Article Manuscript for Publication in the Journal of Physical Chemistry B

Plutonium and Americium Alpha Radiolysis of Nitric Acid Solutions

Gregory P. Hornea,b,*, Colin R. Gregsonc, Howard E. Simsa,c,d, Robin M. Orrc, Robin J. Taylora,c, and Simon M. Pimblotta,b,*

aThe University of Manchester, Dalton Cumbrian Facility, Westlakes Science and Technology Park, Cumbria, CA24 3HA, UK.bThe University of Manchester, School of Chemistry, Oxford Road, Manchester M13 9PL, UK.cNational Nuclear Laboratory, Sellafield Central Laboratory, Sellafield, Seascale, Cumbria, CA20 1PG, UK.dNational Nuclear Laboratory, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK.

* Corresponding authors.

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 measure 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 behaviour 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.

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 multi-component radiation field (γ-rays, α-particles, β-

particles, neutrons, and fission fragments). This radiation field induces radiolytic degradation

of the components of the solvent system (aqueous nitric acid, specialised 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 behaviour 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

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 + H2O Eo(25 oC) = 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

H2O ⇝ eaq−, Haq

+, H, OH, H2, H2O2 (2)

Nitrate radiolysis

NO3− ⇝ NO3

−* → NO2− + O (3)

NO3− ⇝ NO3

−* → NO3 + e− (4)

Nitric acid radiolysis

HNO3 ⇝ HNO3* → HNO2 + O (5)

Diffusion-reaction chemistry of nitrogen species

NO3− + epre

− → NO32− k6 = 1 × 1013 dm3 mol−1 s−1 (6)

NO3− + eaq

− → NO32− k7 = 9.7 × 109 dm3 mol−1 s−1 (7)

2

NO3− + H → HNO3

− k8 = 1.0 × 107 dm3 mol−1 s−1 (8)

NO32− + H2O → NO2 + 2OH− k9 = 1.0 × 103 dm3 mol−1 s−1 (9)

HNO3− → NO2 + OH− k10 = 2.0 × 105 s−1 (10)

NO2 + NO2 ⇌ N2O4 k11f = 4.5 × 108 dm3 mol−1 s−1

k11b = 6 × 103 s−1 (11)

N2O4 + H2O → HNO2 + HNO3 k12 = 18 dm3 mol−1 s−1 (12)

HNO2 ⇌ NO2− + Haq

+ pKa = 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 acid22,28-30, 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 between radiation

quality (radiation type and energy) and redox effects.

3

EXPERIMENTAL

Chemicals

HNO3 (99.995% trace metals basis), N-(1-napthyl)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. Ultra-pure 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), which contained the isotopes 238Pu, 239Pu, 240Pu, 241Pu, 242Pu, and 241Am, determined by a combination of inductively coupled plasma mass spectrometry and

high resolution gamma spectrometry techniques; the amounts and properties of the respective

isotopes are outline in Table 1. Each plutonium experiment used an average of 0.24 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.

Table 1. Initial isotopic composition of the plutonium nitrate stock solution and their respective radioactive properties.

isotopeinitial isotope concentration(mg mL−1)

half-life(years)

predominant decay mode

particle energy(keV)

238Pu 0.08 87.7 α 5593239Pu 16.03 24110 α 5244240Pu 6.54 6563 α 5255241Pu 0.96 14.35 β− 21242Pu 0.37 373300 α 4984241Am 0.23 432.2 α 5638

4

Americium solutions were prepared from an on hand americium nitrate (3.88 g dm−3 Am in

0.91 mol dm−3 HNO3) stock solution at the National Nuclear Laboratory Sellafield Central

Laboratory, 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 stock-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

analysed. 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.

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 reference 35. The procedure involved

the sequential addition of sulfanilamide solution (5.8 ×10−2 mol dm−3) and N-(1-

napthyl)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 fibre 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%.

5

RESULTS

The dependence of HNO2 concentration on absorbed alpha dose, from the radioactive decay

of plutonium and americium, is given in Figure 1 and Figure 2, respectively. In both figures

the 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 to 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 (k s=k ×[Scavenger ]) of NO3− for epre

−, eaq−, and H, leading to the formation of

more HNO2. For HNO3 concentrations ≥1 mol dm-3, contributions from direct radiation

effects increasingly dominate.

Figure 1. Concentration of HNO2 as a function of dose, from plutonium α-decay in HNO3

solutions: 0.1 (), 1.0 (), 4.0 (), and 6.0 mol dm−3 () HNO3; fitted lines for visual aid only.

The radiolytic formation of HNO2 exhibits a nonlinear dose-dependency, in which the

gradient for HNO2 formation decreases with increasing absorbed dose. This nonlinear

behaviour is a consequence of secondary bulk homogeneous reactions, involving the

oxidising products of water radiolysis, e.g. (14) to (17).21,24,34,35,37-39

NO2− + OH → NO2 + OH− k14 = 1.0 ×1010 dm3 mol−1 s−1 (14)

6

HNO2 + OH → NO2 + OH− k15 = 2.0 ×109 dm3 mol−1 s−1 (15)

HNO2 + H2O2 ⇌ ONOOH + H2O k16f = 7.17 × 105 mol dm−3 s−1

k16b = 300 mol dm−3 s−1 (16)

NO32− + H2O2 → NO3− + OH− + OH k17 = 1.6 ×108 dm3 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 2. Concentration of HNO2 as a function of dose, from americium α-decay in HNO3

solutions: 0.1 (), 1.0 (), 4.0 (), and 6.0 mol dm−3 () HNO3; fitted lines for visual aid only.

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. Firstly, the alpha radiolysis yield of HNO2 induced by plutonium α-decay

7

is significantly greater than those from americium α-decay and gamma radiolysis. Secondly,

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

<1 mol dm−3, contribution from direct radiation effects are negligible.20 These differences

may be interpreted as differences in radiation quality (α-particles v.s. γ-rays) and radionuclide

identity (americium v.s. plutonium).

The HNO2 production induced by americium α-decay is in agreement with the polonium α-

decay studies performed by Savel’ev et al.31, and may be explained by the difference in

radiation quality between α-particles and γ-rays, i.e. the effect of linear energy transfer (LET)

upon the primary products of aqueous HNO3 radiolysis, as α-particles possess a considerably

greater LET than γ-rays.40

Figure 3. Concentration of HNO2 as a function of dose received by 0.1 mol dm−3 HNO3

solutions: gamma radiolysis data from unpublished work ()34, plutonium α-decay (), americium α-decay (), and PuO2 α-decay by Kazanjian et al. ()28; fitted lines for visual aid only.

As the LET of a radiation species increases, the separation between energy deposition events

decreases, leading to more densely populated radiation tracks. As a consequence, there is a

proportional increase in the extent of interspur reactions, leading to an increase in radical

recombination. Ultimately, the radiolytic yields of radicals decrease while those of molecular

species increase with increasing LET. As outlined by reactions (6) to (8), HNO2 is

8

predominantly a product of NO3− scavenging key radical species from the radiolysis of water.

Furthermore, it has been previously established that H2O2 is important in the radiation

chemistry of HNO3 and the yield of HNO2. This process has a significant influence on the

yield of HNO2 with dose and can be expected to have a greater effect during alpha radiolysis

due to the higher radiolytic yield of H2O2, for example Gα(H2O2) = 1.6 [41], and Gγ(H2O2) =

0.7 [21]. On this basis, it would be expected that the radiolytic yield of HNO2 will decrease

with increasing LET, which is reflected by the yields obtained from americium α-decay in

this work and those yields from the polonium studies reported by Savel’ev et al.31 The

negligible α-radiolysis yield of HNO2 from 0.1 mol dm−3 HNO3 may suggest that the

concentration of NO3− is not sufficiently high to compete with ultrafast radical recombination

processes occurring within the radiation chemical track. However, the HNO2 yields measured

by Kazanjian et al.28 (G(HNO2) = 0.51) indicate that HNO2 is formed in relatively substantial

amounts. Kazanjian et al. used insoluble, inert plutonium dioxide (PuO2) microspheres,

comprising 80% 238Pu, to induce alpha radiolysis of HNO3 solutions. Their solutions were

spiked with p-nitroaniline to scavenge and prevent further reaction of radiolytically produced

HNO2. Consequently, their reported HNO2 yields represent an upper limit indicative of the

amount produced within the radiation track. Therefore, the negligible yields of HNO2

measured for americium and polonium at low acidities reflect the extent to which secondary

bulk homogeneous chemistry consumes HNO2 and its precursors in 0.1 mol dm−3 HNO3

solutions.

Although LET effects explain the differences induced by americium α-decay relative to

gamma in 0.1 mol dm−3 HNO3, they do not address the fact that the alpha radiolysis yield of

HNO2 from plutonium α-decay is substantially greater than both. This behaviour suggests

chemical involvement of plutonium in the HNO3 system. The work by Kazanjian et al.

should be considered separately, as their source of plutonium was rendered chemically inert

and insoluble. In aqueous acidic solutions, plutonium coexists as four different oxidation

states (Pu(III), Pu(IV), Pu(V), and Pu(VI)), as a consequence of possessing similar redox

potentials, a tendency for Pu(IV) and Pu(V) to undergo disproportionation, and the slower

rate of Pu-O bond rupture relative to electron transfer.42 This equilibrium between different

plutonium oxidation states is normally expressed by (18).

Pu4+ + PuO2+ ⇌ Pu3+ + PuO2

2+ (18)

9

However, the dynamics of the system are considerably more complex and composed of

numerous equilibria. Although Pu(IV) is relatively stable in HNO3 solutions due to

complexation with NO3¯, it is still susceptible to disproportionation (19).41

3Pu4+ + 2H2O ⇌ 2Pu3+ + PuO22+ + 4Haq

+ (19)

The extent of Pu(IV) disproportionation decreases with increasing acid concentration, to the

point at which it is essentially completely inhibited in strongly acidic solutions.41-45 The

disproportionation of Pu(IV) is composed of two sequential bimolecular equilibria involving

Pu(V) (reactions 20 and 21).41-44

2Pu4+ + 2H2O ⇌ Pu3+ + PuO2+ + 4Haq

+ (20)

Pu4+ + PuO2+ ⇌ Pu3+ + PuO2

2+ (21)

These equilibria are further complicated by the disproportionation of Pu(V) (22).41,46

3PuO2+ + 4Haq

+ ⇌ Pu3+ + 2PuO22+ + 2H2O (22)

The extent of Pu(V) disproportionation rapidly increases with increasing acid concentration.

As with Pu(IV), this equilibrium is also composed of a series of individual equilibria

(reactions 23 to 25). The pathway by which Pu(V) undergoes disproportionation is dependent

upon the presence and proportions of other plutonium oxidation states.41,45

2PuO2+ + 4Haq

+ ⇌ Pu4+ + PuO22+ + 2H2O (23)

PuO2+ + Pu4+ ⇌ PuO2

2+ + Pu3+ (24)

PuO2+ + Pu3+ + 4Haq

+ ⇌ 2Pu4+ + 2H2O (25)

It is evident from the above equilibria that what was once a single valency Pu(IV) system

progressively becomes a multi-valency system reflective of (21), containing Pu(III), Pu(IV),

Pu(V), and Pu(VI) in various proportions. All these plutonium oxidation states have been

found to interact with the radiolysis products of aqueous HNO3 to some extent.41,47-50 As

previously highlighted, H2O2 plays a significant role in determining the radiolytic yield of

HNO2 and can also be slowly reduced by Pu(III) and oxidised by Pu(IV) and Pu(VI)

(reactions 26 to 28).47,51,52

Pu3+ + H2O2 → Pu4+ + OH− + OH k26 = 5.5 × 10−2 dm3 mol−1 s−1 (26)

Pu4+ + H2O2 → Pu3+ + HO2 + Haq+k27 = 0.1 dm3 mol−1 s−1 (27)

PuO22+ + H2O2 → PuO2

+ + HO2 + Haq+ k28 = 6.2 × 10−3 dm3 mol−1 s−1 (28)

Consumption of H2O2 by plutonium indirectly promotes the formation of HNO2, in addition

to directly yielding Pu(III), Pu(IV), and Pu(V). The resulting Pu(IV) and Pu(V) can be

expected to slowly undergo disproportionation to ultimately yield Pu(VI) and Pu(III). The

excess Pu(III) may provide some explanation as to why the initial yield of HNO2 (<400 Gy)

in Figure 3, from plutonium α-decay, is as high as the upper limit established by Kazanjian et

10

al. Pu(III) exhibits an autocatalytic process involving nitrogen peroxide (N2O4), a precursor to

HNO2, given by reaction (29).14-19

Pu3+ + N2O4 → Pu4+ + NO2− + NO2 (29)

Pu(III) is oxidised by N2O4 to yield NO2−, NO2, and Pu(IV). NO2

− undergoes protonation to

yield HNO2 (12), NO2 is either consumed by secondary bulk homogenous chemistry or

recycled to yield another molecule of N2O4 (reaction 12), and the Pu(IV) can be expected to

disproportionate to Pu(III) and yield Pu(VI) (reactions 19 and 20), and/or react with H2O2

(reaction 27). Ultimately, this process provides a pathway by which significantly more HNO2

can be formed relative to radiolytic processes alone. Therefore, the measured yields of HNO2

from plutonium α-decay in 0.1 mol dm−3 HNO3 may be explained by the removal of H2O2 by

reaction with Pu(IV), and the autocatalytic reaction between Pu(III) and N2O4. The

subsequent attainment of a steady-state HNO2 yield at higher doses (>400 Gy), is most likely

due to a combination of secondary bulk homogeneous chemistry and potentially the reduction

of Pu(IV) and Pu(VI) by the accumulating HNO2, reactions (30) and (31) respectively.49

Pu4+ + HNO2 ⇌ Pu3+ + NO2 + Haq+ k30 ~ 3.3 × 10−2 dm3 mol−1 s−1 (30)*

PuO22+ + HNO2 ⇌ PuO2

+ + NO2 + Haq+ k31 = 3 × 10−2 dm3 mol−1 s−1 (31)

These latter processes potentially counteract the earlier action of scavenging H2O2, and in

combination with the dynamic plutonium redox equilibria may well provide an explanation

for the unusual radiochemical behaviour of HNO2 in 0.1 mol dm-3 HNO3 plutonium solutions.

However, this unique behaviour is inhibited with increasing acidity and/or HNO3

concentration.

* Extrapolated from Vladimirova, M. V., J. Alloys Comp., 1998, 271, 723-723.

11

Figure 4. Concentration of HNO2 as a function of dose received by 4.0 mol dm−3 HNO3

solutions: gamma radiolysis data from unpublished work ()34, plutonium α-decay (), and americium α-decay (); fitted lines for visual aid only.

Alpha Radiolysis of Concentrated (1.0 – 6.0 M) Nitric Acid. As the concentration of HNO3

increases beyond 0.1 mol dm−3, the yields of HNO2 from plutonium and americium α-decay

begin to converge, demonstrated by the 4.0 mol dm−3 HNO3 data sets given in Figure 4.

Despite differences in radiation quality, the concentration of HNO2 increases almost linearly

with absorbed dose for all three data sets. Both plutonium and americium α-decay yield

comparable amounts of HNO2, which are significantly greater than that from gamma

radiolysis. This is in stark contrast to the behaviour exhibited for the corresponding

0.1 mol dm−3 HNO3 data sets given in Figure 3. As the concentration of HNO3 increases,

Pu(IV) is progressively stabilised through complexation with NO3−.Hence, the redox

chemistry of plutonium is less important at higher acidities with Pu(IV) as essentially the

only stable oxidation state, similar to the situation with Am(III). Consequently, HNO2

generation is then due to the radiolysis induced by plutonium or americium α-decay.

Overall, a combination of LET effects and dynamic redox processes are proposed to provide

a qualitative explanation for the radiochemical behaviour of HNO2 from the alpha radiolysis

of HNO3 solutions, induced by the α-decay of plutonium and americium. However, further

investigations are necessary to provide sufficient insight to support the above postulations,

for example:

12

i. Helium ion accelerator experiments to distinguish between LET effects and redox

chemistry.

ii. Complementary multi-scale modelling with the inclusion of actinide redox chemistry

would provide a more quantitative mechanistic description of the events occurring

within these complex actinide systems.13,35

CONCLUSIONS

The presented research has shown that the radiolytic yield of HNO2, from the alpha radiolysis

of HNO3 solutions by α-decay of plutonium or americium, generally increases with

increasing HNO3 concentration and absorbed dose.

For 0.1 mol dm−3 HNO3 solutions, the radiolytic behaviour of HNO2 appears to be strongly

dependent upon the identity of the radionuclide responsible for inducing α-radiolysis. The α-

radiolysis yield of HNO2 induced by americium α-decay is essentially zero, whereas for

plutonium α-decay the yield is considerably greater than that from americium and gamma

radiolysis. The change in HNO2 behaviour induced by americium α-decay is due to the effect

of LET upon the primary radiolysis products of aqueous HNO3. In particular, the reduction in

available radical precursors (epre−, eaq

−, and H) and the enhancement of H2O2 yields, which

oxidises HNO2 to HNO3. Whereas the enhanced HNO2 yields from plutonium α-decay are a

consequence of interactions between the various oxidation states of plutonium and the

radiolysis products from HNO3, in particular reactions involving H2O2 and N2O4. This

radiochemical behaviour is HNO3 concentration dependent, with differences between

plutonium and americium α-radiolysis decreasing with increasing HNO3 concentration. This

may be interpreted as a combination of α-radiolysis direct effects and acidity influencing

plutonium oxidation state distributions, which in turn affects the radiation chemistry of the

system. However, further investigations are necessary to provide a definitive quantitative

understanding of the chemistry occurring.

Overall, the presented research highlights the complexity of understanding the radiolysis of

the reprocessing solvent systems in the presence of radionuclides, and the importance of

understanding the interactions between redox active elements and primary radiolysis

products. Furthermore, the presence of alpha-emitters provides a means of generating

significant quantities of HNO2 under industrial conditions, and thus reprocessing systems and

HLW storage tanks can be expected to experience the more extreme degradation and

corrosion effects associated with HNO2.

13

AUTHOR INFORMATION

Corresponding Authors:

E-mail: gregory.p.horne@gmail.com.

simon.pimblott@manchester.ac.uk

ACKNOWLEDGEMENTS

This research has been funded though by the Engineering and Physical Sciences Research

Council (EPSRC) (EP/F013809/1, EP/I002855/1 and EP/I034106), the National Nuclear

Laboratory (NNL), the Nuclear Decommissioning Authority (NDA), and the Dalton

Cumbrian Facility, a joint initiative of the NDA and the University of Manchester. G. P.

Horne was supported by a Ph.D. studentship from the EPSRC Nuclear FiRST Doctoral

Training Centre at The University of Manchester (EP/G037140/1).

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14

Article Manuscript for Publication in the Journal of Physical Chemistry B

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