Physical and chemical effects of natural zeolites additives on the cementitious stabilization of cesium and barium isotopes in boric acid liquid waste  

Abstract

Boric acid is used in pressurized water reactor (PWR) systems as an efficient neutron absorber for activity control which aids in the maintenance of steady state operating temperature control. The boric acid liquid waste produced from the system might contain low concentrations of nuclear fission produced radioisotopes such as 137Cs and its metastable decay product 137mBa (also named as 137mCs). Adequate repository storage of this high-volume liquid waste has become environmentally important for study as these radioisotopes can become bio-available in the natural systems if not effectively immobilized. Our research main aims are to assess the geochemical effects of natural zeolite (i.e., clinoptilolite and mordenite) additives in sulfoaluminate cement (SAC) and ordinary Portland cement (OPC) blends for optimizing chemical and mechanical stability for immobilizing boric acid liquid waste and contained fission isotopes, mentioned above in solidified cement paste waste forms. To accomplish our research goal, the natural zeolite additives at ratios ranging from 0%, 5%, 10%, 15% and 20%, respectively, were added to a benchmarked OPC to SAC blend (i.e., 80% OPC to 20% SAC) based on results from our previous experiments. The samples were then mixed with simulated boric acid liquid waste containing Cs and Ba isotopes and allowed to cure for 28 days. Standardized reference leaching test was carried out on the cured solid waste forms for 11 days and then they were analyzed by standardized compressive strength test, scanning electron microscopy (SEM) for morphology and X-ray diffraction (XRD) for mineralogy. While the liquid waste solutions (leachate) from the leaching test were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectroscopy (ICP-MS) to assess elemental and isotopic changes.