Binders for radioactive waste forms made from pretreated calcined sodium bearing waste

Abstract

Radioactive waste generated during the reprocessing of fuel rods by the U.S. Department of Energy (DOE) is stored in underground tanks at Hanford, Savannah River and INEEL. The liquid fraction commonly referred to as sodium bearing waste (SBW), is a highly alkaline solution containing large amounts of sodium hydroxide, sodium nitrate and sodium nitrite. It has been shown that SBW can be mixed with a reducing agent and metakaolin and then calcined at 500°–700°C to form a calcine containing sodium aluminosilicate phases such as zeolite A, hydroxysodalite and/or cancrinite. Although calcination of the pretreated SBW produces a reasonable waste form in its own right, existing regulations require that granular calcines must be solidified before they can be shipped off site. It is possible to solidify the calcine in a number of ways. The calcine can be mixed with additional metakaolin and NaOH solution followed by mild curing (90°–200°C). The solid that forms (aka hydroceramic) has both strength and suitably low leachability. The current study examines the feasibility of using a more conventional Portland cement binder to solidify the calcine. Although strength was adequate, the leachabilities of the Portland cement solidified samples were higher than those of companion samples made with metakaolin. The zeolitic phases in the calcine acted like pozzolans and reacted with the Ca(OH)2 in the Portland cement binder forming additional calcium silicate hydrate (C—S—H). Typically C—S—H is unable to host large amounts of sodium ions in its structure, thus a majority of the sodium present in the zeolites became concentrated in the pore solution present in the Portland cement binder and readily entered the leachant during PCT testing. In this instance metakaolin mixed with NaOH proved to be a superior binder for solidification purposes.