Fabrication, chemical and thermal stability studies of crystalline ceramic wasteform based on oxyapatite phosphate host LaSr(PO)O for high level nuclear waste immobilization.

Reprocessed high-level nuclear waste (HLW) contains range of radioactive components. Crystalline oxyphosphate apatite ceramic of the formula LaSr(PO)O [LSS] was investigated as a host for HLW immobilisation. The systematic study of solid solubility limit of individual rare earth ion substitution leads to the formulation of simulated wasteform of the formula LaPrNdSmGdSr(PO)O (WF1) with the waste loading of 17.95 wt% of rare-earth ions. Both parent and WF1 were synthesized by precipitation method. The thermal stress and groundwater inventory at the repository site can severely affect the wasteform performance, in addition to radiation and mechanical effects. Hence, the fabricated composition with high-level nuclear waste loading must be screened basically for chemical, thermal and radiation resistance. The present study investigated the thermal stability (by TGA), thermal expansion behaviour (by HT-XRD) and chemical durability (MCC-5) of the composition (WF1). The weight loss of WF1 being 2.2% and the average thermal expansion co-efficient (α) of 10.7 ± 1.2 × 10 K in the temperature range (298-973 K) were comparatively lower than the parent phase, LaSr(PO)O. The WF1 showed resistance to leaching of RE and P with only the leaching of Sr ion whose leach rate was of the order 10-10 gmd.