China finished loading 177 fuel assemblies into the second Hualong One reactor at San’ao

Operators at San’ao unit 2, in China’s Zhejiang Province, finished loading the reactor core on 2 November 2026 after moving 177 fuel assemblies into it. China General Nuclear (CGN), the plant’s lead owner, described the milestone as the unit’s official entry into a new stage of nuclear-powered operation and commissioning. The reactor is the second of six Hualong One units planned for the site.

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A reactor core is a precise puzzle, and the order in which assemblies go in matters. At San’ao 2 the work was done with semi-automatic equipment, which let staff program the loading sequence ahead of time instead of steering each move by hand. The step comes after years of civil construction and months of cold and hot testing, and it sits directly in front of the first chain reaction.

Semi-automatic loading and the floating light gun

According to World Nuclear News reporting on the milestone, specialised fuel operators transferred the 177 fuel assemblies from racks in the spent fuel pool to the core, working through the sequence the staff had programmed. Each assembly is a bundle of fuel rods, and the Hualong One design needs 177 of them to make up a full initial core. The unit is the second of the six Hualong One reactors planned at San’ao.

CGN also pointed to a small piece of in-house engineering: a floating light gun, developed by the company itself, which lights up the core underwater so operators and cameras can confirm that every assembly sits where the sequence says it should. Underwater visibility inside a flooded reactor is poor without dedicated lighting, so a purpose-built lamp that rides on the water is more than a curiosity.

Loading is a gate rather than a finish line. Once the assemblies are seated, the vessel is closed up and the unit moves into start-up testing that ends with the first self-sustaining chain reaction. Unit 1 shows how long that road can be: its first criticality came on 14 February 2026, grid connection followed on 12 March, and commercial qualification arrived on 29 April, which works out to 74 days from criticality to commercial status and 48 days from the grid to the same point. Applying a similar run to unit 2 is a reasonable expectation, though CGN has not published a unit 2 schedule by month.

Construction, cold tests and hot tests

The unit’s history is short by Western standards. China’s State Council approved units 1 and 2 as the first phase of San’ao on 2 September 2020, and the National Nuclear Safety Administration issued the construction permit on 30 December 2020. First concrete for unit 2 followed on 30 December 2021, as recorded in the report on the start of its construction. Cold functional tests were completed in October 2025, according to a separate account of the cold testing, and hot functional tests, the dress rehearsal at operating temperature and pressure, wrapped up in April 2026, as the hot-test report describes. From first concrete to fuel load, then, the schedule ran just under five years. Unit 2 is scheduled to begin supplying electricity in 2027, so the plant still has the final commissioning stages ahead: criticality, grid connection and a long power-ascension test run.

The testing record also gives a sense of how closely China’s newer reactors follow a repeated script. Each Hualong One at San’ao goes through the same ladder of cold functional tests, hot functional tests, fuel loading, criticality, grid connection and a commissioning run, and the dates for unit 1 were reported at each step. For observers outside China, the repetition is the story: the same design, built on the same site, passes the same checkpoints in sequence, which is the pattern a standardised fleet is supposed to produce.

What unit 1’s commercial run set up

Unit 1 provides the template. It reached first criticality on 14 February 2026 and connected to the grid on 12 March. CGN said it completed all commissioning on 29 April 2026 and was qualified for commercial operation, a point covered in the account of its entry into commercial service. That first reactor has a net capacity of 1,116 MWe, and its start lifted CGN’s operating nuclear capacity from 33,040 MW to 34,248 MW. If unit 2 matches the sister unit’s pace from fuel load to commercial operation, it would be the second reactor on the plant’s books during 2027.

The San’ao plant is also the first nuclear project in the Yangtze River Delta region to use the Hualong One design, which matters to a region with heavy industry and limited room for new coal. CGN’s own figures for the finished six-unit plant, published through its corporate channels, put output at more than 54 TWh a year, with savings of over 16 million tonnes of standard coal and more than 51 million tonnes of carbon dioxide annually.

CGN’s environmental claims for the finished plant are large, and they are CGN’s own: more than 16 million tonnes of standard coal not burned each year, and more than 51 million tonnes of carbon dioxide not emitted. Both figures are tied to the six units running at the full 54 TWh output, so they describe the plant at completion rather than the single reactor now loaded with fuel. Unit 2 alone will contribute a fraction of them once it is running.

Units 3 to 6 and the rest of the build-out

Units 3 and 4 were approved by the State Council in August 2024 as part of a package of 11 reactors, and first concrete for unit 3’s nuclear island was poured on 19 December 2025. Units 5 and 6 are planned but not yet under construction. The World Nuclear Association keeps a reactor record for San’ao 2 that tracks each of these dates as the unit moves toward grid connection.

The open question is timing. CGN has said only that unit 2 will supply power in 2027, not which month. Readers watching the second reactor should look for CGN’s criticality announcement, the next dated step that the company’s sources will confirm.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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