ITER cooling-tower fans.

ITER operation would not be possible without several cooling systems, because heat generated in the plasma and in auxiliary equipment must be continuously removed and ultimately rejected to the environment.

Heat Dissipation

The ITER Tokamak and its auxiliary systems will generate an average of about 500 MW of heat during a typical plasma pulse cycle, with a peak of more than 1,100 MW during the plasma burn phase. All of this heat must ultimately be dissipated to the environment.

Cooling

Other ITER systems must operate at very low temperatures. The superconducting magnets are cooled to about 4 K, while cryogenic vacuum pumps use cryosorption panels cooled to similarly low temperatures to trap gases. Some diagnostic systems also require cooling for reliable operation. ITER therefore uses two major cooling systems: an extensive water-cooling system and a large cryogenic system.

Water Cooling System

Fan cylinder of one of the ITER cooling-tower cells.

The cooling water system removes heat from the vacuum vessel, blanket and divertor, as well as from auxiliary systems including radio-frequency heating and current drive, the cryogenic system, chilled-water systems and the coil power-supply and distribution equipment.

The system comprises multiple closed cooling loops and heat exchangers belonging to the Tokamak Cooling Water System (TCWS), Component Cooling Water System (CCWS) and Chilled Water System (CHWS), together with the open-loop Heat Rejection System (HRS). Altogether, the cooling-water installations contain about 36 kilometres of piping. Components of the TCWS must meet the applicable French nuclear pressure-equipment requirements.

As an experimental device, ITER will not use this heat to generate electricity. Instead, it is transferred through several cooling loops to an induced-draft cooling-tower system and rejected to the atmosphere. Because evaporation concentrates dissolved minerals in the cooling water, part of the water is continuously discharged and replaced with fresh water supplied from the Canal de Provence.

Before discharge, the water is tested for parameters including temperature, pH, hydrocarbons, chlorides, sulphates and tritium. Only water that meets the prescribed environmental limits is released into the Durance River. During plasma operation, the combined flow rate of the ITER cooling-water loops will be approximately 33 m³/s, through pipes with nominal diameters of up to about 1.6 metres.

Cryogenic System

Liquid-helium quench tanks for the ITER cryoplant.

The ITER cryogenic system creates and maintains the very low temperatures required by the superconducting magnets, cryogenic vacuum pumps and some diagnostic systems. The magnets operate at about 4 K (−269 °C), a temperature far below the range achievable with water cooling, so forced-flow supercritical helium is used as the coolant.

Liquid nitrogen at about 80 K is used mainly to pre-cool the helium refrigeration system. The cryogenic system can provide cooling at three principal temperature levels: about 80 K, 50 K and 4 K. The ITER cryoplant contains three helium refrigerators with a combined average cooling capacity of 75 kW at 4.5 K and a maximum liquefaction rate of 12,300 litres per hour.

Nitrogen is produced from atmospheric air and processed in two nitrogen refrigerators with a combined cooling capacity of about 1,300 kW at 80 K. It provides pre-cooling for the helium refrigerators and supports the thermal-shield cooling system. Cryogenic fluids are distributed from the cryoplant to the Tokamak Building through approximately 5.5 kilometres of vacuum-insulated cryolines.

Approximately 45 percent of the cryogenic cooling power is required by the magnets, 40 percent by the thermal shields and 15 percent by the cryopumps. The ITER cryogenic system has a total helium inventory of about 25 tonnes, which is stored and circulated throughout the installation at different cryogenic temperature levels.