Pulse radiolysis study on the reactivity of NO3˙ radical toward uranous(iv), hydrazinium nitrate and hydroxyl ammonium nitrate at room temperature and at 45 °C

Concentrated nitric acid solutions subjected to radiation produce radicals of extreme importance in the reprocessing of spent nuclear fuel. Knowledge of the different rate constants of the reactions involved in this chemistry is needed to improve the efficiency of the process and to define safe operating practices. Pulse radiolysis measurements are performed to find the rate constant of the reaction between NO3˙ radicals and U(iv) in highly concentrated nitrate solution. The optimal stabilization conditions toward thermal oxidation are defined for the considered solutions at room temperature and at 45 °C by adding anti-nitrous agents such as hydrazinium nitrate (HN) and hydroxyl ammonium nitrate (HAN). The decay of the NO3˙ radical is monitored and its reaction rates with HN, HAN and U(iv) are found to be 1.3 × 105, 1.5 × 107 and 1.6 × 106 M-1 s-1 at room temperature. The latter value is more than 10 times lower than the one currently used in numerical codes for simulation of the long-term radiolytic degradation associated with the reprocessing and storage of spent nuclear waste. At 45 °C, conditions similar to the reprocessing of spent fuel, the values of the rate constants of NO3˙ radical toward HN, HAN and U(iv) increase and are found to be 2.6 × 105, 2.9 × 107 and 9.3 × 106 M-1 s-1.

Potential applications of sonochemistry in spent nuclear fuel reprocessing: a short review

The industrial treatment of spent nuclear fuel is based upon a hydrometallurgical process in nitric acid medium. In order to minimize the volume of radioactive waste it seems interesting to generate the reactive species in situ in such solutions using ultrasonic irradiation without addition of salt-forming reagents. This review summarizes for the first time the versatile sonochemical processes with uranium, neptunium and plutonium in homogeneous nitric acid solutions and heterogeneous systems. The dissolution of refractory solids, ultrasonically driven liquid-liquid extraction and the sonochemical degradation of the volatile products of organic solvent radiolysis issued from PUREX process are considered. Also the guidelines for required further work to ensure successful application of the studied processes at industrial scale are discussed.

Structural Investigation of Pd(II) in Concentrated Nitric and Perchloric Acid Solutions by XAFS

XAFS spectra of palladium(II) in concentrated HNO3/HClO4 acid mixtures have been recorded and analyzed. Structural parameters of the Pd(H2O)4(2+) complex and the mixed nitric Pd(NO3)2(H2O)2 complex, for the first time, were determined by the XAFS method. For pure 5 M HClO4 and for mixtures (0-0.3 M HNO3), the XAFS spectra of the 0.02 M Pd solutions are indeed very similar and originated from four Pd-O(w) equivalent distances. For the Pd(H2O)4(2+) square-planar aqua ion in strong perchloric acid, the use of an FEFF6 theoretical approach led to a first-shell Pd-O(w) distance of 2.00 (1) A and a Debye-Waller (DW) factor of sigma2 = 0.0030 (3) A2. Four water molecules are tightly bound to the Pd2+ ion in the equatorial plane, while two (or one) axial water molecules are weakly bound to the metal ion at 2.5 A with a DW factor of 0.015 (5) A2. For highly concentrated mixtures (4-6 M HNO3) and for pure concentrated (4-6 M) nitric acid as well as for crystalline powder Pd(NO3)2(H2O)2, the XAFS spectra are very similar and are determined by the mixed nitric complex Pd(NO3)2(H2O)2: four Pd-O near-equivalent distances of 2.01 (1) A from two H2O and two NO3 molecules with a total DW factor of sigma2 = 0.0037 (3) A2. Moreover, two Pd---N distances of 2.8-2.9 A were determined in the second coordination shell. Finally, for intermediate mixtures (1-3 M HNO3 in 5 M HClO4), the XAFS spectra are a superposition of the XAFS of Pd(H2O)4(2+) and Pd(NO3)2(H2O)2 complexes. The mean ligand number NO3(-) around Pd2+ has been calculated, and the XAFS results at pH close to zero confirm the spectrophotometric results previously published.

Scavenging of OH(*) radicals produced from H(2)O sonolysis with nitrate ions

The scavenging of OH(?) radicals formed during H(2)O sonolysis with nitrate-ions was studied in HNO(3)/NaNO(3) mixture at the constant NO(3)(-) ions concentration ([HNO(3)]+[NaNO(3)])=1 M in Ar atmosphere. Small amounts of N(2)H(5)NO(3) was added to solutions to avoid HNO(2) accumulation due to HNO(3) sonolysis. It was shown that the increase of [H(+)] causes the increase of H(2)O(2) formation rate (W(H(2)O(2)). (W(H(2)O(2)) values reach the plateau at [HNO(3)] approximately 1 M. The (W(H(2)O(2)) ratio in solution with [H(+)]=1 M and pure water was found to be equal to 2.4+/-0.4. It was assumed that (W(H(2)O(2)) increase in nitric acid medium is related to the changing of H(2)O(2) formation mechanism. In pure water H(2)O(2) is formed due to the OH(*) radicals recombination. In HNO(3)+NaNO(3) mixture the mechanism of H(2)O(2) formation consists in conversion of OH(*) radicals to NO(3)(*) radicals followed by NO(3)(*) radicals hydrolysis. Results obtained show that OH(*) radicals recombination mainly occurs in the liquid phase surrounding the cavitating bubble.

Kinetics of hydrazinium nitrate decomposition in nitric acid solutions under the effect of power ultrasound

The effect of ultrasound (f = 20 kHz) on the decomposition of hydrazinium nitrate was investigated in a nitric acid medium. The kinetics of N2H5+ decomposition and initial HN3 formation increase in a linear manner with the HNO3 concentration (from 1 to 6 M) and with the ultrasonic intensity (from 0.5 to 3.1 W cm-2). Both rates were equal to that of HNO2 formation in the absence of N2H5+, indicating that the N2H5+ decomposition mechanism is the same as observed without ultrasound between HNO2 and N2H5+. The variation of the steady-state HN3 concentration with the HNO3 concentration and the ultrasonic intensity suggests the existence of a nonexplosive HN3 thermal decomposition mechanism in the cavitation bubble under the effect of ultrasound. It was also observed at ultrasonic intensities exceeding 3.5 W cm-2 that the decomposition of HN3 led to the accumulation of NH4+ in solution.

Preliminary results on the effect of power ultrasound on nitrogen oxide and dioxide atmosphere in nitric acid solutions

The effect of ultrasound (20 kHz, 3 W cm-2) on the kinetics of HNO2 and H2O2 formation was investigated in a 1 M HNO3 medium for NO2-Ar and NO-Ar gas mixtures in various volume fractions (f(NO2) < 1.7 vol% and f(NO) < 1.1 vol%, respectively). The H2O2 formation rate measured in 1 M HNO3 in the presence of N2H5NO3 was observed to be much lower than that of HNO2 without N2H5NO3, and was relatively independent of the NO2 or NO gas volume fractions in the argon atmosphere. The HNO2 formation rate increased under ultrasound, and was higher with NO than with NO2. The induction period observed without ultrasound disappeared when ultrasound was applied. The first step in the sonochemical mechanism of HNO2 formation in the presence of NO2 involves thermal decomposition of NO2 into NO within the cavitation bubble. In the second step of HNO2 formation, NO reacts either with HNO3 in the cavitation bubble, or with NO2 in the cavitation bubble or at the bubble/solution interface.

Kinetics of nitrous acid formation in a two-phase tri-n-butylphosphate-diluent/aqueous nitric acid extraction system under the effect of power ultrasound

The kinetics of nitrous acid formation were investigated in two-phase tri-n-butylphosphate (TBP)-diluent/HNO3 (1.5-6.0 mol l-1) systems, where diluent is n-C16H34, n-C12H26, n-C9H20 and i-C8H18, under the effect of power ultrasound at 20 kHz frequency under Ar atmosphere. The rate of HNO2 sonochemical formation decreases with the rise in diluent vapor pressure. The HNO2 formed is distributed between the aqueous and organic phases due to its extraction with TBP. The kinetics of HNO2 sonochemical formation in the two-phase system exhibits induction periods due to NOx (NO + NO2) gas reactions in the HNO3 medium. This induction period decreases with increasing HNO3 concentration and ultrasound intensity. The HNO2 steady-state concentration was obtained under long-time sonication as the result of HNO2 sonochemical decomposition. HNO2 decomposes faster under sonication in the aqueous phase than in the organic phase.

Kinetics of nitrous acid formation in nitric acid solutions under the effect of power ultrasound

Sonochemical nitrous acid formation was investigated in 0.1-4.0 mol dm(-3) aqueous nitric acid solutions under the effect of power ultrasound with 20 kHz frequency. HNO2 steady-state concentration was obtained under long-time sonication; the excess HNO2 formed is decomposed and evoluted from the solution as NO and NO2 gases. The HNO2 steady-state concentration and the HNO2 initial formation rate depend linearly on the HNO3 concentration and acoustic intensity (1.8-3.5 W cm(-2)) and decrease with rising temperature in the range 21-50 degrees C. The HNO2 formation rate depends on the type of saturating gas as follows: Ar > N2 > He > air. NO and O2 are the major gaseous products of HNO3 sonication. The NO2 accumulation of in the gas phase is observed only when the decomposition of HNO2 formed becomes noticeable. The gaseous products formation rates depend on the HNO3 concentration, acoustic intensity and the type of saturating gas. The mechanism of HNO2 sonochemical formation is assumed to be the thermal decomposition of HNO3 in the gaseous vicinity of collapsing bubbles or in the overheated liquid reaction zone surrounding the cavitational bubbles.