Impacts of Preferential Flow on Tc-99and Np-237 Vadose Transport in Soils at the Savannah River Site

Author

Josh ParrisFollow

Date of Award

5-2023

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Hydrogeology

Committee Chair/Advisor

Dr. Brian Powell

Committee Member

Dr. Lawrence Murdoch

Committee Member

Dr. Christophe Darnault

Abstract

Since the 1950s, the United States has produced approximately 90,000 metric tons of spent nuclear fuel (SNF) (Office of Nuclear Energy, 2022); however, no long-term storage solutions are available. Technecium-99 and neptunium-237, two fission products found in SNF, readily form highly mobile species in oxidizing conditions (Hu, 2008; Bondietti, 1979) and have respective half-lives of 2.13 x 10⁵ and 2.14 x10⁶ years (Hu, 2010). Considering these characteristics, ⁹⁹Tc and ²³⁷Np are two risk-driving isotopes found in SNF storage. The process of macropore-facilitated preferential flow, transport through cracks within a soil matrix, has been recognized to increase radionuclide mobility (Bundt, 2000). The aim of this study is to clarify the influence of macropore structures (orientation and geometry) on the transport of ⁹⁹Tc and ²³⁷Np in unsaturated sand and sandy clay loam (SCL) soils.

Unsaturated column flow experiments were completed with Br (tracer), ⁹⁹Tc, and ²³⁷Np on homogeneous and macroporous soil columns. CT scans of soil columns were acquired to generate 2D and 3D imaging of macropore networks. Retardation factors for Tc⁹⁹ breakthrough were 1.30 in homogeneous SCL and ranged from 1.28 to 1.41 in macroporous SCL, indicating that macropore-facilitated bypass flow is limited in unsaturated conditions. However, ⁹⁹Tc exhibited earlier initial arrival (IBV) and longer attenuation in macroporous SCL soils. Increasing dispersivity estimates, determined by fitting the two-region nonequilibrium convection dispersion equation (MIM), well describe the anomalous breakthrough trends observed in macroporous SCL soils. From these findings, we conclude that macropore structures oriented transverse to unsaturated flow act as capillary barriers (resulting in longer radionuclide attenuation). If vertically oriented, macropore structures act as capillary boundaries that isolate matrix flow to a fraction of the pore network (increasing the average porewater velocity and IBV).

Recommended Citation

Parris, Josh, "Impacts of Preferential Flow on Tc-99and Np-237 Vadose Transport in Soils at the Savannah River Site" (2023). All Theses. 4040.
https://open.clemson.edu/all_theses/4040