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Radiation-Damage Effects in Borated Graphite

J. M. Davidson, L. O. Gates, and R. E. Nightingale

Nuclear Science and Engineering / Volume 26 / Number 1 / Pages 90-98

September 1966

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Radiation effects were determined in samples of borated graphite used as a neutron shield in the Enrico Fermi Power Plant. The material nominally contained 5 or 7 wt% boron as boron-carbide particles in a nuclear-graphite matrix. The graphite from the center of the graphitizing furnace had a shiny, grey appearance. Microscopy studies showed that the boron carbide had melted and the graphite particles were recrystallized. The remaining material had the usual dull black appearance of nuclear graphite., Most irradiation tests were conducted at 370 and 500°C to a total thermal-neutron dose of 2.5 × 1021 n/cm2 in a predominantly thermal-neutron spectrum. Dimensional changes and other radiation effects were much larger than those in nonborated materials. One grey sample expanded 3.3%, but dimensional changes and other property changes in the black materials were generally less., The radiation effects have been attributed primarily to carbon-atom displacements caused by the energetic lithium and helium atoms in the 10B(n,α)7Li reaction. The faster rate of damage in the grey material is believed to have been due to the finer dispersion of boron in the matrix. This finer dispersion would allow more of the helium and lithium atoms to escape from the boron-carbide particles and produce carbon-atom displacements., Preliminary tests in a neutron spectrum, where the ratio of thermal-to-fast neutrons was less than 1% of that in the flux utilized in the above experiments, produced much smaller changes for comparable fast-neutron doses. This is further evidence that most damage is caused by thermal neutrons.

 
 
 
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