Nuclear Site Remediation - ACS Publications - American Chemical

Chapter 21

Radiation and Chemistry in Nuclear Waste: The NO System and Organic Aging Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: November 29, 2000 | doi: 10.1021/bk-2001-0778.ch021

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1

2

Dan Meisel , Donald M . Camaioni , and Thom M . Orlando

3

1

Notre Dame Radiation Laboratory, and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556 Energy and Health Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352

2

3

We describe results that advance the understanding o f radiation effects in high level waste (HLW) stored at D O E sites. The scientific issues on which we focus include: a) reactions o f primary radicals (e , O H , and H) o f water radiolysis with NO3-/ NO2- b) redox chemistry ofNO radicals and ions, c) degradation mechanisms and kinetics of organic components of H L W , and d) interfacial radiolysis effects in aqueous suspensions and at crystalline N a N O interfaces. Understanding these effects and the chemical reactions they induce have contributed to resolving safety issues and setting waste management guidelines at Hanford. -

x

3

Several safety issues concerning stored high level waste ( H L W ) at the Hanford Site have been identified prior to the start of the E M S P . The aging of organics in the waste had been shown to lead to several safety concerns such as flammable and noxious gases generation ( H , N 0 , N H volatile organic chemicals). Possible runaway reactions of organics (largely chelates) mixed with 2

342

2

3 }

© 2001 American Chemical Society In Nuclear Site Remediation; Eller, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

343

nitrate salts or organic liquids pooled on the surface of the waste were another significant concern in H L W tanks. We embarked on a coordinated effort to understand the processes that are initiated by radiation and lead to aging of H L W . Our main objective in this project is to assist the safety programs at Hanford and other D O E sites in resolving outstanding safety issues. H L W invariably contain high concentrations, often above saturation levels, of nitrate and nitrite salts. Because of the high efficiency of scavenging of the primary radicals from water radiolysis by N 0 7 N 0 " , a majority of the radiolytically generated radicals are of the N O family, reactions 1-4 (7, 2).

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3

2

x

1 0

O H + N 0 " - » N 0 + OH~ ki = 1 . 0 x l 0 M " V 2

(1)

2

and at high p H 8

1

1

O" + N 0 " (+H 0) - » N 0 + O H " kia = 3.1xl0 M " s" 2

2

2

2

9

1

1

e " + N 0 " - » N 0 " k = 9.7xl0 M" s" aq

3

3

(2)

2

2

4

1

1

N 0 " (+ H 0 ) - » N 0 + 2 0 H " k = 5.5xl0 M " s" 3

2

2

(la)

3

(3)

At high nitrite concentrations, H is primarily converted to N O , reaction 4. s

1

1

H + N 0 " -> N O + O H " k4 = 7.1x10 M" s" 2

(4)

2

The initial reduction product of nitrate, N 0 " , is expected to be a strong reductant, but N 0 and N O are oxidants ( E ° ( N 0 / N 0 " ) = 1.04 V ) (3). Therefore, the early events that follow the absorption of radiation, and the subsequent redox chemistry of N O radicals and ions will determine the redox environment in H L W . Thus, they are highly relevant to understanding radiolytic aging of wastes. 3

2

2

2

x

Results and Discussion In this report we focus on the following processes; a) reactions of primary radicals (e", OH, and H) of water radiolysis with N 0 7 N0 ~, b) redox chemistry of N O radicals and ions, c) degradation reactions of organic components, and d) interfacial radiolysis effects in aqueous suspensions and at crystalline N a N 0 interfaces. We used pulse radiolysis in conjunction with time resolved spectrophotometric, EPR, and conductivity detection techniques. Continuous γ radiolysis and N 0 gas contacting of H L W simulants, with subsequent product analyses by ion chromatography and N M R techniques. Computation of the structure and energies of intermediates using ab initio theory and solvation models was used in support of the experimental effort. Ultra-high vacuum 3

2

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In Nuclear Site Remediation; Eller, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

344

surface analysis techniques were applied to measure electron beam interfacial and bulk damage mechanisms of N a N 0 crystallites. In a separate chapter of this volume we detail our studies on the effects of interfaces of oxide powders and colloidal suspensions in H L W radiolysis. 3

2

Formation, Reactions and Properties N 0 " Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: November 29, 2000 | doi: 10.1021/bk-2001-0778.ch021

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Reaction 3 is a bottleneck in the transformation from a reducing environment to an oxidizing one. To our knowledge, e ~ is the only radical that reduces nitrate ion. Mezyk and Bartels showed that H atoms do react with N 0 ~ , but they concluded that the product is directly N 0 (4). As long as the N 0 radical exists in solution, the potential for the generation of H from water cannot be excluded. Therefore, we studied the relevant chemistry of N0 ~. The absorption spectrum of N 0 " peaks at X =260 nm with ε=2.0χ10 M " cm" . It decays within a few με to generate a weakly absorbing species X =390 nm attributed to N 0 . A small fraction of N 0 dimerized on the same time window to N 0 (5), which leads to a band at X =270 nm. The charge of the initial product was verified to be -2 by measuring the ionic strength effect (controlled by varying the concentratiçn of N a N 0 ) on the rate of electron transfer from N 0 " to methyl viologen ( M V ), reaction 5. aq

3

2 _

2

3

2

2

3

2

3

3

1

max

1

max

2

2

2

4

max

3

3

2-

N0

3

2+

+ MV

+

-»N0 - + MV

(5)

3

A least squares fit to the Bronsted-Bj errum modified Debye-Huckel equation, Eq. 6, yields: the rate constant at zero ionic strength, k =(1.3±0.1)xl0 M " s" , and an effective reaction distance d=2.48±0.2 Â. The product of the charges, Z Z = -4.2±0.2, confirms the proposition that the radical is indeed N 0 " . 10

1

1

0

2

a

b

3

k

1Μ*Ζ Ζ [μ α

όΛ

(6)

Reactions of the Precursor of Hydrated Electrons The nitrate ion is one of the most efficient scavengers for the precursor to the hydrated electron e"*. The concentration at which only 37% of the initially produced thermal electrons escape scavenging, is C 7 = 0.45 M (6, 7). 3

e

~th

+ N

° -->P 3

(2a)

Because of the low C 7 and because the concentration of nitrate in H L W may be extremely high (1-5 M), the identity of the product, Ρ in reaction 2a, is 3


345

+

of considerable significance. To test that question we measured the yield of M V at increasing [NaN0 ] and at several doses per pulse. Up to 30% decrease in the yield of M V was observed at 5 M of N a N 0 . Some of the radiation is absorbed directly by the nitrate salt at such high concentrations. This "direct effect" produces directly N 0 , and perhaps N O , but not the reducing N 0 " (8). Thus, it may be concluded that the thermalized precursor to e" , labeled e"h in reaction 2a, reduces nitrate to N 0 \ 3

+

3

2

2

3

aq

t

2


3

Acid-Base Chemistry of NO3 ~ 2

The lifetime of N 0 " was determined across a broad p H range by following its characteristic absorption at 270 nm. The lifetime decreases with p H and with total buffer concentration as previously reported. This trend has been ascribed to acid-base equilibria of reactions 7 and 8 with the protonated forms assumed to have shorter lifetimes than N 0 " (7, 9). 3

2

3

H N0 2

^

3

H + H N O 3 - (pK = 4.8) +

(7)

al

+

HN0 "

2

H + N 0 " (pK = 7.5)

3

3

(8)

a2

2

N 0 ' + H A H N 0 " + A "

(9)

H N O 3 " -> N 0 + O H " (k = 2 . 3 x l 0 V )

(10)

3

3

2

10

s

1

H N 0 - » H 0 + N 0 (k„ = 7.0x10 s") 2

3

2

2

(11) 2

Table I lists rate constants for the reactions of various "acids" with N 0 " . A l l were obtained from the dependence of the rate of decay of the radical dianion on the concentration of the acid. Details of the kinetic analysis are given elsewhere (7(9), but they indicate that the protonated intermediate is either very short-lived or a transition state structure for reactions 3 and 10, or 11. This conclusion concurs with that of Mezyk and Bartels (4). Addition of H atom in reaction 12 would lead to the same intermediate discussed above (reactions 7, 8). 3

H + N 0 -->N0 + O H 3

2

(12)

Furthermore, the fast reaction of the various acids (e.g., boric acid, which is an OH" acceptor and not H donor) with the radical dianion suggests that the reaction is not simply a proton transfer. It seems better described as an O " transfer, even in the case of water, reaction 3. +

2


346

Table I. Rate Constants For the Reaction of Various "Acids" with N 0 k M's'

HA H

+


NH

+ 4

B(OH)

3

HCO3"

CAPSH NH

a

10

>2.0xl0

(5.3±0.5)xl0

8

5.0x10

s

1

(3.6±0.4)xl0

8

9.25

2.0x10

s

11

(8.4±0.8)xl0

7

9.24

2.8X10 +

Ref.

10

(4.5±0.5)xl0

H2PO4"

Lit. k M's'

pK (of parent)

7.21

(l.l±0.1)xl0

11

12

10.3

7

3

10.4

7

(3.4±0.6)xl0°

3

MeOH

(1.2±0.4)xl0

i-BuOH

N O + 2 0 H " 2

(15)

2

2

The redox potential of the N 0 7 N 0 " couple was determined in a similar method to that described above to be E = -0.47±0.02 V under similar conditions. Thus, reaction 14 is thermodynamically feasible. Attempts were made to measure the rate constant of reaction 14 using competition between the bipyridinium ions and nitrite for N 0 ~ . Only an upper limit for the rate constant, k i < 5 x l 0 M ' V , could be estimated. Whereas this is a relatively slow rate constant, we cannot rule out the possibility that the reduction equivalents are partially converted to N O via reaction 14, 15 in highly concentrated nitrite solutions. 2

2


m

2

3

4

1

4

2

Other Routes to N0 ' 3

From the one-electron redox potential E ° ( N 0 / N 0 " ) = 1.04 V and the twoelectron redox potential of nitrate, E ° ( N 0 7 N 0 " = 0.01 V , one estimates for Reaction 3: K ~ lxlΟ" M . Thus, it is conceivable that at high OFT concentrations the oxide ion, O ", may react with N 0 to produce N 0 " . Such a proposition is unexpected. If oxide ions add to NOs, a strongly oxidizing environment is converted to a strongly reducing one. Another previously unreported route to the production of N 0 " is the addition of O" to N 0 ~ . Using pulse radiolysis with time-resolved E S R detection, we obtained recently unequivocal evidence that Ο adds, at least partially, to nitrite ions, reaction 16. 2

3

2

2

2

3

2

2

2

3

2

3

2

2

N0 ~ + 0 ~ - » N 0 " 2

(16)

3

2

As can be seen in Figure 1, the E S R signal of N 0 " can be produced from solutions that contained only nitrite (and N 0 ) at high base concentrations. The same E S R signal is obtained in nitrate containing neutral solutions via reaction 2. Also shown i f Figure 1 are the results of simulations of the known processes that occur in that system, summarized in Table IL In order to obtain reasonable fit to experimental results, both reaction 16 and the reverse of reaction 3 are necessary. However, the evidence for reaction -3 is weak because adjustments to other rate constants could eliminate it from the scheme. 3

2



348

To conclude our studies of the NO3 " radical, it seems clear that it is more prevalent in irradiated solutions of nitrate and nitrite than initially thought. It is a strong reductant that interconverts with a strong oxidant, the N 0 radical. However, its importance in H L W is still questionable. Because of its relatively short lifetime and because of the fact that it is a single electron redox reagent, its participation in gas generation processes is probably minimal. On the other hand, its successor, the N 0 radical is relatively long lived and participates in oxidative degradation of organic compounds in H L W tanks. 2

2

2

Reactions of Model Organic Complexants with N 0

2

The radiolysis of homogeneous aqueous H L W simulants generate primarily O H / O " and NCb. The reactions of N 0 with organic chelates and carboxylate salts were studied in order to develop a quantitative model for their degradation. In these experiments we contacted aqueous alkaline solutions of organic species with N containing 10-100 ppm N 0 . The latter dissolves in the solution, it dimerizes and hydrolytically disproportionates to nitrite and nitrate. In the presence of the organic solute N 0 may be reduced directly to nitrite. We use the competition between these two pathways to estimate the rate of the organic oxidation. Below we discuss the results from several of these organic solutes. 2

2

2

2



349

Table I L Kinetic Model For Simulating the Pulse Radiolysis of Alkaline Nitrite Solutions Forward (Back) Rate Constants

Reaction

No. 1

e" +N 0->N

2

e" + N 0 ~ (+ H 0 ) -> N O + OH"

3.5x10 M" s"'

3

OH + N 0 " ->N0 + OH'

1χ10 Μ-ν 1.3x10'° M" s" (9.4xl0 s"')

a q

2

aq

2

2

2

9.1x10 M " V

2

s

13

s

+ O~

2

1

1

OH + OH" #

5

Η + OH" -4 e' + H 0

·0" + H 0 2

aq

7

0- + N0 "(+H 0) ->N0 + 20H'

7

0~ + N 0 " - > N 0 3 " "

2

1

7

4 . 7 x l 0 M ' ' s" 7

2

1

3

8

N0 " + H 0 # N 0 + 20H"

9

2N0 #

10

N 0 (+ H 0 ) -> N 0 " + N 0 " +2H

2

2

2

1

2

11

2

2

4

4

2

NO + N0 (+20H") 2

3

2

->2N0 " 2

+

1

a a a, forward rate: 10,1

1

1

1000 s" 9

la

1

1

8

N 0

2

15

1

1.8X10 M" s' l x l O M ' s" (500 M " s" ) 5 x l 0 M ' ' s" (6800 s"')

2

3

14

2.0x10 M ' s"

6

2

1

7

2

2

2

1 0

2

4

Notes/Refs.

1

l x l O M" s"'

16,5 16,5 1, 5, 16

a

Rate constants for Reactions 6 and -£ were adjusted to give the best fit to the curves in Figure 1.


350

Formate Ion Formate ion degrades cleanly to carbonate ion upon reaction with N 0 (reactions 18, 19). N 0 + H C 0 " -> N 0 " + C 0 " + FT 2

2

2

2


(18)

2

N 0 + C 0 " + H 0 -» N 0 " + C0 " + 2H 2

2

2

2

2

+

(19)

3

Figure 2 shows results from contacting 25 and 100 ppm N 0 in N with a 0.1M formate in 1M N a O H solution. Both [N0 "] and [NO3"] increase linearly with time, but [N0 ~] grows faster indicating that oxidation of formate ion occurs competitively with N 0 hydrolysis (Reaction 20) (7 7, 18) 2

2

2

2

2

2 N 0 + H 0 -> H N 0 + HNO3 2

2

(20)

2

TJme (Days)

Figure 2. Concentrations of NO 3' and NO 2" in solution vs. time upon contact with NO2/N2 gas. NO 2'grows faster than ΝΟ3' due to reduction by formate. Diamonds: nitrite, Squares: nitrate.

1J

The stoichiometry of formate oxidation was verified using C-labeled formate. Conversion to C-carbonate after the contact experiments follows the stoichiometry shown in reactions 18, 19. A model to describe the competition kinetics of the contact experiments was developed and is described elsewhere. 13


351

Glycolate, Glycine, Iminodiacetictate (IDA), and Nitrilotriacetate (ΉΤΑ) !

1 3

We utilized ion chromatography and H and C N M R spectroscopies in analysis of products from these compounds following the reaction with N 0 . Glycolate degrades mainly to formate and oxalate. N T A and I D A degrade via stepwise loss of - C H C 0 ~ groups, producing mainly formate, carbonate and 2


2

2

N(CH C0 ") 2

2

N 3

°

2

) HN(CH C0 ") 2

N

H NCH C0 " 2

2

2

°

2

2

N 2

2

° >

) HC0 "

(21)

2

oxalate. Formate and carbonate dominate over oxalate indicating that oxidative decarboxylation is the dominate reaction mechanism. Note that in 1 M N a O H solutions used for these studies, formaldehyde and glyoxal convert to formate and oxalate, respectively, producing H gas (reaction 25) (79, 20). 2

R N C H C 0 " + N 0 -» R NCH - + C 0 + N 0 " 2

2

2

2

2

2

2

(22)

2

+

R NCH « + N 0 - » R N = C H + N 0 " 2

2

2

2

2

(23)

2

+

R N = C H + HO" + H 0 - » R N H + H C ( 0 H ) 2

2

2

2

2

(24)

2

H C ( O H ) + O H " -> H C 0 " H + H 0 2

2

2

2

(25)

2

Oxalate results from oxidation at the methylene group. R N C H C 0 " + N 0 -> R NCH(«)C0 " + N 0 ~ + H 2

2

2

2

2

2

+

2

(22a)

Studies with 1- and 2- C-labeled glycine show it to degrade to formate, oxalate and carbonate ions. The branching ratio is -60/40 formate/oxalate. Therefore, glycine is functionally similar to IDA and N T A , and glycolate except that it tends to give higher fractions of oxalate. The details of N 0 attack on these compounds are unknown. In the case of aminocarboxylate ions, we consider the possibility that reactions occur by electron transfer from Ν followed by, or concerted with, decarboxylation of the radical. A precedent for such a path is found in the autoxidation of trialkylamines in alkaline aqueous solutions (27), the decarboxylation of amino acid anions by hydroxyl radical (22, 25), and decarboxylation of anilinoacetate radical cations (24). Even the pathway that yields oxalate may originate from attack at N . For example, branching reactions may follow electron transfer from the amino group (reaction 26). 2



352

The aminomethyl radical, H N C H (path a, reaction 26), and the nitrogencentered radical, N H C H C 0 ~ (path b, reaction 26), may abstract H from another glycine molecule to give carbon-centered radical, H N C H C 0 " (23). Alternatively, the N H C H C 0 " may undergo water assisted 1,2 Η-shift to form H N C H C 0 " . For glycolate ion, analogous paths following electron transfer oxidation at the O H group are possible. Alternatively, the N H C H C 0 ~ may undergo water assisted 1,2 Η-shift to form H N C H C 0 " . For glycolate ion, analogous paths following electron transfer oxidation at the O H group are possible. The dimer, N 0 , may also reacts with the organic substrates. Challis and coworkers reported that secondary amines (25) and glycylglycine (26) are nitrosated on contact with N 0 in alkaline aqueous solutions. They suggest nucleophilic attack on N 0 , reaction 27. 2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

4

2

2

4

R N H + N 0 -> R N N O + N 0 " 2

2

4

2

(27)

3

Depending on conditions, other adducts may also form during oxidation of glycine and glycolate. We identified hydroxyaspartate ion on oxidation of glycine by N 0 and by γ radiolysis of alkaline nitrate solutions that contained glycine. However, when 1 M N 0 ~ is present in the solution, production of hydroxyaspartate (and other adducts) is substantially reduced and formate, oxalate and carbonate ions amount to -90% of the products. Thus, it seems that the presence of high nitrite concentrations may prevent the radiolytic formation of higher molecular weight products in tank wastes. Relative reactivities of organic substrates with N 0 were determined by competition with C-formate ion. Conversions of the unlabeled substrate to Re formate and labeled formate to C 0 " were measured by N M R and used to calculate the rates. The trend obtained is formate:glycolate:glycine:IDA:NTA ~ 1:19:7:11:19. Because the absolute rate for N 0 reaction with formate is ~1 M ' s", the relative reactivities may also be equated to the absolute rates. 2

2

2

13

Ij

2

3

1

2


353


γ-Radiolysis Aging of Organic Solutes Waste aging studies were performed at the Hanford Site to explain waste characterization data and determine the effect of organic aging on combustion hazards of H L W (27). These studies simulated waste aging by γ-irradiating non radioactive waste simulants. The relative rates of complexants disappearance during irradiation are summarized in Table III. The compounds are the main chelates used in Hanford processes and some degradation intermediates. Where direct comparisons can be made, the relative reactivities are similar to those obtained from the N 0 contact experiments. Significantly, formate is substantially less reactive than aminocarboxylate ions and glycolate ions. This selectivity is counter to that expected for OH/O" (2). 2

Table III. Relative Rates of Disappearance of Organic Compounds in Waste Simulants upon γ Radiolysis a

1

Substrate *

kj re

u-Ethylenediaminediacetic (EDDA)

13

s-EDDA

13

IDA

12

NTA

10

HEDTA

14

Glycine

7

Glycolate

5

EDTA

6

Formate

Γ

Citrate

0.7

Acetate 0.7 Reproduced from Ref. 27. In 3.75 M NaN0 , 1.25 M NaN0 , 2 M NaOH and at 20°C. "Defined value. b

3

2

For the Hanford waste aging studies (27) we also measured the radiolytic yield of carbonate from formate in the simulants. The yield increases with the concentration of formate reaching a maximum value of - 2 molecules/100 eV (see Table IV). Since the necessary rate constants were measured we were able to model the degradation processes. Results are compared with experimental observations in Table IV.


354

Table IV. Yields and Fractions of Carbonate Ion Produced from H , O H , Ο and N 0 During γ-Radiolysis of H L W Simulants Containing Formate Ion 3

2


Concentration M Dose Rate OH'

HC0 ~

Rad/h

2

1.0

4.6xl0

2

2 2

0.03

0.1

0.1

2

0.1

Model 5

4.6x10

0.1

2

G(COt)

1.9

s

% Distribution of C0 " 3

Expt 2.0

4

4

NO,

H

13

13

5

7

87

5

1

3

96

0

2

32

22

5

44

52

OH

o-

57

0.8

0.8

9.3x10"

2.5

C

4.6x10

s

0.2

42

4.6xl0

2

1.5

3

2.2

v

1 c

"Simulant: 1.25 M N a N 0 , 3.75 M N a N 0 , 2 M NaOH. Ref. 27. 0.03 M N a N 0 , 0.1 M N a N 0 ref. 28. 2

2

3

3

Table IV also details the fraction of carbonate resulting from attack by O H , H , 0~, and N 0 . The model shows that 0 7 0 H is the dominant oxidant of formate in the high radiation fields used in the experiments. However, dose rates in the tank wastes are much smaller; they range between 10 -10 Rad/h (29). The last line in Table IV shows that reducing the dose rate increases the yield of carbonate and the fraction of oxidation by N 0 . This behavior is expected since N 0 oxidation of formate competes with second-order radical recombination reactions. 2

2

4

2

2

Role of NO in Flammable Gas Generation In radiolysis of formate, N O mainly terminates with N 0 . Radiolysis of simulants containing formate as the only organic solute does not generate H and N 0 gases. However, the complexants do (30). If N O couples with organic radicals from these compounds, R N O compounds are formed. These are expected to undergo solvolytic and ionic reactions to form carbonyl compounds and hydroxylamine. The latter ultimately produce H , N 0 , N , and N H in waste simulants (19, 20,31). However, simulations show that as longj as^nitrite reacts with the organic radicals with rate constants of >10 M~ s" , then termination with N O cannot compete. The organic radicals can react with nitrite by electron transfer and/or addition reactions. Electron transfer will produce N O and aldehydes that liberate H in strongly alkaline solutions, but electron transfer will not lead to N 0 , N and N H . The latter path produces metastable radical anions (RN0 ") that may live long enough to terminate with N O . Accordingly, R N O and gases may arise by this indirect but kinetically favorable path. 2

2

2

2

2

2

4

2

2

2

3

2


3

1

355

Radiation Effects on Crystalline N a N 0

3

Sodium nitrate is a wide band-gap ionic material with the calcite-type rhombohedral lattice. It is a major component of H L W wastes currently in underground storage tanks. As part of our study on radiation-induced processes within porous solids and at interfaces, we have performed a detailed study of electron interactions with N a N 0 single crystals using the tools and techniques of modern surface science (52). Solution-grown N a N 0 single crystals were annealed for 2-3 hours at 430 Κ in an ultrahigh vacuum (lxlO" Torr) chamber. The sample temperature was then varied from 100-450 K . A Kimbal Physics ELG-2 low-energy electron gun was utilized as the electron-beam source. The incident energy was variable from 5-100 eV. The pulsed beam supplied an electron fluence of - 10 electrons per c m per pulse; the continuous irradiation was applied at the current density of - 1 0 μ A / c m and a maximal dose of - 1 0 electron/cm . Yields of neutral desorbates were obtained utilizing quadrupole mass-spectrometer (QMS), which was programmed to detect 10 masses. The time-of-flight (TOF) and thus the velocity distributions of the neutral desorbates were measured by phase-locking the Q M S to the electron-beam frequency. The A E S spectra and S E E M images of the damaged N a N 0 single crystals were taken using a Physical Electronics 680 Auger Nanoprobe. Figure 3 is a S E E M image of the N a N 0 surface showing the effects of electron irradiation. The square area (center portion) irradiated with 10-keV electrons is topologically rough with a large amount of disorder and deep cone like damage features. The A E S spectrum of the damaged area demonstrates the dominance of the oxygen and sodium peaks. This clearly indicates a sizable depletion in both nitrogen and oxygen, pointing to presence of N a 0 as one of the main surface damage products. The Electron-Stimulated Desorption (ESD) of all these molecular products is thermally activated in the temperature region between 110-440 Κ and the most significant temperature effect can be observed at temperature above 300 K . The normalized data for N O and O? E S D yields have essentially identical temperature dependencies. Recent studies report N O and Ο ( Pj) fragments as primary desorption products under low-dose nanosecond-pulse electron- and UV-photon irradiation (55, 34). The molecular products released from the N a N 0 single crystal surface under microsecond pulses of the 100 eV electronbeam irradiation are N O and 0 with a small amount of N 0 . The temperature dependence of the E S D yields of these molecules is presented in Arrhenius coordinates in Figure 4. The data can be approximated by a sum of two Maxwell-Boltzmann-type equations of the form: I = loi exp(-E /kT) + I exp(-E /kT) where Ei and E are activation energies, 0.16±0.03 and 0.010±0.004 eV, respectively; I i and I are constants. The solid line in Figure 4 is a fit using this equation. The similar temperature dependencies for N O and 0 ESD, and the constant yield ratio,


3

3

10

9

2

2

1 6

2

3

3

2

J

3

2

2

t

02

2

2

0

2


02


356

Figure 3. SEEM image of the NaN0 surface, whose central area has been irradiated with 10- keV electron-beam. (Reproducedfrom Ref 32 with permission from the American Chemical Society) 3

_ 5.5

1

5

ΟΝΟ

2 φ

4

>r

I

3.5

m 3 ο 2.5

3

4

5

6

7

8

1

1000/T ( Κ ) Figure 4. Temperature dependence of ESD yields of NO (open circles) and 0 (filled circles) in Arrhenius coordinates. The yields are normalized for the sake of comparison. (Reproducedfrom Ref 32 with permission from the American Chemical Society) 2


357

indicate that these molecules are produced via the same process and likely involve a common precursor. The N 0 , 0 and N O TOF distributions resulting from pulsed (1 μβ, 20 Hz, 100 eV) electron-beam irradiation of N a N 0 crystals held at 423 Κ are shown in Figure 5. 2

2


3

Figure 5. Velocity distributions for NO (open circles), O2 (solid circles) andNOi (squares) following 100 eV electron irradiation ofNaNΟ 3 surfaces at 423 K. The intensities are normalized. (Reproducedfrom Ref. 32 with permission from the American Chemical Society)

The lines are the calculated 423 Κ Maxwell-Boltzmann distributions for 0 , N O and N 0 respectively assuming instantaneous desorption during the electron pulse. To facilitate comparisons of the leading edges, all intensities are normalized. The experimental NO2 velocity distribution fits the MaxwellBoltzmann approximation, but the emission of the N O and 0 is extended for several milliseconds after the electron pulse and the delayed N O and 0 emission kinetics are similar. We suggest that the observed N O and 0 E S D temperature dependence is due to the temperature dependence of N 0 " production. The N 0 " is a source of holes, which migrate to the surface and recombine with Ν Ο Γ to produce N 0 precursors. The Έ ' excited state of N 0 can undergo unimoleeular dissociation yielding N O ( Π ) and 0 ( I " , A ) molecules due to vibronic coupling to the N 0 ( Α ' ) ground state. This excited state is long-lived, dissociating on the time scale of hundreds of microseconds in the gas phase. This proposed mechanism does not need a fundamental ionization event and can explain delayed N O and 0 emission induced by 5 and 6.4 eV 2

2

2

2

2

2

2

3

3

2

3

2

2

3

2

2


l

g

g

358

photons. Therefore, thermal annealing of irradiated N a N 0 crystals, which activates the decay of bulk radiation defects, produces oxygen that diffuses to the surface. With respect to H L W , the results suggest that radiolysis of the bulk and interfacial N a N 0 crystalline components may be a source of 0 in the wastes. Allowing for continuous dissolution-reprecipitation of N a N 0 due to thermal gradients and seasonal temperature swings of the wastes, 0 and nitrite ions formed in the bulk crystals may be released to the solution. Assuming that surface radiolysis is analogous to that at aqueous-crystalline interfaces, the interfacial radiolytic process nay be a direct source of NO. 3

3

2

3


2

Conclusion With respect to issues of relevance to D O E ' s Environment Management operations, we conclude that: •

2

The probability that H can be formed from N 0 * is rather small. Experimentally, the yield of H decreases upon increasing N 0 " concentrations, even though NO? " is efficiently produced in these solutions. The generation of H from water requires two reduction equivalents, but in the absence of catalysts N 0 " is able to provide only one equivalent before it dissociates. The protonated form is much shorter lived, i f it exists at all. Thus, it is converted to N 0 even faster in the presence of Lewis acids, reducing the probability of fuel generating reduction processes. 2

3

2

3

2

2

2

3

2

•

N 0 contact experiments corroborate the proposition that N 0 is generated during radiolysis of waste simulants. Thus, it contributes to oxidative degradation of organic solutes in H L W .

•

N O radicals, which are produced by reduction of nitrite (by H atoms and organic radicals), or by direct effect on crystalline N a N 0 , may accumulate to relatively high steady state levels. They can then terminate with various organic radicals. The products RNO compounds lead to H , N 0 and N H .

2

2

3

2

•

2

3

Nitrite ions scavenge organic radicals thus promoting organic oxidation. However, they also facilitate gas generation as described in the previous item. They may also convert N 0 " to N O , thereby reducing the efficiency of oxidatively degrading organic waste components. 2

3

These insights and some of the quantitative rate determinations have been incorporated into safety analyses at the Hanford site. In that context they contributed to the resolution of several of the safety issues at the site, including the organic tanks and gas generation issues.


359

Acknowledgement This is a summary of the efforts of many of our colleagues. Their contributions are much appreciated. This report describes coordinated efforts by three projects of the Environmental Management Science Program ("The N O System in Nuclear Waste," "Mechanisms and Kinetics of Organic Aging in High Level Nuclear Wastes,"' and "Interfacial Radiolysis Effects in Tanks Waste"). Support from the E M S P of these projects is acknowledged. The insight developed here would not be possible without many years of support by the Office of Basic Energy Sciences - Chemical Sciences Division. Our interactions with Richland Operations and the Hanford and Savannah River sites during those years provided the impetus to these studies. This is document N D R L No. 4193 from the Notre Dame Radiation Laboratory. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the U . S . Department of Energy under Contract DE-AC06-76RLO 1830.


x

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