Fluctuations in Thermal Conductivity and Specific Electrical Resistance within a Batch of Reactor Graphite Sigri AS 2-500 Measured at Temperatures between 100 and 1000 °C

[Automatically extracted and translated] Sigri AS 2-500 is an extruded reactor graphite based on pitch coke. The aim of extensive investigations is to prove its suitability as a material for block-shaped HTR fuel elements and reflector structures. These investigations focus on capturing production-related variations of properties of interest, not only in the unirradiated state but especially after neutron irradiation, as this is the only way to ensure a safe and economical design of HT reactor components. As part of these investigations, measurements of thermal conductivity and specific electrical resistance were compiled for 56 unirradiated graphite samples depending on temperature. The samples were taken from a 25-ton batch by Sigri according to a specially developed plan and marked with corresponding signatures. Each signature consists of a number and a letter placed before or after it. In the first case, the axis of the cylindrical samples is oriented parallel to the preferred orientation of the graphite grains (pressing direction), in the second case perpendicular to it. It is planned to irradiate the samples at piecewise constant temperatures in the HFR/Petten and to measure them during irradiation breaks. The compilations on pages 4 to 6 contain the sample grouping for the first irradiation experiments and the respective irradiation temperature. In the last column of these representations, page references to the measurement data compiled here were made. Regarding the measurement method, a well-tested modified Kohlrausch method was used. The cylindrical samples have outer dimensions of 6 mm diameter and 32 mm length, and the specified measurement values refer to the direction of the sample axis. The representations on pages 7 and 8 give an impression of the magnitude of the measured fluctuation range in thermal conductivity and specific electrical resistance. It was attempted to represent the entirety of the measurement points here. However, for clarity, the measurement point coordinates given in the following tables had to be somewhat 'transformed', as the concentration at the points 100, 200, 300, etc., would otherwise have become too large. The representations clearly show that the maximum and minimum values occurring at any arbitrarily selected measurement temperature differ by about a factor of 1.5. If one forms average curves from the measurements carried out parallel and perpendicular to the preferred orientation of the grains, an anisotropy factor of about 1.15 results for both the thermal conductivities and the specific electrical resistances, independent of the measurement temperature (the Y-values measured on sample 90 H, which stand out strongly, were not considered here). It might seem sufficient to specify typical measurement values (batch mean values) together with the respective bandwidth. However, this is not correct, as there is a possibility that the relative change of a measurement value during neutron irradiation depends on its pre-irradiation value, and this point should receive special attention. Therefore, it is sensible to specify the measured individual curves and accept the associated scope of this report.