A study has been made of the effect of radiolytic oxidation by carbon dioxide on the porosity of 'A' grade pile graphite ('P.G.A.') and also a material of lower initial porosity designated CPsRsG. Use of in-pile rigs operated at 735 lb/in* (gauge) enables weight losses up to 29% to be obtained. Results indicate that up to 40% of the originally closed pore volume becomes accessible by 2% weight loss, the pores thus opened probably being similar in character and origin to the original open pores. Most of the variation between specimens is eliminated at this stage. The remaining closed porosity (ca. 3% of the bulk vohune for P.G.A. and 6% for CP&G)
is removed only in proportion to the graphite consumed, and is tentatively identified with cleavage cracks of the type revealed by electron micrography and also plugged pores with thick cell walls in the case of CPsRaG. Proportionality between oxidation rate and accessible pore volume (as found in earlier studies) is again observed except in the case of heavily oxidised specimens. This qualification suggests that ineffective radiolysis of gas occurs at the centre of large pores in these cases.