Influence of Temperature and Pressure on the Beryllium Pebbles Bed Electrical Resistivity

The electrical resistivity behaviour of a beryllium pebble bed has been studied as a function of the temperature and pressure. At room temperature the resistivity of a single size 2 mm pebble bed decreases drastically from 2·10⁻² Ωm to 10⁻⁴Ωm by applying an external pressure. After this first drop, the resistivity shows an almost linear decrease with the applied pressure. The same trend appears for a single size 0.1–0.2 mm pebble bed, but the resistivity values are about one order of magnitude higher than in the case of the 2 mm pebbles. At room temperature, the lowest resistivity values were found for the case of a binary pebble bed. After a mechanical cycling the electrical resistivity of the bed never reaches its initial value for zero pressure but it remains about one order of magnitude below the original value. After the first loading cycle the following loading/unloading resistivity curves do not show any significant change. The temperature dependence of the mixed pebble bed was investigated in air at 300 °C, 450 °C and 550 °C. The resistivity behaviour of the pebble bed with the applied pressure is, at high temperature, qualitatively the same as that observed at room temperature. For the same applied load the pebble bed electrical resistivity increases almost linearly with the temperature. Measurements of the oxyde content of the pebbles before and after the heating show a higher beryllium oxide content for the heated pebbles than for the not heated ones.