Solidification of low-level radioactive wastes in masonry cement. [Masonry cement-boric acid waste forms]
Portland cements are widely used as solidification agents for low-level radioactive wastes. However, it is known that boric acid wastes, as generated at pressurized water reactors (PWR's) are difficult to solidify using ordinary Portland cements. Waste containing as little as 5 wt% boric acid inhibits the curing of the cement. For this purpose, the suitability of masonry cement was investigated. Masonry cement, in the U.S. consists of 50 wt% slaked lime (Ca0H2) and 50 wt% of Portland type I
... nt* Addition of boric acid in molar concentrations equal to or less than the molar concentration of the alkali in the cement, eliminates any inhibiting effects. Accordingly, 15 wt% boric acid can be satisfactorily incorporated into masonry cement. The suitability of masonry cement for the solidification of sodium sulfate wastes produced at boiling water reactors (BWR's) was also investigated. It was observed that although sodium sulfate -masonry cement waste forms containing as much as 40 wt% Na2SOi4. can be prepared, waste forms with more than 7 wt% sodium sulfate undergo catastrophic failure when exposed to an aqueous environment. Waste form volume increase is so severe that swelling, cracking, exfoliation and crumbling occurs during the first several days of water immersion. It was determined by X-ray diffraction that in the presence of water, the sulfate reacts with hydrated calcium aluminate to form calcium aluminum sulfate hydrate (ettringite). This reaction involves a volume increase resulting in failure of the waste form. Formulation data was identified to maximize volumetric efficiency for the solidification of boric acid and sodium sulfate waster. Measurement of some of the waste form properties relevant to evaluating the potential for the release of radionuclides to the environment included leachability, compression strengths and chemical interactions between the waste components and masonry cement.