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Alpha-radiolysis of nitric acid aqueous solution irradiated by 238Pu source

NUCLEAR CHEMISTRY, RADIOCHEMISTRY, RADIOPHARMACEUTICALS AND NUCLEAR MEDICINE

Alpha-radiolysis of nitric acid aqueous solution irradiated by 238Pu source

Zhe Liu，
Zhong Fang，
Liang Wang，
Hui He，
Ming-Zhang Lin
Nuclear Science and TechniquesVol.28, No.4Article number 54Published in print 01 Apr 2017Available online 03 Mar 2017
DOI：10.1007/s41365-017-0200-4
47900

Alpha radiolysis of nitric acid aqueous solution by a 238Pu source is investigated experimentally and theoretically. The time dependence of the nitrous acid yield on dose rate, nitric acid concentration, and nitrate ion concentration is studied. A novel kinetic model for the α-radiolysis of nitric acid aqueous solution is established, by considering the direct and indirect effects,. The simulation results agree well with the experimental data, indicating the validity of our model to treat the reaction paths for generation and consumption of nitrous acid. It is shown that the redox reactions involving Pu cannot be neglected in the α-radiolysis of the solution. The results provide a better understanding of the α-ray radiolysis of aqueous nitric acid.

α-radiolysisNitric acidPlutoniumAqueous solutionKinetic modelNitrous acid

1. Introduction

Nitric acid is the most common inorganic acid used in the PUREX (Plutonium Uranium Recovery EXtraction) process, under intense α, β and γ radiations [1-4]. The radiolytic products of aqueous nitric acid, such as nitrous acid, hydrogen peroxide, hydrogen and oxygen affect the organic phase degradation, valence of actinides, corrosion of the container, and the reductants in PUREX [4, 5]. Thus investigation on the radiolysis of nitric acid attracts increasing attention [5-10].

Experiments on radiolysis of solid nitrates were began in 1970s [11-15], and the direct radiolysis effect was proposed. For nitric acid aqueous solution, the reactions involved are more complex due to the influence of water radiolysis. The formation of gases (e.g., H2 and O2) in the solution was investigated [15-18]. It was proved that both the direct radiolysis of solutes and the reactions between radiolytic products and water contributed to the production of gases. However, while there are many experimental and theoretical works on γ-ray radiolysis of aqueous nitric acid [7-9, 19-21]., just limited investigations using an α-radiation source have been reported [16-18, 22-25], largely due to the lack of an α irradiation facility and the operation difficulties. The yield of HNO2 was measured by Miner et al. with the 238Pu microsphere as an α-radiation source [26]. The survey on HNO2 yield is important for understanding the mechanism of α radiolysis of aqueous nitric acid and for the application in PUREX, considering that HNO2 can consume the reductant for Pu(III) in PUREX. However, the concentration gradient of nitric acid in Ref.[22] is too large to be applied for practical use. On the theoretical side, the redox reactions about Pu and some radiolytic products of nitric acid were proposed in Refs. [27, 28], though the primary yields of some radicals and several reactions involving nitric acid were absent. To the best of our knowledge, there is still no complete kinetic model for the α-radiolysis of aqueous nitric acid.

In this paper, experiments on is conducted production of NO2− using 238Pu as the α-radiation source (238Pu decays, in a half-life of 87.7 years, by emitting α particles in averaged energy of 5.49 MeV). A novel complete kinetic model for α-radiolysis of aqueous nitric acid is established using MCPA FACSIMILE Version 4.0 (MCPA Software Ltd., UK) [29]. FACSIMILE is a powerful modeling tool designed to efficiently solve differential equations in engineering and scientific researches, and is widely used in chemical kinetic simulation [30, 31]. The experimental and simulation results should benefit for a better understanding of the radiolysis of aqueous nitric acid in nuclear industry, since the concentration of nitric acid in our work is within the range of that in the typical PUREX process.

2. Materials and Methods

2.1. Materials

The 238Pu in the form of PuO2 powder and the deionized water (18.25 MΩ·cm) were provided by China Institute of Atomic Energy.. The nitric acid and sodium nitrate were analytical pure (Sinopharm Group Co. Ltd.).

2.2. Preparation of stock solution

A 238Pu solution， instead of the 238Pu microsphere [26], was employed, in consideration of radiation uniformity, convenience of dose rate determination (nearly all radiation energy was absorbed by the solution), and the less amount (just mg or less) of α emitter required for the experiments [26].

The long-term stock solution was prepared by dissolving 238PuO2 powder in aqueous nitric acid catalyzed by Ag+-Ag2+ cycle under constant-current electrolysis. The 238Pu(IV)-NO3− stock solution was purified with No. 2606 anion exchange resin packed column (offered by China Institute of Atomic Energy). During the purification, Pu was adsorbed in the exchange column and the Ag+ remained with the solution. The Pu was eluted by 0.35 M HNO3 solution. It is worth noting that there should be radiolytic resin products in solution, since the Pu is a strong radiation source. Thus, before the experiments, the small amount of stock solution to be used was purified again with the same method. In the second purification, the radiation source was less and the purification time was shorter, with negligible radiolytic products.

2.3. Preparation of experimental solutions

The stock solution was adjusted to desired conditions. Different dose rates were achieved by adjusting the Pu concentration, noted as c(Pu), and the variation of NO3− concentration was realized by adding sodium nitrate. The concentration of nitric acid used in the experiments was within the range of PUREX process (about 1.0–5.0 M), thus the results are suggestive for understanding of the actual α-radiolysis of aqueous nitric acid.

2.4. Characterization

The concentration of nitrous acid was determined by spectrophotometry (SPECORD S600 Spectrophotometer, Analytik Jena AG, German), using a 1-cm quartz cuvette (YiXing JingKe Optical Instrument Co. Ltd.). Spectrophotometry was used to continuously measure the production of nitrous acid.

2.5 Kinetic model

There are many reaction paths for the α-radiolysis of aqueous nitric acid. Firstly, the radiolytic products of water react with nitric acid or nitrate ion [30]:

•OH+HNO3→•NO3+H2O, k=1.4×108M-1·s-1 (1) •H+NO3-→•HNO3-, k=1.0×107M-1·s-1 (2)

The effect of direct radiolysis of solute also plays an important role for concentrated solution, as the solute absorbs a large fraction of the radiation energy [21]:

NO3--\/\/\→NO2-, O(1D), O(3P) (3) O(1D) + H2O→H2O2 (4) O(3P)+ NO3-→NO2-+O2 (5)

For concentrated nitric acid solution, radiolysis of the un-dissociated nitric acid molecule should not be neglected [20, 21]:

HNO3-\/\/\→HNO2,O(D),O(P) (6) HNO3-\/\/\→NO3+ e-aq+H+ (7)

In addition, the actinides can react with radicals in aqueous nitric acid [27, 35]. The redox reactions involving actinides may also affect the generation and consumption of nitrous acid, for example [27]:

Pu(III)+•NO2→Pu(IV)+NO2- k=2.5×103 M-1·s-1 (8)

Therefore, in order to get an overall picture about the radiolysis of aqueous nitric acid under α-radiation, we set up a kinetic model including the primary yields of the radicals for the radiolysis of both pure water and nitric acid, and the reaction sets for the reaction between these radicals and nitric acid, water, and Pu.

The radical yields of pure water, nitric acid, and nitrate ion are essential to the simulation for α-radiolysis of nitric acid aqueous solution. The primary yields for the α-radiolysis of pure water at α-particle energy of 4.6 MeV were reported by Eriksen et al., [40]. This energy is close to the averaged α-particle energy for 238Pu (5.5 MeV), so the radical yields in Ref. [40] were used in this work with slight adjustment. Due to the rapid scavenging reaction between pre-solvated electron and nitrate ion, NO32− should replace e−aq. LET (linear energy transfer) effect was also considered. The radical yields for pure water are given in Table 1. The γ-radiolysis of nitric acid/nitrate ion was studied by Jiang et al. [22]. For α-radiolysis, the decomposition paths should be the same and the radical yields should be smaller. The radical yields used in this work (the right rows of Table 1) are adjusted by fitting with experimental data.

Table 1
The primary radical yields (in 100 eV−1) of pure water, un-dissociated nitric acid, and nitrate ion. Due to the rapid reaction between presolvated electron and nitrate ion, •NO32− is expressed instead of the hydrated electron [6].
Radiolysis of pure water Radiolysis of un-dissociated nitric acid Radiolysis of nitrate ion
Radicals G value Radicals G value Radicals G value Radicals G value
GH2O(H•) 0.21 GH2O(•HO2) 0.22 GHNO3(HNO2) 0.5 GNO3-(NO2−) 0.7
GH2O(H2) 1.23 GH2O(•NO32-) 0.2 GHNO3(•O(P)) 0.5 GNO3-(•O(P)) 0.7
GH2O(•OH) 0.24            
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The yield of certain radical in solution is described by:

Gtot(Xi)=∑jGYj(Xi)fj, (9)

where, Xi is the ith radical; Yj is the jth component (water, molecular nitric acid, nitrate ion, etc.) of solution; and fj = Nj/Ntot is the electron fraction of that component, with Nj and Ntot being the electron number of the corresponding component and total electron number of the solution, respectively. The absorbed energy for corresponding component (Ej = Etotfj) varies as the concentration of solute changes. Etot represents the total energy absorbed by the solution. Thus the primary radical yield varies as the concentrations of solutes change.

In order to build a kinetic model, the reaction sets and corresponding rate constants about water, nitric acid and Pu are needed. The reactions between radicals produced in pure water radiolysis have been studied by many researchers. Buxton et al. summarized the reactions and determined the selected values for reaction rate constants Ref. [32]. Elliot and Bartels reported the reaction rate constants in water radiolysis with temperature from room temperature to 300℃ based on the experimental data in Chalk River Laboratory [38]. The reactions about pure water radiolysis can be obtained from the two works. There are various nitrogenous radicals and corresponding reactions in the radiolysis of nitric acid aqueous solution [39-42]. In Pu+HNO3 system, reactions involving Pu have been studied by Vladimirova [27, 28]. All the reactions and rate constants used in this model are summarized in Table 2.

Table 2.
The reaction set and corresponding reaction constants between the nitric acid, water, Pu, and radicals. The units of reaction rate constants for second order reactions are M−1·s−1, and those of rate constants for first order reactions are s−1.
No. Reaction Rate constant No. Reaction Rate constant
R001 [32] eaq−+ H2O→H• + OH− 1.9×101 R113 [51] •OH + •NO2 →ONOOH 4.5×109
R002 [32] eaq−+ eaq−→H2 + 2OH− 5.5×109 R114 [39] H2O2 + HNO2→ONOOH + H2O 7.17×105
R003 [32] eaq−+ H•→H2 + OH− 2.5×1010 R115 [39] ONOOH + H2O→H2O2 + HNO2 3×102
R004 [32] eaq−+ •OH→OH− 3.0×1010 R116 [42] ONOOH →H+ + NO3− 0.9×100
R005 [32] eaq−+ •O−→2OH− 2.2×1010 R117 [39] •H + •NO2→H+ + NO2− 1.0×1010
R006 [32] eaq−+ H+→H• 2.3×1010 R118 [39] eaq− + •NO2→NO2− 1.0×1010
R007 [32] eaq−+ H2O2→OH−+ •OH 1.1×1010 R119 [32] eaq− + NO2−→NO22− 3.5×109
R008 [32] eaq−+ HO2−→2OH−+ •OH 3.5×109 R120 [51] •H + NO2− →•NO + OH− 1.6×109
R009 [32] eaq−+ O2→O2•− 1.9×1010 R121 [32] •OH + •NO22− →NO2− + OH− 3.0×109
R010 [32] eaq−+ O2•−→O22− 1.3×1010 R122 [32] H+ + •NO22− →•NO + OH- 2.0×1010
R011 [32] H• + H2O→H2 + •OH 1.0×101 R123 [32] H2O + •NO22−→ •NO + 2OH− 1.0×105
R012 [32] H• + H•→H2 7.8×109 R124 [42] ONOOH→H+ + ONOO− 5.0×104
R013 [32] H• + •OH→H2O 7.0×109 R125 [42] H+ + ONOO− →ONOOH 5.0×1010
R014 [32] H• + OH−→eaq- + H2O 2.2×107 R126 [52] 2HNO2→•NO + •NO2 + H2O 1.34×101
R015 [32] H• + H2O2→•OH + H2O 9.0×107 R127 [52] •NO + •NO2 + H2O →2HNO2 1.68×108
R016 [32] H• + O2→HO•2 2.1×1010 R128 [42] ONOOH→•NO2 + •OH 3.5×10−1
R017 [32] H• + HO•2→H2O2 1.14×1010 R129 [39] HNO2 + HNO3→2•NO2 + H2O 4.5×10−2
R018 [32] •OH + •OH→H2O2 5.5×109 R130 [52] 2•NO2 + H2O→HNO2 + HNO3 8.4×107
R019 [32] •OH + •O−→HO2− 7.61×109 R131 [32] •OH+ HNO2→ •NO2+ H2O 2.6×109
R020 [32] •OH + H2→•H + H2O 4.2×107 R132 [33] •H + HNO2 →•NO + H2O 4.5×108
R021 [32] •OH + OH− →•O−+ H2O 1.3×1010 R133 [53] •NO3 + HNO2→•NO2 + HNO3 2.0×108
R022 [32] •OH + H2O2→H2O+ •HO2 2.7×107 R134 [39] HNO2 →H+ + NO2− 4.6×106
R023 [32] •OH + HO2−→OH− + •HO2 7.5×109 R135 [39] H+ + NO2− →HNO2 1.0×1010
R024 [32] •OH + H2O2+→H3O+ + O2 1.2×1010 R136 [54] HNO3 à H+ + NO3− 2.0×1010
R025 [32] •OH + HO2•→H2O + O2 6.0×109 R137 [54] H+ + NO3−→HNO3 4.4×108
R026 [32] •OH + O2−• →OH- + O2 8.0×109 R138 [21] NO3− + O(P) →NO2− + O2 2.2×109
R027 [32] •O−+ H2O→OH−+ •OH 1.8×106 R139 [41] •NO2 + HO2 →HNO2 + O2 1.8×109
R028 [32] •O−+ •O-→O22− 9.0 ×108 R140 [41] •NO2 + O2−→NO2− + O2 4.5×109
R029 [32] •O−+ H2→OH−+ H• 8.0×107 R141[This work] NO→ NO(g) 9.0×10−2
R030 [32] •O−+ H2O2→O2−• + H2O 7.08×108 R141 [30] NO3 + H2O→HNO3 + •OH 3.0×102
R031 [32] •O−+ HO2−→O2−+ OH− 4.0×108 R142 [30] NO3 + H2O2→HNO3 + •HO2 7.1×106
R032 [32] •O−+ O2→O3− 3.6×109 R143 [30] NO3 + HO2→HNO3 + O2 3.0×109
R033 [32] •O−+ O2•−→2OH− + O2 6.0×108 R201 [27] Pu(IV) + eaq− →Pu(III) 1.0×1011
R034 [38] HO•2→O2•−+ H+ 6.62×105 R202 [27] Pu(IV) + •H →Pu(III) + H+ 1.0×108
R035 [38] O2•− + H+→HO•2 4.52×1010 R203 [27] Pu(VI) + eaq− →Pu(V) 6.5×1010
R036 [38] H2O2→H+ + HO2− 6.49×10−2 R204 [27] Pu(VI) + •H →Pu(V) + H+ 6.5×107
R037 [38] H+ + HO2→H2O2 4.52×1010 R205 [27] Pu(VI)+HNO2→Pu(V)+•NO2+H+ 1×10−1
R038 [38] H2O2→H2O + (1/2) O2 8.29×10−8 R206 [27] Pu(V) + •NO2 →Pu(VI) + NO2- 1.9×104
R101 [32] NO3− + eaq−→NO32− 9.7×109 R207 [27] Pu(IV)+HNO2 →Pu(III)+ •NO2+H+ 7×10−3
R102 [33] NO3− + H•→OH− + •NO2 1.4×106 R208 [27] Pu(III) + •NO2 + H+ →Pu(IV) + HNO2 2.5×103
R103 [39] HNO3 + H•→H2 + •NO3 1.0×107 R209 [27] Pu(V) + •NO3 + H+ →Pu(VI) + HNO3 1×109
R104 [32] NO32− + H+→OH−+ •NO2 2.0×1010 R210 [27] Pu(III) + •NO3 + H+ →Pu(IV) + HNO3 2.5×108
R105 [32] NO32−+ H2O→2OH− + •NO2 5.6×104 R211 [27] Pu(IV) + •NO3 + H+ →Pu(V) + HNO3 6×102
R106 [32] NO32−+ O2 →•NO3− + •O2− 2.4×108 R212 [27] Pu(V) + Pu(IV) →Pu(VI) + Pu(III) 1.2×101
R107 [32] NO32−+ •OH →•NO3− + OH− 3.0×109 R213 [27] Pu(VI) + Pu(III) →Pu(V) + Pu(IV) 2.4×101
R108 [20] HNO3 + •OH →•NO3 + H2O 1.4×108 R214 [27] Pu(V) + Pu(III) →2Pu(IV) 3.5×100
R109 [32] •NO3 + •OH →•NO2 + •HO2 2.9×107 R215 [27] 2Pu(IV) →Pu(V) + Pu(III) 3×10−6
R110 [50] 2•NO3→2•NO2 + O2 7.9×105 R216 [27] 2Pu(V) →Pu(IV) + Pu(VI) 1×10−2
R111 [32] NO2−+ •OH→•NO2 + OH− 9.6×109 R217 [27] Pu(IV) + H2O2 →Pu(III) + •HO2 +H+ 2×103
R112 [32] NO2− + •NO3→•NO2 + NO3− 1.2×109 R218 [27] Pu(VI) + H2O2→Pu(V) + •HO2 + H+ 3×10−2
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The numerical calculations in the simulation for the radiolysis of aqueous nitric acid were conducted with MCPA FACSIMILE [29]. All the primary yields and reaction rate constants keep invariant while the dose rate, pH value and nitrate ion concentration vary.

3. Results and Discussions

3.1. Time dependence of absorbance curve

The typical absorbance curve for the aqueous nitric acid solution of 238Pu as a function of irradiation time (up to 300 min) is illustrated in the upper part of Fig. 1, with the wavelength from 340 to 500 nm. The absorbance peaks of Pu(III), Pu(IV), Pu(V) and Pu(VI) appear at 600, 476, 569 and 831 nm, respectively [43-45]. Their corresponding molar absorption coefficients are 36, 68, 20, and 158 M−1·cm−1, respectively (The molar absorption coefficients of Pu vary with the HNO3 concentration. Only typical values are shown here.). In Fig. 1, the absorbance peak of Pu(IV) is distinct and has little variation. The peaks for Pu(V) and Pu(IV) are not observed and that for Pu(III) is quite weak (not shown in Fig. 1). This indicates that Pu ions are mostly tetravalent during experiment.

Fig. 1.
Evolution of absorbance curve for aqueous nitric acid solution of 238Pu with time. c(Pu)=1.7 g/L; c(HNO3)=4.0 M; optical path 1.0 cm.
pic

The absorbance curve changes obviously around 371 nm, so the area near 371 nm is enlarged in the lower part of Fig. 1. One of the five absorbance peaks of nitrous acid is located at 371 nm. As the concentration of Pu(IV) is nearly invariant, the change of absorbance around 371 nm can be completely attributed to the production of nitrous acid. The equilibrium constant for dissociation of nitrous acid is 10−3.27 M [46]. The solution is strongly acidic in experiment, with the concentration of HNO3 ranging from 1.0 to 5.0 M. So nitrous acid hardly dissociates. The absorbance at 371 nm represents the changes in nitrous acid concentration, noted as c(HNO2). The absorbance A=εcl, where ε is the molar absorption coefficient, c is the concentration, and l is the optical path. The molar absorption coefficient of nitrous acid is taken from Ref.[47]: ε(HNO2, 371 nm) = 60.46 M−1·cm−1 and ε(NO2−, 371 nm) = 16.18 M−1·cm−1.

3.2 Valence of Pu

The simulated oxidation states of Pu as a function of irradiation time is shown in Fig. 2. It’s obvious that the valences of Pu hardly change during experiment. Pu is predominantly tetravalent. This is in agreement with experimental results.

Fig. 2.
The percentages of Pu(III) and Pu(IV) vs. the irradiation time, at c(Pu) = 1.7 g/L and c(HNO3) = 3.0 M. The percentages of Pu(V) and Pu(VI) are not shown in the figure as they are two orders of magnitude smaller than Pu(III).
pic

What should be emphasized is that, the radiolysis behavior of aqueous nitric acid takes place as early as PuO2 powder is suspended in stock solution (0.35 M nitric acid). However, the reactions following the complete dissolution of PuO2 powder in aqueous nitric acid is complex and unpredictable as the inhomogeneity of solution and impurities are involved. As shown in experiment, Pu is mainly tetravalent in the stock solution, and the valences of Pu have little change during experiment. For simplicity, Pu is assumed to be tetravalent totally. Besides, the variation instead of amount of c(HNO2) is concerned in this work. Without loss of generality，the c(HNO2) in stock solution is set to be zero in simulation.

3.3. Dose-rate effect

Since the radiation energy is almost totally absorbed by the solution, the absorbed dose is proportional to the concentration of Pu. The dose-rate effect on the production of nitrous acid is investigated by varying the concentration of Pu. Fig. 3 shows the experimental and theoretical results of absorbance at 371 nm as a function of the irradiation time. The dose rate of solution using 1 g/L Pu as radiation source is 32.5 Gy·min−1. (The exact value of dose rate varies with the solution density. Such effect is taken into consideration in our calculation.)

Fig. 3.
Radiation time dependence of absorbance at 371 nm with different concentrations of 238Pu; c(HNO3) = 3.0 M; optical path 1.0 cm.
pic

This value is calculated from the half-life of 238Pu (87.7 years) and the averaged energy of α particles (5.5 MeV) assuming that all the energy of α particles are absorbed by the solution. The calculated value has good consistency with the measured data using scintillation spectrometry within the experimental time range. The experimental method was described in our previous work [48]. Thus the kinetic model is suitable to describe the α-radiolysis of nitric acid aqueous solution using 238Pu as the radiation source. The production of nitrous acid increases with increasing dose obviously.

3.4. Variation of the concentration of nitric acid

Fig. 4 shows the time dependence of absorbance at 371 nm with different concentrations of nitric acid. The calculation results fit fairly well the experimental data. The nitrous acid production increases with the concentration of nitric acid. Together with the results about dose-rate effect (shown in Fig. 3), one can see that the radical yield, instead of nitric acid concentration, is vital for the nitrous acid yield, within the range of nitric acid concentration and dose rate in this experiment. Thus the roughly overall reaction about nitrous acid yield, namely NO3−↔NO2− + O•, is far from being desired. Besides, as the nitric acid concentration rises, the electron fraction of each component varies, hence the changes in radical yield. Such change of radical yield has little effect on the nitrous acid yield, either. That can be roughly explained by the competition among the radiolysis of pure water, un-dissociated nitric acid, and nitrate ion.

Fig. 4.
Time dependence of the absorbance at 371 nm with different concentrations of nitric acid. c(Pu) = 1.7 g/L; optical path 1.0 cm.
pic
3.5. Variation of concentration of nitrate ion

Both the concentrations of molecular nitric acid and nitrate ion change with c(HNO3). Fig. 5 shows the measured and simulated absorbance at 371 nm with different c(NaNO3) as a function of irradiation time, at c(HNO3)=1.0 M and c(Pu)=2.9 g/L. The two results agree well with each other. It is seen that, the yield of HNO2 increases with c(NO3−) slowly. Since the radiolysis of pure water produces more radicals, the increase of c(NO3−) only increase the yield of radicals slightly. Thus the production of HNO2 has an unconspicuous increase with c(NaNO3) from 1.41 to 3.53 M.

Fig 5.
Time dependence of absorbance at 371 nm with different concentrations of NaNO3; c(HNO3)=1.0 M；c(Pu) =2.9 g/L; optical path 1.0 cm.
pic
3.6. Effect of redox reactions about Pu

The influences of the reactions involving Pu (R201–R218 in Table 2) on the production of HNO2 were analyzed. In order to check the effects of R201–R218, Fig. 6(a) shows the simulated absorbance at 371 nm with the model that excludes certain reactions, at c(HNO3)=3.0 M and c(Pu)=1.7 g/L. Without all the reactions about Pu, the curve deviates significantly from the experimental data, indicating the importance of reactions involving Pu.

Fig. 6.
(a) The measured and simulated absorbance at 371 nm vs. irradiation time, together with the results simulated without certain redox reactions involving Pu (R201–R218 are the reactions in Table 2). (b) The simulated absorbance at 371 nm with different values of c(Pu)react, the concentration of Pu that participate in the redox reaction. c(HNO3) = 3.0 M. c(Pu) = 1.7 g/L; optical path 1.0 cm.
pic

Then the important reactions among all the 18 reactions involving Pu were picked up. As shown in Fig. 6(a), without R217, the production of HNO2 is much less than the experimental data and close to results without all reactions involving Pu. R217 is the vital reaction, which is expressed as:

Pu(IV)+H2O2→Pu(III)+•HO2+ H+, k=2×103M-1·s-1 (10)

Pu(IV) is reduced to Pu(III) by H2O2. H2O2 also consumes HNO2 rapidly through the reaction:

HNO2+H2O2→ONOOH+H2O, k=7.17×105 M-1·s-1 (11)

Thus R217 benefits the production of HNO2. Pu(III) produced in R217 can be oxidized to Pu(IV) in R208:

Pu(III)+•NO2→Pu(IV)+NO2-, k=2.5×103M-1·s-1 (12)

Here •NO2 was reduced to NO2−. This reaction, together with its reverse reaction R207, also has obvious influence on the production of HNO2. All the other reactions about Pu, such as the disproportionate/symproportionate reactions (R212– R216), are of either too small reactant concentrations or reaction rate constants. Their contributions are negligible (less than 0.1 %), and are not plotted in Fig. 6(a).

For further discussion about the effect of reactions involving Pu, c(Pu)react is defined as the concentration of Pu involved in the chemical reactions. The simulation results of different c(Pu)react are plotted in Fig. 6(b), at c(HNO3)=3.0 M and c(Pu) =1.7 g/L. The yield of HNO2 increases with c(Pu)react at c(Pu)react<0.5 g/L, and keeps nearly invariant at c(Pu)react> 0.5 g/L. This indicates that, Pu(IV) should be treated as catalyst instead of reactant/oxidant for the production of HNO2 on the whole. Its effect can be represented roughly as:

H2O2+NO2→Pu(IV)HNO2+HO2 (13)

Besides, the reaction rate constants of Pu depend on c(HNO3) [27, 28]. However, the rate constant for the most important reaction, Pu(IV) + H2O2→Pu(III) + •HO2 +H+, is invariant with c(HNO3). Therefore, the variation of rate constants with c(HNO3) has little effect on the yield of HNO2.

3.7. Mechanism for the production of nitrous acid

The intrinsic mechanism of nitrous acid generation and consumption was comprehensively investigated. With this model, the reaction rates for the reactions that directly generate or consume nitrous acid was calculated at c(HNO3)=3.0 M and c(Pu)=1.7 g/L. The results of the six most important reactions for generation and consumption of HNO2 are shown in Fig 7. The rates of other reactions are more than one order of magnitudes lower than that of the selected ones. The most important reactions that contribute to generation of nitrous acid are given as below:

Fig 7
The generation and consumption rates of six important reaction paths for the generation (a) and consumption (b) reactions of HNO2, respectively (Referring the Table 2 for the number of reactions). c(HNO3) = 3.0 M, and c(Pu) = 1.7 g/L.
pic
R127: •NO+•NO2+H2O→2HNO2, k=1.8×101M-1·s-1 (14) R130: 2•NO2+H2O→HNO2+HNO3, k=8.4×107M-1·s-1 (15) R115: ONOOH+H2O→H2O2+HNO2, k=5.4×100M-1·s-1 (16)

And the three reactions that dominate the consumption of nitrous acid are also collected here:

R126: 2HNO2→•NO+•NO2+H2O, k=1.34×101M-1·s-1 (17) R129: HNO2+HNO3→•NO2+•NO3+2H+, k=4.5×10-2M-1·s-1 (18) R114: H2O2+HNO2→ONOOH+H2O, k=7.17×105M-1·s-1 (19)

The two sets of reactions are reverse reactions. It is seen that, NO2, NO and H2O2 are the vital intermediate products for production of HNO2. Obviously, these reactions and intermediate productions in the PUREX process is paramount in that they actually show us how to manipulate the concentration of nitrous acid. Although the reaction constant rates in Eqs.(14)–(19) differ by 7 orders of magnitude, their contribution to the generation/consumption of HNO2 is comparable, as there are significant difference between the concentrations of reactant.

4. Conclusion

In this work, the α-radiolysis of aqueous nitric acid with 238Pu as an irradiation source is studied, and a kinetic model is established accordingly. The influence of dose rate, nitric acid concentration, and nitrate ion concentration on the yield of nitrous acid are investigated experimentally. Our results have implications for the practical reprocessing of spent fuel, as the nitric acid concentration used in the experiment purposely simulates that in PUREX process. A complete kinetic model including the indirect effect and direct effect is provided. With this model, the time dependences of the yield of nitrous acid with different dose rates, nitric acid concentrations, and nitrate ion concentrations are calculated. For each condition, the theoretical results agree well with the experimental data. Additionally, six reactions and intermediate products that dominate the generation and consumption of nitrous acid are proposed based on this model. Moreover, the calculation results imply that the redox reactions involving Pu are important components among all the complex reactions occurred in α-radiolysis of aqueous nitric acid solution. The experimental results and kinetic model are valuable for understanding the α-radiolysis of aqueous nitric acid.

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Recommended Articles

1. Introduction

2. Materials and Methods

2.1. Materials

2.2. Preparation of stock solution

2.3. Preparation of experimental solutions

2.4. Characterization

2.5 Kinetic model

3. Results and Discussions

3.1. Time dependence of absorbance curve

3.2 Valence of Pu

3.3. Dose-rate effect

3.4. Variation of the concentration of nitric acid

3.5. Variation of concentration of nitrate ion

3.6. Effect of redox reactions about Pu

3.7. Mechanism for the production of nitrous acid

4. Conclusion

References

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Nuclear Science and Techniques
Nuclear Science and Techniques