Pulse Radiolysis Studies of Liquid Heavy Water at Temperatures up to 250 Oc

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PULSE RADIOLYSIS STUDIES OF LIQUID HEAVY WATER AT TEMPERATURES UP TO 250 OC

Craig R. Stuart, Denis C. Ouellette and A. John Elliot

Reactor Chemistry Branch Atomic Energy of CanadaLtd. Chalk River Laboratories Chalk River, ON CANADA KOJ lJ0

2002 September

AJKL-12107 EACL

&TUDES DE RADIOLYSE PULSJkE DE L’EAU LOURDE LIQUIDE SELON DES TEMPIhATURES ALLANT JUSQU’A 250 “C

Par

Craig R. Stuart, Denis C. Ouellette et A. John Elliot

RlhUMl?

Ce document Porte sur les constantesde vitesse et les energiesd’activation connexesen ce qui concerne les reactions des especesradicalaires primaires (ew-, OD et D), lesquelles sont formees au cows de la radiolyse de l’eau lourde B des temperaturesse situant entre 20 et 250 “C. Ces donnees sur l’eau lourde ont 6tCcomparees aux renseignementscorrespondants en ce qui concerne l’eau ordinaire. Ces donneescinetiques font partie de la base de donneesqui est necessaireB la modelisation de la chimie sous rayonnementen milieu aqueux qui survient dans le cceurd’un reacteur CANDUMD refroidi et mod&e par eau lourde.

Chimie des reacteurs finergie atomique du Canadalimitee Laboratoires de Chalk River Chalk River ON CANADA KOJ 1JO

septembre2002

CANDUMD est une marque de commerce dCpos6e d’hergie atomique du Canada limitee (EACL).

AECL-12107 AECL

PULSE RADIOLYSIS STUDIES OF LIQUID HEAVY WATER AT TEMPERATURES UP TO 250 “C

Craig R. Stuart, Denis C. Ouellette and A. John Elliot

ABSTRACT

This report documents the rate constantsand associatedactivation energies for the reactions of the primary radical species,eaq-, SOD and .D, which are formed during the radiolysis of heavy water within the temperaturerange 20 to 250 “C. Theseheavy-water data have been compared with the corresponding information for light water. Thesekinetic data form part of the database that is required to model the aqueousradiation chemistry that occurs within the core of the heavy-water cooled and moderatedCANDU@ reactor.

Reactor Chemistry Branch Atomic Energy of CanadaLtd. Chalk River Laboratories Chalk River, ON CANADA KOJ lJ0

2002 September

CANDUO is a registered trademark of Atomic Energy of Canada Limited (AECL),

AECL-12107 TABLE OF CONTENTS

SECTION PAGE 1. INTRODUCTION ...... 1 2. EXPERIMENTAL...... 1 3. RESULTS AND DISCUSSION...... 2 3.1 Spectral Characteristicsof the Hydrated Electron in Heavy Water...... 2 3.2 Reactions of the Hydrated Electron ...... 3 3.2.1 Self-Reaction of the Hydrated Electron in Alkaline Heavy Water...... 3 3.2.2 Reaction of the Hydrated Electron with the Deuteroxyl Radical...... 6 4. REACTION OF THE HYDRATED ELECTRON WITH DEUTERIIJM PEROXIDE ...... 7 5. REACTION OF THE HYDRATED ELECTRON WITH MOLECULAR OXYGEN...... 8 5.1 Reaction of the Hydrated Electron with D+...... i...... 9 6. REACTIONS OF THE DEUTERIUM RADICAL ...... 10 6.1 Reaction of the Deuterium Radical with Deuterium Peroxide...... 10 6.2 Reaction of the Deuterium Atom with Oxygen...... 11 6.3 Reactions of the Deuteroxyl Radical...... 13 6.3.1 Dissociation Constant for the Deuteroxyl Radical ...... 13 6.3.2 The Reaction of the Deuteroxyl Radical with Molecular Deuterium ...... 13 6.3.3 The Reaction of the Deuteroxyl Radical with Deuterium Peroxide ...... 15 7. CONCLUSIONS .:...... 17 8. ACKNOWLEDGEMENTS ...... 18 9. REFERENCES ...... s...... , ...... 18 LIST OF TABLES Paae Table 1. Arrhenius parametersfor reactions in heavy water at elevated temperatures...... 22 Table 2. Arrhenius parametersfor the light-and heavy-waterreactions of .OH/*OD + Hx/Dz in light and heavy water...... 22

LIST OF FIGURES Page

Figure 1. The temperature dependenceof the absorption coefficient at hmax for the hydrated electron...... 23

Figure 2. Arrhenius plot of the rate constant for the self-reaction of the hydrated electron ...... 23

Figure 3. Arrhenius plot of the rate constant for the reaction of the hydrated electron with the deuteroxyl radical...... 24

Figure 4. Arrhenius plot of the rate constant for the reaction of the hydrated electron with deuterium peroxide ...... 24

Figure 5. Arrhenius plot of the rate constant for the reaction of the hydrated electron with oxygen...... ,...... 25

Figure 6. Arrhenius plot of the rate constant for the reaction of the hydrated electron with the deuterium ion ...... _...... 25 Figure 7. Arrhenius plot of the rate constant for the reaction of the deuterium atom with deuterium peroxide ...... 26

Figure 8. Arrhenius plot of the rate constant for the reaction of the deuterium atom with the permanganateion ...... 26

Figure 9. Arrhenius plot of the rate constant for the reaction of the deuterium atom with ferricyanide ...... *...... 27 Figure 10. Arrhenius plot of the rate constant for the reaction of the deuterium atom with oxygen...... 27 Figure 11. Temperature dependenceof dissociation constantsin heavy water ...... 28 Figure 12. Arrhenius plot of the rate constant for the reaction of the deuteroxyl radical with ferrocyanide ions .,...... f...... 28 Figure 13. Arrhenius plot of the rate constant for the reaction of the hydrated electron with nitrous oxide ...... 29 Figure 14. Arrhenius plot of the rate constant for the reaction of the deuteroxyl radical with molecular deuterium ...... 29

Figure 15. Solubility of nitrous oxide at 1 atmospheredetermined in D20 ...... 30

Figure 16. Arrhenius plot of the rate constant for the reaction of the deuteroxyl radical with deuterium peroxide ...... *...... 30

Figure 17. pD dependenceof the effective rate constant for the reaction of -OD/-O- with deuterium peroxide ...... *...... 31

Figure 18. pH dependenceof the effective rate constant for the reaction of -OH/-O- with hydrogen peroxide ,...... f.. 31 1. INTRODUCTION

Over the past two decades,radiation chemical yields and kinetics associatedwith the radiolysis of light water, H20, have been studied extensively at temperaturesup to 300 “C in various laboratories. As a result of these studies, it is now possible to model the radiation chemistry within a variety of light-water reactor systems[l]. Modelling the radiolysis of water in a reactor circuit provides a better understandingof the processesinvolved in controlling the water chemistry. With the correct control of aqueouschemistry, it is possible to reduce material degradation of systemcomponents [ 11.

A radiation chemistry databasefor light water has been developed at Chalk River Laboratories (CRL) and has been published in an available report [2]. To assistin the development of experimental techniques and protocols associatedwith this light-water program, there was a large body of well-established room temperature(and someelevated temperature) data with which to compare results. The availability of the light-water information was a direct result of the majority of nuclear reactors being light water cooled.

BecauseCANDU@ reactors operatewith heavy water, D20, in the heat transport system (HTS) and the moderator, a researchprogram is ongoing at CRL to collect the required data for modelling the radiolysis of heavy water [3] at temperaturesup to about 310 “C. The experimental approachtaken has been to measurethe kinetics of radical reactions in heavy water using measurementtechniques established during the light-water studies [2].

This report documentsmeasurements of the temperaturedependence of selected reactions of the hydrated electron (e,,‘), deuterium atom (*D) and the deuteroxyl radical (-OD) formed when heavy water is pulse-irradiated (seeReaction (1)).

D20 -‘v’+ eaq-,.OD, .D, D202, D2, +D02,D+ (1)

2. EXPERIMENTAL

The chemicals used for the experimental portion of this program were of AR grade or better and were used as supplied. The heavy water was reactor grade (i.e., 299.8% D) and was redistilled once from alkaline permanganatesolution; sodium peroxide was used to adjust the pH of the alkaline permanganatesolution. When solutes containing exchangeableprotons were used, the heavy water purity never fell below 98.4% D.

Most of the experimental proceduresused in this laboratory (sample degassing,pulse radiolysis, etc.) have been described in earlier publications [4, 51. The high-temperature pulse radiolysis apparatus(HTPRA), which is used for high-temperaturekinetic measurements,has also been described in detail in other publications [4,6]. For certain experiments in the HTPRA, it was necessaryto saturatethe solutions with a single gas or a mixture of gasesat pressuresup to

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10.3 MPa. For these experiments, the solution was placed in an open reservoir vessel, rather than the syringe, The HTPRA was first thoroughly flushed of air prior to pressurization and was then pressurizedwith the required gas atmosphere. It is essentialto use high-purity gasesto pressurizethe HTPRA when the open reservoir is used to minimise dissolved trace impurities. To ensure an efficient equilibration of the pressurizing gaseswith the solution to be irradiated, the solution was stirred with a PTFE-coveredmagnetic stirrer, which broke the solution surface.

The dosimeter used was an oxygen-saturatedKSCN solution (0.01 mol dm“), where G((SCN);) x ~7s is equal [7] to 2.59 x lOA m2 J-l. (This is equivalent to GE = 2.50 x 10” for G in units of # per 100 eV and E in dm3 mol-’ cm-‘). This GE value is 4.6% higher than that previously used at CRL [4].

Historically, the term “G-value” has been used to representthe yield of a speciesin units of “number of speciesformed (or destroyed)per 100 eV of energy absorbed”. Currently, the S.I. unit for yields, mol J-‘, has also been referred to as a “G-value” [8], which can be confusing if careful note of the units is not taken. In this paper, yields are given in units of mol J-*. In this report “g-value” is only used for the primary yield of species-inReaction (1) above; “G-value” is used for all other yields.

The kinetics simulation packageFACSIMILE [9] has been used to model or fit some of the kinetic data in this study. As an input for the computer simulations, the appropriate temperature dependencesof the radiolysis yields of the primary products (Reaction (1)) were taken from Reference [4].

3. RESULTS AND DISCUSSION

The Arrhenius kinetic parametersmeasured for the reactions in heavy water described in this section are summarisedin Table . When a reaction in light water is being referred to, the same reaction number for the equivalent heavy-waterreaction is used, but with the addition of a “prime”.

3.1 Spectral Characteristics of the Hydrated Electron in Heavy Water

The spectrum of the hydrated electron was measuredin heavy water at temperaturesbetween 20 and 200 “C by irradiating a helium-purged solution containing 10e3mol dmm3sodium tetraborate. This solution was irradiated using a nominal 0.5 ps duration electron pulse that delivered a dose of about 2.5 Gy. Where appropriate, the observedspectra were corrected for decay during the pulse using the computer program FACSIMILE [9]. The decay correction used a simplified reaction set, the pulse time profile and the radiation dose.

The wavelength of maximum absorption (LX) for the hydrated electron in heavy water was the same as that found in light water [lo], within experimental error, for the temperature range 20 to 200 “C (see Figure 1). As the temperaturewas increased,the total observed yield of the hydrated electron (G(eaq-))increased due to an increasedprimary hydrated electron yield [4] coupled with 3 the formation of the hydrated electron through Reaction (2) below, in the alkaline tetraborate solution:

*D + OD- + eaq-+ D20 (2)

For a borate-buffered light-water solution, as the temperatureincreases to 200 “C, the pH at temperature remains reasonablyconstant, and so the concentration of OH- must increase to maintain acid-baseequilibrium as set by the pKa of water [ 111. Similar pD buffering can also be expected in heavy water [ 121,such that, at 180 “C, Reaction (2) will be complete within the pulse in the borate-buffered solution used for this experiment.

Up to 100 “C, Reaction (2) did not contribute significantly to the observedyield formed during the pulse, so the absorption coefficient of the hydrated electron in heavy water was based on the temperature-dependentvalue of g(eq,? from Reference[4]. Above about 180 OC,the total yield of eaq‘was taken to be equal to g(e,,) + g(*D). The temperaturedependencies of the yields for eaq-and *D were calculated from the polynomials given in Equations (VIII), (X) and (XI) in Reference [4]. The temperaturedependence of the absorption coefficient, E(eaq-),at the wavelength of peak absorption is shown in the inset of Figure 1 and is given by

E = 2327.7 - 1.301 x t (E/ m* mol“) (t / “C) (I) It can be seenthat the absorption coefficient for the hydrated electron in light water decreases with increasing temperature at a greaterrate than in heavy water. The reader should note that with the revised dosimetry value used in this report, the data for &(eBq-)in light water are 4.6% higher than the values reported earlier by Elliot and Ouellette [lo].

3.2 Reactions of the Hydrated Electron

3.2.1 Self-Reaction of the Hydrated Electron in Alkaline Heavy Water

The bimolecular self-reaction of the hydrated electron producesmolecular deuterium through Reaction (3). All direct measurementsof this reaction and the equivalent reaction in light water, for temperatures>lOO “C, have been made in alkaline solution ([OH-/ODJ > lo4 mol dm-“) and used dissolved H2 or D2 to convert the *OH/GD formed in Reaction (1) into hydrated electrons (see Reactions (4) and (5) below) [ 131.

In order to obtain the required Hz/D2concentrations of about 10-l mol dm‘“, the HTPRA was used with the open-topped reservoir and stirrer as describedin Section 2. Initial measurements of Reaction (3) made in D20 used HZ rather than DZ (seeReaction (4) below); comparisonswith later experiments using D2 saturatedsolutions showed no change in the measuredkinetics of the system.

eaq-+ eaq- D20> D2 + OD- + OD 4

.OD + Dz (Hz) -+ .D (.H) + D20 (HDO) (4) -D (.H) + OD- + eaqW+ D20 (HDO) (5)

The bimolecular decay of absorption due to the hydrated electron was studied in both light and heavy water. The dose ranged from about 1.5 to 70 Gy and the OD-/OH‘ concentration was (l-3) x 10s2mol dms3. The pressureof Hz or D2 was 10.3 MPa (1500 psig), which gave a hydrogen concentration of 8.3 x 10e2mol dmW3at 20 “C in Hz0 or a deuterium concentration of 0.113 mol dm” at 20 “C in D20 (see Reference [ 141). As the solution reservoir remains at room temperature throughout the experiment, the H2/D2concentration in the reservoir also remains constant.

Simple second-orderfits of the observed decaysgave dose-dependentrate constants, indicating the hydrated electron was also reacting with trace impurities in the solution. In order to separate the self-decay kinetics from the impurity reactions, the kinetic data were analysedby computer fitting using the computer code FACSIMILE 191,with the appropriate reaction set for light- or heavy-water radiolysis (which included a reaction where the hydrated electron reacted with an impurity [lo]). In the heavy-water case,with the exception of the reaction of the hydrated electron with the deuterium atom, Reaction (6) below, all the significant reactions associated with determining k3 have been determined:

eq- + *D D20 > D2 + OD-

By analogy with the light-water reaction [2], it was assumedthat Reaction (6) is primarily diffusion controlled. Thus, kg was estimatedby reducing the value for the equivalent reaction in light water [3] by a factor of 1.3 at all temperatures;this factor is the ratio of the self-diffusion coefficients of Hz0 and D20 [l&16].

From the dose dependence,a consistent initial impurity concentration and rate constant for the reaction of eaq-with the impurity was determined for all dosesat each temperature studied. For reactions up to 150 “C, at high dose per pulse values (50-70 Gy), about 90% of the hydrated electrons reacted through Reaction (3), although this value fell to about 30% at low dose per pulse values (1 S-3 Gy). At 200 “C, the percentageof electrons that underwent self-reaction was lower, becausethe rate of the impurity reactions continues to increasewith increasing temperature as the rate of self-reaction decreases(see below). In D20 at 200 “C it was not possible to define a consistent impurity reaction for all dosesstudied. Therefore, the data point plotted in Figure 2 at 200 “C (i.e., l/T = 2.11 x 10m3R’) is the result of simple second-order kinetic fits of the high dose per pulse data, and therefore representsa maximum value for ks at this temperature.

The rate constant (2k3) for Reaction (3) was determinedto be 7.1 x lo9 dm3 mol-’ s-’ at 19 “C, with an activation energy of 19.5 kJ mol“. An Arrhenius plot of the self-reaction of the hydrated electron in D20 is shown in Figure 2 together with the comparabledata for light water. The Arrhenius parametersassociated with Reaction (3) below 130 OCare listed in Table 1. The 5 activation energy for the reaction of 19.5 kJ mol’ is similar to that for diffusion of the hydrated electron in heavy water [ 171(19.76 f 0.21 kJ mol“).

The light-water study was undertaken to confirm the results of Christensenand Sehested[ 131, who reported that the value of ks’ (as measuredby simple second-orderkinetics) for alkaline solutions increasedwith temperatureup to 150 “C, but then decreasedas the temperaturewas further increased. Figure 2 shows the results of Christensenand Sehestednormalized to the hydrated electron absorption coefficients used in this report. Simple second-orderfits to the current data for a comparable absorbeddose gave values for k? in Hz0 that are consistent with the normalized data of Christensen and Sehestedover the whole temperaturerange studied. However, computer fitting of the current data with a complete light-water reaction set [2], including impurity reactions, was undertakento provide consistent fits over the dose range studied. Compensation for the impurity reaction results in values of k3*in Hz0 that were lower than those obtained by simple second-orderkinetic fits (seeFigure 2).

For heavy water, the difference between the simple second-orderfit values for k3 and those derived from the computer fits was less than that for the light-water study, as a result of improved experimental techniques that resulted in lower impurity concentrations. At temperaturesgreater than about 130 OC,k3 declined sharply in alkaline solution in a similar manner to the equivalent reaction in H20, except that the turnover point occurred at a lower temperature in D20. The mechanismbehind this inverse temperaturedependence above 130 “C is not understood in either Hz0 or D20. It has been suggestedthat the formation of a dielectron intermediate [ 18, 191may be involved (see Reaction (3a) below):

eaq- + ewe G+ (2eq)2- + D2 + 20D-

For this to be the case, it has to be assumedthat the activation energy for the dissociation of the dielectron to re-form two hydrated electrons is greater than that of the forward reaction to form D2 and OD-, and that, therefore, dissociation dominates at elevated temperatures.

The above results are for measurementsmade in alkaline solutions. It still needsto be establishedwhether there is a “turnover” of the rate constant in neutral solution. Establishing the behaviour of this reaction at near neutral pD/pH at elevatedtemperature is difficult, because Reaction (5) below proceedstoo slowly to effectively covert *D to e4-, and it is difficult to separatethe cross-reactionsbetween *D and eq- so that the value for ks can be extracted. To date, preliminary studies using formate ions as *OD scavengershave not proved successfulin either Hz0 or D20, probably due to the introduction of impurities and other cross-reactionsinvolving COz- into the system.

Measurementof the reaction of the hydrated electron with the deuteroxyl radical (see Reaction (7) below) at elevated temperatures,described in the next section, has provided insights into the self-reaction of the hydrated electron at elevated temperaturesin near neutral solution.

There have been two other reports in the literature regarding Reaction (3). Hart and Fielden [20] measuredthe bimolecular rate constant (2ks) for the self-reaction of the hydrated electron in D20 6 to be 1.2 x 10” dm3 mol“ s-r at ambient temperature(23 + 2 “C). Hickel [21] measuredk3 over the temperaturerange 5-65 “C and measured2k3 to be 9 x lo9 dm3 mol-’ s“ at 26 “C and measuredan activation energy of 20.0 kJ moi-’ . The data presentedby both Hart and Fielden [20] and Hickel [2 l] were calculated using simple second-orderkinetic fits. Agreement between these two sets of data is fair, but both studies used an absorption coefficient that was 20% lower than the value determined in this current study. When corrected to the current absorption coefficient, both values are significantly higher than the second-orderrate constantsmeasured in this study, suggestingthe presenceof impurities in the previous studies.

3.2.2 Reaction of the Hydrated Electron with the Deuteroxyl Radical

The temperaturedependence of the rate constant of the reaction of the hydrated electron with the deuteroxyl radical (see Reaction (7) below) has been determined in heavy water at temperatures up to 200 “C. A heavy-water solution containing 4 x 10” mol dmm3borate buffer was irradiated with a dose per pulse between about 1.5 and 25 Gy. Using the dose-dependentdata, a consistent impurity reaction was determined during FACSIMILE [9] computer fitting for all doses,at each temperature studied. The method for the determination of k7 is the sameas the method used in light water, which was describedin Reference [lo].

eaq-+ SOD + OD- (7)

The rate constant for Reaction (7) was determined to be 3.0 x lOI0 dm3 mol-’ s-t at 22 “C, with an activation energy of 7.5 kJ mol“ between 20 and 200 “C. The data for this reaction are shown in Figure 3 together with the data for the reaction of the hydrated electron with the hydroxyl radical in light water. The light-water results reported from this laboratory [lo] have been recalculated to reflect revised dosimetry values, the absorption coefficient for the hydrated electron and the revised rate constant for the self-reaction of the hydrated electron in light water. This recalculation has resulted in rate constantsthat are slightly higher (about 5%) than those previously published by Elliot and Ouellette [lo] for light water.

It was noted in Reference [lo] that, although data in light water had been collected at 200 “C, it was not possible to acceptably fit the kinetic traces to the radiolysis reaction set with any degree of confidence. Re-examining the light water kinetic tracesat 200 “C from the earlier study [lo], it has been found that acceptablefits to the dose-dependentdata could be obtained by using the rate constant for the self-reaction of the hydrated electron obtained by extrapolating the Arrhenius relation for values below 150 to 200 “C (----a----- in Figure 2). Such a continuing Arrhenius behaviour is plausible in a more neutral solution. Without this adjustment, the fits at 200 “C were poor and the fitted value for k7’ was significantly higher than that predicted by the Arrhenius fit through the data between 20 and 150 “C. Christensenet al. [22] also measuredthe rate constant for Reaction (7’) in Hz0 at elevated temperaturesusing a similar experimental method. Using their measuredtemperature behaviour for the self-reaction of the hydrated electron in alkaline solutions [ 131,they also obtained poor fits to the data for k7’ above 150 “C. Additionally, the reported rate constantsfor Reaction (7’) given in Reference [22] at temperatures above 150 “C were also higher than those predicted by the Arrhenius dependenceof the data below 150 “C. The value of k7*obtained from fitting the kinetic traces is critically dependenton 7 the selectedvalue of k3’; if the rate constant for Reaction (3’) is increased,then the rate constant for Reaction (7’) must be decreasedto compensate. Changing to a value for kspbased on an Arrhenius extrapolation of the values below 150 “C also improved the quality of the fits to the time-dependentloss of the hydrated electron.

The two independent experimental studies of the reaction of the hydrated electron with the hydroxyl radical from References[ 10,221 suggestthat the rate constant for the self-reaction of the hydrated electron up to 200 “C may follow the continuance of the 20-150 “C Arrhenius dependencein a borate-buffered light-water solution ([OH-] cl.2 x low3mol dm” at 200 “C).

In contrast to light water, for heavy water, acceptablefits to the experimental data for Reaction (7) were possible at 200 “C using the measuredvalue for the rate constant of the self- reaction of the hydrated electron in alkaline solution. The 200 “C value for k7 lies on the continuation of the Arrhenius line through the data at temperaturesbelow 150 “C. The difference between light and heavy water values is thought to be due to the fact that the borate-buffered solution has a higher pD value in D20 when comparedwith the pH in H20, and thus has a lower concentration of D’ (about 4 times lower) when comparedwith the concentration of H+ in the above studies. Such behaviour suggeststhat the formation of products from the “dielectron” postulated in Reaction (3a) is dependentupon the D+ concentration:

eaq- + eqe * (2eaJ2- + D2 + 20D-

Further experiments are planned to confirm the behaviour of the hydrated electron in near-neutral solution.

4. REACTION OF THE HYDRATED ELECTRON WITH DEUTERIUM PEROXIDE

Previously, the rate of reaction of the hydrated electron with deuterium peroxide has been determined to be (1.18 & 0.08) x 10” at 23 “C by Fielden and Hart [23] by following the decayof absorbancedue to the hydrated electron at 700 nm:

eaq- + D202 + SOD + OD- (8) In the current study, the rate constant for Reaction (8) was measuredup to 150 “C by following the loss of absorbancedue to the hydrated electron. The wavelength of observation was at h,,, of the hydrated electron for the temperatureof the irradiation. Reaction (8) was studied at a minimum of three different deuterium peroxide concentrationsbetween 2.5 and 5.5 x 10e5mol drnh3using a dose per pulse between 1.5 and 2.5 Gy. The pseudo first-order decaysof absorbancedue to the hydrated electron were measured;plotting these first-order rate constantsagainst peroxide concentration compensatedfor the reaction of the hydrated electron with other solutes and itself. Solutions were helium-purged to remove dissolved oxygen prior to radiolysis. 8

Due to the thermal instability of D202, the rate of reaction of the hydrated electron with deuterium peroxide was only determined up to 150 “C. Care was taken at the upper end of the temperaturerange to ensure that thermal degradation of the peroxide was minimised by flushing fresh solution into the irradiation cell and allowing just enough time for equilibration (up to 2 minutes, but generally less) to the desired temperatureprior to irradiation. Unirradiated solution from the reservoir syringe in the HTPRA was assayedfor peroxide concentration using the Ghormley method [24] before and after a pulse radiolysis session.

At 22 “C, the rate constant (ks) for Reaction (8) was determined to be (1.07 + 0.05) x 10” dm3 molt s-*, which is consistent with the earlier determination by Fielden and Hart [23]. The temperature dependenceof ks measuredin this study has an activation energy of 16.0 kJ mole’, similar to the light-water analogue,but with an absolute rate constant value about 30% lower than in Hz0 at 20 “C [2,22]. An Arrhenius plot showing the data for kg, and the light- water analoguecan be found in Figure 4. The Arrhenius parametersfor the linear fit in Figure 4 are listed in Table 1 together with data at 25 OCfor D20.

5. REACTION OF THE HYDRATED ELECTRON WITH MOLECULAR OXYGEN

Fielden and Hart [23] determined the rate constant for the reaction of the hydrated electron with molecular oxygen in heavy water (seeReaction (9) below) to be (1.54 + 0.11) x 10” dm3 mol“ s-’ at 23 OC,by following the loss of absorbancedue to the hydrated electron at 700 nm:

eaq‘ + 02 + 02.‘ (9)

In the current study, kp was measuredby following the decay of the absorbanceof the hydrated electron (at h,,,) as a function of dissolved oxygen concentrationsbetween 1.0 and 3.7 x 10m5mol dmm3.The different concentrationsof oxygen were prepared by mixing fully oxygen-saturatedsolution with helium-purged solution in the appropriate ratio. All solutions contained 4 x 10m3mol dmm3borate buffer; the dose per pulse was about 6 Gy.

The value of kg has been determined to be 1.45 X 10” at 19 OC,which is in good agreementwith the room temperaturevalue of Fielden and Hart 11231.The activation energy for Reaction (9) in D20 is 14.0 kJ mol“. The temperature-dependentdata for kg are shown in Figure 5 together with data for the equivalent reaction in light water.

In order to refine the relative rate constantsbetween key reactions for reactor radiolysis, a series of steady-statey-radiolysis “benchmarking” experimentsusing oxygen-saturatedwater have been performed, and the time dependenceof the hydrogen peroxide yield was subsequentlycomputer modelled [25]. Such benchmarking experimentswere designedto focus on the kinetics of a number of reactions rather than just one, as is generally the casein pulse radiolysis experiments. In this way, the relative rate constantsfor a number of reactions can be determined and the individual reaction rates refined. Modelling the radiolysis of benchmarking experiments 9 suggestedthat the reported rate constant for the reaction eq- with 02 in Hz0 was too low at ambient temperature [25]. As a result, the rate constant for the reaction of the hydrated electron with oxygen in light water has been reinvestigatedand is now revised from the previously published value 121.

The values of kgpin Hz0 reported from our recent work supersedeprevious measurementsfrom this laboratory [2,26,27]. The rate constant for Reaction (9’) has been determined to be (2.11 + 0.08) x 10” dm3 mol-’ s“ at 20 “C, which is consistent with the original value given by Wood [28] (2.17 x 10” dm3 mole1s“ at 20 “C). The current data for the reaction of the hydrated electron with oxygen are shown in Figure 5 together with the results for heavy water. The activation energy of (11.9 f 0.7) kJ molt measuredover 20-200 “C in Hz0 is lower than the previously determined light-water value [27] of 14.17 kJ mol“ and the activation energy measuredin D20 of 14.0 kJ mol-‘.

5.1 Reaction of the Hydrated Electron with D+

The reaction of the hydrated electron with the deuteron forms one half of the equilibrium between the hydrated electron and the deuterium atom (seeReaction (10) below). When studying the forward reaction, care must be taken to ensurethat sufficient D+ is available to ensure complete conversion of hydrated electrons to .D atoms.

There are two different determinationsfor the forward rate constant for Reaction (10) reported in the literature. Fielden and Hart [23] measureda value of 1.71 x 10” dm3 mol-’ s-’ at 23 “C for kto, whilst Singh et al. [29] determinedkio to be (8 + 2) x lo9 dm3 molt s-’ at 21 “C. In this study, kto has been determined to be 9.5 x lo9 dm3 mol-’ s“ at 23 “C, which is more consistent with the findings of Singh et al. 1291,but which is lower by a factor of two than that of Fielden and Hart [23]. The source of the discrepancyin the value of Fielden and Hart is thought to arise from the use of a pH meter to estimateD+ concentrationsrather than titration (see below).

In the current study, the rate constantfor Reaction (10) was measuredby following the decay of the hydrated electron in solutions containing (2.5 - 6.0) x lo-’ mol dms3D+ added as DzS04. The concentration of D+ was measuredby titration of stock solutions with NaOH. The required concentration was prepared by dilution of the stock solution. It was found that estimating the pD using pH meter measurements,along with the appropriate corrections, did not give the required accuracy,possibly due to the low ionic strength of the solution. In the experiments, the D+ concentration was sufficiently high to ensurethe essentially complete conversion of eaq‘to -D through Reaction (10). From the Arrhenius plot for forward Reaction (lo), an activation energy of 11.8 kJ molt has been determined,which is higher than the value of 9.5 kJ mol-’ for the equivalent reaction in light water measuredin these laboratories [2]. The light-water data of Shiraishi et al. [30] are also shown in Figure 6. Shiraishi et al. [30] observed an increasing activation energy at temperaturesabove 100 “C; this phenomenonwas noteobserved in the current study in either light or heavy water over the temperatureand concentration range studied. Shiraishi et al. used a proton concentrationrange around lo-fold lower than the studies described 10 here. Our experience would suggestthat the high-temperatureresults of Shiraishi et al. may have been compromised by the effects of competing impurity processes.

6. REACTIONS OF THE DEUTERIUM RADICAL

6.1 Reaction of the Deuterium Radical with Deuterium Peroxide

The reaction of the deuterium atom with deuterium peroxide (seeReaction (11) below) has been measuredbetween 18 and 80 “C in heavy water:

.D + D202 + SOD f D20 (11)

Kinetic absorption spectroscopywas used to follow the build-up of the absorption of Cl~a-,which effectively measuresthe rate of production of the *OD produced in Reaction (11). Absorption due to Cl2.- was measuredat 360 nm (hmaXin D20) in a de-oxygenatedheavy-water solution containing 0.1 mol dme3DCl, 0.9 mol dme3HCl04 (1 mol dme3total D’) and (1.1 - 2.2) x 10m2mol dmW3D202. The use of 0.9 mol dme3HC104 resulted in the minimum isotopic content of heavy water encounteredin this study, 98.4%. The dose per pulse was about 4 Gy.

Following Reaction (l), the hydrated electron was converted, within the pulse duration, to deuterium atoms through Reaction (10):

D+ + eaq- * SD (10)

The .OD radicals produced in Reaction (1) produce a prompt yield of Clz*-through Reaction (12), below. The SODproduced through Reaction (11) also reactswith Cl- to give a further build-up of absorption due to Cl*-- over a longer timescale.

SOD + Cl- “- > C12*- + OD- (fast kt2 >109 dm3 mof’ s-t) (12)

As the rate constant for Reaction (12) is a factor of about lo3 greater than kt 1under the conditions listed above, the observedgrowth of Cl2.-absorption, when corrected for the simultaneous decay of Clz.-, is a measureof the rate of Reaction (11).

The corrected slow growth of absorption due to Clz*-represents the pseudo-first-order rate constant (kl 1[D202]). The rate constant for Reaction (11) was determined to be (2.1 -r-0.4) x lo7 dm” mol-’ s-l at 18 “C; an activation energy of 16.2 kJ mol-’ was estimated.

The temperature dependenceof the rate constant for the reaction of SDwith D202 has also been determined by Mezyk and Bartels [3 l] using electron paramagneticresonance free induction decay (EPRFID) to measurethe reaction rate between the SDatom and peroxide from the dependenceof the rate of loss of spin polarisation of .D atoms as a function of peroxide 11 concentration. Mezyk and Bartels report a value of 1.82 x lo7 dm3 mol-’ s-’ at 16.7 “C with an activation energy of 16.2 k.Jmol-‘; the results from both studies are consistent, although two completely different measurementtechniques were employed. The EPRFID method measures the rate of loss of spin polarisation in the reactant *D in solution, while kinetic absorption spectroscopymeasures the rate of product formation.

The Arrhenius plot showing both data sets, as well as the equivalent light-water data, is shown in Figure 7; both sets of data for the reaction in light water [26,3 l] were measuredby the same methods as those described for heavy water.

6.2 Reaction of Deuterium Atom with Oxygen

The reaction of deuterium atoms with oxygen in D20 (see Reaction (13) below) has been studied over the temperaturerange 19-100 “C using two competition kinetics methods, which gave slightly different results at elevated temperatures. In earlier studies [26], both methods gave good agreementfor the corresponding reaction in light water.

.D + O2 -+ aDO2 ( T- ‘0; -I- D’) (13)

The first competition method used the permanganateion as a competitor for SD,and the loss of absorption due to Mn04‘ was measuredat 525 nm in D20:

.D + MnOd- + Mn04’ + D’ (14)

As the rate constant for Reaction (14) had not been determined in heavy water, kt4 was measured at temperaturesup to 125 “C by monitoring the loss of permanganateion absorption at 525 nm in acid solution ( 10e2mol dm-” D+ using D2S04). The permanganateion concentration was varied between 1.5 and 6 x 10e5mol dm” at room temperature. The temperaturedependence of Reaction (14) was found to be non-Arrhenius; it is describedby a second-orderpolynomial fit given by Equation (II), below. The Arrhenius plot for the correspondingreaction in light water was also non-linear [27]. At 19 “C, k14is 1.85 x 10” dm3 mol“ s-‘. The Arrhenius plot for Reaction (14) is shown in Figure 8 together with the equivalent light-water data [27].

log k = 11.4950 + 67.6954/T -124812.5/T2 (T in K) m The measuredkt4 value was used in the measurementof the rate of Reaction (13) using competition kinetics. In all cases,solutions contained 10e2mol dms3Df to produce *D through scavengingeaq‘ (see Reaction (10)); the permanganateconcentration was (3-6) x 10m5mol dme3 and the oxygen concentration in the competition solutions was (6.5-7.0) x 10e5mol dm-“. The reference solutions were helium-spargedto remove oxygen.

The rate constant for Reaction (13) was also determinedby using a secondcompetition kinetic schemewith ferricyanide ion, Fe(CN)h3‘,as the competitor. The loss of absorption due to Fe(CN)b3-was measuredat 420 nm over a range of oxygen concentrations: 12

*D + Fe(CN)b3- + Fe(CN)64- + D’ (15)

The rate constant for Reaction (15) in heavy water was first determined at temperaturesup to 75 “C in heavy water containing 0.1 mol dmV3HClO4 by observing the absorbancechange at 420 nm due to the loss of ferricyanide ions. The concentration range of ferricyanide ion was varied between 5 x 10W5and 12 x 10V5mol dmm3.All solutions contained low2mol dme3 2-methyl-propan-2-01to scavenge.OD, thus preventing the ferrocyanide ions produced in Reaction (15) from being oxidized (seeReaction (16) below) to re-form ferricyanide ions. The temperature range studied was limited by the stability of the ferricyanide ions; the 2-methyl-propan-2-01concentration was chosen to maintain the temperature stability of the ferricyanide ions [26]. The rate constant for Reaction (15) was measuredto be 4.08 x lo9 dm3 mol-’ s-l at 20 “C and the activation energy has been determined to be 10.2 kJ mol-’ . The Arrhenius plot for Reaction (15) is shown in Figure 9 together with the corresponding light-water results [26].

SOD + Fe(CN)h4- + Fe(CN)e3- + OD* (16)

The competition experiments to determine kt3 were carried out in heavy-water solutions containing 0.1 mol dm” HCl04; low2mol dmq32-methyl-propan-2-01 to scavengeSOD radicals; lOA mol dmS3ferricyanide; and (6-30) x 10m5mol dmm3oxygen. Oxygen concentration was varied by mixing oxygen- and helium-saturatedsolutions; the reference solutions were spargedwith helium to remove dissolved oxygen.

At room temperature, results from both competition techniquesare in good agreementand give an averagevalue for kt3 of 9.4 x lo9 dm” mol-’ s-t at 20 “C, but at higher temperaturesthe data from the ferricyanide experiments were consistently lower than those determined using the permanganatemethod-(see Figure 10). Therefore, the full line through the competition kinetics data is only accurate to about 15%, with an activation energy of 10.4 kJ mol“ likely to be a good approximation.

Han and Bartels [32] also determinedthe rate constant for the reaction of the deuterium atom with oxygen by using the EPRFJDmethod to measurethe rate of loss of SDfrom solution between 5 and 70 “C. The value of kt3 determined by this method was 1.53 x 10” at 20 “C, which is higher than the value of 9.4 x lo9 dm3 mol-‘s-l measuredin this current study. A similar disparity exists with the equivalent reaction in light water. All the data for this reaction in both light and heavy water are given in Figure 10. The disparity between the data for Reaction (13) generatedby competition kinetics and by EPRFID methods can be rationalized based on the fact that the reaction involves the interaction of a doublet state deuterium radical with a triplet state oxygen molecule. The EPRFID technique measuresthe depolarisation rate of the reactant *D. Depolarization is a function of the encounterrate of the deuterium atoms with oxygen rather than a measureof the true reaction rate [32]. Not every encounter between the two components necessarilyresults in reaction, so the lower rate measuredby a competition kinetics method reflects the actual reaction rate, i.e., the formation of products. 13

Although the absolute rate constantsmeasured by Han and Bartels [32] are higher than data presentedfrom the current studies, the activation energy of 11.O f 1.2 kJ mol“ for the reaction of *D with 02 in 90% D20 is consistent with the current work.

6.3 Reactions of the Deuteroxyl Radical

6.3.1 Dissociation Constant for the Deuteroxyl Radical

The acid dissociation constant for the deuteroxyl radical (pK.on) has been measuredbetween 20 and 100 “C by monitoring the pD dependenceof the reaction of *OD/.O- with ferrocyanide. The same technique was used in this laboratory [5] to measurethe corresponding pK.on in light water:

.OD -O- -t D+ (17)

*OD + OD- e -O- + D20 (18)

It was assumedthat the reaction of *O-with ferrocyanide does not proceed over the time window (up to 40 us) of interest here (i.e., ki6 >>> k19):

*OD f Fe(CN)G4- -+ Fe(CN)h3‘ + OD- (16)

-0. + Fe(CN)h4- j+ (1%

Thus, the observed rate of formation of ferricyanide (Fe(CN)e3-)is a measureof the proportion of *OD that has not dissociatedto form *O-at a particular OD- concentration. At high OD‘ concentrations (0.01 <[OD-] cl mol dmm3),allowance was made for the change of pK,(D20) with ionic strength using the data of Shoesmith and Lee [33]. The behaviour of PK.~~ (see Figure 11) has a similar temperaturedependence between 20 and 100 “C to that shown by pKoH [2]. Thus, the O-300 “C temperaturedependence data for pK.ou have been scaled to fit the available pK.oo data, and have been plotted in Figure 11 along with data for pK, (D20) [33-351 and p& (WA) [361.

The Airhenius plot for Reaction (16) is shown in Figure 12 at temperaturesbetween 20 and 100 “C together with the light-water results [26].

6.3.2 Reaction of the Deuteroxyl Radical with Molecular Deuterium

Reaction (20) below is the key reaction involved in suppressingthe radiolytic production of oxygen in the heavy-water coolant in the HTS of a CANDU reactor [37] as the dissolved deuterium scavengesthe *OD radical, which is a precursor to radiolytic oxygen formation. Maintaining an oxygen-free environment is important in minimising corrosion processesin the reactor.

*OD + Dz + .D + D20 (20) 14

To measurethe rate of Reaction (20) at temperaturesbetween 18 and 250 “C, the method developed by Christensenand Sehested[38] for the analogouslight-water reaction was used. The hydrated electrons formed in Reaction (1) are converted to *OD radicals by reaction with N20 (see Reaction (21) below) during the pulse:

eaq- + N20 + D++ N2 + SOD(fast) (21)

The rate constant for Reaction (21) has been measuredup to 250 “C. The value of k21was 8.5 x lo9 dm3 mof ’ s-’ at 25 “C and the activation energy was 16.8 kJ mol-‘. The Arrhenius plot for Reaction (21) is shown in Figure 13 together with light-water data from Reference [26].

The concentration of dissolved deuterium is set so that the SODradicals react with the dissolved deuterium to form deuterium atoms (see Reaction (20)) on a timescale that is much longer than the pulse duration. Becausethe deuterium atoms have only a weak optical absorption [39], sufficient oxygen was also added to the solution to ensurethe prompt conversion of deuterium atoms to the more strongly absorbing superoxideradical (see Reaction (13)).

SOD + DZ + *D + D20 (rate-limiting) (20)

.D + O2 + -DO2 ( * Oz.- + D+) (fast) (13)

As the rate-limiting step in the formation of Oz.-in this schemeis Reaction (20), the pseudo-first- order growth of 0 2~~absorption at 250 nm is a measureof k20[D& The rate constant for Reaction (20) has been determined to be 1.17 x lo7 dm3 mol-’ s-’ at 18 “C, and the temperature dependenceis shown in Figure 14. Fielden and Hart [23] have also reported a room temperature value for k20of 1.6 x lo7 dm3 mol-’ s-’ at 23 “C, but do not give full details of the experimental procedures.

The experimental measurementswere carried out in heavy water buffered at pD -7 with 10m2mol drnT3phosphate buffer. Careful procedureswere developed to ensure that at no time was a flammable gas mixture produced in the HTPRA vessel. The HTPRA was pressurizedto a total pressureof 10.3 MPa (1500 psig) as follows: about 100 kPa of 02 (about 15 psig), about 310 kPa of N20 (about 45 psig) and the required amount of He (7.9-9.3 MPa) followed by between 0.69 and 2.1 MPa D2 (100 and 300 psig). The concentrationsof dissolved gasesat 20 “C were 1.3 x 10” mol drnm302; 7.5 x 10m2mol dm” N20; and (0.8-2.3) x 10m2mol dm” D2. Scharlin and Battino [ 141have determined the solubility of oxygen, hydrogen and deuterium gasesin both light and heavy water over the temperaturerange 15-45 “C. The solubility of nitrous oxide gas in heavy water was determined at temperaturesup to 35 “C as part of this study. The solubility of N20 in heavy water is shown in Figure 15 together with the solubility of N20 in light water [40].

The above mixture of dissolved gasesenabled the measurementof k20at temperaturesup to 250 “C. The rate constant for the reaction of *OH with H2 in light water, together with the reaction of SOD+ HZ in D20 and -OH + D2 in Hz0 (see Reactions (20’), (20a) and (20b) below), 15 were also determined by following the absorption due to the formation of the superoxide radical. The observed temperature-dependentdata for thesereactions have a linear Arrhenius dependence and are also shown in Figure 14 together with data for Reaction (20’) from Reference [38]. The Arrhenius parametersfor all four reactions are given in Table 2.

-OH -t H2 -+ .H + Hz0 (in H20) (20’)

SOD -I- Hz + .H + HDO (in D20) GOa)

*OH + D2 -+ *D + HDO (inH20) (2Ob)

The rate constant for the reaction of SODwith D2 (seeReaction (20)) was also measuredat room temperatureby following the disappearanceof the weak absorption due to SODat 250 nm in oxygen-free nitrous oxide saturatedsolution. The rate constant was consistent with the value measuredfrom the growth of Oz.‘, although it had a higher uncertainty associatedwith it due to the poor signal to noise ratio of the experimental decayprofiles.

6.3.3 Reaction of the Deuteroxyl Radical with Deuterium Peroxide

Reaction (22) below also plays an important role in the radiolytic production of oxygen in heavy water:

SOD + D202 + *DO2 + D20 (22)

The rate constant for Reaction (22) has been measuredbetween 17 and 125 “C by monitoring the growth of 02- in helium-purged peroxide solutions containing 10e2mol dm” equimolar phosphate buffer. The temperaturerange studied is limited by the stability of deuterium peroxide in D20. In all casesthe peroxide concentrationwas between 0.02 and 0.045 mol dm”. The deuterium peroxide concentration was measuredin the unirradiated solution before and after the experimental sessionto verify that the peroxide concentration did not drop significantly over the duration of the experimental session. In these solutions the hydrated electron formed in Reaction (1) reacts with the deuterium peroxide during the pulse to form SODradicals (see Reaction (8)).

eaq- + D202 + .OD + OD- (8) It is assumedthat the SODradicals react with the deuterium peroxide to initially form sD02, which quickly dissociatesto form 02~~under the pD conditions used. Absorption due to the formation of 02~~was monitored at 250 nm.

The rate constant for Reaction (22) was determined to be (9.1 & 1.0) x lo6 dme3mol-’ s“ at 23 “C and the activation energy for this reaction has been determined to be 15.7 kJ mol“. An Arrhenius plot for Reaction (22) reaction is given in Figure 16 together with the corresponding data for light water [2,41]. 16

Christensen et al. [41] have determined the rate constant of the reaction of the hydroxyl radical with hydrogen peroxide in light water between 20 and 150 “C. These light-water determinations were performed in the absenceof a buffer and also assumedthat the deprotonation of HO2 to form Oz.- was not the rate-limiting step. During the course of the current studies, the work of Christensen et al. in light water was repeatedwith the addition of 10m2mol dmd3equimolar phosphate buffer (Christensenet al. reported a room temperaturepH of 7.8 but used no buffer [41]). The use of a buffer increasesthe rate of deprotonation of HO2. Results obtained for the reaction of *OH with Hz02 from this buffered systemare in agreementwith the results of Christensen et al. between 20 and 100 “C. The results of Christensenet al. [41] above 125 “C are suspectedto have been affected by thermal decomposition of the hydrogen peroxide.

In alkaline solution, both the deuteroxyl radical and deuterium peroxide deprotonate to form 0.’ (see Reaction (17)) and DOi (see Reaction (23)), respectively. The temperature dependenceof the dissociation constantsfor both thesereactions are shown in Figure 11.

.OD G== 0.- + D+ (17)

D202 - D02- + D+ (23)

In order to provide an effective overall rate constant for the reaction of these speciesregardless of protonation, the pD dependenceof the reaction of *OD/*O-with Dz02/D02* has been measuredat temperaturesup to 100 “C (seeReactions (24) to (26) below). The corresponding reactions in light water were also measured.

.OD + DO; + 02.- + D20 (24)

*O-+ D202 + 02.- + D20 (25)

-O- + D02- -+ 02.- + OD- w5)

The rate of formation of 02.‘ was measuredin both buffered and unbuffered solutions at room temperature pDs between 7.5 and 13.8. The results of thesepD-dependent experiments are shown in Figure 17 at 20,50,75 and 100 “C together with fit lines to the data at these temperatures. The pD units plotted at elevated temperaturerepresent -log [D’] as calculated using the FACSIMILE [9] computer simulation package. Symbols in Figure 17 joined by dotted lines indicate one set of experimental conditions. In each experiment, the pD of the solution was either controlled by a buffer system,or allowed to change as the temperaturewas increased. The fits were generatedusing the following relation:

k(.0D+D202) + k(*OD+DOi)xB + k(*O-+D202)xA + k(.O-+DOz)xAxB k(“.OD + D20;‘) = AxB m 17 where k(“.OD + D202”) is the rate of reaction for +OD/.O-with D202/D02- at a particular pD

A = ( 1()pD-pK(D202) + 1)

B = (~@-P’W’W + 1)

In fitting the dependenceof k(“*OD + D202”) on pD (seeFigure 17), the dissociation constants for both *OD and D202 were assumedto be equal at all temperaturesbased on the data in Figure 11. Due to this similarity in pKas for SODand D202, the rate constants for Reactions (24) and (25) cannot be deconvoluted. The behaviour noted here for the pD dependenceof the overall reaction rate of Reactions (24) to (26) is consistentwith the behaviour reported by Christensenet al. [41] in light water (see Figure 18).

As highly alkaline solutions are not relevant for reactor operation, limited effort was spent generating reliable experimental data at pD values above 12. Thus, the isothermal lines given in Figure 17 are not well defined at high pD values and, as such, reliable temperature-dependent values for Reaction (26) cannot be determined.

Similar temperature- and pH-dependentstudies have been performed in light water, and the results between 20 and 100 “C are shown in Figure 18. As with the study in D20, the temperature dependencesof the reactions in highly alkaline solutions are not well defined.

7. CONCLUSIONS

In general, temperature-dependentkinetics in heavy water follow similar patterns to those in light water, although the heavy water rate constantstend to be slightly lower than the corresponding light water rate constants. As the isotope effect for each individual reaction is different, it will be necessaryto model the radiolysis of light- and heavy-watersystems using the appropriate rate constant and g-value database.

In order to complete the determination of the rate constantsfor the reaction sets of the radiolysis of light and heavy water at elevated temperatures,a few reactions still need to be studied:

1. the self-reaction of the hydrated electron at elevatedtemperatures in near-neutral water, 2. the reaction of the hydrated electron with *H and .D, and 3. reactions of DOZ*(HO~*)and OZ.- at elevated temperatures.

Once the complete reaction setsare available, the databasehas to be validated. One approach being used in these laboratories is to perform steady-statey-radiolysis experiments on well- defined aqueoussolutions, followed by modelling the observedtime profile of product formation. When these more complex radiolytic systemscan be satisfactorily modelled at elevated temperatures,including the role of trace impurities, the models can then be used to reliably predict the radiation chemistry within CANDU systems. 18

8. ACKNOWLEDGEMENTS

The authors would like to thank M.P. Chenier for technical assistancein producing the data in this report. Helpful discussionswith D.M. Bartels, G.V. Buxton and D.R. McCracken are acknowledged. This work was principally funded by AECL Components and SystemsProgram 32, with early work funded by the CANDU Owners Group (COG).

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36. Brodovitch, J.C.; Percival, P.W.; Poteryko, D. Determination of the Dissociation Constant of D202 in D20 by a Conventional Method and Via Muoniom Kinetics. Can J. Chem. 1988,66,2410. 21

37. McCracken, D.R.; Rasewych,J.B.; Shorter, W.R. Coolant Radiolysis and Boiling in Water-Cooled Reactors. Water Chem. Nucl. React. Sys. 1989,5, 107.

38. Christensen, H.; Sehested,K. Reactions of Hydroxyl Radicals With Hydrogen at Elevated Temperatures.Determination of the Activation Energy. J. Phys. Chem. 1983, 87, 118.

39. Nielsen, S.O.; Michael, B-D.; Hart, E.J. Ultraviolet Absorption Spectra of eaq-,H, OH, D and OD From Pulse Radiolysis of Aqueous Solutions. J. Phys. Chem. 1976,80,2482.

40. Young, C.L. SoZubiEityData Series; Volume 8, Oxides of Nitrogen, 1981; 2.

41. Christensen, H.; Sehested,K.; Corfitzen, H. Reaction of Hydroxyl Radicals with Hydrogen Peroxide at Ambient and Elevated Temperatures.J. Phys. Chem. 1982,86, 1588. 22

Table 1. Arrbenius parameters for reactions in heavy water at elevated temperatures.

kat25OC Temperature I Reaction / dm3 mol’* s-l E, / kJ mol-l Range Studied /“C 19 - 130” I 7 I e,- + SOD I 3.26 x 10” 1 18 - 200 1 8 1 can-+ Da02 I 1.11 x 1o’O I 18 - 150 9 eaqm+ 02 1.68 x 10” 14.0 19 - 200 10 eaq-+ D+ 1.03 x 1o’O 11.8 16 - 200 21 easy+ N20 8.48 x log 16.8 20 - 250 I 11 I .D + D202 I 2.72 x lo7 1 16.2 18 -80 .D+O? 1 9.63 x 10” 1 19 - 250 14 .D + MnOr 2.07 x 10” Non-Arrhenius’ 19 - 125 15 SD+ Fe(CN)h3- 4.33 x 10” 10.2 20 - 75 16 .OD + Fe(CN)c3- 8.92 x 10” 12.4 17 - 100 I zor .OD + D2 I 1.35 x lo7 1 16.3 18 - 250 I 22 I *OD + D,O? I 9.59 x lo6 I 15.7 17 - 125 * behaviour non-Arrhenius above 130 “C in alkaline solution ’ temperature dependencedescribed by log kt4 = 11.495 + 67.6954/T - 123812.5/T (T in K)

Table 2. Arrhenius parameters for the light-and heavy-water reactions of l OH/*OD + H2/D2 in light and heavy water.

Reaction Solvent k (20 “C) / dm3 mol-’ s-’ E, / kJ mol-’ .OD + D2 D20 1.22 x lo7 16.3

.OD I- H2 D20 2.24 x lo7 16.6

*OH + D2 H20 1.60 x lo7 17.3

*OH -t HZ H20 3.27 x lo7 17.5 23

\ -- E 1800 - “E ‘s, 1600 -

0 50 100 150 200 250 .n_

Figure 1. The temperature dependence of the absorption coefficient at LX for the hydrated electron in heavy water (@) and light water (0). The inset shows the temperature dependence of the wavelength of the absorption maximum (LX) in heavy water (A) and light water (A).

Figure 2. Arrhenius plot of the rate constant (k3,k3#)for the self-reaction of the hydrated electron from computer fitting in heavy water(O), and in light water (0). Also shown are simple second-order rate constants from Reference [13] in Hz0 corrected to the absorption coefficients in Figure 1 (A). 24

3x10’0 -

\ 2XlW-’ .“‘. *‘n”*‘*“*‘. o.co20 0.0025 o.cm30 0.0035

KIT

Figure 3. Arrhenius plot of the rate constant (k7) for the reaction of the hydrated electron with the deuteroxyl radical in D20 (a) or the hydroxyl radical in Hz0 (0).

L L 10” g “E =Az) 0 0” + ‘CT op lz

10’0

0.0020 0.0025 0.0030 0.0035 K/T

Figure 4. Arrhenius plot of the rate constant (kg) for the reaction of the hydrated electron with deuterium peroxide in DzO (a), or hydrogen peroxide in Hz0 (a); Hz0 data from Reference [22] (0). 25

7 ” IO” B E

% ?G 0 + 07 3i’

10’0 0.0020 0.0025 0.0030 0.0035 WT

Figure 5. Arrhenius plot of the rate constant (kg) for the reaction of the hydrated electron with oxygen in D20 (0). Revised data for the reaction in light water (A).

Figure 6. Arrhenius plot of the rate constant (klo) for the reaction of the hydrated electron with the deuterium ion in D20 (U) and from Reference [29] (A). Data for the reaction of eaq‘ with H+ in light water are included for comparison from this study (O), Reference [30] (0) and Reference [29] (A). 26

‘;u, -r- E” “E % 108 0 Iii

6 zz

10’ 0.0020 0.0025 0.0030 0.0035 KIT

Figure 7. Arrhenius plot of the rate constant (kll) for the reaction of the deuterium atom with deuterium peroxide from the current work (0); from Reference [31] (V). Data for the reaction of l H with Hz02 in light water are included for comparison: Reference [26] (0); Reference [31] (V).

1.. - .I. . ..I.. ..I.

Figure 8. Arrhenius plot of the rate constant (k14)for the reaction of the deuterium atom with the permanganate ion in D20 (a). Data for the reaction of l H with permanganate in Hz0 from Reference [27] (0) are included for comparison. 27

4x10'0

3x10'0

I -k 2x10'0 E "E e r 62 9x10910'0 x 8x109 lL 7x109 ;: 6x109 k 5x109 4x109

3x109

Figure 9. Arrhenius plot of the rate constant (k15)for the reaction of the deuterium atom with ferricyanide in D20 (0). Data for the reaction of l H with ferricyanide in Hz0 from Reference [26] (0) are included for comparison.

IO” .* ...... , . , . d -. * .

h 7 5 E

3 2 0” + n Y&

0.0020 0.0025 0.0030 0.0035

K/T

Figure 10. Arrhenius plot of the rate constant (k13)for the reaction of the deuterium atom with oxygen in D20: using permanganate as a deuterium atom scavenger (m), using ferricyanide as a deuterium atom scavenger (+), and EPRFID data from Reference [32] (A) (see text). Data for reaction of l H with 02 are included for comparison: permanganate [27] (El), ferricyanide [26] (0), and EPRFID [32] (A). 28

TIC

Figure 11. Temperature dependence of dissociation constants in heavy water. pK,(DzO) from Reference [33] (V), Reference 1341(A), and Reference [35] (0). p&n measured in the current study (W); pKn202 from Reference [36] (0).

0.0020 0.0025 0.0030 0.0035

Figure 12. Arrhenius plot of the rate constant (k16)for the reaction of the deuteroxyl radical with ferrocyanide ions in D20 (a). Data for the reaction of *OH with ferrocyanide ions in Hz0 from Reference [26] (0) are included for comparison. 29

0.0020 0.0025 0.0030 0.0035 KIT

Figure 13. Arrhenius plot of the rate constant (kzl) for the reaction of the hydrated electron with nitrous oxide in D20 (0) and in Hz0 (0).

.I.. .., . . . *, . rn . . , . 10

Figure 14. Arrhenius plot of the rate constants (k zo’,km,.,, k20a, kzo) for the following reactions: ------(0) *OH + HZ in Hz0 (0) Reference 38, ------(V) -OH + D2 in H20, ----- (0) l 0D + Hz in D20, - (A) l OD + D2 in D20. 30

0 5 10 15 20 25 30 35 40 TIC

Figure 15. Solubility of nitrous oxide at 1 atmosphere determined in DzO (A) and Hz0 (A). Compiled data from Reference [40] are given for comparison (-).

0.0020 0.0025 0.0030 0.0035 K/T

Figure 16. Arrhenius plot of the rate constant (k& for the reaction of the deuteroxyl radical with deuterium peroxide in D20 (V). Data for the reaction of *OH with Hz02 in Hz0 from Reference [2] (V) and Reference [41] (0) are included for comparison. 6 7 6 9 10 11 12 13 14 15 pDalTemperature

Figure 17. pD dependence of the effective rate constant for the reaction of l OD/*o’ with deuterium peroxide at 20 (O), 50 (A), 75 (‘I) and 100 “C (+). pD is defined as -log[D+] at the measurement temperature. Symbols joined by dotted lines indicate one set of experimental conditions. pKas for Dz02 and l OD at various temperatures are represented by the symbols in the box attached to the x-axis.

10’0t....,....,....,....,....,....,....,.rr7

IO' 6 7 6 9 10 11 12 13 14 pH at Temperature

Figure 18. pH dependence of the effective rate constant for the reaction of -OH/-O- with hydrogen peroxide at 20 (O), 50 (A), 75 (V) and 100 “C (+). pH is defined as -log[H+] at the measurement temperature. Symbols joined by dotted lines indicate one set of experimental conditions. Open symbols at 20 “C (0) are from Reference [41]. pKas for Hz02 and *OH at the various temperatures are represented by the symbols in the box attached to the x-axis. AECL-12107

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