Journal of NUCLEAR SCIENCE and TECHNOLOGY, 22[3J, pp. 225-232 (March 1985). 225 TECHNICAL REPORT Irradiation Behaviors of Nuclear Grade Graphite in Commercial Reactor, (II) Thermal and Physical Properties Hideto MATSUO and Tamotsu SAITO Department of Fuels and Materials Research, Tokai Research Establishment, Japan Atomic Energy Research Institute* Received April 26, 1984 Revised September 7, 1984 Thermal conductivity, electrical resistivity and stored energy were measured for Pechiney nuclear grade graphite irradiated in the temperature range 220~400dc up to the maximum neutron fluence 2.2x1020 n/cm2 (5>0.85 MeV) in the environment of a carbon dioxide in a commercial reactor. Thermal conductivity decreased, electrical resistivity and stored energy increased owing to neutron irradiation and their changes were larger for the samples irradiated at lower temperatures. A linear relation between stored energy and fractional change in thermal resistivity was obtained for the irradiated samples and it was found that its proportional constant is about two times of that reported previously. The relation between thermal conductivity and electrical resistivity is discussed for irradiated samples as well. KEYWORDS: graphite, irradiation, radiation effects, co:nmercial reactors, thermal conductivity, thermal resistivity, electrical resistivity, stored energy, neutron beams I. INTRODUCTION An irradiated graphite has significant thermal propoperties that should be carefully considered beforehand in the design and safety analysis of a graphite moderated reactor which operation temperature is low. Stored energy is one of the properties and special attention has been paid to the stored energy of the graphite used in a low temperature reactor since the accident of the Windscale reactor, which was caused owing to the release of stored energy in 1957. It has also been reported that stored energy is related to the change in thermal conductivity for the irradiated Pile Grade A graphite(1), however there is no report on the relation for other graphite materials. One of the objectives of the present study is to confirm whether the relation holds for Pechiney nuclear grade graphite or not. It is known that thermal conductivity has a linear relation to electrical resistivity measured at room temperature for unirradiated graphites(2)(3), and one of the authors has already reported the relation of the graphite irradiated to low neutron fluence(4)(5). The other objective of this study is to confirm whether the relation holds for the graphite material irradiated to higher neutron fluence. * Tokai-mura, Ibaraki-ken 319-11. 61
226 TECHNICAL REPORT (H. Matsuo, T. Saito) J. Nucl. Sci. Technol., This is the second one of a series of papers reporting the results of irradiation behaviors of the nuclear grade graphite. Dimensional changes and thermal expansion were discussed in the first report"). The present paper describes the effect of neutron irradiation on thermal conductivity, electrical resistivity, stored energy, and the relation between those properties for the graphite material irradiated in the commercial reactor. II. EXPERIMENTAL PROCEDURE 1. Sample The properties and forming method of Pechiney nuclear grade graphite used in the present experiment have already been reported in detail in this journal(6). Specimens for the measurements of thermal conductivity and electrical resistivity were cut parallel or perpendicular to the extrusion direction. The size of the specimens was 6.35 mm in diameter and 76.2 mm long. Sample for the measurement of stored energy was prepared by pulverizing an irradiated specimen of 12.7 mm in diameter and 50.8 mm long. The particle was smaller than 100- mesh. 2. Irradiation Irradiation was carried out in the Tokai Nuclear Power Station of Japan Atomic Power Co. (JAPCO) and irradiation conditions for the samples have already been reported in detail in this journal(6). 3. Measurements (1) Thermal Conductivity Kohlrausch method was applied to the measurement of thermal conductivity at 20dc and the schematic diagram is shown in Fig.1. Electrical current passing through a sample during the measurement was adjusted to be 5~20 A, which depended on electrical resistivity, in order to keep the mean temperature of the sample 20dc. Temperatures were measured at three points, one is at the center of the sample and the others were at the two points aparting 30 mm from the center of the sample. The temperature differences between the center and the other two positions were about 8dc and the mean temperature of the sample was controlled to be 20dc. The environment of the sample was in a vacuum of about 0.13 Pa in order to reduce the heat loss from the sample in the radial Fig. 1 Schematic diagram for measurement of thermal conductivity at room temperature 62
Vol. 22, No. 3 (Mar. 1985) TECHNICAL REPORT (H. Matsuo, T. Saito) 227 direction. Measurements were carried out three times on one sample and the average of these measured values were taken to be the typical measured value for the sample. 2) Electrical Resistivity Electrical resistivity was measured at room temperature by using a potential drop method. Measurements were carried out three times on one sample, changing the measured positions, and the average value of three measurements was taken. 3) Stored Energy ( Stored energy was obtained from the difference of heat of combustion for an irradiated and an unirradiated samples. The heat of combustion of the samples was measured by using a bombcalorimeter. Some 0.6 g of the sample and 0.2 g of liquid paraffin to promote burning the sample were burned at the same time in the oxygen atmosphere of 2.9 MPa. Measured value for an unirradiated sample was 7,799+-8 cal/g. Five measurements were done for one irradiated sample, on the average, and the average of three measurements were taken to be the typical measured value for the sample, excluding the maximum and minimum value. III. RESULTS AND DISCUSSION ( 1. Thermal Conductivity Fractional changes in thermal resistivity for parallel and perpendicular cut specimens are presented in Fig.2(a) and (b) as a function of thermal neutron fluence. Thermal resistivity increased owing to neutron irradiation and its rate of increment gradually decreased with neutron fluence. The increment was higher for the samples irradiated at lower temperature. Figure 3 shows the change in anisotropy ratio of thermal conductivity with neutron fluence. While the scattering of the measured values are large, the anisotropy ratios were almost constant up to the maximum neutron fluence for all samples and the changes did not depend on irradiation temperature as well. 2. Electrical Resistivity Figure 4(a) and (b) show the fractional changes in electrical resistivity for parallel and perpendicular cut specimens with neutron fluence. Electrical resistivities showed sharp increments in the early stage of irradiation and then (a) Parallel direction (b) Perpendicular direction 2(a), (b) Change of thermal Fig. conductivity in parallel and perpendicular directions to extrusion as a function of neutron fluence 63
228 TECHNICAL REPORT (H. Matsuo, T. Saito) J. Nucl. Sci. Technol., became almost constant values with increase of neutron fluence. The rate of the increment was a little larger for parallel cut specimens than perpendicular ones, however they showed almost a similar tendency with neutron fluence. Changes in anisotropy ratio of electrical resistivity are shown in Fig.5. No change was observed, though the scattering of the measured values is large just like the case of thermal conductivity shown in Fig.3. 3. Stored Energy Stored energy accummulated by neutron irradiation at low temperature is suddenly released when the stored energy is sufficiently high and the release rate of the stored energy exceeds the specific heat of a graphite, and consequently the graphite is spontaneously heated to higher temperatures without external heating. Therefore stored energy has been carefully studied. The experimental results on stored energy are presented in Fig. 6 as a function of neutron fluence. Newgard has reported that stored energy can be expressed using the following semiempirical equation"' : Fig. 3 Change in anisotropy ratio of thermal conductivity as a function of neutron fluence (a) Parallel direction E = A(1-exp (-BF)), (1) where A and B are constants, and F is the neutron fluence in the unit of 1020 n/cm2. The parameters for the above equation were obtained from the present experimental results and they are presented in Table 1. It is obvious from the table that stored energy is larger for the samples irradiated at lower temperatures. (b) Perpendicular direction Fig.4(a), (b) Change of electrical resistivity in parallel and perpendicular directions to extrusion as a function of neutron fluence 64
Vol. 22, No. 3 (Mar. 1985) TECHNICAL REPORT (H. Matsuo, T. Saito) 229 4. Relation between Thermal Conductivity and Electrical Resistivity Electrical resistivity of an unirradiated nuclear grade graphite at room temperature has a linear relation with its thermal resistivity(2)(3). Because phonons contributing to heat conduction, and electrons and holes contributing to electrical conduction are scattered at the crystallite boundaries. Therefore, thermal conductivity, of which measurement method is complex and difficult, is sometimes obtained from electrical resistivity of whose measurement method is very simple and easy. Examination was carried out in the present study in order to clarify whether the relation holds for neutron irradiated graphite or not. Figure 7(a) and (b) show the Fig.5 Change in anisotropy ratio of electrical resistivity as a fundtion of neutron fluence relation between thermal resistivity Fig.6 Stored energy as a function of neutron fluence and electrical resistivity for the samples irradiated in the temperature range Table 1 Parameters in Eq.(1) obtained from 220~400dc up to the maximum neutron present experimental results fluence 2.2x1020 n/cm2, parallel in Fig.7(a) and perpendicular direction to extrusion in 7(b), respectively. A linear relation Fig. holds in the case that the measured values of thermal and electrical resistivites are small, however it does not hold when the values become larger. 1 hese changes are the same for both parallel and perpendicular cut specimens. Two reasons are considered for this. One is that the effect of neutron irradiation on conduction of phonons, electrons and holes are different, resulting in the changes in concentration of electrons and holes and their different scattering mechanism due to defects induced by irradiation in addition to the scattering at crystallite boundaries. The other is that the saturation of changes in the electrical resistivity occurred in the early stage of irradiation compared with the change in thermal conductivity. 5. Relation between Thermal Conductivity and Stored Energy It has been reported that stored energy is related to fractional change of thermal conductivity for neutron irradiated nuclear grade graphite and the relation was used to analyze the kinds of defects formed owing to irradiation(7). 65
230 TECHNICAL REPORT (H. Matsuo, T. Saito) J. Nucl. Sci. Technol., Figure 8(a) and (b) show the relation between stored energy and fractional change of thermal resistivity for Pechiney graphite, parallel in Fig.8(a) and perpendicular to the extrusion in Fig.8(b), respectively. A dotted line is the relation for the Pile Grade A (PGA) graphite reported by Bell et al.(1) In those figures the relation was obtained assuming that a linear relation holds, and the proportional constant was obtained by using a least square method, leading to the following relations : (a) Parallel cut specimens E=12.0(K0/K-1) for parallel direction, (2) E=14.2(K0/K-1) for perpendicular direction, (3) E=12.9(K0/K-1) for both directions. (4) The proportional constant depended on the direction which the thermal conductivity was measured. It leads to 12.9 if all data for parallel and perpendicular direction were taken into consideration (b) Perpendicular cut specimens Fig. 7(a), (b) Relation between thermal and electrical resistivity for parallel and perpendicular cut specimens at the same time. These relations do not coincide with the one reported previously(". The proportional constants are approximately two times the value(1). Kelly'(8) estimated the energy of formation of the vacancy Efv from the previous experimental relation(1) E=6.5(K0/K-1), giving Efv~1.3eV for PGA, which does not agree with the present results. It is therefore considered that the present results leads to have an effect on discussion for the formation energy of defects analyzed by Kelly. If the relations are universal, the formation energy of vacancy becomes about two times as large as that reported previously for a nuclear grade graphite irradiated in a similar condition. It is not clear whether the relations are different for all nuclear grade graphites or not At the present time the relations have been obtained only for two kinds of graphite, one is for the PGA graphite and the other is for the Pechiney graphite. While it was clarified in the present study that the anisotropy ratio of thermal conductivity did not change owing to irradiation as shown in Fig.3, the proportional constant may depend on the kind 66
Vol. 22, No. 3 (Mar. 1985) TECHNICAL REPORT (H. Matsuo, T. Saito) 231 of graphite in addition to preferred orientation of crystallites. The ratio of the proportional constant is about 1.20. This value is equal to the anisotropy ratio of thermal conductivity. This means that stored energy is dependent on crystallinity and independent of preferred orientation of crystallites, however change of thermal conductivity depends on porosity varying from graphite to graphite in addition to the crystallinity. Therefore, it seems that it is not preferable to discuss the fundamental formation energy of defect such as that of vacancy in graphite by using the relation between stored energy and fractional change in thermal conductivity for neutron irradiated nuclear grade graphite. (a) Parallel cut specimens IV. SUMMARY Experimental results and discussions on thermal conductivity, electrical resistivity and stored energy are summarized as follows : (b) Perpendicular Cut specimens 8(a), (b) Relation between stored Fig. energy and fractional change in thermal resistivity for parallel and perpendicular cut specimens (1) Thermal resistivity increased owing to neutron irradiation, and the rate of increment being higher for low irradiation temperature became smaller with increasing neutron fluence. The changes were almost similar to both parallel and perpendicular cut specimens. (2) Electrical resistivity showed a rapid increase in the early stage of irradiation and then saturated with increasing neutron fluence. The changes were almost similar in both parallel and perpendicular cut specimens. (3) The anisotropy ratios of thermal conductivity or electrical resistivity did not change owing to neutron irradiation. (4) Stored energy depended strongly on irradiation temperature. It was larger for the samples irradiated at lower temperatures. ) Thermal conductivity has (5a linear relation with electrical resistivity in the early stage of irradiation, however the relation breaks down for heavy irradiation. 67
232 TECHNICAL REPORT (H. Matsuo, T. Saito) J. Nucl. Sci. Technol., (6) Stored energy has a linear relation with the fractional change of thermal resistivity. Its proportional constant was approximately two times that obtained previously. REFERENCES (1) BELL, J. C., et al.: Phil. Trans. Roy. Soc., London, A254, 361 (1962). (2) NIGHTINGALE, R. E. : "Nuclear Graphite", 123 (1962), Academic Press, New York. (3) MASON, I. B., KNIBBS, R. H. : AERE-R 3973, (1962). (4) MATSUO, H. : J. Nucl. Mater., 42, 105 (1972). (5) MATSUO, H., HONDA, T.: ibid., 45, 79 (1972/1973). (6) MATSUO, H., SAITO, T. : To be published in J. Nucl. Sci. Technol., 22C4), (1985). (7) NEWGARD, J. J. : J. Appl. Phys., 30, 1449 (1959). (8) KELLY, B. T.: "Chemistry and Physics of Carbon", (Ed. WALKER P. L., Jr.), Vol. 5, 119 (1969), Marcel Dekker, New York. 68