active, self-sustaining system for the first time.

That distinction matters because the remaining steps are where the real output arrives. After criticality, specialists run low-power evaluations and inspections before output can climb. Power is then raised in staged intervals, checked at each level, until the unit connects to the grid and enters trial operation ahead of a formal commercial declaration.

The IAEA nuclear plant commissioning guidelines define first criticality as the point at which regulatory authorisation is required before power ascent can begin, establishing it as a formal gating checkpoint rather than a routine engineering step.

The commissioning stages between criticality and commercial operation

The full sequence from a loaded core to a working power plant runs as follows:

1. Fuel loading (completed for Unit 7 on 18 August 2026)
2. First criticality (achieved approximately 28 September 2026)
3. Low-power testing and inspections
4. Incremental power ascent through staged intervals
5. First grid connection
6. Trial operation
7. Commercial operation

As of late September 2026, Unit 7 sits at step two. That places it near the start of the power-generating phase, not the end.

The timeline gives this milestone immediate commercial weight. Rosatom Director General Alexey Likhachev has stated the commercial operation target for Unit 7 remains 2026, with Unit 8 following in 2027. Unit 8 completed its cold functional tests, which verify the leak-tightness of the primary circuit, in August 2026.

For anyone tracking nuclear timelines, the read is straightforward. First criticality is a verified milestone, but grid-connected output is still several stages away, and the 2026 target compresses those final stages into weeks rather than months. Whether that target holds is the near-term test.

Tianwan’s scale: eight reactors, nine gigawatts, and a 57-million-tonne carbon equation

Unit 7 is one reactor in a site built to hold eight. Understanding what its commissioning means requires stepping back to the scale of the whole facility.

The Tianwan plant hosts three distinct reactor generations. Units 1 to 4 are Russian-supplied VVER reactors that entered commercial service between June 2007 and December 2018. Units 5 and 6 use the Chinese-developed ACPR1000 design. Units 7 and 8 are Rosatom’s VVER-1200 (V-491) Generation III+ reactors, each rated at roughly 1,100 MWe, currently under commissioning.

Once every unit is running, the numbers become substantial. Total installed capacity is projected to pass 9 GWe, with annual electricity output expected to exceed 70 TWh. Units 7 and 8 alone are set to contribute around 18.8 TWh each year.

The environmental figure is where the scale lands hardest.

Projected annual CO2 reduction (full site): approximately 57.4 million tonnes

A single site displacing 57.4 million tonnes of carbon dioxide a year is not a rounding error in China’s decarbonisation arithmetic. It is a material line item. That figure is what turns Tianwan from a regional power source into a national policy asset, and it explains why commissioning milestones here draw sustained political attention from both Beijing and Moscow.

Ownership reinforces the point. The plant is held by China National Nuclear Corporation (CNNC) at 50%, China Power Investment Corporation at 30%, and Jiangsu Guoxin Group at 20%, a structure that keeps the facility firmly within China’s state energy apparatus.

CNNC’s upstream uranium strategy has expanded well beyond China’s borders as the organisation secures fuel supply for a reactor fleet growing faster than domestic mining can support, a pattern that runs in parallel with the commissioning of new capacity at sites like Tianwan.

Two decades of Rosatom-CNNC partnership: how Tianwan became a model for nuclear cooperation

The relationship behind Unit 7 is older than the reactor itself. Likhachev has noted that Units 1 and 2, built to Russian specifications, have operated at the site for roughly twenty years, with Units 3 and 4 running for nearly a decade. That gives Rosatom a multi-generational presence at a single location, an unusual depth of continuity in the reactor export business.

What began as a reactor supply arrangement has since been elevated into something more strategic. The contracts for Units 7 and 8 were prepared under a strategic package of agreements signed during President Vladimir Putin’s June 2018 visit to China, covering both these units and reactors at the Xudapu site. Construction start in May 2021 was marked by a joint virtual ceremony with Xi Jinping and Putin.

Putin characterised the Tianwan and Xudapu projects as marking “a significant part of the comprehensive strategic partnership of coordination for a new era between the two countries.”

Chinese nuclear officials have described Tianwan Phase IV as “a model project of Sino-Russian nuclear energy cooperation,” language that positions the plant as a diplomatic reference point rather than a routine commercial deal.

The scope of the arrangement runs across the full reactor life cycle. Rosatom’s remit spans design, construction, and fuel supply, extended by a maintenance agreement covering existing Units 1 to 4, the new Units 7 and 8, and Xudapu Units 3 and 4.

Sino-Russian nuclear agreements have increasingly been structured to embed Russian technology and fuel supply across the full reactor lifespan, rather than as one-off construction contracts, a pattern that gives Rosatom a durable commercial presence in China’s energy system long after concrete work is finished.

The key milestones in the partnership read as a clear chronology:

• 2007-2018: Units 1-4 enter commercial service
• June 2018: Strategic package of agreements signed during Putin’s China visit
• May 2021: Unit 7 construction start and joint groundbreaking ceremony
• December 2025: Unit 7 hot functional tests completed
• September 2026: Unit 7 first criticality

For investors, the implication is worth weighing. The move from construction contractor to life-cycle service partner means Rosatom’s involvement with this facility persists for the multi-decade operational lifespan of the reactors, well into the second half of the century. That embeds Russian fuel fabrication and service capabilities into China’s nuclear ecosystem, a dependency that carries both supply-security and technology-sovereignty questions.

China’s nuclear build-out in numbers: where Tianwan fits in the world’s fastest-expanding fleet

Tianwan is one plant in a national programme moving at a pace few countries can match. Pinning down China’s exact capacity is complicated by measurement differences: some sources report net capacity, others gross, and snapshot dates vary. The ranges still tell a consistent story.

The International Atomic Energy Agency’s Power Reactor Information System listed 60 reactors and 58,721 MW(e) of net operating capacity as of September 2026. The World Nuclear Association reported roughly 62 GWe gross operating capacity and about 43 GWe under construction at the end of 2025. The US Energy Information Administration counted 28 reactors totalling 30 GW under construction in early 2026.

The decade comparison is the number that matters most.

According to EIA data, China’s nuclear capacity rose from 31.4 GW in 2016 to 58.7 GW in May 2026, an 87% increase in ten years. New additions came in at 1.1 GW during 2025 and a further 2.2 GW through May 2026.

China’s fleet-based construction model, which treats dozens of simultaneous reactor builds as a single managed system rather than separate projects, is a core reason Tianwan’s commissioning stages have proceeded more predictably than comparable programmes in Western Europe or North America.

China’s fleet is deliberately diverse, spanning several large reactor designs:

• CPR-1000 and ACPR1000: earlier and upgraded domestic pressurised water reactors
• Hualong One (HPR-1000): China’s flagship domestic Generation III design
• AP1000: the US-origin Westinghouse design
• VVER-1000 and VVER-1200: the Russian-supplied reactors, the latter deployed at Tianwan Units 7 and 8

That mix lets operators benchmark the VVER-1200 directly against domestic designs on cost, safety, and performance. For anyone tracking uranium demand, the signal is structural rather than incremental. An 87% capacity gain in a decade, with 28 to 43 reactors still under construction, points to acceleration, not a plateau, and Unit 7’s first criticality is a concrete confirmation that this pipeline is advancing on schedule.

What Unit 7’s commissioning signals for nuclear energy’s next phase

A single reactor reaching criticality is, on its own, a technical event. Read against the wider picture, it is an indicator of three forces converging: bilateral diplomacy, clean energy policy, and commercial nuclear competition. Tianwan Unit 7 sits at that intersection.

The near-term proof points are specific and measurable. These are the variables worth tracking over the coming months:

1. Unit 7 first grid connection
2. Unit 7 trial operation announcement
3. Unit 7 commercial operation declaration, targeted for 2026
4. Unit 8 commissioning progress, with commercial operation targeted for 2027

The broader signal reaches beyond China. Tianwan exemplifies Rosatom’s full life-cycle export model, the same design-build-fuel-service approach deployed at Akkuyu in Turkey, El Dabaa in Egypt, and Kudankulam in India. That a technically advanced nuclear power still finds this model attractive says something durable about its competitiveness.

Global nuclear build-rate constraints mean that even as China adds capacity faster than any other country, the worldwide pace of new reactor construction still falls well short of what climate modelling suggests is needed by mid-century, a gap that makes each confirmed commissioning milestone more significant as a data point for the broader energy transition.

For investors weighing uranium demand or Rosatom’s global footprint, Unit 7’s progression toward commercial operation is the most immediate evidence available that China’s nuclear timeline is holding. First criticality is the milestone that makes commercial operation real rather than planned. Watch whether the 2026 target is met, and you have a clean read on whether the world’s fastest-expanding nuclear fleet is keeping to schedule.

This article is for informational purposes only and should not be considered financial advice. Investors should conduct their own research and consult with financial professionals before making investment decisions. Financial projections and forward-looking statements are subject to change based on market developments and various risk factors.