Design of the cryogenic distribution system for the ITER disruption mitigation based on shattered pellet injection

• The cryogenic distribution system for the ITER disruption mitigation and the 28.5 mm diameter cold cell assemblies were designed based on laboratory experimental results and validated numerical models. • The cold cell assembly can de-sublimate and launch good quality H-pellets in < 20 min and fulfil the inter-pulse readiness time requirement of < 30 min. • Materials, components and integration solutions were identified to guarantee the operation and maintenance of the disruption mitigation system in the ITER severe magnetic and nuclear environment. The ITER tokamak will be equipped with a machine protection system to avoid or mitigate the damage to its in-vessel components in the event of plasma disruptions. The Disruption Mitigation System (DMS) will be based on Injection of Shattered Pellets (SPI) made of hydrogen, neon or their mixtures into the plasma, to convert the plasma energy into radiation while avoiding the formation or dissipate the energy of runaway electrons and to minimize the electromagnetic loads by controlling the plasma current quench. To achieve the disruption mitigation requirements and fulfill the pulse rate for the ITER Research Plan, the DMS Cryogenic Distribution System (CDS) shall form cylindrical pellets with a diameter of 28.5 mm, a length of 57 mm and good integrity, by de-sublimation of gases inside a Supercritical helium (SHe) cooled Cold Cell (CC), in ≈1200 s (20 min) for hydrogen, and maintain their availability over several plasma pulses. The DMS CDS was integrated into the ITER baseline at a late design stage, with limited SHe cooling capacity supplied in parallel to the cryopumps for vacuum vessel, cryostat and neutral beam injectors. Seven Cold Distribution Boxes (CDBs) dedicated to the DMS equatorial and upper port locations were introduced, each equipped with a Joule-Thompson (JT) expansion valve and a liquid helium vessel, to supply the SHe flow to 27 CCs at a stable temperature of ∼5 K for pellet formation and preservation. The CC design was supported by de-sublimation numerical modelling and experiments to optimize the pellet shape and integrity and to minimize the CC cooling requirement to form pellets within an acceptable time. The cryogenic system design aimed at minimizing heat losses while considering the very challenging environmental (magnetic field, nuclear, seismic) and complex integration requirements. This paper presents the DMS CDS description, following the final design review in 2024, focusing on the CC novel design supported by CFD models and laboratory experiments.