Article pubs.acs.org/JPCB
Plutonium and Americium Alpha Radiolysis of Nitric Acid Solutions Gregory P. Horne,*,†,‡ Colin R. Gregson,§ Howard E. Sims,†,§,∥ Robin M. Orr,§ Robin J. Taylor,†,§ and Simon M. Pimblott*,†,‡ †
The University of Manchester, Dalton Cumbrian Facility, Westlakes Science and Technology Park, Cumbria, CA24 3HA, U.K. The University of Manchester, School of Chemistry, Oxford Road, Manchester, M13 9PL, U.K. § National Nuclear Laboratory, Sellafield Central Laboratory, Sellafield, Seascale, Cumbria, CA20 1PG, U.K. ∥ National Nuclear Laboratory, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, U.K. ‡
ABSTRACT: The yield of HNO2, as a function of absorbed dose and HNO3 concentration, from the α-radiolysis of aerated HNO3 solutions containing plutonium or americium has been investigated. There are significant differences in the yields measured from solutions of the two different radionuclides. For 0.1 mol dm−3 HNO3 solutions, the radiolytic yield of HNO2 produced by americium α-decay is below the detection limit, whereas for plutonium α-decay the yield is considerably greater than that found previously for γ-radiolysis. The differences between the solutions of the two radionuclides are a consequence of redox reactions involving plutonium and the products of aqueous HNO3 radiolysis, in particular H2O2 and HNO2 and its precursors. This radiation chemical behavior is HNO3 concentration dependent with the differences between plutonium and americium α-radiolysis decreasing with increasing HNO3 concentration. This change may be interpreted as a combination of α-radiolysis direct effects and acidity influencing the plutonium oxidation state distribution, which in turn affects the radiation chemistry of the system.
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INTRODUCTION Nitric acid has many uses in the nuclear industry from being used as the aqueous solvent in the reprocessing of spent nuclear fuel (SNF) to being the storage medium for high level liquid waste (HLW) originating from reprocessing operations, which is housed in stainless steel storage tanks. Unlike most other industrial scale chemical engineering systems, SNF reprocessing solvent systems are subject to an intense multicomponent radiation field (γ-rays, α-particles, βparticles, neutrons, and fission fragments). This radiation field induces radiolytic degradation of the components of the solvent system (aqueous nitric acid, specialized extractant ligands, and the organic diluent), resulting in the formation of numerous deleterious products.1−6 Many of these degradation products have undesirable properties that are detrimental to either the performance of a reprocessing process2−6 or the plant materials, enhancing corrosion of plant structural materials such as stainless steels.7−9 Under conditions used in the storage of HLW, radiolytic degradation of nitric acid adversely influences the dissolution of fission products through denitrification and deacidification, and leads to degradation of the stainless steel storage tanks housing the HLW.7−9 This complex behavior presents challenges to the management, performance, development, and engineering of reprocessing technologies and storage of subsequent nuclear waste materials. Understanding the fundamental radiation chemistry of nitric acid is paramount in mitigating the degradation of the materials comprising SNF reprocessing plants and HLW storage tanks. © 2017 American Chemical Society
Nitrous acid (HNO2) is the principal radiolytic degradation product of nitric acid and a significant contributor to altering both the physical and chemical properties of nitric acid media, and its corrosion potential.8,10−12 HNO3 + 2H+ + 2e− ⇌ HNO2 + H 2O E°(25 °C) = 934 mV/SHE
(1)
Furthermore, nitrous acid exhibits complex redox relationships with a number of actinides, of which its reactions with plutonium and neptunium are of concern to the performance of reprocessing solvent systems.3−5,13−19 The radiolytic formation of nitrous from aerated nitric acid can be described by the following simple reaction scheme.20−27 Water radiolysis: H 2O ⇝ eaq −, Haq +, H•, OH•, H 2 , H 2O2
(2)
Nitrate radiolysis: NO3− ⇝ NO3−* → NO2− + O
(3)
NO3− ⇝ NO3−* → NO3• + e−
(4)
Received: November 30, 2016 Revised: January 9, 2017 Published: January 9, 2017 883
DOI: 10.1021/acs.jpcb.6b12061 J. Phys. Chem. B 2017, 121, 883−889
Article
The Journal of Physical Chemistry B
between radiation quality (radiation type and energy) and redox effects.
Nitric acid radiolysis: HNO3 ⇝ HNO3* → HNO2 + O
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(5)
EXPERIMENTAL SECTION Chemicals. HNO3 (99.995% trace metals basis), N-(1naphthyl)ethylenediamine dihydrochloride (≥98%), sulfanilamide (≥99%), and HCl (ACS reagent grade) were obtained from Sigma-Aldrich. Plutonium nitrate and americium nitrate were supplied by the National Nuclear Laboratory. All chemicals were used as received without further purification. Ultrapure water was used to make up all aqueous solutions. Actinide Solution Preparation. All handling of active solutions was performed in designated active fumehoods and negative pressure glove boxes in a nuclear licensed facility in compliance with all relevant regulations and procedures for handling radioactive elements. Plutonium solutions were prepared from a plutonium nitrate stock solution (23.98 g dm−3 Pu in 1.3 mol dm−3 HNO3), the isotopic distribution of which was determined by a combination of inductively coupled plasma mass spectrometry and high resolution alpha and gamma spectrometry techniques; amounts and properties of the respective isotopes are outlined in Table 1. Each plutonium experiment used an average of 0.24
Diffusion-reaction chemistry of nitrogen species: NO3− + e pre− → NO3•2 − k6 = 1 × 1013 dm 3 mol−1 s−1
(6)
NO3− + eaq − → NO3•2 − k 7 = 9.7 × 109 dm 3 mol−1 s−1
(7)
NO3− + H• → HNO3− k 8 = 1.0 × 107 dm 3 mol−1 s−1
(8)
NO3•2 − + H 2O → NO2 • + 2OH− k 9 = 1.0 × 103 dm 3 mol−1 s−1
(9)
HNO3− → NO2 • + OH− k10 = 2.0 × 105 s−1
(10)
NO2 • + NO2 • ⇌ N2O4
Table 1. Initial Isotopic Composition of the Plutonium Nitrate Stock Solution and Their Respective Radioactive Properties
k11f = 4.5 × 108 dm 3 mol−1 s−1 k11b = 6 × 103 s−1
(11)
N2O4 + H 2O → HNO2 + HNO3 k12 = 18 dm 3 mol−1 s−1
isotope 238
Pu Pu 240 Pu 241 Pu 242 Pu 241 Am
(12)
239
HNO2 ⇌ NO2− + Haq + pK a = 3.2
(13)
This reaction scheme is not exhaustive, as nitrous acid and its precursors are subject to a plethora of secondary reactions.20 A number of research groups have investigated the radiolytic formation of nitrous acid induced by the gamma radiolysis of nitric acid;22,28−30 however, there have been very few studies using alpha radiation.28,31,32 Alpha particles contribute a significant fraction of the activity of SNF, and in the context of the present work, the key radionuclides recovered by reprocessing are predominantly alpha emitters (i.e., uranium and plutonium, and neptunium, americium, and curium if minor actinides are also separated). The chemistry of alpha irradiation is known to be significantly different from that of gamma irradiation due to underlying differences in the radiation track structure. Consequently, it is necessary to understand the effects of alpha radiolysis, as distinct from gamma radiolysis, on the radiolysis of nitric acid. The presented research focuses on the radiolytic yield of nitrous acid from the alpha radiolysis of aqueous nitric acid solutions containing plutonium or americium, as a function of absorbed dose and nitric acid concentration (0.1, 1.0, 4.0, and 6.0 mol dm−3). Plutonium and americium were chosen as the alpha emitters for this research because: (i) plutonium is the key alpha emitter extracted during SNF reprocessing; (ii) americium is important in the separation of minor actinides by processes such as i-SANEX and EXAm;33,34 and (iii) plutonium is redox active, whereas americium remains in its trivalent state under typical reprocessing conditions, potentially allowing for differentiation
initial isotope concentration (mg mL−1)
half-life (years)
predominant decay mode
particle kinetic energy (keV)
0.08 16.03 6.54 0.96 0.37 0.23
88 24110 6563 14 373300 432
α α α β− α α
5593 5244 5255 21 4984 5638
mL of stock solution, made up to 100 mL using the appropriate concentration of HNO3, i.e., 0.1, 1.0, 4.0, and 6.0 mol dm−3. This dilution provided an average initial dose rate of 2.54 × 107 MeV s−1 mL−1 (4.07 × 10−3 Gy s−1), calculated from the respective activities of the various isotopes. Americium solutions were prepared from an americium nitrate (3.88 g dm−3 Am in 0.91 mol dm−3 HNO3) stock solution, with an isotope distribution of >99% 241Am (with traces of U, Np, and Pu). Each americium experiment used an average of 63 μL of stock solution, made up to 70 mL using the appropriate concentration of HNO3, providing an initial dose rate of 2.57 × 107 MeV s−1 mL−1 (4.12 × 10−3 Gy s−1), calculated from the respective activity of 241Am. Each sample consisted of 5 mL of aqueous actinide (plutonium or americium) nitric acid solution sealed in a 20 mL ground glass socketed vessel equipped with a stop-cock, leaving a 15 mL (±1 mL) head space volume of air. Irradiation Procedure. Self-irradiation of the test solutions occurred from the α-decay of the plutonium or americium isotopes. Samples were left until the desired dose had been attained and immediately analyzed. Dose rates were calculated from the respective activities of the actinide solutions and were corrected for decay of the various isotopes and the ingrowth of additional radionuclides. 884
DOI: 10.1021/acs.jpcb.6b12061 J. Phys. Chem. B 2017, 121, 883−889
Article
The Journal of Physical Chemistry B Analytical Methods. Measurement of nitrous acid concentrations was performed by a modified version of the Shinn method, the full details for which can be found in ref 35. The procedure involved the sequential addition of sulfanilamide solution (5.8 × 10−2 mol dm−3) and N-(1-naphthyl)ethylenediamine dihydrochloride solution (3.9 × 10−3 mol dm−3) to 5 mL diluted samples of the irradiated actinide solutions. The absorption of the resulting azo-dye was measured using a PerkinElmer Lambda 35 UV−vis-NIR spectrophotometer equipped with fiber optic cables and an Ocean Optics cell holder. An extinction coefficient (ε) of 4.6 × 104 M−1 cm−1 was determined at λmax = 543 nm. Each of the presented nitrous acid concentration data points is an average of three independent solutions, with each average possessing a standard deviation error of less than ±10%.
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Figure 2. Concentration of HNO2 as a function of dose, from americium α-decay in HNO3 solutions: 0.1 (black ■), 1.0 (red ●), 4.0 (blue ▲), and 6.0 mol dm−3 (green ▼) HNO3; fitted lines for visual aid only.
RESULTS The dependence of HNO2 concentration on absorbed alpha dose, from the radioactive decay of plutonium and americium, is given in Figures 1 and 2, respectively. In both figures, the
NO2− + OH• → NO2 • + OH− k14 = 1.0 × 1010 dm 3 mol−1 s−1
(14)
HNO2 + OH• → NO2 • + OH− k15 = 2.0 × 109 dm 3 mol−1 s−1
(15)
HNO2 + H 2O2 ⇌ ONOOH + H 2O k16f = 7.17 × 105 mol dm−3 s−1 k16b = 300 mol dm−3 s−1
(16)
NO3•2 − + H 2O2 → NO3− + OH− + OH• k17 = 1.6 × 108 dm 3 mol−1 s−1
(17)
Significant quantities of HNO2 are formed from α-radiolysis within the investigated dose range for HNO3 concentrations ≥4.0 mol dm−3 (>1.5 × 10−4 mol dm−3 HNO2). These HNO3 concentrations coincide with the typical aqueous nitric acid concentration range for the extraction of actinides in reprocessing and storage of highly active raffinate. 3−5 Consequently, reprocessing systems and HLW storage tanks experience the highest radiolytic yields of HNO2, and can thus be expected to be subject to the more extreme degradation and corrosion effects associated with HNO2.
Figure 1. Concentration of HNO2 as a function of dose, from plutonium α-decay in HNO3 solutions: 0.1 (black ■), 1.0 (red ●), 4.0 (blue ▲), and 6.0 mol dm−3 (green ▼) HNO3; fitted lines for visual aid only.
concentration of HNO2 increases with increasing HNO3 concentration and absorbed dose. These trends are the same as those seen for complementary gamma radiolysis experiments,36 and are a consequence of the increasing contributions of direct (reactions 3 and 5) and indirect (reactions 6−8) radiation effects on the production of HNO2.22,29,30 As the concentration of HNO3 is increased, there is a proportional increase in the scavenging capacities (ks = k × [Scavenger]) of NO3− for epre−, eaq−, and H•, leading to the formation of more HNO 2 . For HNO 3 concentrations ≥1 mol dm−3, contributions from direct radiation effects increasingly dominate. The radiolytic formation of HNO2 exhibits a nonlinear dosedependency, in which the gradient for HNO2 formation decreases with increasing absorbed dose. This nonlinear behavior is a consequence of secondary bulk homogeneous reactions, involving the oxidizing products of water radiolysis, e.g, eqs 14−17.21,24,36,37,37−39
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DISCUSSION Alpha Radiolysis of 0.1 M Nitric Acid. The differences exhibited by the HNO2 yields between americium and plutonium for 0.1 mol dm−3 HNO3 solutions, shown in Figure 3, are of particular interest. First, the alpha radiolysis yield of HNO2 induced by plutonium α-decay is significantly greater than those from americium α-decay and gamma radiolysis. Second, the alpha radiolysis yield of HNO2 from americium α-decay is essentially zero, whereas that from gamma radiolysis increases with absorbed dose. As the concentration of HNO3 is
