Hydride Bed Isotopic Exchange

Savannah River Tritium Enterprise (SRTE) has used LaNi_4.25Al_0.75 (LANA75) hydride beds to store hydrogen isotopes for over two decades. A benefit of using LANA75 is that the ³He generated from tritium decay is retained in the hydride material, allowing the hydride beds to deliver high-purity product gas. A disadvantage is that the ³He accumulates in the LANA75 material over time, which forms a heel that cannot be removed under normal operating conditions. The heel traps hydrogen in the bed, slowly reducing the operational capacity of the bed as the heel grows. Eventually, the ³He begins to release from the material, preventing the delivery of high-purity product.

The hydride beds are replaced when (1) operational capacity is reduced such that it is impactive to routine operations and/or (2) product purity is not maintained due to ³He release. Prior to replacing and disposing of the beds, it is necessary to isotopically exchange the gas on the bed to recover as much tritium as possible. Isotopic exchange involves repeatedly absorbing deuterium onto the bed and desorbing hydrogen isotopes from the bed until a predetermined criterion has been met. The isotopic exchange process represents a significant additional load on routine operations both in time and in the amount of waste gas that requires further processing.

A set of beds was recently prepared for replacement. The isotopic exchange method used by SRTE is presented, along with results of the most recent isotopic exchange. Lessons learned during the recent isotopic exchange process led to modifications that reduce isotopic exchange duration and corresponding waste gas produced while increasing the amount of tritium recovered.