Amending the story of helium-3

In his Issues and Events story about the helium-3 supply (Physics Today, June 2010, page 22 ), David Kramer did not mention a pending huge drain on the world’s tritium resource before it decays into ³He—namely, fuel for fusion research devices. The ITER fusion energy test facility alone will require an initial on-site inventory of 2 kg of tritium, about 10% of the expected worldwide nonweapons tritium inventory after 2020. Other tritium-consuming fusion devices—for example, Ignitor (Physics Today, June 2010, page 27 )—are in various approval stages.

Even if there is insignificant fusion burn of tritium in the ITER plasma, the facility will have an ongoing tritium demand of nearly 0.5 kg per year to compensate for its decay into ³He and its inevitable long-term trapping in the vessel walls and pumping systems. Achieving advertised deuterium-tritium plasma performance could raise ITER’s tritium replenishment need above 1 kg per year, about half the world’s annual tritium manufacture beyond 2020 unless some fission reactors are dedicated to tritium production.

In principle, ITER’s tritium requirement is somewhat compatible with ³He demand. Only a fraction of the injected tritium will be reacted even with multiple recycling, so much of the ³He from tritium decay might be recovered eventually from the vacuum pumping system, the plasma chamber wall, and the tritium recycling systems.

Fortunately for ³He users, the planned date for ITER to start up with only hydrogen plasmas continues to recede, and tritium usage is likely two decades away. Nevertheless, this looming conflict should highlight to energy-supply planners that despite claims of fusion being an infinite energy source, as a practical matter it is dependent on fission reactors for its fuel supply.