Changjiang Unit 4 Clears Pressure Test, Enters Commissioning Phase

A single number just moved Changjiang Unit 4 across the line that separates a construction site from a nuclear power plant in the making. On 8 September 2026, engineers sequentially verified primary circuit pressure up to 22.3 MPa, completing cold functional testing and placing the reactor formally inside the commissioning phase.

That threshold is not a routine progress update. It is a defined technical gate, and crossing it means the physical build of Unit 4 has now been pressure-tested and signed off.

The timing matters because Unit 4 is not alone at this site. Its twin, Changjiang Unit 3, connected to the electrical grid on 1 August 2026, meaning two Hualong One reactors, China’s third-generation pressurised water reactor design, are now moving through different stages of the same delivery pipeline. The CNY 40 billion Phase II project, roughly US$5.9 billion, is visibly converging on completion, with Unit 4 as the trailing unit.

What follows maps the steps between this milestone and Unit 4’s eventual grid connection, and what that trajectory tells you about China’s nuclear delivery pace.

What cold functional testing actually confirmed, and why it matters

Cold functional testing is a high-pressure verification of a reactor’s primary circuit conducted at cold, non-operational temperatures. It confirms that the pressure vessel, pipework, and valves can hold pressures well above what they will ever face in normal service, all before a single fuel assembly enters the system.

The test is not a single squeeze. It is a deliberate escalation, with pressure held and evaluated at six defined points.

1. 2.7 MPa: Initial low-pressure hold to confirm baseline integrity of the circuit.
2. 7.0 MPa: First meaningful stress step, checking early leak paths.
3. 10.0 MPa: Intermediate hold verifying weld and joint performance under load.
4. 15.4 MPa: Approaching operating-range pressure, testing installation quality.
5. 17.13 MPa: Verification near and beyond standard service conditions.
6. 22.3 MPa: Final peak hold, stress-testing the circuit above operating pressure.

Each step is designed to surface welding defects, installation errors, or leak paths before they can matter. A circuit that holds at every stage has demonstrated it can contain the coolant that will eventually carry the reactor’s heat.

The headline verification threshold The primary circuit was pressure-tested to 22.3 MPa, comfortably above the pressure it will experience in normal operation. That tells you the structural risk layer of commissioning has now been cleared.

Successful completion is a one-way gate with regulatory standing. It closes out the construction verification phase and formally opens commissioning. For anyone tracking Hualong One delivery timelines, this is the point where schedule risk shifts from construction execution to commissioning performance.

How Unit 4’s pre-commissioning sequence unfolded

The cold test did not arrive in isolation. It sat at the end of a tightly sequenced run of milestones through the northern summer of 2026.

Nuclear loop flushing began on 22 June 2026, clearing the pipework and verifying leak tightness. The main control room was commissioned on 31 July 2026, entering full duty shifts. The secondary loop combined hydrostatic test followed on 6-7 August 2026, framed as the core prerequisite for cold testing.

That order, loop flushing to control room commissioning to hydrostatic testing to cold functional testing, matches the standard pressurised water reactor commissioning sequence. Unit 4 has tracked its programme without any reported deviation, which is itself a signal of delivery discipline.

What the commissioning pipeline ahead looks like before grid connection

Cold testing clears the structural gate. It does not clear the operational one. The next major hurdle, hot functional testing, is where commissioning schedules most often absorb slippage.

Hot functional testing brings the primary coolant system up to full operating temperature and pressure, simulating the thermal conditions the reactor will face in service. For the first Hualong One unit, Fuqing-5, roughly 200 distinct tests were conducted over several weeks before fuel loading was authorised. Thermal stresses, piping vibration, and control system behaviour all have to be verified and, where needed, corrected.

From here, the sequence is well defined, with each step a prerequisite for the next.

1. Hot functional testing: Coolant system taken to operating temperature and pressure.
2. Fuel loading: Nuclear fuel introduced once hot testing is signed off.
3. First criticality: The first self-sustaining chain reaction.
4. Grid connection: The unit begins feeding electricity to the network.
5. Commercial operation: Full operational status after sustained performance testing.

Unit 3 offers a fresh reference point for how quickly these steps can follow one another. It reached first criticality on 10 July 2026 and grid connection on 1 August 2026, a matter of weeks between the two.

The timeline has moved. Commercial operation for the Phase II units was initially targeted for the end of 2026, but industry coverage now points to early 2027 for both Units 3 and 4 to reach full operational status. That shift is not a red flag. It tells you Unit 4’s remaining path is measured in months, not years, and that the trailing unit is close behind its twin.

Changjiang’s wider significance as a multi-reactor nuclear technology hub

Zoom out from Unit 4, and the full Changjiang site comes into view. What is being built here is not two reactors but a demonstration cluster spanning three reactor technologies, and that carries strategic weight well beyond its generating capacity.

The site’s reactor portfolio spans four large units plus a small modular reactor:

• Units 1 and 2: CNP-600 pressurised water reactors, commercially operational since 2015 and 2016.
• Units 3 and 4: Hualong One reactors, currently in commissioning.
• Linglong One (ACP100): a 125 MWe small modular reactor, designed for 1 TWh of electricity per year, in pre-commissioning.

The financial architecture behind the expansion is the CNY 40 billion Phase II investment, with China Huaneng holding a 51% majority stake. That ownership structure ties one of China’s largest power groups directly to the site’s build-out.

A world first at Changjiang Linglong One is described as the world’s first land-based commercial small modular reactor demonstration project. Its primary circuit cold functional test was completed on 16 October 2025, and it remains in pre-commissioning.

For energy investors, the co-location matters. Changjiang is the clearest single-site evidence that China’s nuclear strategy is a technology-stacking model, where each reactor generation de-risks the next, and where the SMR sits alongside large baseload units rather than replacing them.

The Hainan Province policy backdrop sharpens the point. Provincial energy directives designate nuclear as the primary baseload source, gas for peaking, and a ban on new coal plants. Because Hainan is an island with limited grid interconnection, the firm capacity that nuclear provides is particularly valuable for system stability.

Where the Hualong One stands in China’s broader nuclear expansion

The Changjiang units are two data points inside a much larger build-out. As of end-2025, China operated 59 commercial nuclear units totalling 62.48 GW, with a further 53 units approved and under construction at 62.93 GW.

Globally, 41 Hualong One units are reported as approved, under construction, or in operation, with per-unit costs targeting approximately US$5 billion. Each unit is estimated to displace around 3.12 million tonnes of coal and avoid roughly 8.16 million tonnes of CO2 annually.

The first Hualong One units achieved a 68.7-month construction period, positioned at the time as the only Gen III nuclear project delivered on schedule globally. That figure is the delivery benchmark China is now working to replicate and improve across sites like Changjiang.

What the Unit 4 timeline means for investors watching China’s nuclear delivery pace

Strip away the individual milestone, and a clearer picture of programme management emerges. Two Hualong One units at the same site, moving through different commissioning gates within weeks of each other, points to a level of pipeline discipline that few nuclear industries can currently match.

One caveat belongs on the record. The 8 September 2026 cold testing completion is reported by the original source, and independent verification through IAEA PRIS databases was pending as of publication. The milestone should be read as reported, not yet independently confirmed.

Even with that caveat, the delivery record is consequential. The 68.7-month construction period for the first Hualong One units stands as a rare on-schedule Gen III benchmark, and Changjiang’s Unit 3 to Unit 4 sequencing suggests that pace is being sustained rather than treated as a one-off.

The scale statistic that matters According to World Nuclear Association data from mid-2026, 32 of the 63 reactors under construction globally are in China.

Three signals from the Changjiang programme are worth holding onto:

• Multi-unit delivery pace: Unit 3 at grid connection and Unit 4 entering commissioning within weeks shows a staggered, repeatable rhythm.
• SMR demonstration co-location: the world’s first commercial SMR demonstration running in parallel with large baseload units on one site.
• Hualong One cost-curve trajectory: a 41-unit global pipeline building the learning curve that drives future cost and timeline improvements.

For investors tracking nuclear sector exposure, that co-location gives Changjiang significance beyond its megawatts. Chinese vendors are accumulating a construction and commissioning learning curve no other nuclear industry currently matches, which feeds directly into the cost and timeline outlook for future units, at home and for export.

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 market conditions, regulatory developments, and various risk factors, and past performance does not guarantee future results.

What comes after cold testing, and what to watch next

The story now sits at its forward edge, and a handful of specific triggers will tell you whether Changjiang’s pace holds or wobbles over the next six months.

Hot functional testing is the milestone to watch first. It is the point at which Unit 4’s commissioning schedule will either hold firm or begin to absorb slippage, and it is where thermal and dynamic performance gets its first full test.

Here is what to monitor at the site in the months ahead:

• Unit 4 hot functional testing commencement: a clean run keeps early-2027 commercial operation on track; delays would signal schedule pressure.
• Linglong One commissioning outcome: a stated 2026 commercial operation target remains unconfirmed at publication, making its progress a second live watchpoint.
• Unit 4 grid connection: the moment the trailing Hualong One unit begins feeding the Hainan grid.
• Hainan clean-energy progress: the province’s 2030 clean-energy island target frames how quickly this firm baseload capacity is needed.

The next six months at Changjiang, spanning Unit 4’s hot functional testing and Linglong One’s commissioning result, will either validate or complicate the narrative that China has mastered nuclear delivery at scale. That makes this single site one of the most information-dense watchpoints in global nuclear right now, and knowing which gate signals what lets you assess the programme with specificity rather than headline-level observation.