Tritium permeation considerations in the MARS direct convertor

The direct convertors located in the end cells of the MARS Tandem Mirror Fusion reactor recover the electrostatic energy of plasma particles that leak out of the center cell region. Since the direct convertors will be implanted in part by escaping high energy (> 40 keV) tritons at fluxes of over 10¹⁶ T cm^-2 s^-1 and low energy (∼30 eV) tritons at fluxes of over 10¹⁸ T cm^-2 s^-1, the possibility of tritium permeation into the water coolant, must be assessed. The current design calls for a TZM (Mo-0.5Ti-0.1Zr) structure; vanadium has also been under consideration. Combined nuclear reaction profiling and thermal desorption spectroscopy measurements on TZM show that deuterium retention is dominated by radiation damage traps with 1-1.5 eV detrapping energy, and bulk chemical traps of ≈ 0.7 eV presumably from the Ti and Zr alloying additions. Analysis of deuterium reemission data from vanadium demonstrates that molecular recombination controls the bulk retention. Values of the recombination constant for vanadium vary from 1.1 × 10^-22 cm⁴ sec^-1 at 500 °C to 3.6 × 10^-24 cm⁴ sec^-1 at 300 °C. Using these results and available literature data, calculations for Be on TZM, V, and V on Cu using DIFFUSE predict steady state permeation rates in excess of 10⁸ Ci day^-1 for bare V, 2000 Ci day^-1 for Be on TZM, and ≈ 300 Ci day^-1 for V on Cu.