Corrosion- and irradiation-induced porosity changes of a nuclear graphitic material

In pristine specimens of a nuclear grade graphitic material the volume of accessible pores with diameters μ m is much smaller (0.9 mm 3 g −1 ) than the total accessible pore volume (82 mm 3 g −1 ) but the former increased with corrosion much more than the latter (36 and 160 mm 3 g −1 , respectively, at a burn-off = 5 wt%). The specimens were oxidized by CO 2 at 900°C. Independent of burn-off the most frequent pore diameter is ∼2 nm, as was determined from N 2 adsorption isotherms at 77 K. Corrosion leads also to a pronounced increase in pores in the range 2–5 nm and in the apparent BET surface area from 0.6 m 2 g −1 (pristine) to 39 m 2 g −1 (burn-off = 5 wt%). The analysis of small angle X-ray scattering (SAXS) curves revealed that at the beginning of corrosion the existing 1 nm pores are enlarged. Additional micropores are developed with further burn-off. A neutron fluence of 5 × 10m 21 cm −2 EDN (equivalent to about 7 dpa) at 950°C caused a marked increase in the volume of pores with 2.5 nm diameter. From a comparison of the SAXS results with the data obtained by xylene impregnation and N 2 adsorption it was concluded that most of the pores generated by irradiation are accessible to gases.