Presentation on theme: "The Behavior of Radionuclides and RCRA Elements in Tank Backfill Grouts Mark Fuhrmann, Jeffrey Gillow, and John Heiser Environmental Sciences Department."— Presentation transcript:
The Behavior of Radionuclides and RCRA Elements in Tank Backfill Grouts Mark Fuhrmann, Jeffrey Gillow, and John Heiser Environmental Sciences Department Brookhaven National Laboratory Environmental Sciences Department Environmental Research and Technology Division
History of HLW Processing at West Valley l 1966-1972, Nuclear Fuel Services at West Valley reprocessed spent nuclear fuel rods. l 1996-2003, vitrified 600,000 gallons of HLW, 277 glass waste forms-2.5 tons each, 24 million Curies. l Tanks need to be decommissioned
Long-Term Disposition? l HLW tanks and several cells for HLW processing are heavily contaminated. l Most wastes to be shipped off-site l Tanks and cells may be partially filled with removable grout for interim stabilization and shielding; with enough stability and retention capacity if DOE decides to leave it in place!
Grout Formulations l Grout 21: 7.6 % cement, 32.6% fly ash, 10% zeolite, 12.5% slag, 37.2% water. l Grout 26: 8.8 % cement, 42.2 % fly ash, 12.0% apatite, 13.0% slag, 23.9% water. Are these formulations optimal or can they be made to better sequester contaminants?
Characterize retention of contaminants in the two Grouts l Isotherms over several orders of magnitude: bracketing waste concentrations for 23 elements 4 What components of the grout provide best retention? l Determine K d and begin to look at mechanisms 4 Long-term condition of grout is fractured and has water moving through it. 4 Assume water contains oxygen 4 Predicted speciation with Geochemist's Workbench. constants NEA Thermochemical Database Project and published reviews (Guillaumont et al., 2003). l What is the long-term performance of these materials?
Zeolite in grout improved Sr Retention in Grout 21. G-21 Kd=110 G-26 Kd= 60 Zeolite sorbs 85 Sr readily even with competing ions of Ca in the contact solution. Kd = Concentration in Solid Concentration in Liquid
Uptake of Tc is very low, less than one in both grouts. Blast furnace slag does remove more Tc than other materials, but the linear isotherm implies adsorption, not precipitation.
Sorption of 125 I on grouts is very weak. Very low retention on grouts implies competition Uptake on cement is significant, but on no other materials.
Sorption of contaminants on different components of the grouts: l Can be measured and can be significantly different. e.g. 4 125 I sorbs weakly on grout, K d around 0.7 4 Cement sorbs a lot of 125 I, with K d around 200, all other materials are much lower. 4 If better sequestration of I is needed, then more cement in the grout mix is worth trying.
Uranium Behavior in Contact with Grouts l Batch Experiments 4 550 days aging 4 55 g Ca/K OH contact solution 4 2 g grout or components 4 Then KHCO 3 added incrementally l Flow-through Column Experiments 4 Simulated groundwater 4 Ran for about 1000 pore volumes
U in solution in batch experiments as KHCO 3 is added: cement Keeping pH above 11.5 helps retain U even with high KHCO 3
Two grouts behave differently after aging. The two data points that are off-scale are from Grout-26. Presence of the fluorapatite causes U to be released from G-26 after aging and KHCO 3 added.
Long-Term batch Experiments (no KHCO 3 added) Slowly Grew Calcite Crystals Containing U, Pb, As, and Se Single crystal XRD done at SUNY-SB by Aaron Celestian
Flow through Uranium experiments set up for tritium slug test to determine pore volumes
U (ppb) in Column Effluent water was simulated site groundwater flow rate was about 200 mL /day; 10 pore-volumes/day. Time pH=12.4 pH = 8
Uranium Leached from Columns ColumnMass of U in Column (mg) Mass of U leached (mg) % U leached G-216.740.6659.7 G-267.270.4966.8
Calcite formed in the columns Fe remains with the grout, As is mobilized from the grout and is incorporated into calcite. So is U and Se. image Fe Ca As NSLS X-27a Grout 21, no U Synchrotron Microprobe : spot size 10 micrometers
UFe As Elemental Distributions in Calcite Grown in HLW Tank Backfill grouts Column experiments, west valley grout G-21, 1000 pore volumes X-27A, NSLS, M. Fuhrmann
Conclusions l Primary sequestration of contaminants takes place on different components of the grouts. l Batch experiments imply that high pH minimizes U concentrations even with high KHCO 3. l In column experiments U in solution was less than 70 ppb for both grouts. l Grout 21- 9.7% of U leached. l Grout 26- 6.8% of U leached. l U, As, and Se are incorporated in calcite as grouts weather.
Acknowledgements Funded by DOE through the West Valley Nuclear Services Co. Dan Westcott, program manager, Washington Group Int. Some activities supported by DOE through the Center for Environmental Molecular Sciences, a DOE/NSF center. Thanks to Richard Reeder, John Parise, David Frumberg, Jenny Huang, Aaron Celestian,Tony Lanzirotti (X-26A), Paul Northrup (X-15A) and James Ablett (X-27A).