Immobilization of Radioactive Wastes in Glasses and Magnesium Phosphate Cements

Abstract

The incorporation of molybdenum (Mo) into borosilicate glasses developed for the immobilization of radioactive waste causes alkali and alkaline earth molybdates to crystallize because of their low solubility. This study investigated the effects of adding MnO and ZnO to a simplified nuclear waste glass on the crystallization of molybdates and structure of the glass. The natural cooling of nuclear waste glasses in canisters was simulated by cooling the molten glass slowly at a rate of 1°C/min. X-ray diffraction and scanning electron microscopy were used to analyze the crystallization behavior, and X-ray absorption spectroscopy was used to identify the local structures of Mn, Zn, and Mo within the glass. The results confirmed that the addition of MnO and ZnO both increased the solubility of Mo in the nuclear waste glass and preferentially inhibited the formation of alkali molybdate crystals, which deteriorate the chemical durability of the glass. Moreover, the solubility of Mo was further improved by the addition of MnO into Nd2O3-bearing borosilicate glasses. Unlike Nd, the Mo-rich phase barely included Mn and Zn. Both Mn and Zn appeared in a tetrahedral structure that comprised a silicate network. On the basis of the bond valence model, it is suggested that MoO42− units can be directly connected to the Mn and Zn. The efficacy of magnesium potassium phosphate cements (MKPCs) as waste forms for the solidification of radioactive concrete powder wastes resulting from the decommissioning of nuclear power plants was evaluated. MKPC specimens that contained up to 50 wt% of simulated concrete powder wastes (SCPWs) were evaluated. The porosity and compressive strength of the MKPC specimens were measured using scanning electron microscopy and X-ray diffraction. The addition of SCPWs reduced the porosity and increased the compressive strength of the MKPC specimens. Struvite-K crystals were well-synthesized, and no additional crystal phase was formed. After thermal cycling and water-immersion tests, MKPC specimens with 50 wt% SCPWs satisfied the waste-acceptance criteria (WAC) for compressive strength. Semi-dynamic leaching tests were performed using the ANS 16.1 method; the leachability indices of Cs, Co, and Sr were 11.45, 17.63, and 15.66, respectively, which also satisfy the WAC. MKPCs cure quickly and have a high heat of hydration, which can be a problem when making large amounts. As a new type of retarder, calcined zinc oxide (ZnO) has been developed. This study investigates the hydration process and ZnO retardation of MKPC suspensions and pastes. Unlike boric acid, a conventional retarder, ZnO strongly retards the initial hydration of MKPCs. Zinc is first precipitated as zinc potassium phosphate in the MKPC system. It was postulated that Zn2+ was adsorbed on the surface of MgO to prevent hydration or to destabilize the hydrated Mg2+ ions (Mg(H2O)62+) to inhibit the dissolution of MgO.