CAREM-25: Argentina's integral SMR

CAREM, Central Argentina de Elementos Modulares, is the power reactor of the Comisión Nacional de Energía Atómica. The CAREM-25 prototype is an integral PWR of 100 MWt and on the order of 30 MWe gross, about 25 MWe net: the entire high-energy primary system, the core, the steam generators, the coolant and the steam dome, sits inside a single forged-steel vessel with stainless cladding, 11 m tall and 3.2 m in diameter, with walls 13 to 20 cm thick. With internals and steam generators it weighs 267 metric tons.

With no primary piping, the largest vessel penetration is 38 mm: the large-break loss-of-coolant accident, which sizes much of the safety case of a conventional PWR, drops off the list of possible events. Twelve helical once-through steam generators, each with 52 tubes 26 m long in six layers, fill the annular space above the core and deliver dry steam at 4.7 MPa with 30 °C of superheat.

There are no coolant pumps. Water enters the core at 284 °C, leaves at 326 °C and rises through a 4.6 m riser; at the top it exits the riser through lateral windows, flows down the annulus where the steam generators sit, gives up its heat and re-enters the core from below. The density difference between the hot and cold columns drives 410 kg/s. The flow rate adjusts itself to the power: if the core generates more heat, more water circulates.

Nor is there a separate pressurizer: the vessel's steam dome sets the pressure at 12.25 MPa by liquid-vapor equilibrium. The core outlet temperature is the saturation temperature at that pressure, and a fraction of steam formed by flashing in the riser makes up for what condenses on the cold structures of the dome. The heaters and sprays of a classic PWR disappear. It is also the most debated point of the design: an internal CNEA report circulated in 2026 questioned the degree of experimental validation of this natural-convection cooling and opened a controversy within the sector.

The core has 61 fuel assemblies of hexagonal cross section and 1.4 m active length, each with 108 rods 9 mm in diameter, 18 guide tubes and one instrumentation tube. The fuel is UO₂ enriched to between 1.8 and 3.1%, with gadolinium oxide as burnable poison in specific rods. No dissolved boron is used in the coolant during normal operation: reactivity is controlled with rods alone, and the prototype has a reference cycle of 14 months with replacement of half the core and a burnup of 24 GWd/t.

The 25 control rod drive mechanisms are hydraulic and sit entirely inside the vessel: a flow of pressurized water holds each silver-indium-cadmium rod cluster in position, and flow pulses move it step by step. Nine are fast-shutdown rods and sixteen are regulating rods. If power to the pump that generates that flow is cut, the rods drop into the core by gravity in about two seconds. CNEA has tested the mechanisms in a high-pressure, high-temperature loop since 2011, and with them the possibility of a rod being ejected from the vessel disappears.

The safety systems need neither electrical power nor operator action to start. Two independent shutdown systems: the first drops the rods by gravity; the second injects borated water from two 1 m³ tanks, also by gravity, in under 35 minutes, and either one alone is enough to shut the reactor down. Two relief valves, each with 100% of the required capacity, protect the vessel against overpressure.

Decay heat is removed by two passive condensers with horizontal U-tubes submerged in pools inside the containment: they condense steam from the dome and return the water to the vessel by natural circulation. With just one of them the core stays covered and cool for 36 hours after a total loss of power, which the design treats as a design-basis event. For loss-of-coolant events, two tanks at 2.8 MPa inject water when the pressure drops below 1.5 MPa. The containment is cylindrical, of the pressure-suppression type, with 1.2 m reinforced-concrete walls, a steel liner, a design pressure of 0.5 MPa and a 0.25 g design earthquake; it provides for hydrogen control and in-vessel retention of the corium.

The secondary side is conventional: feedwater at 200 °C enters through small nozzles, rises inside the helical tubes in counterflow to the primary and leaves as dry steam at 4.7 MPa toward a turbine, a generator and a condenser. The steam leaves superheated, the steam generators are designed to withstand primary pressure with the secondary side depressurized, and the entire secondary side up to the isolation valves withstands primary pressure if a tube ruptures.

The prototype is being built on the Atucha site, at Lima, Zárate district, Buenos Aires province. The reactor building covers 18,500 m², of which about 14,000 m² belong to the nuclear module. Around 70% of the qualified supplies, components and services come from Argentine companies: the vessel was contracted in December 2013 for 298 million pesos of the time, and the engineering, the steam generators and the control rod drives were developed with local industry. That supply chain is today the base on which INVAP promotes the ACR-300.

The concept was presented in 1984 at an IAEA conference on small reactors in Lima, Peru, and it is one of the first integral SMRs in the world. In 2009 CNEA submitted the preliminary safety report to the Nuclear Regulatory Authority; in September 2013 the ARN authorized construction and in February 2014 first concrete was poured. Start-up was planned for 2017, then 2020, 2022 and finally late 2027. The civil contractor, Techint, stopped work in November 2019 over payment delays and design changes; work resumed in 2020 and 2021, and in October 2023 Nucleoeléctrica signed a technical assistance agreement with CNEA.

In May 2024 CNEA's new leadership put the design through a 60-day critical design review, focused on the internals not yet manufactured. In September of that year the three contractors laid off 153 workers, 470 over the year, and the site came to a halt; in early 2025 CNEA formally suspended construction to concentrate the budget on engineering. Physical progress stood at 63% at the end of 2024, with civil works far advanced. During 2025 the preservation team shrank from 30 to 7 people, and in April 2026 CNEA announced it would downgrade the project's management. At least US$750 million has been invested. There is no restart date; the commercial version, of 100 to 120 MWe, never left the drawing board.