An Overview of Selected Radiation Chemical Reactions Affecting Fuel Cycle Solvent Extraction

Abstract

Aqueous solvent extraction for the recovery of uranium and plutonium from dissolved nuclear fuel has been used successfully since the advent of the nuclear age. Following decades of PUREX operating experience in several countries, new solvent extraction processes are now being developed worldwide to extract additional long-lived radionuclides , including especially the minor actindes. These must be partitioned from the lanthanides in what is an especially challenging separation. These new processes rely on specialty ligands as metal complexing agents. They must be reasonably stable to hydrolytic and radiolytic degradation in the acidic, irradiated biphasic system. Solvent system degradation may lead to decreases in ligand concentration, production of interfering degradation products, and changes in solvent viscocity and phase separation parameters. Therefore, research is underway in the USA, Europe and Asia with a goal of understanding these affects on solvent extraction efficiency for specific systems. This chapter presents an overview of the important reactions that are common to all solvent extraction systems with reference to recent research findings at Idaho National Lab (INL) and elsewhere. It is shown that indirect radiolysis by reaction with radiolytically produced reactive species such as hydroxyl radical, radical cations and nitrous acid is responsible for much ligand and diluent degradation.

Gamma radiolytic stability of CyMe4BTBP and the effect of nitric acid

“…For the development of processes on an industrial scale, one main issue to consider is the detailed knowledge of radiolytic stability of the chemicals used since a solvent would be in contact for longer times with highly radioactive process streams. Degradation of the solvent may lead to the production of interfering degradation products, decreases in ligand concentration and changes in solvent viscosity as well as phase separation parameters [13]. This degradation may result in losses in selectivity and affi nity for target metal extraction.…”

TRUEX Radiolysis Testing Using the INL Radiolysis Test Loop: FY-2012 Status Report

“…The major reactive species formed [3,4] Due to relatively low ligand concentrations employed most decay energy is absorbed by the diluent and ligand radiolysis is expected to occur by indirect, rather than direct mechanisms. [5] For example, the proposed TRUEX solvent contains 0.2 M CMPO as the ligand, while the tributylphosphate (TBP) modifier and dodecane diluent concentrations are about 1.4 M and 3 M, respectively. Consequently, most ligand damage will be due to reaction with reactive species created by energy deposition in the balance of the organic phase, or in the acidic aqueous phase in contact with that organic phase.…”

A comparison of the alpha and gamma radiolysis of CMPO

“…1 Irradiation of the ligand occurs due to the decay energy of actinides and fission products in the dissolved nuclear fuel solution. The radiation types are predominantly low linear energy transfer (LET) beta/gamma radiation from fission product decay, and high LET alpha radiation from actinide decay.…”

“…Due to relatively low ligand concentrations employed most decay energy is absorbed by the diluent and ligand radiolysis is expected to occur by indirect, rather than direct mechanisms. 1 For example, the proposed TRUEX solvent contains 0.2 M octylphenyl-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) as the ligand, while the dodecane diluent and tributylphosphate (TBP) modifier concentrations are about 3 M and 1 M, respectively. Consequently, most ligand damage will be due to reaction with reactive species created by energy deposition in the balance of the organic phase, or in the acidic aqueous phase in contact with that organic phase.…”