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70. Some studies on irradiation creep of graphite
ABSTRACTS
214
height, layer spacing and anisotropy were measured. Significant increases in preferred orientation resulted from irradiation at temperatures above 1100°C. The crystallite dimensional changes were derived from the measured dimensional changes and the preferred orientations. Comparison of the results with previously published data for carbons and graphites shows that at irradiation temperatures below N 500°C the crystallite dimensional change rates show no dependence on crystallite size and decrease with increasing irradiation temperature. Above N 500°C dimensional change rates increase with increasing temperature and are higher for carbons with smaller crystallite sizes. l
This work wu supported by tbe U.S. Atomic Energy Commission under Contract AT (04-3)-167, Project Agreement No. 12.
69. A model
to describe
neutron-induced
dimensional
changes
in pyrolytic
carbon.*
J. C. Bokros and A. S. Schwartz (GeneralAtomicDim’sionof GeneralDynamicsCorporation,San Diego, California). A model has been developed that describes the dimensional behavior of pyrolytic carbon during irradiation by fast neutrons when the neutron exposure is less than 3 x 1051nvt. The model uses crystallite averaging methods to describe the anisotropic distortion of the bulk in terms of its preferred orientation and the shape change of the individual crystallites. The densification, a simple first-order rate process, was assumed to be independent of the shape change and superimposed isotropically on it. The validity of the approach has been checked by comparing the predictions of the model with the dimensional changes observed in twelve carbons irradiated in the range 680”-980°C. The present results together with data already published show that the densification constant kd for carbons with L, parameters in the range 24 A to 45 A isindependent of temperature and equal to 3.3 x 10-n (nvt)-1. When L, is between 155 A and 175 A, k,,is also independent of temperature and equal to 1.1 x 10-aa (nvt)-I. The value of k, increases with temperature for carbons with intermediate values of L,. l
This work was supported by the U.S. Atomic Energy Commission under Contract AT@-3)-167,
70. Some studies
on irradiation
Project Agreement No. 12 and No. 17.
creep of graphite.
J. E. Brocklehurst and R. G. Brown (U.K. Atomic Energy Authority, Reactor Materials Laboratory,Culcheth,Nr. Wawington,Luncs., England). The irradiation induced creep of graphite under constant tensile and compressive stresses in BR-2 is considered. It is shown that over a fast neutron dose range of 50 x lOa n. cm-2 there is no difference in creep strain between specimens irradiated at 650°C and specimens irradiated at 4OO”C, and there is no large difference between tensile and compressive creep behaviour over the dose range examined. Specimens containing compressive and tensile creep strain show a different behaviour on thermal annealing and these results are discussed in terms of current ideas on dimensional changes in graphite. 71. Dimensional and 300” to 14w%.*
property
changes
of impregnated
and
isotropic
graphites
irradiated
at
G. B. Engle (General Atomic Division of Gemral Dynamics Corporation,San Diego, California). When anisotropic graphites are irradiated in the range 880”-1270X, to doses of 2-3 x lOa nvt (E > 0.18 MeV), the linear contraction starts to saturate in the direction normal to the layer planes. The isotropic graphites do not show this characteristic when irradiated at the same temperatures and doses. Changes in bulk thermal expansion coefficients were small or nonexistent except for graphites made with Texas-coke filler and samples irradiated at 300”-400”C. All graphites studied exhibited a single linear relationship between thermal expansion and bulk dimensional changes in the range 320”-615C. In the range 840”-149O”C, a separate linear relationship was observed for anisotropic or isotropic graphites, At high temperatures the linear relationship is shifted toward higher contraction for the isotropic graphites. The presence of a nongraphitizing impregnant carbon in the pores of an anisotropic needle-coke graphite increased its volume contraction, reduced the anisotropy of contraction, and retarded “turnaround” in the perpendicular direction when irradiated in the range 880”-1230%. l
Th& work W.XY supported by the U.S. Atomic Energy Commission under Contract AT(O4-3)-167, Project Agreement No. 17.
72. The effect of irradiation of impregnated
at 960”-1500°C on the thermal graphites.*
conductivity
and electrical
resistivity
G. B. Engle and K. Koyama (GeneralAtomicDivision of GeneralDynamicsCorporation,San Diego, California). The thermal conductivities of all graphites were reduced when measured at room temperature after irradiation at 300”1500°C. Removal of the irradiation defects by thermal annealing during measurement was accomplished
214
height, layer spacing and anisotropy were measured. Significant increases in preferred orientation resulted from irradiation at temperatures above 1100°C. The crystallite dimensional changes were derived from the measured dimensional changes and the preferred orientations. Comparison of the results with previously published data for carbons and graphites shows that at irradiation temperatures below N 500°C the crystallite dimensional change rates show no dependence on crystallite size and decrease with increasing irradiation temperature. Above N 500°C dimensional change rates increase with increasing temperature and are higher for carbons with smaller crystallite sizes. l
This work wu supported by tbe U.S. Atomic Energy Commission under Contract AT (04-3)-167, Project Agreement No. 12.
69. A model
to describe
neutron-induced
dimensional
changes
in pyrolytic
carbon.*
J. C. Bokros and A. S. Schwartz (GeneralAtomicDim’sionof GeneralDynamicsCorporation,San Diego, California). A model has been developed that describes the dimensional behavior of pyrolytic carbon during irradiation by fast neutrons when the neutron exposure is less than 3 x 1051nvt. The model uses crystallite averaging methods to describe the anisotropic distortion of the bulk in terms of its preferred orientation and the shape change of the individual crystallites. The densification, a simple first-order rate process, was assumed to be independent of the shape change and superimposed isotropically on it. The validity of the approach has been checked by comparing the predictions of the model with the dimensional changes observed in twelve carbons irradiated in the range 680”-980°C. The present results together with data already published show that the densification constant kd for carbons with L, parameters in the range 24 A to 45 A isindependent of temperature and equal to 3.3 x 10-n (nvt)-1. When L, is between 155 A and 175 A, k,,is also independent of temperature and equal to 1.1 x 10-aa (nvt)-I. The value of k, increases with temperature for carbons with intermediate values of L,. l
This work was supported by the U.S. Atomic Energy Commission under Contract AT@-3)-167,
70. Some studies
on irradiation
Project Agreement No. 12 and No. 17.
creep of graphite.
J. E. Brocklehurst and R. G. Brown (U.K. Atomic Energy Authority, Reactor Materials Laboratory,Culcheth,Nr. Wawington,Luncs., England). The irradiation induced creep of graphite under constant tensile and compressive stresses in BR-2 is considered. It is shown that over a fast neutron dose range of 50 x lOa n. cm-2 there is no difference in creep strain between specimens irradiated at 650°C and specimens irradiated at 4OO”C, and there is no large difference between tensile and compressive creep behaviour over the dose range examined. Specimens containing compressive and tensile creep strain show a different behaviour on thermal annealing and these results are discussed in terms of current ideas on dimensional changes in graphite. 71. Dimensional and 300” to 14w%.*
property
changes
of impregnated
and
isotropic
graphites
irradiated
at
G. B. Engle (General Atomic Division of Gemral Dynamics Corporation,San Diego, California). When anisotropic graphites are irradiated in the range 880”-1270X, to doses of 2-3 x lOa nvt (E > 0.18 MeV), the linear contraction starts to saturate in the direction normal to the layer planes. The isotropic graphites do not show this characteristic when irradiated at the same temperatures and doses. Changes in bulk thermal expansion coefficients were small or nonexistent except for graphites made with Texas-coke filler and samples irradiated at 300”-400”C. All graphites studied exhibited a single linear relationship between thermal expansion and bulk dimensional changes in the range 320”-615C. In the range 840”-149O”C, a separate linear relationship was observed for anisotropic or isotropic graphites, At high temperatures the linear relationship is shifted toward higher contraction for the isotropic graphites. The presence of a nongraphitizing impregnant carbon in the pores of an anisotropic needle-coke graphite increased its volume contraction, reduced the anisotropy of contraction, and retarded “turnaround” in the perpendicular direction when irradiated in the range 880”-1230%. l
Th& work W.XY supported by the U.S. Atomic Energy Commission under Contract AT(O4-3)-167, Project Agreement No. 17.
72. The effect of irradiation of impregnated
at 960”-1500°C on the thermal graphites.*
conductivity
and electrical
resistivity
G. B. Engle and K. Koyama (GeneralAtomicDivision of GeneralDynamicsCorporation,San Diego, California). The thermal conductivities of all graphites were reduced when measured at room temperature after irradiation at 300”1500°C. Removal of the irradiation defects by thermal annealing during measurement was accomplished