Localised solution environments drive radionuclide fractionation in uraninite

Area of Science:

• Geochemistry
• Environmental Science
• Nuclear Science

Background:

• Uraninite is the primary ore of uranium and a common accessory mineral in uranium-rich rocks.
• Understanding radionuclide (RN) behavior in the uranium decay series is crucial for managing environmental and engineered risks.
• Secular disequilibrium among RNs can occur during the leaching of uraninite.

Purpose of the Study:

• To investigate the influence of different solution environments (chloride brines, acid mine drainage, groundwater) and pore pressure on radionuclide mobility and disequilibrium from uraninite.
• To determine the primary factors controlling the release and mobility of RNs from uraninite.
• To understand the geochemical behavior of RNs in the U-decay series for improved risk prediction and management.

Main Methods:

• Leaching experiments using uraninite in various solution environments (chloride, sulfate, carbonate).
• Analysis of radionuclide distribution (surface vs. bulk) within uraninite grains.
• Assessment of the impact of pore pressure on radionuclide mobility.

Main Results:

• Solution environment, particularly carbonate-bearing fluids, significantly influences RN mobility and causes disequilibrium.
• Intermediate RN daughters (Ra-226, Pb-210, Po-210, Th-234/230) are primarily in the uraninite bulk, while end products (Pb-206, Pb-207) are at grain surfaces/edges.
• Pore pressure had a minimal effect on RN mobility compared to the solution environment.
• Carbonate fluids induced significant fractionation of RNs due to variable complexation and sorption.

Conclusions:

• The geochemical behavior of RNs in the U-decay series is strongly influenced by the solution environment during uraninite leaching.
• Significant RN fractionation occurs, especially in carbonate-rich fluids, impacting mobility and distribution.
• Accurate long-term RN behavior modeling requires incorporating this differential fractionation and geochemical interaction with host rocks.