Auto-irradiation effects in DT pellets

Abstract

FUSION reactors based on magnetic containment and operating in a quasi steady-state will need to be refuelled during their burn period. The ballistic injection of solid pellets of hydrogen isotopes, first suggested in 1954¹, still seems to be the most promising method, and two satisfactory techniques for fabricating solid hydrogen pellets have been developed. In one, a continuous extrusion of solid hydrogen is thermally chopped into cylindrical pellets². In the other, a jet of liquid hydrogen is acoustically broken into uniform droplets which are then frozen by reducing the pressure to below the triple point value³. Both techniques have worked well with deuterium, but fusion reactors will need DT pellets, and we consider here how the radioactivity of tritium will affect the situation. Two effects are apparent with characteristic times which may be short enough to be of practical significance: thermal stress arising from the deposition of decay heat throughout the pellet and electric stress caused by the charging of the pellet due to the escape of β particles from its surface. For a pellet of radius a, the thermal stress varies as a² and the electric stress as a⁻², which leads to both upper and lower size limits at radii where the effective stress approaches the yield stress of solid hydrogen. Fortunately the range of pellet radii envisaged in current reactor concepts (10⁻²–10⁻⁴ m) falls mid-way between the two limits.