Plutonium reactive transport in fractured granite: Multi-species experiments and simulations

Xiaoying Zhang; Zheng Wang; Paul Reimus; Funing Ma; Mohamad Reza Soltanian; Baoshan Xing; Jianzheng Zang; Yu Wang; Zhenxue Dai

doi:10.1016/j.watres.2022.119068

Plutonium reactive transport in fractured granite: Multi-species experiments and simulations

Water Res. 2022 Oct 1:224:119068. doi: 10.1016/j.watres.2022.119068. Epub 2022 Sep 7.

Authors

Xiaoying Zhang¹, Zheng Wang², Paul Reimus³, Funing Ma², Mohamad Reza Soltanian⁴, Baoshan Xing⁵, Jianzheng Zang⁶, Yu Wang⁷, Zhenxue Dai⁸

Affiliations

¹ Institute of Intelligent Simulation and Early Warning for Subsurface Environment, Jilin University, Changchun 130026, China; College of Construction Engineering, Jilin University, Changchun 130026, China.
² College of Construction Engineering, Jilin University, Changchun 130026, China.
³ Los Alamos National Laboratory, Los Alamos, NM 87545, United States.
⁴ Departments of Geology and Environmental Engineering, University of Cincinnati, Cincinnati, OH, United States.
⁵ Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, United States.
⁶ Institute of Nuclear and New Technology, Tsinghua University, Beijing 100084, China.
⁷ Institute of Nuclear and New Technology, Tsinghua University, Beijing 100084, China. Electronic address: yuwang16@mails.tsinghua.edu.cn.
⁸ Institute of Intelligent Simulation and Early Warning for Subsurface Environment, Jilin University, Changchun 130026, China; College of Construction Engineering, Jilin University, Changchun 130026, China; Key Laboratory of Groundwater Resources and Environment, Ministry of Education, Changchun 130026, China. Electronic address: dzx@jlu.edu.cn.

PMID: 36103780
DOI: 10.1016/j.watres.2022.119068

Abstract

Plutonium (Pu) in the subsurface environment can transport in different oxidation states as an aqueous solute or as colloidal particles. The transport behavior of Pu is affected by the relative abundances of these species and can be difficult to predict when they simultaneously exist. This study investigates the concurrent transport of Pu intrinsic colloids, Pu(IV)_(aq) and Pu(V-VI)_(aq) through a combination of controlled experiments and semi-analytical dual-porosity transport modeling. Pu transport experiments were conducted in a fractured granite at high and low flow rates to elucidate sorption processes and their scaling behavior. In the experiments, Pu(IV)_(aq) was the least mobile of the Pu species, Pu(V-VI)_(aq) had intermediate mobility, and the colloidal Pu, which consisted mainly of precipitated and/or hydrolyzed Pu(IV), was the most mobile. The semi-analytical modeling revealed that the sorption of each Pu species was rate-limited, as the sorption could not be described by assuming local equilibrium in the experiments. The model was able to describe the sorption of the different Pu species that occurring either on fracture surfaces, in the pores of the rock matrix, or simultaneously in both locations. While equally good fits to the data could be achieved using any of these assumptions, a fracture-dominated process was considered to be the most plausible because it provided the most reasonable estimates of sorption rate constants. Importantly, a key result of this work is that the sorption rate constant of all Pu species tends to decrease with increasing time scales, which implies that Pu will tend to be more mobile at longer time scales than observations at shorter time scales suggest. This result has important implications for predicting the environmental impacts of Pu in the safety assessments of geologic repositories for radioactive waste disposal, and we explore potential mechanistic bases for upscaling the sorption rate constants to time and distance scales that cannot be practically evaluated in experiments.

Keywords: Fractured rock; Pu species; Reactive transport; Sorption mechanism; Upscaling.

MeSH terms

Colloids / analysis
Plutonium* / analysis
Radioactive Waste*
Silicon Dioxide

Substances

Colloids
Radioactive Waste
granite
Plutonium
Silicon Dioxide