(528b) Aspen/Oli Model of Waste Evaporation and Neutralization Process for Mox Waste Solidification Building

The Mixed Oxide Fuel Fabrication Facility (MOX or MFFF) at the Savannah River Site includes a Waste Solidification Building (WSB), which was built to house the process for converting aqueous wastes from the MFFF and the proposed Pit Disassembly and Conversion Facility (PDCF) into cements for long-term storage in either onsite repositories or at the Waste Isolation Pilot Plant (WIPP).  The WSB process design required flowsheet modeling of the mixing tanks, batch evaporators, and neutralization tanks included in the WSB process. 

 A steady state computer simulation was developed, using Aspen Plus to model the mixing tanks and evaporators and OLI to model the chemistry.  In the Aspen Plus simulation, the batch evaporators are modeled as multistage flash evaporators to account for changes in the vapor overheads composition during each batch.  The WSB process includes separate mixing tanks and evaporators for low activity and high activity wastes.  Limits are placed on the uranium concentration in the condensate from the low activity waste evaporator and on the total salt concentration in the condensate from the high activity waste evaporator to ensure that solids do not precipitate in either the evaporators or the transfer lines from the evaporators to the neutralization tanks.  In addition, the chloride concentration in the condensate from the low activity waste evaporator is limited to control corrosion.  To eliminate the need for recycle loops, feed forward controls are used to apply these limits.  The OLI chemistry was modified to add precipitation chemistry for sodium diuranate, which was absent from the standard OLI databank.  The simulation was used to predict flow rates, compositions, and heating duties for feeds from the PDCF and alternate feedstocks.  The simulation is configured so that both inputs and outputs appear on a Microsoft Excel spreadsheet.