Development of SYNROC melting process for conditioning of partitioned HLW

In the frame of the high level waste (HLW) partitioning concept, HLW from spent fuel reprocessing must be processed with formation of short-lived (Cs-Sr) and long-lived (actinide) fractions to be treated and stored separately. Short-lived fraction to be stored for thousands of years can be incorporated in either borosilicate glass or silicate-based rock-type materials. The latter waste form seems to be more preferable due to higher long-term and radiation stability. Among silicate waste forms, pyroxene-, sphene-, and garnet-based materials were considered. These materials were produced using an inductive melting in a cold crucible. Long-lived fraction requires millions of years to be stored. Vitreous waste forms can not guarantee long-term stability for such long period and crystalline hosts, whose stability during geological periods has been proven, must be applied only. Among crystalline host phases for rare earths and actinides, Synroc phases (zirconolite and perovskite), murataite, pyrochlore, and zircon were considered. Ceramic materials based on above-listed phases were produced by cold pressing and sintering. The target phase content was found to be ≥90 vol.%. Zirconolite-, murataite-, pyrochlore-rich ceramics and melted zirconia were also produced in the cold crucible. Zircon was not produced through melting due to its incongruent melting and formation of zirconia and high- silica glass. The cold crucible melting process variables were determined. Materials obtained were examined with X-ray diffraction, SEM/TEM, optical microscopy, and electron microprobe analysis. Materials contained simulated long-lived fraction of HLW were almost fully crystalline with grain size between fractions of microns and hundreds of microns. Uranium, cerium, neodymium, and gadolinium introduced as actinide surrogates were found to enter zirconolite, pyrochlore or murataite. Proposals on waste treatment technology include HLW partitioning and incorporation of short-lived and long-lived fractions into rock-type materials using the cold crucible melting. (author)