Introduction

For recycle of unused and bred nuclear fuel materials, solvent extraction based PUREX process is currently employed at industrial scale of operations. For mutual separation of uranium and plutonium in the streams separated from fission products during aqueous route of reprocessing operations, Pu valance change either by chemical means or electro-redox route is employed. Naylor [1] presented U–Pu separation techniques in aqueous route in detail. Miles [2] had reported scavenging action of hydroxyurea on nitrous acid during Pu(IV) to Pu(III) reduction. However, conventionally hydrazine was used as nitrous acid scavenger. This aqueous complexation of Pu(IV) is possibly due to action of –NHOH group. Zhaowu et al. [3] used hydroxyurea (HU) in place of acetohydroxamic acid (AHA) for mutual separation of U/Pu as well as U/Np. Zhaowu et al. [4] also studied kinetics of Pu reduction by hydroxyurea. Recently, Sivakumar et al. [5, 6] presented experimental evaluation of hydroxyurea as an aqueous partitioning agent during mutual separation of U/Pu in fast reactor fuel reprocessing as well as decontamination of recovered uranium from Pu traces.

Thermophysical and thermochemical data on aqueous solutions of hydroxyurea is scarce. Recently, Kumar et al. [7] listed few properties of aqueous hydroxyurea solutions like density, apparent molal volume, vapour pressure and thermodynamic water activity values, which were derived from in-house experiments.

It is a well-known fact that –NHOH containing compounds like hydrazine, hydroxylamine and related salts behave like energetic materials with huge potential for explosion in confined spaces. In past, several incidents [8, 9] were reported for hydroxylamine. For hydroxyurea, one similar incident was reported in literature [10, 11]. The reported incident happened during the vacuum concentration of aqueous hydroxyurea solution at 323.15 K (50 °C) and at 50 mm Hg pressure. Since aqueous solution of hydroxyurea at temperature slightly above the ambient was involved in the incident, to evaluate the safety envelope, authors investigated thermal decomposition of aqueous and nitric solutions of hydroxyurea and resulting pressurization in the well-controlled adiabatic calorimeter in close-vent conditions.

In this paper, results of related experiments are presented and discussed in detail.

Experimental work

Hydroxyurea (HU 98%+, M/S Sigma Aldrich) and conc. nitric acid (69–72%, M/S Chemspure) were used as received without further purification. Aqueous solutions of hydroxyurea and nitric acid were prepared by dissolving the appropriate quantity in the ASTM grade-I water from a Millipore Simplicity system. Both nitric acid and hydroxyurea solutions were mixed in equal volumes just before start of experiment and typically 15–25 mg quantities of mixed solution were taken for experiments in an adiabatic calorimeter (Advanced Reactive System Screening Tool also known as ARSST from M/S Fauske and Associates Inc.). A 10 mL glass test cell was wrapped with a 24 W belt heater and was put in insulated outer cover and finally in a 350 mL containment vessel, rated for 750 psi. A small piece of stainless steel foil was added to simulate metallic surface as in the actual systems. The containment vessel was pressurized to the tune of ~60 psi with compressed air to minimize vapourization of the contents during heating. The temperature of the hydroxyurea/nitric acid solution generally increased at the programmed rate of ~2 °C/min until approximately 250 °C. The initial temperature for each of the experiment was room temperature.

Results and discussion

The generated data was processed in MATLAB for analysis and plotting. Since original nitric acid solutions were of strength 2N, 4N and 8N, therefore, after mixing with equi-volume aqueous solutions of hydroxyurea, the concentration of acid could be approximated as 1, 2 and 4N respectively. The first solution used was aqueous solution of HU at zero acidity. Thermal decomposition experiments were completed for hydroxyurea solutions of acidities of 0, 1, 2 and 4N respectively. Figure 1a–d shows temporal variation of temperature for these four solutions. Since there were heavy oscillations in pressure readings for solution no. 3 (H+ ~2 N), due to safety considerations, corresponding experiment was terminated after nearly 100 min of operation. Figure 1 shows a steady increase in temperature with time till heating continues and steady fall in pressure after that. Figure 2a–d shows temporal variation of pressure generated per g mass of hydroxyurea. It may be observed that in Fig. 2a, b, reaction peak at ~50 min of elapsed time is clearly visible where as in Fig. 2c, d there is no distinguishable reaction peak. Thus, it may be inferred that at higher acidity, decomposition of hydroxyurea is completed sooner as compared to solutions at lower acidities.

Fig. 1
figure 1

Temporal variation of temperature during thermal decomposition study. a Hydroxyurea-water system. b Hydroxyurea-1N aqueous nitric acid solution. c Hydroxyurea-2N aqueous nitric acid solution. d Hydroxyurea-4N aqueous nitric acid solution

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Fig. 2
figure 2

Temporal variation of pressure generated per g of hydroxyurea during thermal decomposition study. a Hydroxyurea-water system. b Hydroxyurea-1N aqueous nitric acid solution. c Hydroxyurea-2N aqueous nitric acid solution. d Hydroxyurea-4N aqueous nitric acid solution

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Figure 3a–d shows variation of pressure generated per gm of hydroxyurea with temperature. From Fig. 3, it is evident that there is a sudden increase in pressurization in solutions of hydroxyurea with water and 1N acidity around temperature 115–120 °C. This sudden increase in pressure is absent in the samples containing 2N and 4N acid. Sudden increase in the pressure is attributed to the thermal decomposition of un-reacted hydroxyurea present in the sample. These results are significantly different from the results reported in the literature [11] where an induction period of 48 h was observed at a temperature of 80 °C. However in our case, rapid reactions were observed around 115–120 °C. Similar temperature was also reported for decomposition of aqueous solutions of hydroxyurea in DSC mode by Lunghi [11]. This peak is absent in the samples containing 2N and 4N acidity as hydroxyurea has already reacted with the acid before reaching this temperature. The total pressurization is nearly same in all four cases and is around 75–85 psi per g of hydroxyurea. So, the pressurization is mainly due to decomposition products of hydroxyurea. Contribution of nitric acid component to the pressurization in these experiments may not be significant.

Fig. 3
figure 3

Variation of pressure with temperature during thermal decomposition study. a Hydroxyurea-water system. b Hydroxyurea-1N aqueous nitric acid solution. c Hydroxyurea-2N aqueous nitric acid solution. d Hydroxyurea-4N aqueous nitric acid solution

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Since experimental results of this study indicate extensive pressurization during heating of aqueous/nitric acid solutions of hydroxyurea in closed-vent conditions, it is advisable not to heat either aqueous or nitric acid solutions of hydroxyurea to ensure safety of operating personal and laboratory.

Conclusions

Hydroxyurea is basically an unstable and reactive compound. It decomposes in aqueous solutions around 115–120 °C and its thermal degradation products contribute to pressurization of around 75–80 psi/g of hydroxyurea in small vessels. System designed for handling these components should be provided with adequate venting to prevent containment failure during accidental pressurization in the event of heating.