PNNL

Abstract

Bulk vitrification (BV) was selected as a potential supplemental treatment to accelerate the cleanup of low-activity waste (LAW) at the U.S. Department of Energy Hanford Site. In the BV process, low-activity waste, soil, and glass forming chemicals are mixed, dried and placed in a metal box lined with a castable refractory block (CRB). Electric current, supplied by two graphite electrodes in the box, melts the waste feed and produces a durable glass waste form. During engineering-scale (ES) tests of BV, a small fraction of radioactive technetium-99 (Tc) (and rhenium [Re], a nonradioactive surrogate) were transferred out of the LAW glass feed and molten LAW glass, and deposited on the surface and within the pores of the CRB. Tc is a primary risk driver for long-term performance of immobilized LAW; therefore, even small fractions of Tc present in a readily leachable form rather than immobilized in a glass matrix can impact long-term performance.