The Hanford Site Tank Farms contain approximately 53 million gallons of radioactive solid and liquid waste currently stored in a combination of 177 single and double-shelled tanks.  The Office of River Protection (ORP) was established in the late 1990's to assist in completing the DOE mission of site closure by providing management of the immobilization, treatment, and final dispose of the high level waste (HLW) and hazardous wastes.   The Waste Treatment Plant (WTP) is under construction at Hanford in order to pretreat and vitrify (i.e., immobilize/solidify) HLW for long-term safe storage, transportation, and final disposal in a federal repository.  Borosilicate waste glass has been chosen as the immobilization technology for HLW processing.  The current Hanford WTP baseline design for HLW vitrification is a joule-heated ceramic melter (JHM).  The baseline approach will result in approximately 9,200 canisters of borosilicate HLW glass that will be stored onsite for later transport and disposal at the Yucca Mountain repository.  

One of the challenges in processing the Hanford Waste is the undesirable effect of the chromium (Cr) content on the joule heated melters and the HLW glass properties.  Cr loaded into glass at a level greater than approximately 1.4 weight percent precipitates as spinels ([Fe,Ni,Mn][Cr,Fe]₂O₄) and eskolaite (Cr₂O₃) crystals in the glass melt during the vitrification process, These particles will potentially clog and/or short circuit the joule heated melters and also destabilize the structure of the glass, thus affecting the long term stability of the vitrified waste.   

The WTP pretreatment facility is currently designed to separate waste into fractions of high level waste (HLW) and low activity waste (LAW).  In the present design, Cr will be leached out of the HLW vitrification facility feed stream into the LAW vitrification feed stream through an 8 hour 0.1 M alkaline leach at 85 °C followed by a 6 hour permanganate (MnO₄^-) leach at 25 °C.  This process has been observed to effectively mobilize Cr(III) as chromate (CrO₄^2-).   

ORP's strategy is to further refine the baseline alkaline/oxidative leaching process by eliminating process steps to reduce operating costs and shorten the process schedule. Alternative oxidant processes which combine the alkaline and oxidative leaching steps are currently being evaluated; these may reduce overall processing time by up to three years.  The oxidant dose requirements are also being refined by incorporating the inherent properties of the waste into the washing and leaching mechanism.  Ionizing gamma radiation produced by radioactive constituents of the waste contributes to the oxidation potential while some ions in the waste are themselves a reservoir of oxidants.  The consideration of these waste properties in the choice of oxidants and dose will provide the WTP operator with a method with which to tailor an optimal treatment scheme for each unique waste batch that is processed through the WTP.  The placement of the oxidative leach step is also being evaluated - the process may be more effective applied in the waste storage tanks rather than in the Pretreatment facility.