1193 J. CHEM. SOC. FARADAY TRANS., 1993, 89(8), 1193-1197

Temperature Dependence of g Values for H,O and D,O irradiated with Low Linear Energy Transfer Radiation

A. John Elliot," Monique P. Chenier and Denis C. Ouellette System Chemistry and Corrosion Branch, AECL Research, Chalk River Laboratories, Chalk River, Ontario, Canada KOJ IJO

The g values for the primary species formed in the y-radiolysis of light and heavy water have been measured as a function of temperature up to 300°C. With the exception ofg(H,) andg(D,), all the g values are consistent with the generally accepted diffusion-kinetic model of spurs, i.e. with an increase in temperature, the g values of the free radicals increased while those of peroxide decreased. The g values for H, and D, increased with tem- perature which suggests that they are formed by other mechanisms in addition to radical-radical reactions in the spur.

Water-cooled nuclear power reactors operate with either light or heavy water as the primary heat-transport medium. Since radiolysis of this water has been associated with corro- sion and hydriding of in-core components, computer pack- ages are being developed to model the radiation chemistry of the water as it passes through the reactor core. The computer models require a knowledge of the g values for the primary species formed from both fast neutron and y-radiations [see reaction (l)] as well as the rate constants for all of the reac- tions involving these species.

radiation H'O ___ - e,, OH, H, H', H'O', HO, (1)

The term ' g value' will be used for the yield of the primary species in reaction (1) CQ. s after the ionizing event and ' G value' will be used for the experimentally determined yields from which g values are deduced. This information is required for the operating temperature range (250-3 15 "C) found in nuclear reactors.

In this paper, estimates of the g values for the primary species formed in reaction (l), for both light and heavy water, have been made as a function of temperature for low linear energy transfer (LET) radiation. The results presented here supersede those given in our earlier paper for light water.'

Experimentally, the estimation of g values at higher tem- peratures is straightforward. The difficulty is in finding chemical systems which are both thermally stable above 100 "C and whose radiation chemistry is well defined. Pulse radiolysis experiments have a further complication ; the molar absorption coefficient of the species formed has to be known at the temperature being studied. Steady-state radiolysis experiments at high temperatures do not have this problem because all analyses are generally made at room tem- perature. The approach taken in this report was to estimate the g values of all the primary products over as wide a range of temperature as possible and then ensure that there was a material balance.

Experimental The chemicals (AK grade or better) were used as supplied. The light water was distilled, passed through a Millipore purification system and then redistilled from alkaline per- manganate. Reactor grade (99.8% D) heavy water was redistilled once from alkaline permanganate. When solutes containing exchangeable protons were added to D 2 0 solu- tions, the D 2 0 will have been degraded to, at worst, 98.5%.

The steady-state radiolysis procedures, sample preparation and gas analysis methods are described in an earlier pub-

lication.' Hydrogen peroxide was analysed by the Ghormley tri-iodide method' using a molar absorption coefficient at 350 nm of 25 500 dm3 mol- ' cm- '. Nitrite ion was analysed by the diazo-method3 using a molar absorption coefficient at 540 nm of 52860 dm3 mol-' cm-'. The presence of heavy water had no effect on these assays.

The high-temperature pulse radiolysis facility has been fully described in a recent r e p ~ r t . ~ In brief, a vessel that is pressurized to 10.3 MPa (1 500 psi) with helium is used to enclose the optical cell (1 cm) which is mounted within an insulated copper heating block, and the syringe which con- tains the experimental solution. The solution in the cell can be changed by allowing the pressure in the vessel to push the syringe plunger so that fresh solution flows through the cell. The waste solution is collected outside the vessel at atmo- spheric pressure. Only the solution in the cell is heated; a feature that helps minimize thermal decomposition of solutes. The temperature of the solution is monitored by a thermo- couple mounted in a thermo-well within the cell. The analys- ing light enters and leaves the vessel through 4.5 cm thick quartz windows; the 0.3 ps electron pulses from the 2.25 MeV? Van de Graaff electron accelerator enter the pressure vessel through a rupture disc (2 cm diameter). The pulse-to- pulse variations in the dose are monitored by the charge col- lected from the fringe of the electron beam before it enters the cell.

KSCN solutions were used for pulse radiolysis dosimetry. The value of G*E used was 2.39 x lo4 at 475 nm5 where G is defined as the number of species formed per 100 eV of energy absorbed and E, the molar absorption coefficient, is expressed in dm3 mol-' cm-'. Although there are a number of values used in the literature,6 the above value of G*E has been confirmed recent- ly by Buxton and S t ~ a r t . ~ In calculating the dose absorbed, changes in the electron and mass densities of the solution were taken into account.8 In general, the experimental pro- cedure was to carry out dosimetry before and after the high- temperature run ; the difference between the two dosimetries was less than 2%. During the high-temperature experiments, data were collected as the temperature was raised and also as it was lowered from the maximum temperature.

The temperature dependence of the molar absorption coef- ficient for the hexacyanoferrate(rI1) ion at 420 nm in light and heavy water was measured up to 85 "C in a thermostatted cell using a Hewlett-Packard HP8450A spectrophotometer.

Oxygen-saturated lo-' mol dm-

t 1 eV z 1.60218 x J.

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1194 J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

Results Radiolysis of Solutions containing Nitrite

The G values for H, and D, were measured from y-irradiated, degassed solutions of potassium nitrite in H,O and D,O. The concentration range of nitrite was (1-500) x mol dmP3 in H,O, and (1-100) x mol dm-3 in D,O. From plots of G(H,) and G(D,) us. the cube root of the nitrite ion con- centration, the values of g(H2) and g(D,) were obtained by extrapolation to zero nitrite concentration.' The values for g(H,) were 0.45 f 0.02 (25 "C), 0.50 f 0.02 (100 "C), 0.54 f 0.03 (200°C) and 0.65 _+ 0.03 (300°C). The values for g(D,) were 0.38 f 0.02 (25 "C), 0.44 f 0.02 (100 "C) 0.51 0.03 (200°C) and 0.60 f 0.04 (300°C). These values are shown in Fig. 1 and 2.

The g values measured at room temperature are in agree- ment with the normally accepted values of 0.45 and 0.36 in light and heavy water, respectively.8 The values for g(H2) are also in agreement with the temperature dependence reported by Kent and Sims," who used a heated small loop for their experiments rather than glass ampoules.

5 c -I

4

a 3 - m D

0 100 200 300 temperature/"(=

Fig. 1 g values for the primary species as a function of temperature. (a) g(0H): G[Fe(CN)z-] from air-saturated lop3 mol dm- j Fe(CN):- (+); G(C0;) from air-saturated 0.01 mol dm-3 HCO; (0). (b) g(ea.J: G(N0;) from degassed rnol dm-3 NO; and 5 x mol dm-3 phosphite (H); G(MV+) from deoxygenated 2.5 x mol dm-3 MV2+ and 0.01 mol dm-3 tert-butyl alcohol with either 2 x lop3 mol dm-3 phosphate buffer (A) or borate buffer (0). (c) g(H,) + g(H): G(H,) from degassed lop3 mol dmP3 NO; and 5 x lop3 mol dm-3 phosphite (A); G(H,) from degassed mol dm-3 acetone and 0.01 mol dm- j methanol (V). (e) g(H,O,): G(H,O,) from degassed 5 x mol dm-3 acrylamide (a). (f) g(H,): G(H,) from degassed 10-3-0.5 mol dm-3 NO; (0). The solid lines represent the appropriate eqn. (1)-(V) in the text. The dashed line (d) is g(H) calculated from the difference between eqn. (IV) and (111).

5

4

a 3 - m D

2

1

0

I

I I 1 1 - 1 1 I I I I I I 1 I 1 I - 0 100 200 300

tempera t urer C Fig. 2 g-values for the primary species as a function of temperature. (a) g(0D): G[Fe(CN)z-] from air-saturated rnol dm-3 Fe(CN):- (+); G(C0;) from air-saturated 0.01 rnol dmd3 DCO, (0). (b) g(ea;): G(N0;) from degassed mol dm-' NO; and 5 x mol dmp3 phosphite (W); G(MV+) from deoxygenated 2.5 x rnol dm-3 MV2+ and 0.02 mol dm-3 tert-butyl alcohol with 2 x mol dm-3 phosphate buffer (A). (c) g(D2) + g(D): G(D,) from degassed rnol dmp3 NO, and 5 x mol dm-3 phosphite (A); G(D,) from degassed mol dm-3 ['Hlacetone and 0.1 mol dm-3 [2H]methanol (V). (e) g(D,O,): G(D,O,) from degassed 5 x loF4 mol dmP3 acrylamide (a). (f) g(D2): G(D,) from degassed 10-'-0.1 mol dmp3 NO; (0). The solid lines represent the appropriate eqn. (VII1)-(XII) in the text. The dashed line (d) is g(D) calculated from the difference between eqn. (X) and (XI).

Radiolysis of Solutions containing Nitrate and Phosphite Ions

In both light and heavy water, degassed solutions containing mol dm-3 sodium nitrate and 5 x mol dm-3

sodium phosphite were y-irradiated at temperatures between 25 and 200°C. (This solution cannot be used above 200°C since hydrogen is generated thermally at 250 "C, although not at 200 "C.) This system" was used to estimate g(ea;), [g(H2)

As G(N0;) decreased slightly over the dose range (4- 30) x 10,' eV kg-', g(ea,J was equated to the value of G(N0;) when extrapolated to zero dose; the results are sum- marized in Fig. 1 and 2. In H,O at 25, 100 and 180"C, g(ea;) was estimated to be 2.65 f 0.05, 2.90 f 0.05 and 3.18 f 0.05, respectively. In D,O at 25, 100 and 200"C, g(ea;) was esti- mated to be 3.00 f 0.05, 3.20 & 0.05, and 3.45 f 0.05, respec- tively. The room temperature values for light and heavy water are in agreement with the accepted values of 2.63 and 2.95, respectively.8

The values of G(H,) and G(D,) in the nitrate-phosphite system are a measure of [g(H,) + g(H)] and [g(D2) + g(D)], respectively. These hydrogen (deuterium) yields were linear over the (10-100) x 10,' eV kg-' dose range studied. The

+ g(W1 and Cg(D2) + s(D)l*

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values of G(H,) were 1.01 & 0.03 (25"C), 1.18 k 0.03 (lOO"C), 1.31 f 0.06 (180°C) and 1.37 k 0.04 (200°C). For heavy water, the values of G(D,) were 0.70 & 0.02 (25"C), 0.85 & 0.03 (100 "C) and 1.04 & 0.07 (200 "C). These are sum- marized in Fig. 1 and 2.

Radiolysis of D,O Solutions containing Acetone and Methanol

As an alternative measure of [g(D2) + g(D)], G(D,) was mea- sured from degassed heavy water solutions containing 10- mol dmW3 C2H]acetone and 0.1 mol dm-3 C2H]methanol over the range 25 to 300°C. The values of G(D,) were 0.73 f 0.03 (25 "C), 0.83 & 0.04 (100"C), 1.11 & 0.03 (200"C), 1.62 (250°C) and 2.7 (300°C) and are shown in Fig. 2. As can be seen, the data are similar in nature to those reported for light water (reproduced in Fig. 1 in our previous paper'); above 200 "C there is a significant increase in deuterium yield. The values for (D,) up to 200°C agree well with the values obtained from the nitrate-phosphite solution described in the previous section (see Fig. 2).

To determine whether the dramatic increase in deuterium yield above 200°C originated from the solutes or the heavy water itself, the fully deuteriated solutes were replaced with the protonated analogues. Ideally, HD or H, can only orig- inate with the solute. The gas composition was analysed by mass spectrometry. No H, was detected and the ratio of D, to total [HD + D,] was 0.36, 0.46 and 0.92 at 25, 200 and 300 "C, respectively, indicating that the excess hydrogen at 300 "C came predominantly from the heavy water. This result invalidates the mechanism proposed earlier' to account for the excess hydrogen production. At present, however, no other explanation can be offered.

Pulse Radiolysis of Solutions containing Methyl Viologen

Deoxygenated solutions containing l,l'-dimethyl-4,4'-bipyri- dinium ion (methyl viologen, MV2+) were used to estimate the hydrated electron yield in both light and heavy water up to 200°C. Above this temperature, the thermal production of MV + prevented accurate data being rneasured.I2 Solutions which contained 2.5 x lop4 mol dmP3 MV2+ and 0.01 mol dm-3 tert-butyl alcohol (0.02 mol dm-3) in D,O were buf- fered with either phosphate or borate. At these solute concen- trations, the conditions were such that the principal reactions were :

ea; + MV2+ -+ MV' (2)

H (D) + MV2+ -+ MV+, products + H, (HD) (3)

OH (OD) + (CH3)3COH (D) -+ *CH,(CHj),COH (D)

+ H 2 0 (HDO) (4)

and scavenging in the spur was minimal. The yield of MV+ was monitored af 605 nm. Reaction (2) can easily be separat- ed from reaction (3); the latter reaction is over an order of magnitude slower than reaction (2) at room temperature.' The yield of MV+ at less than 1 ps was taken as the hydrated electron yield. A slower growth associated with reaction (3) could be seen at longer times. The spectra of MV' in light and heavy water had the same bandshape at the same temperature.

From the G*&,,, measured at 25°C for MV', an estimate for g(eai) of 2.75 :t 0.07 in H,O and 3.28 & 0.04 in D,O can be made using &So5 = 13 700 dm3 mol-' cm-', as determined by Watanabe and Honda14 for MV+ in light water. The light water g value is close to the accepted value of 2.638 whereas the heavy water g value is considerably higher than the nor-

mally accepted g(ea;) of 2.95.8 The heavy water results may simply reflect an error in the molar absorption coefficient used for MV' radicals. As it is the temperature dependence of g(ea;) that is required, rather than the absolute g values, the relative change with temperature has been determined.

Shirashi et al. l 2 have measured the relative dependence of &605 in water and reported that it decreased by 8 4% over the temperature range 25-200°C, although they did note the difficulties in producing a fixed concentration of MV+ to study as MV+ could either decay or be produced thermally depending on the conditions. When this dependence was applied to the G*~,o5 data, the relative increase of g(ea;) with temperature was greater than that observed from the nitrate- phosphite system. On the other hand, if it is assumed that &So5 is temperature independent, then the relative tem- perature dependence is the same as for the nitrate-phosphite system. This is shown in Fig. 1 and 2 for the MV+ data nor- malized to the nitrate-phosphite data at room temperature. Some support for a temperature invariant &605 comes from the work of Watanabe and Honda.14 They studied, at room temperature, MV + in water, methanol, ethanol and acetoni- trile by a spectroelectrochemical technique and found the molar absorption coefficient of the peak near 605 nm was, within experimental error, constant in all four solvents. As it appears that this molar absorption coefficient is not very sen- sitive to its environment, this suggests that raising the tem- perature to 200°C in water should not significantly change '605 .

Radiolysis of Degassed Solutions containing Acrylamide

The primary hydrogen peroxide yield was measured using 5 x mol dm-3 acrylamide as a scavenger to prevent the primary radicals from destroying the peroxide." For any given temperature, the measured G(H,O,) or G(D,O,) decreased slightly over the dose range (2-12) x 10,' eV kg- ' ; g(H202) and g(D202) quoted here are the values of G(H202) and G(D,O,) extrapolated to zero dose. For H,O, the values measured for g(H,02) at 25, 70 and 100°C were 0.69 & 0.02, 0.61 & 0.04 and 0.58 & 0.03, respectively. For D 2 0 , the values for g(D,O,) at 25 and 100°C were 0.70 & 0.03 and 0.61 & 0.03, respectively. The results are shown in Fig. 1 and 2. As can be seen, g(H,O,) and g(D,O,) both decrease with increasing temperature. Thermal decomposition of the per- oxide was not a factor in these experiments since the samples were quenched in ice-water within 5 min of completion of the irradiation; the half-life for the thermal decomposition of hydrogen peroxide in 5 x mol dmW3 acrylamide solu- tions at 100°C is greater than 16 h.

In light water, the decrease in g(H,O,) with increasing temperature has been confirmed over the temperature range 25-270°C by Kent and Sims" in high-temperature loop experiments using slightly alkaline solutions containing iodide ions and N,O.

Pulse Radiolysis of Aerated Solutions containing Hexacyanoferrate(I1)

Aerated solutions containing mol dm-3 potassium hexacyanoferrate(1x) were pulse-irradiated over the 20-105 "C temperature range in order to estimate the hydroxyl radical yields in light and heavy water. We are restricted to this tem- perature range as the solution is not thermally stable at higher temperatures. In this case, ea; and H(D) were scav- enged by oxygen, reactions (5) and (6), and the hydroxyl radical reacted to form the hexacyanoferrate(1xx) ion by

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1196 J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

reaction (7)'

ea; + 0, -+ 0;

H (D) + 0, + HO', (DO',) -+ 0, + H + ( D f ) (6)

OH (OD) + Fe(CN):- -+ Fe(CN);- + OH- (OD-) (7)

The molar absorption Coefficient of the hexacyano- ferrate(rr1) ion at 420 nm was found to be 1040 dm3 mol-' cm-' at 23 "C and decreased linearly with temperature at the rate of 3.08 dm3 mol-I cm-' "C-' in both light and heavy water. Based on these molar absorption coefficients, and their temperature dependence, the values for G[Fe(CN): -3 mea- sured in light water were 2.80 f 0.05 (22"C), 3.20 & 0.05 (80 "C) and 3.40 k 0.05 (105 "C); for heavy water they were 2.80 f 0.05 (22 "C), 3.30 f 0.05 (80°C) and 3.50 f 0.05 (105 "C). The agreement with the generally accepted values for g(0H) of 2.72 and for g(0D) of 2.84 at room temperature is reasonable.' These results are shown in Fig. 1 and 2.

Pulse Radiolysis of Potassium Hydrogencarbonate Solutions containing Oxygen

Aerated or oxygen saturated solutions containing 0.10 mol dm - hydrogencarbonate were pulse irradiated to estimate the yield of the hydroxyl radical up to 300°C. In these solu- tions, the ea; and H (D) reacted with oxygen [reactions (5) and (6)] and the hyroxyl radical reacted as follows:

OH (OD) + HCO, (DCO,) -, COY + H,O (D20) (8)

At these hydrogencarbonate concentrations, the yield of CO, will be a measure of the yield of the hydroxyl radical that escapes the spur. Values for G*E were obtained at 600 nm for light and heavy water for the temperature range 20-300 "C. Over this temperature range, the spectral band shapes for CO, in light and heavy water were identical. This is shown in Fig. 3 for the spectra recorded at room temperature and at 280-290°C. As can be seen (Fig. 3), the spectra broadened slightly on the high-energy side of the peak as the tem- perature was increased and, at the highest temperature, the Amax appeared to shift to 595 nm from 600 nm. The band- widths (in eV) at half-height were 0.66 (25 "C), 0.695 (75 "C), 0.72 (150 "C), 0.77 (280 "C).

The G*&600 data for the COT radical could be determined up to nearly 300 "C from potassium hydrogencarbonate solu- tions. Provided the temperature dependence of the E600 is known, an estimate of the temperature dependence of g(0H) and g(0D) can be made. For COT, the reported room tem- perature valueI6 for E~~~ of 1860 dm3 mol-' cm-' should be

I 1 I I I I I 1

4 fl

X 0

I I I 1 I I !b LOO 500 600 700 800

wavelengt h/nm Fig. 3 Spectrum of CO; at room temperature in H,O (0) and D,O (m); at 280-290 "C in H,O (A) and D,O (V)

revised to 1934 dm3 mol-' cm-'. In the original study,I6 csOO for CO, was measured relative to the molar absorption coefficient for the hexacyanoferrate(rI1) ion at 420 nm being 1 OOO dm3 mol-' cm-' rather than 1040 dm3 mol-' cm-l. Using the 0.1 mol dm-3 hydrogencarbonate data and the new molar absorption coefficient, a G(C0,) of 2.64 & 0.10 is calculated for both light and heavy water at room tem- perature. Both values are slightly lower than the generally accepted g values for OH of 2.72 and for OD of 2.84.*

Fig. 1 and 2 show the temperature dependence of g(0H) and g(0D) assuming that E600 does not change with tem- perature and normalizing the data to G[Fe(CN):-] at 25 "C. Within the experimental scatter, the agreement between the CO, and hexacyanoferrate(I1r) data is satisfactory. However, as an alternative approach, if it is assumed that the integrated absorption coefficient is constant with temperature, then the E~~~ temperature dependence should follow the inverse of the spectral bandwidth at half-height." Using the bandwidth data given above, this would predict values for g(0H) and g(0D) based on COT which are 7, 13 and 18% greater at 100, 200 and 300"C, respectively, than the results shown in Fig. 1 and 2. However, these g-values were found to be incon- sistent with a material balance for the total radiolysis.

Discussion The temperature dependences of g values for both light and heavy water are summarized in Fig. 1 and 2. The lines in the figures are considered to be the best g values. For e,;, the G(N0,) values were used; for OH and OD, the G[Fe(CN);-] values were used. The MV+ and CO; results were considered only as confirmation of the other results.

The temperature dependences of the g values for light water as shown in Fig. 1 are given by the following equa- tions :

g(ea;) = 2.56 + 3.40 x lop3 t (1)

g(0H) = 2.64 + 7.17 x t (11)

g(H2) = 0.43 + 0.69 x t (111)

g(H2) + g(H) = 0.97 + 1.98 x t (IV)

g(H,O,) = 0.72 - 1.49 x t (V)

where t is the temperature in "C.

perature dependence of g(H) can be calculated as: From the difference between eqn. (IV) and (111), the tem-

g(H) = 0.54 + 1.28 x t (VI)

The criterion for a consistent set of g values is a material balance such as eqn. (VII),

Gred = = g(-H20) (VII)

where Gred is given by [g(ea;) + g(H) + 2g(H,)] and Go, by [g(OH) + 2g(H20,)]. For light water the material balance, based on the experimental g values, was excellent; at 25"C, Gred = 4.1 1 k 0.10 and Go, = 4.18 & 0.09 while at 100 "C,

For heavy water, the temperature dependences of the g values as given by the lines in Fig. 2 are described by the following equations :

(VIII)

Gred = 4.58 & 0.10 and Go, = 4.56 f 0.11.

g(ea;) = 2.94 + 2.57 x l op3 t

g(0D) = 2.62 + 8.47 x t (IX)

g(D2) = 0.36 + 0.77 x t (X)

g(D,) + g(D) = 0.65 + 1.94 x t (XI)

(XII) g(D,O,) = 0.73 - 1.20 x lop3 t

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The temperature dependence of g(D) can be calculated as:

g(D) = 0.29 + 1.17 x t (XIII)

For heavy water the material balance, while satisfactory, was not as good as for light water. At 25"C, Gred = 4.08 f 0.09 and Go, = 4.20 f 0.1 1 while at 100 "C, Gred = 4.49 +_ 0.10 and

Note that the above equations only describe the experi- mental g values of the individual primary species as a func- tion of temperature, they do not give an exact material balance for the loss of water.

The above results, with the exception of g(H2) and g(D,), conform to the generally accepted diffusion-kinetic model of spurs." The effect of temperature on the rate of many of the near-diffusion-controlled spur reactions in light water has now been studied. For reactions (9)-(12),

Go, = 4.71 +_ 0.11.

ea; + ea; -, H, + 2 0 H -

ea; + H,O, -+ OH + OH-

H + OH -,H,O

OH + OH -+ H 2 0 ,

(9)

(10)

(1 1)

(12)

the increase in rate of these reactions in the spur is less than that of diffusion of the reactants out of the spur." The net result of this is, as the temperature increases, that g( -H,O) increases, the free radical yield that escapes the spur increases, and the yield of the molecular products decreases. This is the pattern that is observed with the exception of

The diffusion-kinetic model of the spur predicts that the dH2) and

molecular hydrogen arises from reactions (9), (13) and (14).

H + H + H , (13)

H + ea; -+ H, (14)

Reaction (9), at least in alkaline solution, slows drastically with an increase in temperature over 150 OC,,' reaction (13) is diffusion controlled' 1*22 and the temperature dependence of reaction (14) has not been reported. However, not all of g(H2) can be predicted by this model at room temperature. A small prompt yield of H, has to be incorporated into the and this yield has been suggested as arising through a number of possible mechanism^.^^ Two of the mechanisms invoke the decomposition of electronically excited water (H,O*) to form H, and two OH radicals either by reactions (15) and (16) or by reactions (17) and (18) where H* is a kinetically hot hydrogen atom.

H20* -, H, + 0

H 2 0 * -+ H* + OH

H* + H,O -+ H, + OH

(15)

(16)

(17)

(18)

0 + H,O -+ 2 0 H

Another mechanism postulates hydride transfer, others talk about H,O as a prec~rsor . ,~

At present, no definitive mechanism can be suggested to account for the increase in g(H2) and g(D2) with tem-

perature. A similar increase in g(H,) has been observed in the effect of temperature on water radiolysis using a 23 MeV ,H+ ion beam and other ion-beam experiments are planned to study g(H,) at higher LETS., Pulse radiolysis experiments are also in progress to complete the characterization of the temperature dependence of all spur reactions. When this is completed, computer simulations of the reactions in the spur will be undertaken to see whether this can resolve the mecha- nism of formation of g(H2).

This work was co-funded by the CANDU Owners Group. The authors thank Dr. G. V. Buxton (Univ. of Leeds), Dr. H Sims (AERE, Harwell) and Dr. D. M. Bartels (Argonne National Laboratory) for discussions during the course of this work. M. James is thanked for the mass spectrometric determinations of the hydrogen isotope ratios.

References 1

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