Overview of fuel retention and recovery in JET deuterium–tritium operation

Tritium inventory build-up is a safety and economic issue for next step fusion devices and power plants. JET with a beryllium wall and tungsten divertor brings a unique contribution on fuel retention and recovery in a metallic device, as it has operated with deuterium, tritium and deuterium–tritium (DT) plasmas. This paper documents the JET fuel retention programme and results focusing on the tritium, DT campaigns and tritium clean-up in 2023. In addition specific experimental details are presented in order to provide lessons learned for fuel retention and inventory assessment procedures for regulators and operators of future fusion devices. Fuel retention results from gas balance and Laser-Induced Desorption with gas detection using Quadrupole Mass Spectrometers (LID-QMS) are discussed. Gas balance has shown that there is no significant isotopic dependence of in-vessel global fuel retention, however a faster decrease in outgassing rate has been observed with increasing mass, likely associated with the difference in concentration and depth profile of tritium and deuterium. LID-QMS data has provided new local in-vessel fuel retention data demonstrating capability for measuring fuel retention, monitoring changes in fuel retention during an operating period as well as providing direct measurement of increased near-surface fuel concentration due to diffusion of hydrogen isotopes to the surface at elevated baking temperature and removal of fuel by inner strike point heating.