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CERN’s newly commissioned primary CO₂ cooling system, installed on the surface, supplies liquid CO₂ to the ATLAS and CMS underground detectors, forming a fully integrated cooling chain. The system replaces traditional hydrofluorocarbon (HFC) refrigerants, which have a significantly higher global warming potential than CO₂. By using CO₂ as a natural refrigerant, CERN aims to reduce its greenhouse gas emissions while ensuring the precise thermal conditions required for the experiments. The project marks a shift toward more sustainable cooling solutions in particle physics.

The development of CERN’s CO₂ cooling technology began in 2008 with the Two-Phase Accumulator Controlled Loop (2PACL) system at the LHCb experiment. CERN’s Detector Technology Group (EP-DT) later expanded the technology to ATLAS and CMS, with Norway’s NTNU contributing thermodynamic expertise. The primary system, designed by EP-DT and built by Infrasolution in Germany, was commissioned in May 2026 after years of prototyping and testing. Collaboration across multiple teams was essential to address technical challenges and ensure system stability.

The primary CO₂ system operates alongside CERN’s existing 2PACL units, which circulate liquid CO₂ through the innermost layers of the ATLAS and CMS detectors. By placing compressors on the surface, the system eliminates the need for underground thermal insulation or intermediate cooling loops, improving efficiency and reducing energy consumption. Bart Verlaat of EP-DT notes that CO₂’s high-pressure properties enable efficient heat transport over long distances, simplifying the overall infrastructure. The system is expected to reduce CERN’s greenhouse gas emissions by approximately 40,000 tonnes of CO₂ equivalent annually.

Extensive testing of the primary CO₂ system will continue during Long Shutdown 3, with the goal of full operational readiness by late 2027. The initiative aligns with CERN’s 2030 target to cut direct greenhouse gas emissions by 50% compared to the 2018 baseline. By replacing HFCs with CO₂, CERN demonstrates how scientific innovation and international collaboration can advance both research capabilities and environmental sustainability.