A3-3 matrix material is the semi-graphitc composite used in high-temperature nuclear reactors to form fuel compounds containing coated particle fuel. In a CO2 cooled reactor, this material would be exposed to radiolytic oxidation in a similar way to that of nuclear graphite in the currently operating UK nuclear fleet, but could be expected to behave differently. Radiolytic oxidation is sensitive to the material pore size distribution, and small pores are known to oxidise more quickly than large pores. SANS will be used to investigate samples irradiated within a CO2 atmosphere at the HOR test reactor at TU Delft to an ionizing dose of 2000 ¿ 4000MGy at 250°C, in order to view the effect of radiolytic oxidation on A3-3 microporosity. Samples of NBG-18 nuclear graphite have also been oxidised within similar conditions and these would be imaged as a comparison alongside unirradiated samples.