ACR-300: INVAP's compact PWR

The ACR-300, Advanced Compact Reactor, is the power reactor of INVAP, the Río Negro company that has designed and exported research reactors, is building the RA-10 at Ezeiza and takes part in PALLAS in the Netherlands. It is a compact Generation III+ pressurized light-water reactor with about 300 MWe per module: at the upper limit of what the IAEA calls an SMR and in the same category as Westinghouse's AP300 or the Rolls-Royce SMR, not that of integral designs such as the CAREM-25.

The difference between a compact PWR and an integral one lies in where the steam generators go. In the integral design they are inside the vessel; in the compact design they stay outside, but attached to it, without the piping loops of a large PWR. INVAP's patent covers designs from 100 to 300 MWe with that architecture. According to the company, basic engineering is under way, with more than 120 engineers and two international design reviews completed; the primary parameters, pressure, temperatures and thermal power, have not been published.

The feature that gives the patent its name is the horizontal arrangement of the steam generators. Each one is a pressure vessel lying on its side, with a bundle of horizontal U-tubes, connected to the reactor vessel by a single nozzle. Horizontal steam generators have a well-known precedent, the Russian VVER, where they are preferred for ease of maintenance and for the large water inventory on the secondary side, but there they are joined to the vessel by large-diameter piping loops.

Laying the generators down changes the building: the height of the complete primary system is, according to the patent, much lower than in a conventional or compact PWR, so the containment can be low. INVAP maintains that the horizontal configuration makes the components up to 40% smaller than those of competing SMRs and cuts the cost of civil works. That figure is the company's, with no published engineering to back it.

In a loop-type PWR, hot water leaves the vessel through a pipe nearly a meter in diameter, passes through the steam generator and returns through another pipe to the pump. In the ACR-300, the single nozzle that supports each steam generator carries a flow divider inside: hot coolant passes through the upper half toward the tubes and returns, now cold, through the lower half. There are no hot or cold legs; the patent explicitly states that it avoids the large-diameter piping that could fail and cause a large-break loss-of-coolant accident.

The same logic applies to the rest of the primary system. The pressurizer is also horizontal and joins the vessel through a single nozzle, with no surge line, with internal insulating panels separating the steam space from the liquid space. The reactor coolant pumps, between two and four according to the patent, attach directly to the vessel through nozzles and lateral connection vessels, located far from the top of the core to avoid cavitation. It is a forced-circulation design; natural circulation is reserved for removing decay heat with the plant shut down.

With a lower primary system, the containment building is lower too, and civil works, the most expensive and slowest part of a nuclear plant, shrink. INVAP projects a plant of about 10 hectares, with modular, prefabricated civil works to shorten construction. Meitner Energy, the company financing the first unit, talks of five years of construction and the lowest construction burden per megawatt among grid-scale SMRs. These are announced targets, not verified design values.

What the ACR-300 has not published is as important as what it has. The thermal power, the primary pressure and temperatures, the fuel enrichment, the cycle length and the passive safety systems are unknown, beyond the general reference to natural-circulation cooling with no human intervention or external power. Some press reports attributed to it natural or slightly enriched uranium fuels inherited from the CANDU; INVAP has not confirmed this.

Each module delivers about 300 MWe through a conventional turbine island. The Argentine Nuclear Plan, presented in December 2024 and detailed in March 2025, proposes four ACR-300s on the Atucha site, at Lima, Buenos Aires, 1,200 MWe in total, with the first unit operating around 2030 and licensing of the design abroad for export. In September 2025 Argentina joined the United States' FIRST program for SMRs.

The timeline is tight: in July 2026 Meitner Energy estimated five years of construction from the approvals, and the license from the Nuclear Regulatory Authority and the environmental approval are still pending. With that, 2030 looks more like a political target than an engineering date. The project replaces Atucha III, the Chinese Hualong One announced in 2022 that never started.

The conceptual design is protected by US patent 12,057,240 B2, Compact reactor with horizontal steam generators and pressurizer, by Pablo Florido, Rodolfo Carlevaris and Alberto Patrignani, assigned to INVAP S.E.: filed on July 2, 2018 and granted on August 6, 2024. It was made public in December 2024, together with the announcement of the Nuclear Plan, and in 2025 it emerged that it would be exploited through a partnership with US investors.

Financing is private. Meitner Energy is a company incorporated in the United States: 60% owned by the Ansari Group, of Iranian-American businessman Hamid Ansari, and 40% by Black River Technology, INVAP's North American subsidiary. On July 2, 2026 the Minister of Economy, Luis Caputo, announced that Meitner had submitted a US$1.2 billion plan for the first unit at Atucha, with about 2,000 direct jobs and entry into the large-investment incentive regime. It would be the first Argentine reactor financed entirely with private capital. In 2026 the ACR-300 is in basic engineering and in licensing before the ARN; no construction has started. Its critics, among them former CNEA president Adriana Serquis, point out that the design still has no detailed engineering.