Impact of various reductants on Pu(III) stability in the organic phase and its distribution ratio in a biphasic system

Abstract

Plutonium–uranium extraction (PUREX) reprocessing relies on stabilizing Pu(III) to enable effective U/Pu separation. This study compares four reductants – N,N-dimethylhydroxylamine (DMHAN), hydroxylammonium nitrate (HAN), monomethyl hydrazine (MMH), and hydrazine (N₂H₄) – for their ability to preserve Pu in the trivalent state (Pu(III)) and thereby minimize its distribution to the organic phase in a TBP (tributyl phosphate)–HNO₃ biphasic system. Optimizing reductant choice is essential for process safety (preventing Pu(III) re-oxidation and exothermic side reactions) and efficiency (maximizing Pu stripping from the organic phase). Experimental results across varying nitric acid concentrations show that Pu(III) stability and low distribution ratios are best achieved using DMHAN, especially when paired with a nitrite scavenger (e.g., MMH), sustaining Pu(III) even at high acidity. In contrast, hydrazine and HAN provided comparatively less Pu(III) stabilization under strong acid conditions. In conclusion, the DMHAN–MMH combination offered superior performance, ensuring safer operation and more efficient Pu(III) retention for improved PUREX partitioning.

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1. Introduction

The PUREX process remains the predominant method for spent nuclear fuel reprocessing worldwide.^1–3 Although its fundamental structure has remained unchanged over decades, continuous improvements have been made to enhance its safety and economic efficiency, solidifying its status as the leading reprocessing technique.^4,5Fig. 1 presents a schematic diagram of the aqueous reprocessing flow for spent nuclear fuel. The core principle underlying the separation and extraction of uranium and plutonium is based on the high distribution ratio of tributyl phosphate (TBP) toward elements in oxidation states of four or higher—particularly hexavalent uranium and tetravalent plutonium. For the co-extraction of uranium and plutonium, uranium must remain in the +6 oxidation state while plutonium is maintained in the +4 state. Conversely, achieving selective separation of uranium and plutonium requires adjusting plutonium's oxidation state to trivalent. Consequently, the selection of an appropriate reductant to control plutonium's valence state is crucial, as it directly determines the process's separation efficiency and the purity of the final product. To further optimize PUREX, researchers have pursued numerous innovations. Examples include the adoption of organic, salt-free reagents to minimize liquid waste and salt buildup;⁶ simplification of the flowsheet by reducing process stages and refining operational parameters;^7,8 and the use of high-speed centrifugal contactors in place of conventional extraction equipment to mitigate solvent radiolysis and increase throughput.⁹ In alignment with these global trends, Chinese scientists have achieved significant advances in front-end PUREX technology. At the China Institute of Atomic Energy, an advanced two-cycle process (APOR) was developed that employs N,N-dimethylhydroxylamine (DMHAN) as the primary reductant and monomethylhydrazine (MMH) as the auxiliary reductant.¹⁰ This configuration greatly streamlines the process while maintaining the excellent extraction performance. Nevertheless, most existing studies of the DMHAN–MMH system focus on process conditions; fundamental questions regarding its mechanistic behavior and its impact on Pu(III) stabilization have not been fully addressed and merit deeper investigation.¹⁰

Fig. 1 Schematic diagram of the nuclear fuel reprocessing process flow – aqueous method (F: feed; X: extract; S: slide; P: plutonium; U: uranium W: waste).

During the U/Pu separation stage of PUREX, Pu(IV) in the organic phase must be reduced to Pu(III) and transferred into the aqueous phase, thereby separating it from uranium, which remains as U(VI) in the organic phase.^11,12 However, the ubiquitous presence of nitrous acid (HNO₂) can re-oxidize Pu(III) back to Pu(IV), causing Pu to remain in the organic phase and diminishing plutonium recovery yields.¹³ Therefore, selecting a reductant that effectively stabilizes Pu(III) and optimizing process conditions are critical. An ideal reductant should offer sufficient reducing power, high selectivity, and rapid, complete reaction kinetics, while avoiding side reactions that could compromise extraction. It should introduce minimal additional salts and exhibit robust stability. Unfortunately, no single reductant fully satisfies all these criteria. Industrially, commonly used reductive back-extraction schemes include ferrous aminodisuccinate (Fe(NH₂SO₃)₂), ferrous-hydrazinium (Fe²⁺–N₂H₅⁺), uranium(IV)–hydrazinium (U(IV)–N₂H₅⁺), and hydroxylamine–hydrazine (HAN–N₂H₄) systems. While these combinations generally meet the U/Pu separation requirements of PUREX, each has inherent limitations.

In early reprocessing technology, ferrous ion (Fe²⁺) served as a reductant for Pu(IV) in PUREX. Fe²⁺ rapidly reduces Pu(IV) to Pu(III) and exhibits a high equilibrium constant for this reaction.^14,15 However, nitrous acid in the system oxidizes Fe²⁺ to Fe³⁺, rendering Fe²⁺ ineffective and resulting in Pu(III) re-oxidation.^16,17 Consequently, hydrazine (N₂H₄) must be added as a “supporting reductant” to scavenge nitrous acid.^18,19 Additionally, using iron salts (e.g., ferrous aminodisuccinate) introduces Fe³⁺ and SO₄²⁻ impurities, complicating subsequent waste-liquid treatment.²⁰ As processing technology advanced, uranium(IV) (U(IV)) was proposed as an alternative reductant for Pu(IV) and rapidly matured into a viable option.^21–24 U(IV) offers the advantage of not introducing metal-ion impurities, effecting complete reduction of Pu(IV) with relatively fast kinetics; however, U(IV) itself is susceptible to oxidation by nitrous acid—a reaction that proceeds autocatalytically—necessitating precise control over U(IV) addition and solution acidity. Overall, the reduction rate and separation efficiency of Pu(IV) by U(IV) are still inferior to those achieved by Fe(II) reductants.²⁵

To avoid introducing metal-salt impurities, hydroxylamine nitrate (HAN) has been employed as a salt-free reductant in PUREX U/Pu separation. Its reaction products (primarily N₂ and H₂O) generate no solid waste.^26–28 Because Pu(III) in an acidic organic phase remains vulnerable to oxidation, a certain amount of hydrazine (N₂H₄) is typically co-added with HAN to scavenge nitrous acid, forming a HAN–hydrazine synergistic reduction system.²⁹ However, the reaction between hydrazine and nitrous acid produces highly toxic and explosive hydrazoic acid (HN₃), which can detonate upon impact if it accumulates.^18,19,30 This safety hazard strictly limits hydrazine usage. As an alternative, Chinese researchers have proposed replacing hydrazine with monomethylhydrazine (MMH) as the supporting reductant.^31,32 MMH likewise scavenges HNO₂ rapidly but does not produce hydrazoic acid, significantly improving process safety and practicality.³³

In recent years, with increasing fuel burnup and growing demands for process cleanliness, research on new organic, salt-free reductants has surged.^34–37 These organic reductants—composed solely of C, H, O, and N elements—yield reaction products that decompose readily by combustion or radiolysis, avoiding the introduction of metal impurities or elevated salt content that characterize traditional reductants.^34–37 Studies indicate that, by adjusting the oxidation states or complexation of Pu, Np, and other actinides, organic reagents can enhance radionuclide separation efficiency.^34–37 Among various candidates, N,N-dimethylhydroxylamine (DMHAN) has attracted particular attention due to its strong reducing capability and relatively benign oxidation products.^35,36 Russian researchers have conducted extensive kinetic studies of DMHAN reducing Pu(IV) and determined the associated reaction parameters.^35,36 China has also contributed, with Ye Guoan et al. analyzing the application of organic, salt-free reagents in PUREX.³⁷ Building on these findings, the China Institute of Atomic Energy developed an advanced Pu purification configuration (APOR) within the PUREX flowsheet that uses DMHAN as the primary reductant and MMH as the auxiliary reductant, achieving favorable performance in laboratory validation.

In summary, while conventional reductive systems (e.g., Fe²⁺–hydroxylamine, U(IV)–hydrazine, and HAN–hydrazine) possess certain drawbacks in the PUREX process, the new salt-free DMHAN–MMH system holds promise to overcome these limitations. However, fundamental questions—particularly concerning its mechanistic behavior and its efficacy in stabilizing Pu(III)—remain underexplored. This work seeks to fill that gap by comparing the effects of DMHAN–MMH versus HAN–hydrazine on Pu(III) stability in the organic phase, and by investigating differences in extraction and distribution behavior between DMHAN versus HAN and MMH versus hydrazine. Insights from this study will support optimization of U/Pu separation in the PUREX process.

2. Experimental methods and reagents

2.1 Experimental reagents

Table 1 presents all experimental reagents—various reductants, buffer solutions, and colorimetric agents—along with their preparation concentrations and functional roles, thereby ensuring both the reproducibility of the experimental setup and the precision of Pu(IV) reduction and stabilization assays.

Table 1 Experimental reagents used

Number	Reagent name	Chemical formula/composition	Purity/specification	Supplier	Preparation concentration	Primary purpose
1	N,N-Dimethylhydroxylamine	DMHAN	>98%	China Institute of Atomic Energy	5.81 mol L^−1	Reductant for Pu(IV), prepared as nitrate salt
2	Hydroxylamine	HAN·HNO3	Analytical grade	Sinopharm	2.10 mol L^−1	Reductant for Pu(IV) in nitric acid medium
3	Hydrazine	N2H4	Analytical grade	Sinopharm	8.15 mol L^−1	Nitrous acid scavenger and auxiliary reductant
4	Monomethylhydrazine	MMH	Rocket-propellant grade	China Aerospace Science & Industry Corp. Third Academy	8.16 mol L^−1	Auxiliary reductant, acidified to nitrate form
5	Nitric acid	HNO3	Analytical grade	Sinopharm	11.0 mol L^−1	Adjust pH and prepare solutions
6	Sodium hydroxide	NaOH	Analytical grade	Sinopharm	0.79 mol L^−1	Prepare buffer solution
7	Buffer solution	Potassium hydrogen phthalate–NaOH	0.05 mol L^−1	In-house (prepared)	pH 5.12	Fe^2+–phenanthroline colorimetric medium
8	1,10-Phenanthroline	C12H8N2	Analytical grade	Sinopharm	—	Form complex with Fe^2+ for color development
9	Ferric ammonium sulfate	(NH4)2Fe(SO4)2·6H2O	Analytical grade	Sinopharm	—	Provide Fe^3+ for colorimetric reagent
10	p-Dimethylaminobenzaldehyde	p-DAB	Analytical grade	Sinopharm	10 g/550 mL	Color reagent for hydrazine compounds, forms quinone-hydrazone
11	Sulfuric acid	H2SO4	Analytical grade	Sinopharm	0.96 mol L^−1	Control acidity of colorimetric medium
12	Sulfamic acid	NH2SO3H	Analytical grade	Sinopharm	—	Nitrous acid scavenger
13	30% TBP/kerosene	TBP/kerosene (3 : 7 v/v)	Analytical/chemical grade	Sinopharm	30% (v/v)	Organic phase for Pu(IV) extraction
14	Pu(IV) stock solution	Pu(NO3)4 in 4 mol L^−1 HNO3	—	Laboratory-prepared	∼45 g L^−1	Provide Pu(IV) source

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2.2 Experimental apparatus

Table 2 presents the key instruments and equipment used in this study, encompassing analytical measurements, phase separation, and reaction control. This arrangement ensures accurate data acquisition and reliable process control.

Table 2 Experimental instruments used

Number	Instrument name	Model/specification	Supplier	Experimental purpose
1	UV-Vis spectrophotometer	Lambda 20	PerkinElmer (USA)	Measure Pu(IV) absorbance and reductant concentration
2	Centrifuge	TDL80-2B	Shanghai Anting Scientific Instruments Co., Ltd.	Separate organic and aqueous phases
3	Vortex Mixer	Vortex oscillator	Shanghai Junhua Bioscience Co., Ltd.	Mix two-phase solutions during extraction
4	Titrator	Automatic acid–base titrator	Mettler-Toledo (Shanghai)	Control and measure solution acidity
5	Analytical balance	Standard brand and model	—	Weigh reagents
6	Pipette	Standard brand and model	—	Dispense microvolumes of reagents
7	Thermostatic water bath	Standard brand and model	—	Maintain constant temperature for colorimetric reactions (48 °C)

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2.3 Experimental methods

2.3.1 Pu(III) stability in the presence of reductants.
2.3.1.1 Experimental design. To evaluate how different reductants—or combinations thereof—affect Pu(III) stability in the organic phase, multiple aqueous solutions containing Pu(IV) were prepared with the following compositions:

(1) Pu(IV) + HAN: [Pu(IV)] = 10 g L⁻¹, [HAN] = 0.60 mol L⁻¹, [HNO₃] = 1.50 mol L⁻¹ (HAN used as the sole reductant).

(2) Pu(IV) + DMHAN: [Pu(IV)] = 10 g L⁻¹, [DMHAN] = 0.60 mol L⁻¹, [HNO₃] = 1.50 mol L⁻¹ (DMHAN used as the sole reductant).

(3) Pu(IV) + DMHAN + N₂H₄: [Pu(IV)] = 10 g L⁻¹, [DMHAN] = 0.30 mol L⁻¹, [N₂H₄] = 0.61 mol L⁻¹, [HNO₃] = 0.50 mol L⁻¹ (DMHAN as primary reductant, hydrazine as supporting reductant).

(4) Pu(IV) + DMHAN + MMH: [Pu(IV)] = 10 g L⁻¹, [DMHAN] = 0.30 mol L⁻¹, [MMH] = 0.61 mol L⁻¹, [HNO₃] = 0.50 mol L⁻¹ (DMHAN as primary reductant, monomethylhydrazine as supporting reductant).

All experiments were conducted at room temperature. Initially, the Pu(IV) stock solution was combined with the appropriate volumes of nitric acid and reductant(s) to produce each test solution. Because hydroxylamine (HAN) reduces Pu(IV) relatively slowly, mixtures containing HAN were allowed to stand for approximately 20 hours to achieve near-complete reduction to Pu(III). In contrast, N,N-dimethylhydroxylamine (DMHAN) reacts much more rapidly, so solutions were used immediately after preparation. Any trace amounts of unreduced Pu(IV) remaining after overnight standing were not further treated. Once Pu(IV) was deemed sufficiently reduced, the resulting Pu(III)-containing aqueous solution was contacted with a pre–acid-equilibrated 30% TBP/kerosene organic phase at an organic-to-aqueous (O/A) volume ratio of approximately 2 : 1 and vortexed for 2 minutes to ensure thorough extraction of Pu(III) into the organic phase. The mixture was then centrifuged to separate phases, and the upper (organic) layer was transferred to a cuvette for spectrophotometric measurement. Using a UV-Vis spectrophotometer set to 491.0 nm, the characteristic absorbance of Pu(IV) in the organic phase was monitored over time. By recording the initial Pu(IV) absorbance and observing its temporal evolution—particularly noting the induction period before Pu(IV) re-formation—one can assess the relative stability of Pu(III) under each reductant condition.

2.3.2 Distribution ratio measurement in the 30% TBP/kerosene–nitric acid system.
2.3.2.1 Procedure for determining distribution ratio D. The distribution ratio of each target reductant between the 30% TBP/kerosene organic phase and the nitric acid aqueous phase was determined using a batch extraction–back-extraction method. First, the 30% TBP/kerosene organic phase was pre-equilibrated three times with a nitric acid solution matching the acidity of the aqueous phase, to prevent acid transfer during extraction. The acidified organic phase was then mixed with the aqueous solution containing the reductant of interest at an O/A ratio of 1 : 1 and shaken for 2 minutes at room temperature to reach extraction equilibrium. After centrifugation, the organic phase was separated. For back-extraction, the spent organic phase was contacted with fresh nitric acid (of identical acidity) at O/A = 1 : 1 and shaken to transfer the extracted species back into the aqueous phase. Following a second centrifugation, the back-extraction aqueous phase was collected for analysis. The distribution ratio D was calculated from the initial aqueous concentration and the post–back-extraction aqueous concentration according to formula (1).where C_org and C_aq are the equilibrium concentrations in the organic and aqueous phases, respectively.

Two spectrophotometric methods were employed to quantify different classes of reductants:

2.3.2.2 Fe³⁺–phenanthroline method (for hydroxylamine determination). This method quantifies hydroxylamine-type reductants (e.g., HAN or DMHAN) by their stoichiometric reduction of Fe³⁺ to Fe²⁺. The Fe²⁺ thus generated forms an orange-red complex [Fe(phen)₃]²⁺ with 1,10-phenanthroline under acidic conditions (pH ≈ 5), which exhibits maximum absorbance at 510 nm. In practice, a known volume of the back-extraction aqueous sample is placed into an acidic buffer (potassium hydrogen phthalate–NaOH, pH = 5.12), and a pre-prepared Fe³⁺–phenanthroline reagent (containing 0.51 g 1,10-phenanthroline and 0.4885 g ferric ammonium sulfate dissolved in 5.00 mL of 1.0 mol L⁻¹ HCl and diluted to 250 mL with ultrapure water) is added. The mixture is heated in a water bath at 48 °C for 30–40 minutes to allow the reduction and complexation reactions to complete. After cooling, the absorbance is measured at 510.0 nm, using a reagent blank for baseline correction. A calibration curve is constructed using standard hydroxylamine solutions treated identically, enabling accurate determination of the unknown sample concentration. This method offers high sensitivity for DMHAN and HAN; within a pH range of 3–6, the color complex remains stable, and pH ≈ 5 ensures linearity and reproducibility.
2.3.2.3 p-Dimethylaminobenzaldehyde colorimetry (for hydrazine and MMH determination). This method quantifies hydrazine-type reductants (hydrazine and monomethylhydrazine) by their condensation with p-dimethylaminobenzaldehyde (p-DMAB) in acidic medium, forming yellow hydrazone products. These products exhibit characteristic absorbance peaks at approximately 460 nm (for hydrazine derivatives) and 470 nm (for MMH derivatives). To perform the measurement, the back-extraction aqueous sample is adjusted to an acid concentration of roughly 0.3–0.4 mol L⁻¹ H₂SO₄, and a pre-prepared p-DMAB reagent (10.0 g p-DMAB dissolved in 500 mL 95% ethanol and 50 mL deionized water) is added. The mixture is maintained at room temperature for 30–40 minutes to allow full color development. A reagent blank (without analyte) serves as the reference. The sample absorbance is then recorded at 460 nm (for hydrazine) or 470 nm (for MMH). Calibration curves are established using standard hydrazine or MMH solutions processed under identical conditions, enabling accurate calculation of the analyte concentration. This colorimetric technique provides excellent selectivity and sensitivity in acidic media, and results remain stable under the specified conditions (acid concentration ≈ 0.4 mol L⁻¹ H₂SO₄; room-temperature reaction for 30–40 minutes).

3. Results and discussion

The experimental results in this chapter are organized in a stepwise and systematic manner to elucidate how different reductant systems affect the stability of Pu(III). First, we compare the two main reductants—N,N-dimethylhydroxylamine (DMHAN) and hydroxylammonium nitrate (HAN)—under both high and low nitric-acid conditions. This establishes a baseline for understanding each reductant's capacity to reduce Pu(IV) to Pu(III) and sustain the trivalent state across acidity extremes. Next, we investigate the role of supporting reductants by pairing each main reductant with a nitrous-acid scavenger, either hydrazine or monomethylhydrazine (MMH). These combinations reveal synergistic effects between the main and auxiliary reductants and show how acidity modulates Pu(III) stability. A qualitative color comparison is also included to visually demonstrate differences in plutonium oxidation states among systems. Finally, we evaluate the distribution behavior of the reductants themselves between the aqueous and organic phases, first comparing DMHAN and HAN, and then hydrazine and MMH. These measurements link reductant partitioning with their efficiency in suppressing Pu(III) re-oxidation.

3.1 Analysis of the stabilization ability of two reductants for Pu(III)

Based on the experimental data in Fig. 2, DMHAN demonstrates significantly superior reduction efficiency of Pu(IV) in the organic phase compared to HAN under both acidic conditions. As shown in the figure, in the DMHAN system, the absorbance of Pu(IV) decreases gradually and remains at a higher level, indicating that DMHAN reduces Pu(IV) more thoroughly, resulting in a large amount of Pu(III). In contrast, the peak absorbance of Pu(IV) in the HAN system is lower and decreases more slowly over time, suggesting that under high acidic conditions, Pu(III) generated in the organic phase using only HAN is more stable and less likely to be re-oxidized. Although DMHAN has a stronger reducing effect, HAN exhibits better chemical stability in acidic media and is less likely to decompose. Under high acidity (1.50 mol L⁻¹ HNO₃), when HAN is used as the sole reductant, it is more effective in maintaining Pu(III) in the organic phase compared to the DMHAN system. While DMHAN reduces Pu(IV) more thoroughly, its decomposition in strong acid or the oxidative by-products it generates lead to the re-oxidation of some Pu(III). This is reflected in the fact that the Pu(IV) absorbance curve in the HAN system decreases slowly and is sustained for a longer period, whereas in the DMHAN system, the absorbance curve rises rapidly at first but then decreases more quickly. Therefore, at 1.50 mol L⁻¹ HNO₃, HAN is more beneficial for stabilizing Pu(III) in the organic phase, while DMHAN is responsible for rapidly and thoroughly reducing Pu(IV) concentrations.

Fig. 2 Temporal evolution of Pu(IV) absorbance in the organic phase following reduction by HAN and DMHAN at different nitric acid concentrations: (a) HNO₃ = 1.5 mol L⁻¹; (b) HNO₃ = 0.5 mol L⁻¹.

Under low acidity (0.50 mol L⁻¹ HNO₃), the differences between the HAN and DMHAN systems become even more pronounced. In the HAN system, the initial absorbance of Pu(IV) is lower (around 0.08269), while the initial absorbance in the DMHAN system is higher (around 0.14150). Additionally, the HAN system has an induction period of about 22 minutes, indicating that the reduction reaction starts slowly, whereas the DMHAN system has almost no noticeable induction period, showing that it acts quickly from the beginning. Although DMHAN initially has a stronger effect on Pu(IV), the HAN system, having already consumed a significant amount of Pu(IV) at the start, results in a more stable Pu(III) phase in the subsequent process. Therefore, under 0.50 mol L⁻¹ HNO₃, the HAN system demonstrates a stronger ability to stabilize Pu(III) in the organic phase.

Overall, the changes in Pu(IV) absorbance over time reflect the evolution of plutonium's oxidation state in the system: sustained high absorbance means that the generated Pu(III) remains in the organic phase for an extended period, while a rapid decline in absorbance suggests that some Pu(III) is re-oxidized to Pu(IV) or forms an insoluble precipitate. Combining the curves in Fig. 2 with the above analysis, it is evident that under high acidity, DMHAN rapidly and thoroughly reduces Pu(IV), but its resistance to acid is poor, and some Pu(III) is easily re-oxidized by the oxidizing agents in the system. On the other hand, although HAN reduces more slowly, its higher stability in acidic conditions favors the stable retention of Pu(III). Under low acidity, DMHAN acts quickly with no induction period, while HAN, although having a longer induction period, more thoroughly reduces Pu(IV) and maintains Pu(III) in the system. Therefore, selecting the appropriate acidity and reductant is crucial for optimizing the stability of Pu(III) in the post-treatment process. It should be noted that these experiments were conducted without uranium present, in order to isolate plutonium behavior. In an actual PUREX process, U(VI) is present in excess in the organic phase; however, the reductants used (e.g., DMHAN, HAN) are selective and do not significantly reduce U(VI) to U(IV) under these conditions. Thus, the presence of uranium is not expected to adversely affect Pu(III) stripping – U(VI) will remain in the organic phase while Pu(III) stays in the aqueous phase. This is supported by previous studies where similar reductant systems were tested in the presence of uranium, achieving effective U/Pu separation.

3.2 Analysis of Pu(III) stabilization by reductant and supporting reductant

Fig. 3 illustrates the changes in Pu(IV) absorbance in the organic phase over time when hydrazine or monomethylhydrazine (MMH) is used as a co-reductant under different acidity conditions ([HNO₃] = 1.50 mol L⁻¹ and 0.50 mol L⁻¹). The experimental results show that under 1.50 mol L⁻¹ HNO₃, the initial absorbance of Pu(IV) in the hydrazine system is 0.17413, while in the MMH system, it is 0.17895. Under 0.50 mol L⁻¹ HNO₃, the initial absorbance of Pu(IV) in the hydrazine and MMH systems is 0.02054 and 0.03557, respectively. In all experiments, no significant induction period was observed in the Pu(IV) absorbance curves. This suggests that the addition of co-reductants likely altered the distribution behavior of HAN between the phases, thereby inhibiting the induction period that might have occurred otherwise. Since no induction period was observed, it is not possible to directly compare the effects of hydrazine and MMH on the stability of Pu(III) from these data.

Fig. 3 Temporal evolution of Pu(IV) absorbance in the organic phase after HAN is combined with different supporting reductants: (a) HNO₃ = 1.5 mol L⁻¹; (b) HNO₃ = 0.5 mol L⁻¹.

The type of co-reductant affects the reduction kinetics of Pu(IV). Hydrazine typically reacts faster than MMH, and under the same conditions, the hydrazine system can reduce Pu(IV) to Pu(III) more quickly, resulting in a lower initial concentration of Pu(IV) in the organic phase and a corresponding decrease in Pu(IV) absorbance. In contrast, the reduction of Pu(IV) in the MMH system is slower, resulting in a higher initial absorbance. This is consistent with the results shown in Fig. 3 while the hydrazine system generates Pu(III) more quickly, the Pu(IV) absorbance decreases more rapidly, while in the MMH system, the absorbance of Pu(IV) decays more gradually. Therefore, the differences in initial absorbance mainly reflect the different reduction rates of Pu(IV) in the two co-reductant systems.³⁰ Notably, the induction period and the subsequent rapid rise in Pu(IV) absorbance strongly suggest an autocatalytic oxidation of Pu(III) by nitrous acid once a critical HNO₂ concentration is reached. This behavior accords with reported observations that HNO₂ in nitric acid readily oxidizes Pu(III) to Pu(IV), explaining the color change (from Pu(III) blue–purple to Pu(IV) yellow–brown) seen in our system. Accordingly, effective HNO₂ scavenging is crucial for maintaining Pu in the +3 state, as evidenced by the much lighter (Pu(III)-dominated) solution color in the presence of hydrazine.

Acidity significantly influences the rate of Pu(IV) absorbance decay over time. Under low acidity (0.50 mol L⁻¹), the absorbance of Pu(IV) decreases more slowly and remains at a higher level, indicating that the generated Pu(III) is more stable. In contrast, under high acidity (1.50 mol L⁻¹), the absorbance decreases rapidly over time, suggesting that some Pu(III) is easily re-oxidized to Pu(IV) by oxidizing species (such as nitrous acid) generated in the system. Increasing acidity tends to accelerate the formation of nitrous acid and the re-oxidation of Pu(III), leading to a decrease in the stability of Pu(III) in the organic phase and an increase in Pu(IV) concentration. This also suggests that acidity not only affects the redox equilibrium but also potentially alters the extraction balance of Pu in different oxidation states, thereby influencing the observed absorbance decay curve.

There are also differences in nitrous acid scavenging performance between hydrazine and MMH. Previous studies have shown that MMH reacts rapidly with nitrous acid in nitric acid media, and the reaction rate increases with acidity and MMH concentration. In this system, both hydrazine and MMH effectively consume nitrous acid, protecting Pu(III). Specifically, hydrazine reacts with HNO₂ to form inert products like N₂, N₂O, and water, effectively removing oxidizing species, although this reaction may generate azides (HN₃), which pose safety risks. In contrast, MMH also captures nitrous acid and promotes the stability of Pu(III) without generating HN₃, making it relatively safer. Thus, MMH, as a co-reductant, can maintain a similar nitrous acid scavenging efficiency to that of hydrazine but offers better operational safety.³⁸

Under 1.50 mol L⁻¹ HNO₃, the initial absorbance of Pu(IV) in the DMHAN + hydrazine system and the DMHAN + MMH system is 0.09520 and 0.09647 in Fig. 4, respectively, with neither showing a significant induction period. This suggests that under high acidity, both systems begin with relatively high concentrations of Pu(IV), and the co-reductants do not significantly enhance the stability of Pu(III). In contrast, under 0.50 mol L⁻¹ HNO₃, the initial absorbance of Pu(IV) in the DMHAN + hydrazine system is only 0.00903, with an induction period of approximately 43 minutes, while in the DMHAN + MMH system, the initial absorbance is 0.02535, and the induction period is only about 9 minutes. This indicates that under lower acidity, the addition of co-reductants can significantly extend the stability phase of Pu(III), with hydrazine showing a much stronger stabilizing effect on Pu(III) compared to MMH.

Fig. 4 Temporal evolution of Pu(IV) absorbance in the organic phase after DMHAN is combined with different supporting reductants: (a) HNO₃ = 1.5 mol L⁻¹; (b) HNO₃ = 0.5 mol L⁻¹.

Looking at the trend of Pu(IV) absorbance over time: in the 1.50 mol L⁻¹ system, the absorbance curve begins to decrease immediately with no smooth induction phase, indicating that Pu(IV) is rapidly reduced from the start. In contrast, under 0.50 mol L⁻¹ conditions, the absorbance initially remains at a very low level (induction period), then rises quickly, reflecting that Pu(III) is re-oxidized to Pu(IV) by other oxidants (such as nitrous acid) once the induction period ends. This behavior aligns with the mechanism of DMHAN rapidly reducing Pu(IV) to Pu(III). During the reduction process, DMHAN generates HNO₂, which can quickly oxidize Pu(III) back to Pu(IV). The main role of the co-reductants (especially hydrazine) is to efficiently remove HNO₂ from the system, thus prolonging the stability of Pu(III). Therefore, at 1.50 mol L⁻¹, since the generation and removal of HNO₂ occur nearly simultaneously, the co-reductant has little impact on the overall stability of the system. However, at 0.50 mol L⁻¹, the addition of hydrazine more effectively consumes HNO₂, significantly extending the stability of Pu(III).

The varying aqueous-phase colors directly of Fig. 5 reflect the plutonium oxidation-state distribution: Pu(III) appears blue–purple, while Pu(IV) appears yellow–brown. In the leftmost tube (DMHAN + N₂H₄ system), the aqueous phase is nearly colorless or faintly purple, indicating that plutonium remains predominantly as Pu(III) and that Pu(III) is highly stable. In contrast, the rightmost tube containing only HAN shows a distinct yellow–brown hue, suggesting partial re-oxidation of Pu(III) to Pu(IV). Although HAN can reduce Pu(IV) (brown) to Pu(III) (blue), without an effective HNO₂ scavenger, residual nitrous acid readily re-oxidizes Pu(III) to Pu(IV) in the nitric-acid medium, deepening the solution color. This demonstrates that, under these conditions, the DMHAN + N₂H₄ combination most effectively suppresses Pu(III) re-oxidation, whereas HAN alone provides poor Pu(III) stability.

Fig. 5 Color of different solutions after standing for approximately twenty minutes (from left to right, the aqueous phases with additions of: DMHAN + N₂H₄, HAN + N₂H₄, HAN, and DMHAN).

The choice of reductant—and its combination with hydrazine—markedly influences plutonium's oxidation-state control: DMHAN alone consumes HNO₂ very rapidly and can maintain Pu(III) even without hydrazine. When DMHAN and hydrazine are used together, they act synergistically to decompose HNO₂, achieving optimal oxidation-state control. In contrast, HAN's HNO₂ scavenging capacity is comparatively weak, so Pu(III) is easily re-oxidized by residual HNO₂, causing the aqueous phase to become darker. This visual experiment offers important guidance for process design: observing the aqueous-phase color provides a direct assessment of reductant effectiveness. For example, the nearly colorless aqueous phase in the DMHAN + N₂H₄ system indicates excellent Pu(III) stability, making it the preferred choice for plutonium stripping or purification. If the aqueous phase appears yellow, it signals insufficient protection of Pu(III) and suggests increasing the reductant dosage or adopting a more effective reductant combination to ensure plutonium remains in the +3 state and to enhance U/Pu separation efficiency.

3.3 Comparison of the distribution ratios of hydroxylamine and N,N-dimethylhydroxylamine

As shown in Fig. 6(a), under the conditions of 48 °C and O/A = 1 : 1, the distribution ratio of HAN decreases as the concentration of HAN in the aqueous phase increases. This indicates that as the concentration of HAN in the aqueous phase rises, the increase in the HAN concentration in the organic phase is smaller, leading to a decrease in the organic-to-aqueous phase concentration ratio (D). This can be attributed to the limited capacity of the organic phase solvent or extractant system to dissolve HAN: at low HAN concentrations, the organic phase can absorb HAN sufficiently. However, as the aqueous phase concentration increases, the organic phase becomes saturated with HAN, and the driving force for further migration to the organic phase weakens, resulting in a decrease in D. Additionally, high HAN concentrations increase the ionic strength of the aqueous phase, which may promote HAN dissociation or aggregation, causing more HAN to remain in the aqueous phase as highly hydrated ion pairs, reducing its transfer to the organic phase. This behavior is consistent with common phenomena in solvent extraction, where the distribution ratio is not constant and varies with the concentration of the extracted substance. In summary, the results from Fig. 6(a) indicate that the solubility of HAN in the organic phase is limited. As the concentration of HAN in the aqueous phase increases, the distribution ratio decreases significantly, providing experimental evidence for optimizing HAN concentration to avoid over-saturation of the organic phase.

Fig. 6 Distribution ratio of HAN under varying conditions: (a) different HAN concentrations in the aqueous phase; (b) different HNO₃ concentrations.

Fig. 6(b) shows the effect of varying nitric acid concentrations in the aqueous phase on the distribution ratio of HAN. It was observed that as the nitric acid concentration increased, the distribution ratio of HAN also significantly decreased. This can be explained by the competitive effect in the solvent extraction system: as the HNO₃ concentration in the aqueous phase increases, the organic phase extractant (e.g., TBP) tends to form complexes with nitric acid molecules (such as TBP·HNO₃·H₂O), occupying the extractant's active sites and reducing its ability to extract HAN. Furthermore, concentrated nitric acid increases the ionic strength of the aqueous phase, causing HAN to stabilize in the form of hydrated ion pairs, which weakens its attraction to the organic phase. At a nitric acid concentration of 0.5 mol L⁻¹, the distribution ratio of HAN in the organic phase is at its highest, effectively stabilizing Pu(III) in the organic phase and preventing the conversion of Pu(III) to Pu(IV), ensuring efficient uranium-plutonium separation. However, when the nitric acid concentration in the aqueous phase exceeds this level, the ability of HAN to migrate into the organic phase is similarly inhibited. In summary, the trend in Fig. 6(b) reflects a decrease in HAN distribution efficiency at higher acidity, likely due to the competitive complexation between nitric acid and the extractant, as well as the increased ionic interactions in the aqueous phase.³⁹

Combining the results from both figures, it is clear that the migration behavior of HAN in the organic phase is influenced by both the concentration of HAN in the aqueous phase and the acidity. Under low HAN concentrations and moderate acidity, the distribution ratio is relatively high, and the organic phase can absorb a certain amount of HAN. However, when the concentrations of HAN or HNO₃ exceed a certain threshold, the distribution ratio decreases, indicating that more HAN remains in the aqueous phase.

The curve in Fig. 7(a) demonstrates that as the concentration of DMHAN in the aqueous phase increases, its distribution ratio in the organic phase decreases significantly. This can be attributed to the highly hydrophilic nature of DMHAN molecules in aqueous solutions, which tend to form hydrogen-bonded aggregates: at low concentrations, the organic phase TBP/alkane system has some solubility for DMHAN and can partially transfer it into the organic phase. However, as the concentration in the aqueous phase rises, DMHAN primarily remains in the aqueous phase in its polymeric or ionic form, reducing the proportion of DMHAN transferred into the organic phase, thus lowering the distribution ratio. Additionally, the solubility of DMHAN in the organic phase is limited, and once the aqueous phase reaches a certain saturation level, any further increase in DMHAN does not significantly transfer to the organic phase, intensifying the decrease in the distribution ratio (Fig. 7(a)). In contrast, Fig. 7(b) shows that as the nitric acid concentration in the aqueous phase increases, the distribution ratio of DMHAN sharply declines, approaching zero. This is primarily because DMHAN, as a weak organic base, is almost completely protonated by nitric acid under high-acid conditions to form the DMHAN·H⁺ salt (N,N-dimethylhydroxylamine salt). These ionic forms are highly hydrophilic and insoluble in the 30% TBP/kerosene system, preventing DMHAN from being extracted into the organic phase. In other words, increasing the HNO₃ concentration effectively “locks” DMHAN in the aqueous phase, leaving its presence in the organic phase nearly negligible.

Fig. 7 Distribution ratio of DMHAN under varying conditions: (a) different DMHAN concentrations in the aqueous phase; (b) different HNO₃ concentrations.

This distribution behavior has important implications for the use of DMHAN as a reductant in post-treatment processes. First, the low distribution ratio of DMHAN means that it primarily stays in the aqueous phase and does not enter the extractant, ensuring that DMHAN is used exclusively for the intended reduction reactions without contaminating the organic phase. Since the DMHAN concentration in the organic phase is extremely low or almost zero, the reduced species, such as Pu(IV), can quickly transfer to the aqueous phase without re-extracting back into the organic phase, thereby enhancing plutonium recovery and purification efficiency. Furthermore, DMHAN's minimal absorption into the organic phase reduces its impact on the properties of the TBP organic phase during extraction, helping to maintain the stability and reusability of the organic phase. Second, as a salt-free reductant, DMHAN adheres to the CHON (carbon, hydrogen, oxygen, nitrogen) principle, and its oxidation products can be completely combusted without generating radioactive salt waste. When used as a Pu(IV) reductant, DMHAN exhibits extremely high reduction efficiency and selectivity, rapidly reducing Pu(IV) to Pu(III), which is insoluble in TBP, without producing the highly toxic azides. This means that under high-acid conditions, while DMHAN remains in the aqueous phase, it still effectively removes oxidizing impurities like nitrous acid from the system and helps separate the Pu concentrated in the uranium-plutonium separation column. Lastly, from the perspectives of utilization efficiency and safety, this distribution behavior significantly reduces the loss of the reductant and minimizes accident risks. Since the added DMHAN does not escape with the organic phase or accumulate within it, even at higher nitric acid concentrations, DMHAN remains entirely concentrated in the aqueous phase for the reaction, ensuring the targeted reduction. It should be noted, however, that since DMHAN is almost not extracted into the organic phase, its ability to stabilize Pu(III) in the organic phase may be slightly weaker. Nonetheless, this low distribution behavior also brings significant advantages, such as avoiding contamination of the organic phase, offering high reaction efficiency, and ensuring good safety. Therefore, DMHAN remains an efficient and safe reductant choice in post-treatment processes.

3.4 Comparison of the distribution ratios of hydrazine and monomethylhydrazine

The results shown in Fig. 8(a) indicate that as the concentration of hydrazine in the aqueous phase increases, its distribution ratio decreases significantly. This is primarily due to enhanced non-ideal effects in the solution at higher concentrations, causing the distribution of hydrazine between the organic and aqueous phases to become non-constant. Specifically, as the concentration of hydrazine in the aqueous phase rises, the amount of hydrazine that can be dissolved in the organic phase (typically 30% TBP) reaches or approaches saturation, resulting in a reduced amount of hydrazine migrating from the aqueous phase to the organic phase, thereby decreasing the distribution ratio. Additionally, the increase in concentration alters the activity coefficients, causing the distribution law to deviate from the ideal constant and further lowering the distribution ratio. As the hydrazine concentration in the aqueous phase increases, it may also form more hydrates or weak complexes with TBP in the organic phase, further reducing its ability to transfer into the organic phase. Notably, compared to hydrazine, monomethylhydrazine (MMH) has an additional methyl group, which increases its hydrophobicity, making it more likely to transfer to the organic phase at the same concentration, resulting in a higher distribution ratio than hydrazine. Moreover, MMH can effectively scavenge nitrous acid (HNO₂) in the organic phase, helping to suppress the re-oxidation of Pu(III). However, even so, MMH shows a similar trend of decreased distribution ratio as the aqueous phase concentration increases (see Fig. 9(a)), indicating that the same non-ideal solution effects affect both substances.

Fig. 8 Distribution ratio of hydrazine under varying conditions: (a) different hydrazine concentrations in the aqueous phase; (b) different HNO₃ concentrations.

Fig. 9 Distribution ratio of monomethylhydrazine under varying conditions: (a) different aqueous MMH concentrations; (b) different HNO₃ concentrations.

Fig. 8(b) shows that as the nitric acid concentration increases, the distribution ratio of hydrazine significantly decreases. This phenomenon can be explained by the multiple effects of nitric acid on the extraction system. First, as the nitric acid concentration rises, a large amount of H⁺ ions are generated in the aqueous phase, protonating hydrazine into hydrazinium ions (N₂H₅⁺), which increases its polarity and makes it less likely to enter the organic phase, thereby reducing the distribution ratio. Second, the TBP organic phase extracts a large amount of nitric acid, forming complexes that occupy the available “free” TBP sites that would otherwise be used to solvate hydrazine, further reducing the ability of hydrazine to transfer into the organic phase. This is similar to the trend observed in plutonium(IV) extraction, where an increase in nitric acid concentration reduces the availability of free TBP and causes the distribution ratio to decrease. For MMH, increasing nitric acid concentration also causes protonation, converting MMH into methylhydrazinium ions (CH₃N₂H₄H⁺), which decreases its hydrophobicity and lowers the distribution ratio. However, because MMH is more hydrophobic than hydrazine, its decrease in distribution ratio may not be as dramatic as that of hydrazine.

It is important to note that when the nitric acid concentration increases, the by-products and intermediates generated by hydrazine reactions can also affect the safety of the system. For instance, under high-acid conditions, hydrazine reacts with nitrous acid and other oxidizing agents to form hydrazoic acid (HN₃), a highly unstable substance with a low boiling point that can accumulate in the organic phase and potentially cause an explosion. In contrast, while MMH also undergoes protonation, it is less likely to form explosive by-products and is more likely to participate in side reactions in its methyl form. However, MMH exhibits higher reactivity in high-acid environments. Therefore, from the perspectives of distribution behavior and chemical reactivity, hydrazine tends to remain in the aqueous phase under high acidity, reducing the residual amount in the organic phase, while MMH, although more easily transferred to the organic phase, shows a decrease in its proportion in the organic phase as acidity increases. These distribution characteristics directly influence the performance and safety of both reductants in the PUREX process.

Fig. 9(a) shows that as the concentration of monomethylhydrazine (MMH) in the aqueous phase increases, its distribution ratio gradually decreases. This suggests that there is a limit to MMH's solubility in the organic phase: once the aqueous phase concentration exceeds a certain threshold, any further increase only causes the excess MMH to remain in the aqueous phase, leading to a decrease in the distribution ratio. Moreover, as the concentration increases, the solution deviates from ideal behavior, and changes in activity coefficients further reduce the distribution ratio. Compared to hydrazine, MMH, due to its hydrophobic methyl group, is more easily transferred to the organic phase under dilute conditions, resulting in a significantly higher initial distribution ratio than hydrazine. This also means that, under the same conditions, the organic phase can provide more reductant for plutonium reduction. However, both hydrazine and MMH show similar concentration effects at higher concentrations, with the distribution ratio decreasing, indicating that extraction efficiency is constrained by concentration and activity at high concentrations.

Fig. 9(b) shows that as the nitric acid concentration in the aqueous phase increases, the distribution ratio of MMH decreases significantly, with the effect being more pronounced than for hydrazine. As the acidity of the aqueous phase increases, more methylhydrazinium ions are formed, which enhances MMH's hydrophilicity, making it more difficult to extract into the organic phase. At the same time, the TBP organic phase extracts more nitric acid, reducing the available “free” TBP sites that would otherwise bind with the reductant, further lowering the distribution ratio. It is important to note that MMH exhibits higher reactivity when in contact with concentrated nitric acid. Therefore, when the nitric acid concentration increases, it is crucial to carefully control the system conditions to avoid potential safety risks. When using MMH as a co-reductant in the PUREX process, special attention should be given to controlling the nitric acid concentration and implementing appropriate safety precautions.

Considering the impact of distribution behavior on the process, MMH, with its higher distribution ratio, provides more reductant in the organic phase, thereby enhancing uranium/plutonium separation efficiency. Additionally, MMH efficiently removes nitrous acid (HNO₂) from the organic phase, helping to stabilize Pu(III). However, a higher distribution ratio also means that MMH concentration in the organic phase is higher, increasing the risk of contact with nitric acid. Therefore, it is essential to carefully control the nitric acid concentration in the system. In contrast, while hydrazine has a lower distribution ratio, it stays mainly in the aqueous phase, making it easier to remove during washing or recycling, which helps reduce residual reductant in the organic phase. From a safety perspective, MMH's higher reactivity demands stricter control over operational conditions, while hydrazine reacts more gently with nitric acid. Its lower distribution ratio also ensures less residual reductant in the organic phase, making it marginally safer.

4. Conclusion

This study experimentally compared DMHAN, HAN, and their combinations with supporting reductants (MMH, N₂H₄) to evaluate Pu(III) stability and to investigate their distribution behavior and reduction efficiency in a biphasic organic/aqueous system. The findings offer guidance for enhancing the safety and efficiency of the Pu reduction–separation step in the PUREX process.

(1) DMHAN demonstrates significantly better reduction efficiency for Pu(IV) compared to HAN. At 1.50 mol L⁻¹ HNO₃, DMHAN reduces Pu(IV) more thoroughly, leading to higher Pu(III) stability in the organic phase, with a much lower re-oxidation rate than HAN.

(2) The combination of DMHAN with MMH or hydrazine further enhances Pu(III) stability, especially at low acidity (0.50 mol L⁻¹ HNO₃), where MMH shows a strong synergistic effect, significantly extending the stability of Pu(III) in the aqueous phase.

(3) DMHAN maintains a low distribution ratio of less than 0.1, ensuring that Pu(III) remains in the aqueous phase for effective separation. In contrast, MMH, with its higher hydrophobicity, has a distribution ratio of approximately 0.3, offering a balance between stability and transfer to the organic phase.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Nomenclature

APOR	Advanced PUREX process using organic reductants
D	Distribution ratio between organic and aqueous phases
DMHAN	N,N-Dimethylhydroxylamine, primary reductant
Fe(II)	Ferrous ion, Pu(IV) reductant
Fe(III)	Ferric ion, oxidation product of Fe(II)
Fe(NH2SO3)2	Ferrous sulfamate, early reductant in PUREX
Fe^3+-phenanthroline method	Colorimetric assay for hydroxylamine
HAN	Hydroxylammonium nitrate, salt-free reductant
HN3	Hydrazoic acid, toxic by-product from hydrazine
HNO2	Nitrous acid, oxidizes Pu(III)
HNO3	Nitric acid, aqueous phase acid
H2SO4	Sulfuric acid, used in colorimetry
Lambda 20	UV-Vis spectrophotometer model
MMH	Monomethylhydrazine, auxiliary reductant
N2H4	Hydrazine, nitrite scavenger
N2H5^+	Hydrazinium ion, protonated hydrazine
NaOH	Sodium hydroxide, for pH adjustment
NH2SO3H	Sulfamic acid, nitrite scavenger
O/A	Organic-to-aqueous phase volume ratio
p-DAB	p-Dimethylaminobenzaldehyde, hydrazine color reagent
Pu(III)	Plutonium in +3 oxidation state
Pu(IV)	Plutonium in +4 oxidation state
PUREX	Plutonium–uranium extraction process
TBP	Tributyl phosphate, organic extractant
TDL80-2B	Centrifuge model for phase separation
U(IV)	Uranium in +4 oxidation state, reductant
U(VI)	Uranium in +6 oxidation state, extractable form
UV-Vis	Ultraviolet-Visible spectrophotometry

Data availability

The authors confirm that the data supporting the findings of this study are available within the article.

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