Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces

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990 Charged Particle and Photon Interactions with Matter

H O HNO H NO H O

2 2 2 3 2

+ → + +

+ −

(34.52)

The loss of nitrous acid by reaction with radiolytically produced hydrogen peroxide is the main

source

of loss of nitrous acid in an irradiated system (Bhattacharyya and Veeraraghavan, 1977).

34.3.1.2

the

roduced

pecies

in the o

rganic

hase

Normal and branched-chain alkanes are the typical organic diluents for the ligands used in nuclear

solvent

extraction. The radiolysis of alkanes is represented by (Spinks and Woods, 1990):

CH CH CH e CH CH CH CH CH CH CH H H

sol 23 2 3 3 2 3 3 2 3

( ) ( ) ( )

n n n

i i i

i− +

+ + + + +

(34.53)

Specic yields for the products depend on the specic alkane irradiated. Branch-chain alkanes have

higher product yields, in the range of 0.2–0.6μmol J

−1

for molecular hydrogen and 0.005–0.1μmol

−1

for methane. Bishop and Firestone (1970) reported a yield of H

•

atom of 0.07μmol J

−1

for C

–C

hydrocarbons. The carbon-centered radical products may also be produced indirectly by hydrogen

abstraction reactions with

•

OH or

•

radicals. Regardless of their origin, these carbon-centered

radicals react with solutes by hydrogen atom abstraction or they may undergo radical–radical addi-

tion

to create higher molecular weight products (Dewhurst, 1958):

CH CH CH CH CH CH CH CH CH

2 2 +2 33 2 2 3 2 3 2

( ) ( ) ( )

n n n

i i

+ → (34.54)

As these higher molecular weight products accumulate, they change the physical characteristics of

the solvent, including phase disengagement time, density, and viscosity. Alkane radicals may also

undergo

disproportionation to produce unsaturated products (Dewhurst, 1958):

CH CH CH CH CH CH CH CH CH CH CH CH CH

2 3 13 2 2 3 2 3 2 3 2

( ) ( ) ( ) ( )

n n n n

i i

+ → +

−



(34.55)

Unsaturated products are susceptible to addition reactions by

•

OH radical or N-centered radical

species.

Among the most important of carbon-centered radical reactions is oxygen addition to produce

peroxyl

radicals (Alfassi, 1997):

R O ROO

i i

+ →

(34.56)

Peroxyl radicals will undergo addition reactions to form tetroxides, which then decompose to pro-

duce aldehydes, ketones, and alcohols from the original compound (von Sonntag and Schuchmann,

1997). They are therefore important intermediates in the oxidative mineralization of organic com-

pounds

by radiolysis.

34.3.1.3

the

ixed

hase

Although the reactive species responsible for radiation chemical effects in solvent extraction systems

can be readily identied, additional factors must be considered to provide a quantitative understand-

ing of radiolysis in the biphasic system. Among these are that reactive species yields and reaction

rates vary with solvents, although most are known only from the aqueous phase. Further, the trans-

fer

of produced reactive species across the aqueous/organic phase boundary must be considered.

Muroya

etal. reported initial yields of solvated electrons of 0.4 and 0.2μmol J

−1

for water and

decanol, respectively (Muroya etal., 2008). Although the yield may be lower, reaction rates for elec-

tron attachment in nonpolar organic solutions are much higher than in aqueous solution (Borovkov,

2008). However, in benzene-, or dodecane-in-water microemulsions, Wu etal. (2001) found that

the yield of aqueous electrons was identical to that of pure water. Electrons, being very mobile in

Radiation Chemistry inNuclear Engineering 991

nonpolar solution, apparently crossed the interface to the aqueous phase. This resulted in a constant

yield

of aqueous electrons despite changes in the proportion of water in the emulsion.

contrast, it was determined in the same study (Wu etal., 2001) that water radiolysis was the only

source of

•

OH radical in irradiated emulsion, indicating that radical cations produced in the organic

phase did not cross the interface to oxidize water. The

•

OH yield in the emulsion was thus proportional

only to the water content. The rate constants for several radical reactions, including the

•

OH radical

reaction with benzene, were found to be similar in pure aqueous and microemulsion aqueous solution.

Some of the most important reactive species produced in the aqueous phase, such as

•

OH or

•

radical, must cross the phase boundary prior to reacting with ligand molecules or their dilu-

ents. This should be relatively easy for neutral species and in fact the solvent extraction system is

designed to move neutral species across the interface by providing intimate phase mixing using

pulsed columns, mixer settlers, or centrifugal contactors. For these emulsions, it may be justiable

to neglect a phase transfer diffusion gradient and to assume uniform radiolysis. However, in reality,

little information is available on the mass transfer rates of these species, and diffusion controlled

regimes may dominate when dose rates are very high or with thick phase layers due to inadequate

mixing (Macášek and Čech, 1984). In practice, the assumption is generally made that reactive spe-

cies created in either phase are available for reaction during phase mixing in solvent extraction. The

investigation

of biphasic radiolysis reactions is an area in need of more detailed investigation.

34.3.2 purex proceSS radiation cheMiStry

34.3.2.1 tbp radiolysis

The PUREX process for the extraction of the major actinides consists of 30% TBP in alkane dilu-

ent. The process can either partition uranium separately or co-extract uranium, neptunium, and/or

plutonium depending on how the valence states of the latter metals are set prior to extraction. The

metal-loaded solvent is then stripped with a mildly acidic aqueous phase and recycled (Schultz and

Navratil, 1984). However, its recycle potential is limited by the radiolytic degradation of TBP and its

diluent. It has long been recognized that the major products of TBP radiolysis are hydrogen, meth-

ane, and dibutylphosphoric acid (HDBP), with monobutylphosphoric acid (H

MBP) and phosphoric

acid produced in lesser amounts. The radiation chemistry of TBP was recently reviewed and the

following

discussion is abbreviated from that source (Mincher etal., 2009a,b,c).

The

accumulation of radiolytic degradation products in the PUREX solvent results in decreased

extraction performance (Lane, 1963; Neace, 1983; Stieglitz and Becker, 1985). The acidic radi-

olysis products are complexing agents that interfere with uranium and plutonium stripping and

ssion product separation factors (Davis, 1984; Tripathi et al., 1999; Tripathi and Ramanujam,

2003). Interfacial crud formation and poor phase separation have been attributed to the formation

of precipitable complexes of zirconium with H

MBP and phosphoric acid (Rochoñ, 1980; Stieglitz

and Becker, 1985; Miyake etal., 1990; Sugai and Munakata, 1992; Egorov etal., 2002, 2005). The

adverse affects of the buildup of these acidic phosphate products in the organic phase are mitigated

during process extractions by solvent washing with aqueous Na

(Blake etal., 1963; Reif, 1988).

However, with continued recycling, washing becomes less effective and the washed solvent shows

increased retention of Pu, Zr, and Ru and increased solution viscosity (Tripathi etal., 2001a). This

has been attributed to the accumulation of higher molecular weight radiolysis products with high

organic phase solubility (Wagner and Towle, 1958; Stieglitz and Becker, 1985). The result is a per-

manently degraded and radioactively contaminated solvent, which is expensive to dispose.

Several mechanisms have been proposed to explain the production of HDBP in irradiated TBP

solutions. Zaitsev and Khaikin (1994) reported that dissociative electron capture resulted in the

production

of the butyl radical and HDBP in irradiated neat TBP:

e C H O PO C H C H O OPO

sol

− −

+ → +( ) ( )

4 9 3 4 9 4 9 2

(34.57)

992 Charged Particle and Photon Interactions with Matter

Jin etal. (1999) attributed the formation of HDBP under these conditions to a combination of dis-

sociative electron capture and decay of excited TBP molecules, while Haase etal. (1973) reported

that electron attachment could also result in free hydrogen atoms and a TBP carbon-centered radi-

cal.

However, the electron-initiated reactions are unlikely to be of consequence in the acidic mixed

phase due to the fast reactions shown in Equations 34.36 through 34.38, and the direct excitation of

TBP becomes less important when TBP is dissolved in a diluent. These may not be important reac-

tions

in the solvent extraction process.

Burr (1958) proposed that HDBP was formed by the decay of the TBP carbon-centered radical:

( ) ( )C H O ( C H O)PO C H C H O OPO

8 8

4 9 2 4 4 4 9 2

→ +

−

(34.58)

This TBP radical could be produced by hydrogen atom abstraction (Burr, 1958) by reaction with

either

the radiolytically produced

•

H atom or

•

OH radical:

( ( ) ( )

HO) H C H O ( C H O)PO H O H

+ → →TBP

4 9 2 4 2 2

M s M i n c h er et al.,= ×

− −

5 0 10 2008

9 1 1

. ( )

(34.59)

M s Mincher et al.,= ×

− −

1 8 10 2008

8 1 1

. ( )

Besides direct decay to HDBP, the TBP radical could also undergo hydrolysis to again produce

HDBP

(von Sonntag etal., 1972):

( ) ( )C H O ( C H O )PO H O C H OH C H O POO H

8 84 9 2 4 2 4 4 9 2

i i

+ → + +

− +

(34.60)

Khaikin (1998) suggested that HDBP was also the stable product of dissolved oxygen addition to the

TBP

radical, which gives the TBP peroxyl radical:

( ) ( )C H O ( C H O)PO O C H O) P(O) O (C H OO

8 9 2 44 9 2 4 2 4 8

i i

+ → – – (34.61)

Superoxide elimination followed by hydrolysis produces HDBP and butyraldehyde, also a measured

product

(Clay and Witort, 1974).

( ) ( )C H O P(O) O (C H )OO O C H O) P(O) O (C H

9 2 44 9 2 4 4 8

– – – –

i i

→ +

− +

(34.62)

( ) (C H O P(O) O (C H ) H O C H O) P(O) O C H CHO H

9 24 9 2 4 2 4 3 7

2– – –+ → + +

− +

(34.63)

Wilkinson and Williams (1961) proposed yet another mechanism for HDBP formation based upon

direct

TBP radiolysis:

( ) (C H O PO e C H O) PO

sol 34 9 3 4 9

− +

(34.64)

( ) (C H O PO C H O) P(OH)OH CH CHCH CH

24 9 3 4 9 2 3

i i+ +

→ + 



(34.65)

( ) (C H O P(OH)OH C H O) POOH H

24 9 2 4 9

+ +

→ +

(34.66)

Radiation Chemistry inNuclear Engineering 993

Intramolecular hydrogen bonding between the phosporyl oxygen and a butoxy hydrogen atom

of the radical cation formed in Equation 34.64 forms a ring structure, which decays as shown

in Equations 34.65 and 34.66. This mechanism, although dependent on direct TBP radiolysis

cannot be entirely discounted since TBP is used at a rather high concentration of 30% in the

PUREX process. Thereactions described above all lead to the production of HDBP, and occur

competitively. Continued irradiation produces H

MBP and phosphoric acid from HDBP via

analogous reactions.

Among the less abundant but still important radiolysis products are the higher molecular weight

acid phosphates. These species with varying alkane chain lengths suggest that radical addition

reactions occur between TBP, HDBP, and alkane solvent radicals, including the production of

TBP dimers (Rochon´, 1980; Adamov etal., 1990). These higher molecular weight compounds are

among those species that are not adequately removed from irradiated solvent by aqueous carbonate

washing.

Additional products of TBP radiolysis in the presence of HNO

are nitrated phosphates, which

also impede stripping efciency. He etal. (2004) proposed that

•

reacts with TBP by hydrogen

atom

abstraction, producing the TBP radical

i i

NO TBP C H O C H O PO HNO

M s M i n c h er et a

3 4 9 2 4 8 3

6 1 1

4 3 10

+ → +

= ×

− −

( ) ( )

. (k ll., 2008)

(34.67)

The TBP radical was then postulated to undergo reaction with additional

•

to produce nitrated

TBP

(He etal., 2004):

i i

NO C H O C H O PO C H O OC H NO PO

33 4 9 2 4 8 4 9 2 4 8

+ →( ) ( ) ( ) ( ) (34.68)

The

•

radical might be expected to add in the same way:

i i

NO C H O C H O PO C H O OC H NO PO

22 4 9 2 4 8 4 9 2 4 8

+ →( ) ( ) ( ) ( ) (34.69)

Methylated, hydroxylated, and nitrated phosphates, resulting from radical addition reactions of

methyl radical, hydroxyl radical, and the nitro-radicals shown above, have been identied in post-

irradiation TBP solutions by numerous investigators (Nowak, 1977; Adamov etal., 1987; Lesage

etal., 1997; Tripathi etal., 2001b). It can be seen that radical addition reactions can create a wide

range

of products, sometimes of high molecular weight.

34.3.2.2

purex diluent

egradation

The TBP alkane diluent undergoes similar radiolytic degradation to generate metal complexing

agents that do not wash out in solvent treatment with alkaline solutions (Lane, 1963). The decompo-

sition of diluents in TBP solvent extraction was reviewed by Tahraoui and Morris (1995). Alkanes

undergo radiolytic nitration of their carbon-centered radicals, in analogy with Equations 34.68

and 34.69 above. Stieglitz and Becker (1985) and Tripathi et al. (2001a) identied both nitro-,

and nitrosoalkanes (RNO

and RONO

) in alkanes irradiated in the presence of nitric acid. These

nitroparafns and their hydroxamic acid reaction products have been implicated in ssion product

complexation in the PUREX process. Nitroparafns are thought to be converted to the complexing

enol form by contact with the alkaline scrub intended to remove the acidic products of TBP decom-

position

(Blake etal., 1963):

RCH N==O O RCH==NO O

( ) ( )↔

−

(34.70)

994 Charged Particle and Photon Interactions with Matter

Hydroxamic acids (Lane, 1963; Egorov etal., 2002) are metal complexing and reducing agents

formed

from nitroparafns by the Victor Meyer reaction:

RCH NO RCONHOH

→ (34.71)

Although hydroxamic acids rapidly hydrolyze in acidic media, small but steady-state organic phase

concentrations have been identied in irradiated solvent (Ohwada 1968; Zaitsev etal., 1987; Huang

etal., 1989). Additional diluent radiolysis products identied in irradiated TBP–dodecane–nitric

acid include alkane oligomers, aliphatic ketones, and acids (Becker etal., 1983). Thus, stripping

difculties and poor separation factors result from a combination of higher molecular weight acidic

phosphates

from TBP radiolysis and from compounds produced by diluent nitration.

Finally,

it should be noted that the use of more stable TBP diluents results in higher yields of

HDBP. This has been attributed to the diluent ionization potential (Tahraoui and Morris, 1995).

Direct diluent radiolysis produces the diluent radical cation, as shown in Equation 34.53. A common

radical

cation stabilization route would be that of charge transfer by reaction with TBP:

[ ( ) ] ( )CH CH CH TBP CH CH CH TBP

3 2 3 3 2 3

i i+ +

+ → +

(34.72)

Thus, the use of very stable diluents with high ionization potential may increase the rate of TBP

degradation. It is not possible to completely eliminate adverse radiation chemical effects in an irra-

diated

solvent extraction system.

34.3.3 radiation cheMiStry in the future fuel cycle

In addition to uranium recovery by the PUREX process, future fuel cycle designs include solvent

extraction steps for the recovery of americium and curium, the so-called minor actinides. The sepa-

ration of the trivalent actinides from the trivalent lanthanides has long been one of the signicant

challenges in radiochemistry. The organophosphorous compound octylphenyldiisobutylcarbamoyl-

methylphosphine oxide (CMPO) has been proposed for the group separation of the trivalent actinides

and lanthanides in the United States (Kalina et al., 1981), whereas in European work tetraalkyl-

diamides have been proposed for the co-extraction of the actinides and lanthanides. The DIAMEX

process uses N,N′-dimethyl-N,N′-dioctyl-hexylethoxy-malonamide (DMDOHEMA) in an alkane

diluent for this purpose (Sorel etal., 2008). Similarly, the tridentate amide N,N,N′,N′-tetraoctyl-3-

diglycolamide (TODGA) has been developed primarily in Japanese work. A 0.1M dodecane solution

of TODGA is capable of extracting U and Pu, as well as Am from nitric acid solution (Sasaki etal.,

2008). Dialkylmonoamides, such as N,N-di-(2-ethylhexyl)isobutyramide (DEHiBA) have also been

developed as suggested alternatives for TBP in U and Pu extraction (Miguirditchian etal., 2008). The

potential advantages and uses of amides as actinide extractants have been reviewed by Gasparini and

Grossi (1986). While the radiation chemistry of these newer solvent extraction processes has not been

characterized as well as for TBP, a picture of their behavior is beginning to emerge.

34.3.3.1 amide

and d

iamide

adiolysis

The general structures of dialkylmonoamides and tetralkyldiamides are shown in Figure 34.21. The

effects of radiolysis on the solvent extraction behavior of these compounds have been investigated

by numerous researchers. For example, Ruikar etal. (1993) reported increasing D

and decreasing

over the absorbed dose range 0–1800kGy for dialkylamides irradiated in dodecane solution

after preequilibration with 3.6M nitric acid. The decrease in amide concentration and ingrowth of

the corresponding acids and amines were monitored by IR spectroscopy (Ruikar etal., 1991). The

decrease in amide concentration satisfactorily explained the decrease in uranium extraction ef-

ciency while increases in D

were postulated to result from the participation of acidic degradation

products in plutonium complexation. The distribution ratio for ssion product zirconium, D

, was also

Radiation Chemistry inNuclear Engineering 995

elevated by irradiation. Increased distribution ratios for zirconium and plutonium may also be due

to the nitrated products of diluent radiolysis, as has been reported for the PUREX solvent (Mincher

etal., 2009a,b,c). The irradiation of 0.5 M of 11 different dialkylamides in benzene solution after

pre-equilibration with 3.6M nitric acid also resulted in degradation to the corresponding amines

and carboxylic acids, with symmetrically substituted dialkylamides being more stable (Mowafy,

2004). In this work also, uranium extraction efciency decreased with increasing absorbed dose,

while

increased.

Mowafy (2007) concluded that within a class of compounds, stability decreased in the order:

symmetrical amides > unsymmetrical amides > branched amides. The pentaalkylpropane

diamides (malonamides) have the general structure (RR′NCO)

CHR″, an example of which is

shown in Figure 34.22. For the malonamide with R = CH

, R′ = C

and R″ = C

more than 30% was decomposed at an absorbed dose of 500 kGy for a 1M t-butylbenzene solution

irradiated in contact with 5 M nitric acid. There was a corresponding decrease in D

, although

and D

were not adversely affected. Cuillerdier etal. (1991) concluded that stability as a func-

tion of R″ increased in the order: H < C

< C

. Long oxyalky

chains appear to protect extraction efciency. Inclusion of a sacricial ether linkage in the mol-

ecule may result in the generation of relatively harmless radiolysis products while maintaining

diamide extraction capability. The radiolytic decomposition of a malonamide to the correspond-

ing acid and amide is shown in Figure 34.23. A review of amide radiolysis is provided in reference

(Mincher etal., 2009c).

34.3.3.2 diamex

and todga p

rocess

adiolysis

The DIAMEX solvent extraction process for recovery of

the minor actinides was developed using DMDOHEMA,

shown in Figure 34.22. The gamma-radiolysis of this com-

pound in alkane solution has been investigated by Berthon

et al. (2001) using a suite of analytical techniques includ-

ing gas chromatography, potentiometry, and mass spec-

trometry. Organic acids were a major diamide radiolysis

product, and alcohols were also created by rupture of the

ether linkages in the R″ group. No alcohols were detected in

compoundswithout an oxygen atom in that chain. Products

R΄

N OH

R΄

N N

R΄

Figure 34.23 Radiolytic decomposition of a malonamide into the corresponding amine and carboxylic acid.

R΄

Figure 34.21 The general structure of dialkylmonamides (left), and tetraalkyldiamides (right).

O O

Figure 34.22 The pentaalk-

ylpropane diamide (malonamide)

dimethyl dioctyl hexylethoxymalon-

amide

(DMDOHEMA).

996 Charged Particle and Photon Interactions with Matter

detected byBerthon etal. (2001, 2004) for DMDOHEMA radiolysis included the monoamide

methyloctylhexyloxybutanamide, the acid amide methoxyoctylcarbamoyl 4-hexyloxybutanoic

acid, the malonamides dimethyloctyl 2-hexyloxyethyl malonamide (DMOHEMA), and methyl-

dioctyl 2-hexyloxyethyl malonamide (MDOHEMA) and the amine methyloctylamine. The loss

of one amide function occurs by rupture of the carbamoyl C–N bond resulting in the amine and

an acidic amide product (Berthon etal., 2001, 2004). This occurs only in the presence of acid,

and is promoted by higher acid concentrations. Although signicant products, monoamides

are rapidly degraded in irradiated solution and the concentrations detected there are low, or

sometimes undetectable.

For DMDOHEMA, with its eight carbon R′ group, the radiolysis products are soluble in the

organic phase. A dose of 690 kGy to 1M DMDOHEMA in dodecane in contact with 4 M nitric acid

resulted in a nal malonamide concentration of 0.59M. The decrease in malonamide concentration

was accompanied by a decrease in D

, D

, and D

. When solutions containing varying amounts

of the major degradation products were prepared, all products were found to interfere with Am

and Nd extraction, with the amine being the most harmful. An acidic solvent wash quantitatively

removed the amine degradation product while an alkaline wash removed about 80% of the acidic

amide (Nicol etal., 2000). The combination of an acid wash, followed by water scrubbing and then

an alkaline wash restored the solvent’s extraction capabilities to that expected for the decreased

malonamide

concentration (Berthon etal., 2004; Bisel etal., 2007).

Modolo

etal. (2008) investigated the post-irradiation solvent extraction performance of 0.2 M

TODGA in hydrocarbon diluent. The D

gradually decreased with absorbed dose for TODGA

and TODGA/TBP mixtures irradiated in the presence and absence of nitric acid, while the D

remained unchanged over the absorbed dose range 0–1000 kGy. The products of TODGA radi-

olysis include dioctylamine, dioctylacetamide, dioctylglycolamide, and dioctylformamide (Sugo

etal., 2002, 2007). The presence of nitric acid did not enhance radiolytic degradation but did

favor cleavage of the C–N bond to form the amine and acidic products over ether bond cleavage

to form acetamide and glycolamide. These results are analogous to those described above for

diamide radiolysis.

34.3.3.3 truex process

adiolysis

The compound octylphenyldiisobutylcarbamoylmethylphosphine oxide (CMPO), used in combina-

tion with TBP in an alkane diluent in the TRUEX (TRansUranic EXtraction) process, is a phos-

phorous-containing amide, and is shown in Figure 34.24. The irradiation of this solvent system in

contact with an acidic aqueous phase results in decreased forward extraction distribution ratios for

Am (D

), but not to the extent predicted by loss in CMPO concentration (Chiarizia and Horwitz,

1986). Stripping distribution ratios exceeded those of the forward extraction following irradiation to

an absorbed gamma-dose of 195kGy. Similar effects were encountered for only 36kGy delivered

from an alpha-source (Buchholz etal., 1996). Cleavage of the C–N bond was proposed to result in

decomposition of CMPO to a carboxylic acid and an amine, followed by decarboxylation of the car-

boxylic acid to a phosphine oxide, in analogy with the reactions for DMDOHEMA discussed above.

Oxidation of the phosphine oxide would produce a phosphinic

acid. Products of this nature have been measured (Chiarizia and

Horwitz, 1986; Nash etal., 1988, 1989; Buchholz etal., 1996).

The occurrence of degradation product phosphine oxides may

explain the reasonably high forward D

despite a loss in CMPO

concentration, while the phosphinic acids would complex Am

to prevent adequate stripping under low aqueous phase acidity

conditions. The kinetic constants for the reactions of the major

radicals produced by diluent radiolysis have not been measured

for CMPO, TODGA, or DMDOHEMA.

Figure 34.24 The structure of

octylphenyldiisobutylcarbamoyl-

methylphosphine

oxide (CMPO).

Radiation Chemistry inNuclear Engineering 997

34.3.4 radiation cheMiStry and actinide oxidation StateS

34.3.4.1 radiolytic production of nitrous acid

The radiolytic production of nitrous acid in irradiated nitric acid has consequences for nuclear solvent

extraction because of its affect on metal valence. Probably, the most important actinide affected is Np.

Highly extractable by the PUREX process in the tetra-, (Np

) and hexavalent (NpO

) states, Np is

inextractable in the pentavalent (NpO

) state. Attempts to co-extract Np with U and Pu, or to prevent

Np extraction during the PUREX process are often confounded by changes in Np valency during the

extraction. The oxidation state of neptunium in aqueous nitric acid depends on the concentration of

nitrous acid according to Equation 34.73 (Siddall and Dukes, 1959):

NpO / H / NO NpO / HNO H O

22 3 2

3 2 1 2 1 2

+ + − +

+ + ↔ + + (34.73)

Neptunium(V) is stable in the absence of nitrous acid, however, it is oxidized to NpO

with small

amounts of added nitrous acid, at increasing rates with increasing nitrous acid concentration. For

example, 5 × 10

−5

M HNO

was found to oxidize neptunium with a half-time of 22min in 3M HNO

(Siddall and Dukes, 1959). However, very high concentrations will shift the equilibrium of Equation

34.73

back to the left, favoring NpO

The direct radiolytic production of nitrous acid was shown in Equations 34.47 and 34.48. Indirect

sources were shown in Equations 34.49 through 34.51. The reaction of the hydroxyl radical with

•

radical, shown in Equation 34.74 is an additional source (Matthews etal., 1972):

i i

OH NO HNO O HNO

+ → → +

3 4 2

(34.74)

Sources of the

•

radical include the reaction of nitrate anion with the hydrogen atom in Equation

34.75 (Buxton etal., 1988), the hydrolysis of the

•

radical, shown in Equation 34.76 (Vladimirova

and Milovanova, 1972), and the

•

radical addition reaction shown in Equation 34.77 (Matthews

etal.,

1972):

NO H NO OH M s

3 2

7 1 1

1 10

− − − −

+ → + = ×

i i

(34.75)

i i

NO H O NO H O

3 2

+ → +

2 2 2

(34.76)

NO NO N O O NO

3 3 2

+ → → +

6 2 2

(34.77)

Production of the

•

radical by any of the above routes is followed by the addition reaction to

produce N

, ultimately resulting in nitrous acid (Sworski etal., 1968; Grätzel etal., 1969): This

was

shown in Equations 34.50 and 34.51.

Radiolytic

reactions also deplete the nitrous acid concentration in irradiated nitric acid.

The hydroxyl radical acts as a sink for nitrous acid produced during radiolysis (Bugaenko and

Roshchektaev,

1971):

i i

OH HNO H O NO M s

+ → + = ×

− −

2 2

9 1 1

2 6 10k .

(34.78)

Hydroxyl radical addition reactions also oxidize

•

radical to nitrate anion in Equation 34.79

(Vione etal., 2003), and add to generate hydrogen peroxide in Equation 34.80 (Buxton etal., 1988):

OH NO HOONO H NO M s

+ → → + = ×

+ − − −

9 1 1

4 5 10k . (34.79)

998 Charged Particle and Photon Interactions with Matter

OH OH H O M s

2 2

+ → = ×

− −

k 5 5 10

9 1 1

(34.80)

The reaction of hydrogen peroxide with nitrous acid oxidizes nitrous acid back to nitric acid, as was

shown in Equation 34.52 (Bhattacharyya and Veeraraghavan, 1977; Vione etal., 2003). Similarly,

•

radical can oxidize nitrous acid (Bugaenko and Roshchektaev, 1971):

HNO NO NO H NO

2 3 3 2

+ → + +

− +

i i

(34.81)

Maximum achievable concentrations of nitrous acid are limited by these oxidation reactions, and by the

reversible decomposition to simple oxides of nitrogen, as shown in Equation 34.82 (Park and Lee, 1988):

2HNO NO NO H O

2 2 2

↔ + +

(34.82)

Park and Lee (1988) modeled that nitrous acid decomposition by the route in Equation 34.82 becomes

signicant at concentrations >10

−5

M. Additional loss of nitrous acid in open systems occurs through

the volatilization of

•

NO and

•

, which have limited nitric acid solubility (Andreichuk etal.,

1984). Andreichuk etal. (1984) found that the loss of nitrous acid was reduced by limiting the vol-

ume of air in contact with the system. Thus, a complicated suite of reactions compete to produce

and remove nitrous acid in irradiated aqueous nitric acid. The ability to model and predict its con-

centration under various solution conditions is limited by the lack of the fundamental bimolecular

rate constants for many of these reactions.

However,

empirical nitrous acid radiolysis yields, G

(HNO

)

, have been measured by several authors.

The yield is affected by total absorbed dose, dose rate, radiation LET, and nitric acid concentration.

Bhattacharyya and Saini (1973) reported that G

(HNO

)

varied linearly with absorbed gamma-dose using

Co source with a dose rate of 3kGy h

−1

. This resulted in a value of ∼0.24μmol J

−1

in 3M HNO

When G

(HNO2)

was evaluated versus nitric acid concentration, it was found to increase with increasing

concentration, as shown in Figure 34.25. The yield was found to be the same in air-free and air-saturated

solution. These data are considered maximum yields, as the produced nitrous acid was reacted in situ

with the colorimetric reagent sulfanilamide for quantitation (Bhattacharyya and Saini, 1973). This

eliminated competition from oxidizing agents that might limit the HNO

concentration and, therefore,

the true yield will be lower due to the reactions shown in Equations 34.52, 34.78, and 34.81.

0.25

0.3

0.2

0.1

0.15

HNO

(μmol J

–1

)

0.05

0 2 4 6 8 10 12

Nitric acid concentration (M)

Figure 34.25 Nitrous acid yield in gamma-irradiated nitric acid. Curve a is data of Bhattacharyya and

Saini (1973) (dose rate = 3kGy h

−1

)in the presence of sulfanilamide. Curve b is data of Vladimirova etal.

(1969)

(Dose rate = 6

kGy

−1

) without scavenger.

Radiation Chemistry inNuclear Engineering 999

Also shown in Figure 34.25 are the data of Vladimirova etal. (1969), collected at a dose rate of

6 kGy h

−1

. They irradiated nitric acid in the absence of sulfanilamide, and the actual G

(HNO

)

was ∼0.06μmol J

−1

, in 3M HNO

, about one-fourth of the value reported by Bhattacharyya and

Saini (1973). The relationship between the yield of nitrous acid and the nitric acid concentration was

linear. Bugaenko and Roshchektaev (1971) reported a linear relationship between total absorbed

dose and nitrite ion concentration. They reported a G

−

of 0.07μmol J

−1

for 3M HNO

at a dose

rate of 36kGy h

−1

. Similar results were reported by Jiang etal. (1994) also using gamma-irradia-

tion. Production of nitrous acid was proportional to absorbed dose over the investigated range of

250–750Gy, and to nitric acid concentration in the range 2–8 M. The maximum measured value

for G

(HNO

)

at a dose rate of 84 Gy h

−1

was 0.1 μmol J

−1

in 8M nitric acid. These authors calculated

a maximum possible yield of 0.32μmol J

−1

. As the concentration of nitric acid increases, the direct

radiolysis of nitric acid becomes more important, resulting in the production of additional nitrous

acid. The nitrous acid yield may be increased in the presence of

•

OH radical scavengers (Vladimirova

and Milovanova, 1972), which would suppress the reactions shown in Equations 34.52 and 34.78

through 34.80. Such scavengers would include the organic diluents and complexing agents used in

solvent extraction. Using the data of Vladimirova etal. (1969), it may be calculated that an absorbed

dose

of 10

kGy

would generate about 7 × 10

−4

M HNO

in 3M HNO

High

LET alpha-radiolysis is of obvious concern in neptunium solvent extraction, and a number

of studies have been performed using

244

Cm irradiation. Bibler (1974) concluded that the direct

radiolysis of nitric acid proceeded in the same fashion for alpha-radiolysis as for gamma-radiolysis,

resulting in the same oxygen (and therefore presumably the same nitrous acid according to Equation

34.47) yield. However, Vladimirova and Milovanova (1972) reported that the production of HNO

was greater for alpha-radiolysis. It depended positively on nitrate ion concentration and dose rate,

but the yield decreased with increasing absorbed dose. The increase in hydrogen peroxide con-

centration with absorbed dose, for which the yield is greater for alpha-radiolysis (Equation 34.35),

probably explains the decrease in nitrous acid generation. Andreichuk etal. (1984) reported similar

results. Nitrous acid concentrations increased quickly with absorbed dose, but then leveled out to

reach an equilibrium concentration that was positively dependent on the nitric acid concentration

and the dose rate. Taking into account the higher hydrogen peroxide yield of high LET radiation,

they calculated equilibrium concentrations of nitrous acid under various conditions. At a dose rate

of 11.5 kGy h

−1

, for example, this varied from 1 × 10

−3

M HNO

in 1M HNO

to 8.7 × 10

−3

M HNO

in 7.5 M HNO

. A value of 6.8 × 10

−3

M HNO

was reported for 3.7 M HNO

. According to Siddall

and Dukes (1959), this concentration of nitrous acid is sufcient to oxidize Np(V) to Np(VI) with a

half-time

of only 6

min

in 4.0

HNO

34.3.4.2 actinide

eactions

with Free r

adicals

The reaction of neptunium with the primary oxidizing and reducing agents generated by water and

nitric acid radiolysis may also alter neptunium oxidation states. The fundamental bimolecular rate

constants for many neptunium (and the other actinide) reactions with radiolytically produced radicals

have been measured (Table 34.8). Using Np as the example, those of interest in irradiated aqueous

nitric

acid are shown in Table 34.8 with a comprehensive collection provided by Mincher and Mezyk

(2009). It can be seen that the aqueous electron is a powerful reducing agent with fast kinetics for

reaction with neptunium cations. However, in the strongly acidic solutions used in solvent extraction,

the reactions of the aqueous electron may be discounted due to its fast reactions shown in Equations

34.34. The produced hydrogen atom is less reactive, but is likely to be a reducing agent for neptunium.

However, its ability to react with neptunium ions in solution will be limited by competition from

oxygen to produce the less reactive hydroperoxyl radical, as shown in Equation 34.39. The hydrogen

atom will also be scavenged by other dissolved metal ions such as uranium, which may be present at

higher concentrations. Hydroxyl radical oxidizes Np

and NpO

. However, its affects on the solvent

extraction system will depend on the extent to which it is scavenged by organic compounds, for which