A DT-plasma in ITER at 15 MA plasma current and at full fusion power will contain a total of thermal and magnetic energy of ~800 MJ, which could be released in case of a disruption, i.e. uncontrolled loss of plasma current and energy, within a time scale of ~10÷100 ms, into conducting structures of the torus and plasma facing components, inducing high mechanical forces and reducing the lifetime of the first wall. ITER will be equipped with the largest ever Disruption Mitigation System (DMS), designed to mitigate the consequences of disruptions by a rapid injection of pellets made of protium, neon or their mixtures. The pellets will be disintegrated (shattered) in mm-size fragments in order to maximise the assimilation in the plasma, and will convert the magnetic and thermal energy into radiation, avoiding the generation or at least mitigating the impact of the relativistic runaway electrons, and controlling the current quench.
 
To achieve the machine protection requirements against disruptions, the DMS will be equipped with 27 injectors, to produce and inject cryogenic pellets in at least two sizes, D=18 mm × L=36 mm and D=28.5 mm × L=57 mm. Each pellet will be formed by de-sublimating the required gas or mixture from the gaseous phase in a dedicated copper cold cell (CC) heat exchanger, cooled by supercritical helium ~5 K and ~5 bar, after being pre-cooled from room temperature to ~ 15÷25 K, which is above the relevant triple points. The pellets might need being stored over several plasma pulses (e.g. hours) at the lowest possible temperature to minimize sublimation when the injector line is exposed to the pumped torus before DMS triggering.
 
An extensive R&D program has successfully demonstrated the de-sublimation, pneumatic dislodgment, and intact acceleration of H and Ne full size pellets in support laboratory environments (CEA-DSBT Grenoble and HUN-REN CER Budapest), proposing few optimized de-sublimation recipes and informing the DMS final design in 2024.
 
The ITER DMS Cryo-Distribution System (CDS) design was optimized for minimizing the heat losses and the pellet de-sublimation time, limited by the ice thermal conductivity, and proposed solutions for allowing the de-sublimation of pellets of different sizes or the sublimation/de-sublimation of pellets in parallel from a common cryogenic manifold.
The CDS is now progressing into the manufacturing phase, including the construction of the first-of-a-kind CC assemblies, the injector cryostats, the cold distribution boxes, the flexible and rigid cryogenic transfer lines. The main manufacturing challenges will be presented, including the development and qualification under nuclear requirements of CC and gas pre-cooler manufacturing technologies, e.g. vacuum brazing, copper electroforming and stainless-steel 3D printing, to ensure for the first time effective cooling performances, nuclear confinement and reliability in an environment made challenging by limited space and access restrictions after nuclear activation.
The integrated functional test of the first CDS prototypes at CEA-DSBT laboratory in 2027 will validate and release the serial manufacturing of the CC, followed by a full DMS single injector assembly, alignment, and test program at HUN-REN CER and ITER.
 
Contributors:

M. Barandiaran(a), O. Barana(a), T. Boujet(b), M. Dibon(a), F. Dhalla(c), S. Jachmich(a), T. Keenan(a), M. Kochergin(a), U. Kruezi(a), N. Jaquet(b), J. Manzagol(b), F. Millet(b), M. Parekh(a), K. Pishchinskiy(a),  X. A. Portela Sobrino(a), L. Qiu(a), C. Weber(e), J. Zeutschel(e), S. Zoletnik(d)

(a) ITER Organization, Route de Vinon Sur Verdon, 13115 Saint-Paul-lez-Durance, France

(b) Univ. Grenoble Alpes, CEA, IRIG-DSBT, F-38000 Grenoble, France

(c) Dhalla Consulting Services, Avenue des Tilleuls 28, 1203 Geneva, Switzerland

(d) HUN-REN, Centre for Energy Research, Institute for Atomic Energy Research, Budapest, Hungary

(e) RI Research Instruments GmbH, D-51429 Bergisch Gladbach, Germany
 

Keywords
ITER|DMS|DISRUPTIONS|COLD CELLS|CRYOGENIC DISTRIBUTION