Preliminary Investigation of the Effects of Neutron Irradiation on the Weibull Modulus of Graphite

The tensile strength distribution of graphite has historically been estimated via the two- and three-parameter Weibull distributions due to the flaw distribution within the material. This is apparent within the American Society of Mechanical Engineers Boiler and Pressure Vessel Code, Section III “Rules for Construction of Nuclear Facility Components”, Division 5 “High Temperature Reactors”, Subsection HH “Class SN Nonmetallic Core Components”, Subpart A “Graphite Materials”, where the materials data sheet (MDS) requires the calculation of both the two- and three-parameter Weibull distributions of the tensile strength for as-manufactured graphite. There is not currently a requirement to determine Weibull distribution of the strength of graphite post-irradiation, mainly because no results have been reported that suggest whether the Weibull parameters, most importantly the Weibull modulus, changes due to neutron irradiation damage. An irradiation program was performed at Oak Ridge National Laboratory where graphite grade ETU-10 was exposed to neutron damage at a range of temperatures and final neutron fluences. Within this experiment, ten strength coupons were irradiated at each temperature and fluence combination which when analyzed can provide information as to whether irradiation damage has altered the Weibull modulus. This paper will focus on the analysis of the post-irradiated equibiaxial flexural strength change via a Weibull analysis of each individual data set and discussion as to whether there appears to be statistically significant changes to the Weibull parameters.