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159. The diffusion of uranium in graphite
401
ABSTRACTS
159. The diffusion of uranium
in graphite*
R. J. Borg, D. R. McKenzie and J. R. Wolfe (Lawrence Radiation Laboratory, University of California, Livermore, California). Using U 232as a tracer, self diffusion coefficients have been obtained as a function of temperature in three varieties of graphite. Measurements were made in the (c) and (a) directions in pyrographite, in 2.T.A.f (p=2.12) and a porous graphite (p== 1X1)1. The determinations spanned at least 400°C in all cases except pyrographite in the (c) direction. The results are summarized by the following equations: Dli,=27.,xexp(-117x 103/RT) Dl1~=6.7~x lO~xexp(-115/RT) DzTA=~.~ x 105x exp(-138.,x 103/RT) D,,,= 5.5 x 105x exp( - 138., x 103/RT) Measurements made in well annealed pyrographite indicate a total absence of volume diffusion. It thus appears that all diffusion proceeds via lattice defects. The diffusion of U in Z.T.A. and ordinary porous graphite are indistinguishable. It is therefore concluded that the diffusion proceeds solely via grain boundaries. *Work was performed tThe
under the auspices of the U.S. Atomic Energy Commission.
authors gratefully acknowledge the generous donation of the Z.T.A.
graphite by the National Carbon Co.
$.The authors gratefully acknowledge the Speer No. 700 graphite by the Speer Carbon Co.
160. Diffusion
and migration of uranium in pyrolytic carbon*
W. V. Goeddel, H. K. Lonsdale and N. L. Sandefur (General Atomic Division, General Dynamics Corporation). Microradiographic techniques have been used to study the migration of uranium through
pyrolytic carbon coatings on UC, fuel particles. Diffusion (migration) coefficients have been determined on actual coated particles by this non-destructive means, and an energy of activation for the process has been obtained over the temperature range 1700-2300°C. Microradiographs of individual coated particles at various times during diffusion anneals have furnished informative “time-lapse” picture sequences of the migration of U through PyC coatings. The penetration has been found to occur radially in localized positions, and then spread circumferentially, creating a bonded structure. These results illustrate the importance of structure of the PyC in regard to diffusion of both fuel (U) and fission product materials. *This work was supported Agreement 12.
by the U.S.
Atomic
Energy
Commission
under Contract
AT(04-3)-167,
Project
161. The threshold curve for the displacement of atoms in graphite-experiments on the resistivity changes produced in single crystals by fast electron irradiation at 15°K M. W. Lucas and E. W. J. Mitchell (J: j? Thomson Physical Laboratory, University of Reading, England). Natural “single” crystals of graphite have been irradiated at 15°K with electrons in the energy
range 0.3-2.0 MeV. Measurements have been made of the change of resistivity with electron dose at all energies. In some cases the magnetoresistance has also been determined. The resistivity changes have been found to vary linearly-with some scatter believed to be associated with temperature fluctuations-with electron dose rates ranging from: Ap/p0=3.76% per 101~ e- cm-a at 2.0 MeV. to Ap/p0<0.3 % per 10’6 e cm-2 at 0.3 MeV. At 2.0 MeV the absolute resistivity change was 1.77 x lO-’ Rem per 1O1”e- cm-*. The variation of these rates with electron energy has been compared with calculations of the displacement of atoms, including for the higher electron energies a correction for the production of secondary displacements. A displacement energy of 60 eV gives agreement (+ 10 eV) with the shape of the experimental curve. Using the
ABSTRACTS
159. The diffusion of uranium
in graphite*
R. J. Borg, D. R. McKenzie and J. R. Wolfe (Lawrence Radiation Laboratory, University of California, Livermore, California). Using U 232as a tracer, self diffusion coefficients have been obtained as a function of temperature in three varieties of graphite. Measurements were made in the (c) and (a) directions in pyrographite, in 2.T.A.f (p=2.12) and a porous graphite (p== 1X1)1. The determinations spanned at least 400°C in all cases except pyrographite in the (c) direction. The results are summarized by the following equations: Dli,=27.,xexp(-117x 103/RT) Dl1~=6.7~x lO~xexp(-115/RT) DzTA=~.~ x 105x exp(-138.,x 103/RT) D,,,= 5.5 x 105x exp( - 138., x 103/RT) Measurements made in well annealed pyrographite indicate a total absence of volume diffusion. It thus appears that all diffusion proceeds via lattice defects. The diffusion of U in Z.T.A. and ordinary porous graphite are indistinguishable. It is therefore concluded that the diffusion proceeds solely via grain boundaries. *Work was performed tThe
under the auspices of the U.S. Atomic Energy Commission.
authors gratefully acknowledge the generous donation of the Z.T.A.
graphite by the National Carbon Co.
$.The authors gratefully acknowledge the Speer No. 700 graphite by the Speer Carbon Co.
160. Diffusion
and migration of uranium in pyrolytic carbon*
W. V. Goeddel, H. K. Lonsdale and N. L. Sandefur (General Atomic Division, General Dynamics Corporation). Microradiographic techniques have been used to study the migration of uranium through
pyrolytic carbon coatings on UC, fuel particles. Diffusion (migration) coefficients have been determined on actual coated particles by this non-destructive means, and an energy of activation for the process has been obtained over the temperature range 1700-2300°C. Microradiographs of individual coated particles at various times during diffusion anneals have furnished informative “time-lapse” picture sequences of the migration of U through PyC coatings. The penetration has been found to occur radially in localized positions, and then spread circumferentially, creating a bonded structure. These results illustrate the importance of structure of the PyC in regard to diffusion of both fuel (U) and fission product materials. *This work was supported Agreement 12.
by the U.S.
Atomic
Energy
Commission
under Contract
AT(04-3)-167,
Project
161. The threshold curve for the displacement of atoms in graphite-experiments on the resistivity changes produced in single crystals by fast electron irradiation at 15°K M. W. Lucas and E. W. J. Mitchell (J: j? Thomson Physical Laboratory, University of Reading, England). Natural “single” crystals of graphite have been irradiated at 15°K with electrons in the energy
range 0.3-2.0 MeV. Measurements have been made of the change of resistivity with electron dose at all energies. In some cases the magnetoresistance has also been determined. The resistivity changes have been found to vary linearly-with some scatter believed to be associated with temperature fluctuations-with electron dose rates ranging from: Ap/p0=3.76% per 101~ e- cm-a at 2.0 MeV. to Ap/p0<0.3 % per 10’6 e cm-2 at 0.3 MeV. At 2.0 MeV the absolute resistivity change was 1.77 x lO-’ Rem per 1O1”e- cm-*. The variation of these rates with electron energy has been compared with calculations of the displacement of atoms, including for the higher electron energies a correction for the production of secondary displacements. A displacement energy of 60 eV gives agreement (+ 10 eV) with the shape of the experimental curve. Using the