A ceramic waste form is being developed at Argonne National Laboratory for
waste generated during the electrometallurgical treatment of spent nuclear
fuel. The waste is generated when fission products are removed from the
electrolyte, LiCI-KCl eutectic. The ceramic waste form is a composite,
fabricated by hot isostatic pressing a mixture of glass frit and zeolite
occluded with fission products and salt. Past work has shown that the
normalized release rate (NRR) is less than 1 g/m2d for all
elements in a Material Characterization Center-Type 1 (MCC-1) leach test run
for 28 days in deionized water at 90°C (363 K). This leach resistance is
comparable to that of early Savannah River glasses. We are investigating how
leach resistance is affected by changes in the cationic form of zeolite and
in the glass composition. Composites were made with three forms of zeolite A
and six glasses. We used three-day ASTM C1220–92 (formerly MCC-1) leach
tests to screen samples for development purposes only. The leach test
results show that the glass composites of zeolites 5A and 4A retain fission
products equally well. The loss of cesium is small, varying from 0.1 to 0.5
wt%, while the loss of divalent and trivalent fission products is one or
more orders of magnitude smaller. Composites of 5A retain chloride ion
better in these short-term screens than 4A and 3A. The more leach resistant
composites were made with durable glasses that were rich in silica and poor
in alkaline earth oxides. The x-ray diffraction (XRD) results show that a
salt phase was absent in the leach resistant composites of 5A and the better
glasses but was present in the other composites with poorer leach
performance. Thus, the data show that the absence of a salt phase in a
composite's XRD pattern corresponds to improved leach resistance. The data
also suggest that the interactions between the zeolite and glass depend on
the composition of both.