Gamma and beta radiolysis of tri-iso-amyl phosphate: Degradation of tri-iso-amyl phosphate and formation of di-iso-amyl phosphoric acid

Highlights

• The γ and β radiolysis of TiAP under various conditions were explored.

• A method for simultaneous determination of TiAP and DiAP was established.

• The effect of diluent and nitric acid on the degradation of TiAP and formation of DiAP was provided.

Abstract

Tri-iso-amyl phosphate (TiAP) is considered a potential alternative solvent to tri-butyl phosphate (TBP) for aqueous reprocessing of molten salt reactors and fast reactor fuels. This study explored the γ and β radiolysis of TiAP under various conditions through the analysis of the radiolytic concentration changes of TiAP and the yields of di-iso-amyl phosphoric acid (DiAP) using gas chromatography. The results indicated that the effects of dose, diluent, and nitric acid on the radiolysis of TiAP under γ-and β-irradiations showed a similar regularity. The degradation rate of TiAP followed a first-order reaction, and the paraffinic diluent was able to sensitize TiAP. Although the presence of nitric acid promoted the radiolysis of TiAP and the formation of DiAP in a neat TiAP system, it did not lead to significant changes in the 36%TiAP/n-DD system, implying that nitric acid is involved in an intricate process. Furthermore, based on these results, possible degradation mechanisms of TiAP were described.

Introduction

Recently, tri-iso-amyl phosphate (TiAP), a homologue of tri-n-butyl phosphate (TBP), has been considered a promising candidate for achieving high-performance extraction of uranium and plutonium from spent fuel (Kong et al., 2013; Kumar et al., 2016; Suresh et al., 2015) because of its excellent physical characteristics and extraction abilities. TiAP has been reported to possess a higher limiting organic concentration (LOC) value than TBP in the extraction of tetravalent actinides. Thus, it exhibits a lower tendency to third-phase formation (Suresh et al., 1994; Rao et al., 2010). In addition, the aqueous solubility of TiAP is less than that of TBP, which can effectively avoid the consequences of organic phase splitting and the formation of “red oil” (Rakesh et al., 2016; Suresh et al., 2009). Therefore, TiAP is being increasing researched upon as a potential alternative extractant for the reprocessing of spent fuels. (Sen et al., 2017; Sreenivasulu et al., 2014).

Spent fuel waste contains long-lived actinides, minor actinides and fission products. Among these nuclides, ^235/238U, ²³⁸Pu, ²⁴¹Am, and ²⁴²Cm can emit high-energy α particles and γ-rays, while ⁹⁰Sr and ¹³⁷Cs can release high-energy β particles and γ-rays (Magill, 2004). Thus, the extraction system in the spent fuel process is irradiated via various rays of strong radiation. Thus, the extractant inevitably undergoes chemical bond breakage or rearrangement, generating a series of radiolytic products and deteriorating its extractive performance. In addition, when radiolytic products accumulate to a certain concentration in the extraction system, the retention value of radionuclides in the organic phase increases. Consequently, these decomposed products may exert adverse effects on the extraction efficiency, separation factors, and solvent-recycle longevity of the extraction system (Adamov et al., 1987; Dzhivanova et al., 2019; Mishra et al., 2015; Zilberman and Chistyakov, 2016). Thus, exploring the interaction between the ionizing radiation and the TiAP solvent system is of great significance for spent fuel reprocessing.

Till data, most studies on radiation stability of TiAP have focused on the influence of γ radiation on physiochemical properties, such as extraction performance, thermodynamic properties, and viscosity of TiAP etc. (Mishra and Pandey, 2019; Sreenivasulu et al., 2016, 2017; Wang et al., 2021). Only the study Wang et al. (Li et al., 2018) performed γ-irradiation experiments to determine the radiolysis rate and the radiolysis product content of TiAP in a 1.1 M TiAP/n-DD-3 M HNO₃ solution. However, these experimental results could not provide sufficient information on the effect of diluents and nitric acid on the radiolysis of TiAP. In addition, the effects of β or α radiation on TiAP degradation have not yet been explored.

The primary reason for the shortage of related irradiation research on TiAP may be the lack of analytical methods for TiAP and its radiolysis products. It is expected that TiAP may radiolytically decompose di-iso-amyl phosphoric acid (DiAP) and mono-iso-amyl phosphoric acid (MiAP). These compounds are believed to be metal chelators and can accumulate in the solvent by successive fuel recycling. However, they cannot be easily measured via titrimetry or potentiometry, even by high-performance liquid chromatography (Boček et al., 1980; Lee and Ting, 1979; Li et al., 2018). Accordingly, the development of reliable methods to analyze TiAP and its radiolysis products can provide certain references for evaluating the radiolytic stability of this extractant while also providing important hints for the study of radiolytic products in other extraction systems. Fortunately, because TiAP and TBP have structural similarities (Fig. 1), the chemical characteristics and radiation behavior of TiAP can be compared to those of TBP. Thus, ideas can be obtained from studies on TBP analytical methodologies and degradation mechanisms.

In this study, TiAP was treated with γ and β radiation under various conditions including the absorbed dose, diluent, and nitric acid concentration. Residual TiAP and accumulation of DiAP were determined using gas chromatography. The radiation chemical yields (G values) of TiAP and DiAP, degradation constants of TiAP, and formation constants of DiAP are discussed. This study systematically explored the radiolytic behavior of TiAP and the formation regularity of DiAP under γ and β irradiation, evaluated the influence of diluent and nitric acidity on the radiolysis of TiAP, and provided valuable data for evaluating the perspective of TiAP in the separation of uranium and plutonium (or thorium) from spent fuel.