Some studies of the effect of thermal and radiolytic oxidation on the neutron small angle scattering from nuclear graphites

Cdm Vol. 16. pp. I?%?03 0 Pergamon Press Ltd.. 1978.

Printed in Great Britain

SOME STUDIES OF THE EFFECT OF THERMAL AND RADIOLYTIC OXIDATION ON THE NEUTRON SMALL ANGLE SCATTERING FROM NUCLEAR GRAPHITES D. G. MARTIN and J. CAISLEY UKAEA,

Metallurgy

Division, Atomic Energy Research Establishment,

Harwell,

Oxon 0x11

ORA, England

(Received IO November 1977) Ah&act-The neutron small angle scattering from electrographite blocks derived from petroleum coke and a gilso carbon has been measured before and after thermal or radiolytic oxidation. For both materials thermal oxidation results in an increase in the scattering whereas radiolytic oxidation causes a decrease. These observations can be explained qualitatively in terms of our present understanding of the respective oxidation mechanisms together with reasonable assumptions concerning the porosity size distribution. In the case of the petroleum coke graphite the addition of trace quantities of methane to the CO2 atmosphere was shown to inhibit to a comparable extent the enlargement of small pores (=2.5-10nm radius, but excluding from consideration the long fine needle shaped pores) and the larger ones (= l-10 pm) which contribute most significantly to the weight loss. 1. INTROOUCTION

The radiolytic oxidation of graphite in a carbon dioxide atmosphere is of technological relevance to the U.K. gas-cooled graphite moderated nuclear power stations. As a result much work has been performed both to find satisfactory methods of inhibiting this oxidation and also to elucidate the fundamental chemical reaction mechanisms [ 1,2]. For the purpose of the present paper it is sufficient to note from Refs. [l, 21 that for a particular nuclear graphite this oxidation can be inhibited by small quantities of impurities such as CO and CH4 in the coolant; also that for a given gaseous composition, the oxidation is dependent on the details of the graphite porosity. In particular Campion et a/.[31 have shown that the radiolytic oxidation of graphite in CO1 containing CH4 + CO additives takes place mainly in pores in the OS-2pm pore entrance diameter range. Porosity measurements based on a variety of techniques were employed in Ref. 131in order to arrive at this conclusion. The present work extends such measurements by using neutron small angle scattering to study the very small pore sizes. This particular technique has been applied to nuclear praphites previously. For example, it can be used to identify the coke source of a particular material [4] and to study porosity changes due to the neutron irradiation[5]. In the present work changes in the 25100A porosity due to the thermal and the radiolytic oxidation of two different types of nuclear graphite are reported.

2. -TAL

=AlIS

The neutron small angle scattering measurements were performed using the Dll camera at I.L.L. Grenoble[6]. A collimated monochromatised beam of 9 A neutrons (i.e. of wavelength sufficiently long to avoid Bragg reflections) was incident on the sample. Scattered neutrons were detected simultaneously in two dimensions; however, since the scattering was observed to be in-

dependent of the azimuthal direction, results from each sample could be condensed into a single curve describing the scattering as a function of angle. The range of scattering vector Q values studied (Q = 4n (sin @)/A, where 2 0 is the scattering angle and A the neutron wavelength) was 1.4-9.4 x 10-l nm-‘. Scattered counts were converted into differential cross sections by means of calibration runs involving a water cell. Samples from two types of moderator quality electrographite blocks were studied, designated in Ref. [3] as FL (derived from petroleum coke) and BMP (derived from gilso carbon). These types of graphite are manufactured by mixing calcined coke with a suitable pitch binder. The mixture undergoes a number of heat treatments and is then converted into a solid bar by either moulding or extrusion. In order to increase the density of this material a series of pitch impregnations followed by heat treatments is often resorted to before the final graphitisation at high temperature. The blocks therefore contain a mixture of graphitised binder, impregnant and coke. In every case the scattering from specimens which had each undergone a particular type of oxidation was compared with similar unoxidised control samples. The effect of three sorts of oxidation of FL graphite was studied:(a) Thermal oxidation at 480°C in air to a fractional weight loss of 5.1%. (b) A 23.9% weight loss due to radiolytic oxidation in CO2 containing 2.1% CO/l90 vpm CH,/210 vpm HzO. (c) Radiolytic oxidation in COZ containing 1OOOvpm CO to two different amounts of weight loss, namely 13.1% and 25.5%. In the case of BMP graphite two different oxidation procedures were employed:(a) Thermal oxidation at 480°C in air to a weight loss of 10.8%. (b) Radiolytic oxidation in COZ containing 0.2% CO and IO-20 vpm Hz0 to an 11.8% weight loss. These

D. G. MARTIN and J. CANBY

200

oxidation procedures are summarised in Table I; note that subsequently for the sake of brevity radiolytic oxidation in atmospheres which contain, or do not contain, CH, will be described as inhibited, or uninhibited, respectively. In addition, samples of BMP graphite, which had been impregnated with a thermally setting resin, together with unimpregnated material and solid samples fabricated solely from the resin, were studied. The open and closed porosity of this particular graphite was 10.6 and 9.3 m3 per 1OOm’ respectively, and 2.3 m’ per 100m3 of this former figure was eliminated by the impregnation. For all the types of material described above specimens in the form of slabs of two different thicknesses, of about 2 mm and 4 mm respectively, were measured. The only exceptions were the uninhibited radiolytically oxidised FL specimens and their controls which were cylindrical, of diameter about 4mm, and mounted such that their axes were perpendicular to the incident beam. 3.RKWL.m A Guinier plot was made of the scattering from each sample, e.g. see Fig. 1. In each case the scattered counts were normalised to a specimen thickness of 5 mm. This normalisation assumed that the scattering is directly proportional to the thickness, an assumption which, from measurements on the two sample thicknesses of the same type of material, was shown to be quite a good approximation. This implies that the results are not affected significantly by multiple scattering effects and that where, as in most cases, measurements on two sample thicknesses have been made, the normalised results constitute two independent measurements on the same type of material. Figure 1 illustrates the effect of oxidation on the scattering from the thicker samples of FL graphite. For the sake of clarity, data points, which all lie either on or extremely close to the lines, have been omitted. It is evident that thermal oxidation increases the scattering, whereas inhibited radiolytic oxidation causes a decrease; although not shown in Fig. 1, it was also found that, in like manner, uninhibited radiolytic oxidation results in a decrease. An increase in the scattering due to thermal oxidation and a decrease due to uninhibited radiolytic Table 1. Fractional changes

Method of oxidation or whether impregnated

Thermal Inhibited radiolvtic

Uninhibitedradiolytic

104

2 02,

Fig. 1. Guinier plot of the small angle scattering from FL graphite. -. as received; ---, thermally oxidised; .. ..., radiolytically oxidised (inhibited). oxidation was found to occur in BMP graphite also. On

the other hand, changes in the scattering from BMP graphite as a result of the impregnation were comparatively smaller, a slight decrease being observed. In an attempt to quantify these differences, tangents have been drawn to each curve whose slopes correspond to pores with radii of gyration of respectively 10 and 2.5 nm. Extrapolation of these tangents to obtain the scattered counts at Q2= 0 then yields cross sections which to a first approximation represent the scattering from pores over a reasonably small range of pore sizes (~30%) around the above values. Changes in the cross sections as a result of oxidation or impregnation will then give a measure of the change in concentration of the 10 and 2.5 nm porosity respectively. Table 1 lists average values of these fractional changes for the different sample thicknesses examined. The accuracy of these ratios is typically 5%. In some cases larger differences occur between nominally identical samples of different

in the

cross sections of 10 and 2.5 nm pores due to oxidation or impregnation

Weight loss (%) FL Graphite 5.1 23.9 13.1 25.5

Ratio of oxidised or impregnated/control cross sections 10nm 2.5 nm 1.66 0.82 0.99 0.87

1.34 0.70 0.92 0.74

1.36 0.91 0.96

1.04 0.92 O.%

BMPGraphite Thermal Unhibited radiolytic Impregnated, fractional porosity change = 1I .6%

nm-’

10.8 11.8

201

Some studies of the effect of thermal and radiolyticoxidation

thickness. However, since no systematic v~ation with thickness was observed it seems reasonable to combine the results, as has been done in Table 1, for the purpose of the subsequent discussion. Values of the 10nm and 2.5 nm cross sections at Q* = 0 for control samples are typically 5000 and 38 barnslsheradian (FL graphite) and 5000 and 48 barnslsteradian (BMP graphite) respectively+ Results from the resin samples are omitted from Table I; this is because the scattering is so small that it may be neglected in the subsequent discussion. Thus the 2.5 nm radius cross section is about 5 barnslsteradian, Also because the scattered intensity varies so weakly with angle, it is not possible to derive a 10 nm radius cross section over the range of Q values studies here. As a result any modification to the scattering due to impregnation will be almost entirely due to the partial elimination of porosity in the graphite and the cont~bution from the resin that has been introduced may be neglected. A DWUSSMIN First of all, it is perhaps worth emphasising that neutron small angle scattering detects both closed and open porosity and that the present measurements observe a rather limited range of the very small pores. Thus, analysing the results in terms of pores with radii of gyration of 10 and 2.5 nm implies that pores whose maximum dimension is greater by a fairly small factor, say three times these figures, will produce a negligible contribution to the neutron scattering over the range of Q values studied here. As a result we need not consider long needle shaped pores, including the narrow ones which are believed to interlink large (3 10 pm) pores [7]. To be strictly accurate the scattering that is observed will originate from regions of disorder in - the 2.5-10 nm size region as well as from pores. However, for the sake of simplicity we shall assume that any such disordered regions are unaffected by the oxidation, so that the effect of oxidation on the scattering is entirely due to modifications in the pore size distribution. It is of interest to compare the changes in scattering due to oxidation with those reported previously as a result of neutron irradiation at 1050°C[5]. These changes due to oxidation and irradiation have been correlated with the accompanying fractional weight losses and volume changes respectively. Now, to a first approximation, changes in both of these parameters can be equated with changes in the overall porosity of the material. However, a 2% volume change due to neutron irradiation alters the neutron scattering by typically a factor 2, whereas a 25% weitht loss due to radiolytic oxidation changes the scattering by only -30%. This shows, therefore, that far more significant changes in the distribution of very small pore sized occur after neutron irradiation than as a result of oxidation. Considering next the present resuIts in total, the most significant overall qualitative conclusion is that thermal oxidation produces an increase in the scattering whereas radiolytic oxidation results in a decrease. These observations can be understood in terms of our current understanding of the two oxidation mechanisms. In the case of thermal oxidation all surfaces can be.

imagined as receiving a comp~able flux of the oxidising species. As a result, for a specific level of oxidation all pores will increase their linear dimensions by the same amount, which implies that the difference between the largest and smallest pore size is un~ected by the oxidation. Now, whether the number of pores increase or decrease within a given size range is determined by the difference between the numbers entering and leaving this particular size range as a result of the oxidation. However, if the pore concentration in the as-received material decreases with increasing pore size, as indeed the shapes of the present scattering curves indicate, an increase in the number of pores within a given size range wilt occur and the scattering thereby enhanced, in keeping with the observations. Figure 2(a) illustrates this schematically. The solid line represents the simplest conceivable pore size distribution in the as-received material and the dashed line the dis~bution after oxidation, while the dotted lines and arrows indicate the way the oxidised distribution was derived. One small modification of Fig. 2(a) is necessary in order to make it more realistic. This figure suggests that after oxidation no pores will occur in the smallest size range; however thermal oxidation can also occur at impurity sites so that these will act as sources for the nucleation and growth of very small pores. By contrast the overall radiolytic oxidation of a pore is believed to be proportional to its volume if the mean free path of the active species is sufficiently greater than the

\ \ Pore size, linear scale

(a)

Pore size,

linear scale

(b) Fig. 2. Schematicillustrationof the changein pore size dist~bution due to (a) thermal and (b) radiotytic oxidation. -, as received;

---9 oxidised.

202

D. G. MARTINand J.

pore size. As a result, for a specific level of oxidation, all pores wih increase their linear dimensions by the same fraction with the result that the range of pore sizes in the solid is increased by the oxidation; accordingly this affects the number of pores within a particular size range. Figure 2(b) illustrates schematically an example of how a very simple pore size distribution is altered. The dotted lines and arrows indicate how the pore size distribution after oxidation is derived; the abscissa value of each point on the as-received distribution is increased by the same fraction as the ordinate value is reduced so that the areas under each distribution are the same. Figure 2(b) illustrates how, for smaller pore sizes, the number of pores over a certain size range decreases with oxidation, and hence the scattering also, in keeping with the observations. It is interesting to compare the changes in scattering due respectively to the inhibited and unhibited oxidation of FL graphite. From Table 1 it is evident that for the same weight loss there are comparable reductions in both the 10 and 2.5 nm radii cross sections. This suggests that the enlargement rate of these small pores, compared with that of the larger pores (=I-lOpm[7]) which contribute more significantly to the weight loss, is little affected by the introduction of methane, i.e. the fractional inhibition of the enlargement of small and large pores is comparable, assuming that scattering from deposits following inhibited oxidation may be neglected over the Q range investigated here. The suggestion that inhibition by methane is virtually independent of pore size has been advanced previously[8]. On the other hand conclusions based on the present work do not invalidate the model advanced in[7] where a significant depletion of the methane concentration in small pores is proposed; this is because Ref. [7] considered only those small pores which had appreciable tortuosity and these are invisible in the present work. As a result of these experimental observations we shah not make any distinction between inhibited and uninhibited radiolytic oxidation in the subsequent discussion. Considering now the thermal oxidation results it is evident from Table 1 that for a particular fractional weight loss a significantly larger change in both the 10 and 2.5 nm porosity occurs in FL compared with BMP graphite. A similar trend has been observed in the 3SOnm BJH pore volumes, and possibly also in the 501OOnm mercury porosimetry volumes, when thermal oxidation effects from these two graphite types are compared(31. It is also noteworthy that these latter two measuring techniques predict increases in the number of such pores as a result of thermal oxidation, in agreement with the corresponding increase in neutron scattering observed here. By contrast the radiolytic oxidation results indicate comparable fractional changes in the 10 and 2.5 nm radii cross sections from the two types of graphite for a given percentage weight loss. In this connection it is relevant to note that mercury porosimetry measurements of FL graphite before and after inhibited oxidation[7] and also of BMP graphite, using an uninhibited atmosphere (P. Schofield, SNL Springfields, private communication),

CAISLEY

have been made. At the smallest pore entrance diameters measureable, (-20 nm), a decrease in the number of pores occurs as a result of oxidation, the effect being rather greater in FL compared with BMP graphite. This is in qualitative agreement with the decrease in neutron scattering after radiolytic oxidation, though not with the comparable cross section changes mentioned above in the two types of graphite. It is evident from Table 1 that there is a greater fractional change in the 10 nm radius cross sections (i.e. the ratio deviates more from unity) compared with the corresponding 2.5 nm radius cross section values as a result of thermal oxidation; by contrast, the situation is reversed after radiolytic oxidation, i.e. there is a smaller fractional change in the 10nm porosity compared with that around 2.5 nm. From a comparison of these observations with the changes in pore size distributions of Fig. 2, it is evident that qualitative agreement is obtained in the radiolytic oxidation case but not after thermal oxidation. However, a qualitative correlation after both types of oxidation is possible if it is assumed that the number of pores as a function of size increases, but at a decreasing rate as the pore size decreases (i.e. the as-received pore size distribution plotted as in Fig. 2 exhibits a negative radius of curvature). Another observation from the very limited evidence of Table 1 is that fractional changes in the cross sections do not appear to vary linearly with weight loss, the tendency being for them to accelerate as the oxidation proceeds. As has already been observed, changes in the cross sections due to impregnation are comparatively small and, indeed, of questionable significance. By analogy with the other results presented here such a small change is not surprising since the fractional porosity change is only 11.6%, whereas the comparable figure relating to the oxidation results is -2O-100%. We conclude therefore that for this particular level of impregnation the major changes in porosity occur at sizes larger than those studied here. 5. SUMMARY AND CONCLUSIONS 1. Neutron small angle scattering measurements have been made on electrographite blocks derived from a petroleum coke and a gilso carbon (designated FL and BMP respectively) before and after thermal and radiolytic oxidation. These measurements enabled changes in porosity over the 2.5-10 nm radii of gyration range to be studied. Note that the radius of gyration concept implies that pores whose maximum dimension is greater than a few times these figures, e.g. long fine needle shaped pores, are not observed. 2. For both types of material thermal oxidation produces an increase in the scattering, whereas radiolytic oxidation causes a decrease. These observations can be explained on the basis of our current understanding of these oxidation mechanisms, together with reasonable assumptions concerning the way the number of pores per unit volume varies with pore size. 3. In the case of FL graphite a comparison between the effects of inhibited and uninhibited radiolytic oxidation was made, i.e. between oxidation in a CO2 at-

Some studies of the effect of thermal and radiolyticoxidation

mosphere which did and did not contain small quantities of methane respectively. It was found that the methane inhibits the oxidation rate of these 2.5-10 nm radii pores and the larger pores which are responsible for most of the weight loss to a comparable extent. 4. A 20% decrease in the open porosity of BMP graphite, produced by impregnating samples with a thermally setting resin, did not signitkantly affect the neutron small angle scattering. 5. A comparison between changes in porosity due to oxidation, observed respectively by neutron small angle scattering and by mercury porosimetry around the small pore detection limit region, reveal some but, as might be expected, not complete correlation. Acknowledgements-The assistance of many people in the supplyof samplesandperformanceof the experimentsis gratefully acknowledgedand, in particular,the following:J. S. Higginsand G. Kostorz (ILL Grenoble), R. J. Blanchard and C. Koch (CEN

203

Grenoble), P. Campion (SNL Springfields) and J. K. Linacre (AERE Harwell). REFERENCES

I. J. Wright, l2fh Biennial Conference on Carbon, Extended Abstracts and Program, July 28-August I 1975.Pittsburg, Pennysylvania,p. 183. 2. C. Tyzak and H. C. Cowen. At. Energy Rw. 14,263 (1976). 3. P. Campion, R. Lind, R. J. Blanchard and C. Koch, 4th London International Carbon and Graphite Conference, Extended Abstracts, 23-27 September 1974 (Society of Chemical Industry, London) p. 149 (1974). 4. D. G. Martin and J: Caisley, 1. Nucl. Mater. 67,318 (1977). 5. D. G. Martin and J. Caislev. Carbon IS. 251 (1977). 6. K. Ibel, J. Appf. Crys!. 9, 296 (1976). 7. P. Campion and R. J. Blanchard, Carbon ‘76, 2nd International Carbon Conference, Baden-Baden Preprints, 27.6-2.7 1976 (Deutsche Keramische Gesellschaft).p. II4 (1976). 8. J. V. Best, A. J. Wickham and C. J. Wood, Co&on ‘76,2nd International Carbon Conference, Baden-Baden, Preprints. 27.6-2.7 1976 (Deutsche Keramische Gesellschaft)p. 109.