Fusion Safety - A Different Set of Variables - 2021

Nuclear Power Industry / Fusion Power Industry

Humrickhouse reported in early 2021 that fusion energy beckons with the prospect of increased safety, reduced radiological hazards, no long-lived radioactive waste, and a virtually inexhaustible fuel supply. Fusion reactors do, however, use and produce some radioactive materials, and mitigating the effects of those materials is a necessary part of fusion reactor design. Fusion safety issues are both like and unlike those encountered in fission, and ongoing research and development supported by the Department of Energy is investigating the critical issues of tritium and radionuclide transport in fusion reactor materials and systems. Experiments and modeling carried out at Idaho National Laboratory’s Safety and Tritium Applied Research (STAR) facility are key components of that research. 

The Deuterium-Tritium (D-T) Fuel Cycle

The radiological hazards associated with fusion devices are closely linked to the select-ed fuel cycle. The deuterium-tritium (D-T) fuel cycle is the choice of most next-step devices worldwide. This fusion reaction combines deuterium and tritium to yield helium and a neutron and has a much larger cross section than other fusion reactions at temperatures approaching 150 million °C, achievable in magnetic confinement devices.

Tritium is radioactive and has no natural supply, so after initial startup, a fusion reactor must produce its own tritium in a lithium-breeding blanket. Fusion neutrons react with lithium to yield helium and tritium, and to achieve a tritium-breeding ratio greater than 1, the addition of a neutron multiplier of either lead or beryllium is typically necessary to offset the loss of neutrons by absorption in surrounding structures and coolants. While the two-stage fusion/breeding reaction consumes deuterium and lithium and directly produces only stable helium, the neutrons inherent to the process lead to the    creation of significant inventories of radioactive materials through neutron activation in the surrounding structures and coolants. The extent of the radiological hazard posed by that activation, as well as the decay heat and the nature and quantity of the waste produced, depend entirely on the materials used in the reactor and the extent to which those materials can become activated.

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