Understanding subsurface migration of actinides is vital for the protection of human and environmental health during disposition of actinide bearing wastes. The sensitivity of actinide elements to redox reactions and the profoundly different geochemical behavior of differing actinide oxidation states complicates our understanding of Pu transport. Thus, data collected under realistic field conditions is vital for model refinement.  The transport of neptunium and plutonium has been monitored for over 2 years in a unique series of field lysimeter experiments at the Savannah River Site.  The field lysimeters (60-cm x 10-cm PVC pipe) were packed with a sandy loam soil and a Np(IV), Np(V), Pu(V),  Pu(IV), or PuO2-nanocolloids, source was placed approximately 30 cm above the base. Then 25 cm of clean soil was packed above the source. The lysimeter design allows for the collection of effluent rainwater from the bottom of the pipe after it has percolated through the soil and source. Quarterly collections of effluent samples are analyzed  for Np, Pu, selected major ions, as well as pH and dissolved oxygen measurements.  As expected based on the low mobility and high solid phase affinity of tetravalent actinides, no measureable Np or Pu has been found in the effluent from lysimeters containing Np(IV), Pu(IV), or PuO2-nanocolloids. However, breakthrough of Np(V) occurred after approximately one year in the field and the concentration of Np in the aqueous effluent has increased steadily for each sampling event thereafter. Unlike Np(V), Similar breakthrough of Pu(V) has not been observed. Thus, is appears that Pu(V) reduction to Pu(IV) is occurring within the lysimeters, consistent with previous observations with pure mineral phases and soils. The data are discussed in terms of conceptual model development and modeled using the 1D transport code HP1 (Hydrus-PHREEQ-1D). These radiological field lysimeter experiments will be continued for a total of 10 years and represent the first of their kind in >30 years.

Transport of plutonium and neptunium of varying oxidation states in the vadose zone using field lysimeters

248th National Meeting (2014)

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