PSI - Issue 23

B.A. Gurovich et al. / Procedia Structural Integrity 23 (2019) 589–594 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

594

6

In addition, it is of interest to estimate the influence of the creep rate at different irradiation temperatures (see Fig. 7 [14], [15]). Fig. 7 shows that the creep coefficient value (B) at a temperature of 250 °C ~ is 3 times sma ller than at temperature of 950 °C. The rate of radiation dimensional change of HOPG (highly oriented pirographite) with the crystallite size corresponding to binder crystallite size of GR-280 graphite at irradi ation temperature of ~ 250°C and ~ 950 °C is practically the same [ 11]. The rate of radiation dimensional change of HOPG with the crystallite size that correspond to the filler crystallite size in GR-280 graphite at irradiation temperature of ~ 250 °C is approximately two times higher than at irradiation temperature of ~ 950 °C [ 11]. This means that the total dimensional change of crystallites in GR-280 graphite under irradiation at ~250 ° С is higher, than at irradiation temperature ~950 ° С . With such features of the creep behavior, the stresses in the GR-280 reactor graphite due to the radiation dimensional change of various regions with different crystallite packing of at irradiation temperature of 250 °C should be higher than at irradiation temperature of 950 °C. In the framework of an alternative approach, the degradation of the properties of graphite should obviously manifest itself earlier at i rradiation temperature of ~250° C. However, under comparable conditions, the properties degradation of the “ two phase ” graphite at the higher irradiation temperature (~950 °C) and higher creep rate is observed at the lower neutron fluence than at l ower irradiation temperature (~250° C). This behavior of graphite at different temperatures indicates that accommodation mechanism is not decisive in the graphite strength properties changing and, ultimately, its degradation. This, in general, indicates that the model of the properties degradation of graphite in the framework of an alternative approach does not explain the actually observed effects at different irradiation temperatures. The results presented clearly indicate that the difference in behavior of the “ two-phase ” GR-280 graphite at low and high irradiation temperatures is solely due to the following. At low irradiation temperatures the effect of the crystallite size on its radiation dimensional change rate is almost absent, while at high irradiation temperatures this effect is significant and increases with the irradiation temperature increase. This temperature dependence is the direct cause of the filler-binder boundaries type fracture and the neutron fluence decrease, at which this process begins with an increase of irradiation temperature. It ultimately causes the graphite degradation as a structural material under irradiation at the operation of graphite-moderated reactor. Thus, the solely factor responsible for the reactor graphite degradation is the magnitude of the difference in radiation dimensional change rates of the filler and binder crystallites. 1. Shtrombakh Y.I. et al. Radiation damage of graphite and carbon-graphite materials // J. Nucl. Mater. 1995. Vol. 225. P. 273 – 301. 2. Platonov P.A. et al. Radiation damage and life-time evaluation of RBMK graphite stack // Specialists meeting on graphite moderator lifecycle behaviour; Bath (United Kingdom); 24-27 Sep 1995 (IAEA-TECDOC--901). International Atomic Energy Agency (IAEA), 1996. 79-90 p. 3. Platonov P.A. et al. Radiacionnaâ degradaciâ grafita reaktorov tipa RBMK [Radiation Degradation of RBMK Graphite] [In Russian] // Vopr. At. Nauk. i Teh. Seriâ Fiz. âdernyh Reakt. [Problems Nucl. Sci. Eng. Ser. Phys. Nucl. React. 2016. № 5. P. 105 – 118. 4. Bradford M.R., Steer A.G. A structurally-based model of irradiated graphite properties // J. Nucl. Mater. Elsevier B.V., 2008. Vol. 381, № 1 – 2. P. 137 – 144. 5. Platonov P. a. et al. Effect of neutron irradiation on the dimensional stability of graphite pre-treated at different temperatures // Radiat. Eff. 1981. Vol. 54, № 1 – 2. P. 91 – 97. 6. Baker D.E. Graphite as a neutron moderator and reflector material // Nucl. Eng. Des. 1971. Vol. 14, № 3. P. 413 – 444. 9. Lebedev I.G., Virgil’ev Y.S. Comparative tests of the radiation resistance of reactor graphite // At. Energy. 1998. Vol. 85, № 5. P. 796 – 801. 10. Lebedev I.G., Kochkarev O.G., Virgil’ev Y.S. Radiation -induced change in the properties of isotropic structural graphite // At. Energy. 2002. Vol. 93, № 1. P. 589 – 594. 11. Gurovich B.A. et al. Structure and properties degradation of the nuclear graphite under neutron irradiation // Advanced Materials Letters 2018, 9(11), 781-788. 12. Engle G.B. Properties and high- temperature irradiation behavior of graphites prepared with uncalcined coke // Carbon N. Y. 1968. Vol. 6, № 4. P. 447 – 454. 13. Ka lyagina I.P., Virgil’ev Y.S. Radiation dimensional stability of graphite with an uncalcined coke - filler // Sov. At. Energy. 1983. Vol. 55, № 3. P. 588 – 591. 14. International Atomic Energy Agency. Irradiation Damage in Graphite due to Fast Neutrons in Fission and Fusion Systems (IAEA-TECDOC 1154). Vienna: International Atomic Energy Agency (IAEA), 2000. 177 p. 15. Barabanov V.N., Virgil’ev Y.S. Radiacionnaâ pročnost’ konstrukcionnogo grafita [Radiation strength of structural graphite] [I n Russian]. Moscow: Atomizdat, 1976. 78 p. References 7. Kelly B.T. Graphite – the most fascinating nuclear material // Carbon N. Y. 1982. Vol. 20, № 1. P. 3 – 11. 8. Cox J.H., Helm J.W. Graphite irradiations 300°– 1200°C // Carbon N. Y. 1969. Vol. 7, № 2. P. 319 – 327.