Modelling of the fissuring mode of fracture in Candu pressure-tube material and, in particular, Stage 1 crack growth (essentially flat J(R) curve) as observed in some irradiated compact toughness specimens, is reported. A preliminary attempt has been made at modelling the characteristics of the fracture process zone associated with a crack that tunnels at the specimen mid-section. Based on the experimentally determined J(R) curve, both the fracture-process zone size and the crack opening displacement at the trailing edge of the zone have been predicted, and their values are seemingly not unreasonable. The initial considerations enable specific issues to be highlighted which need to be addressed before a complete picture is obtained of the fissuring mode of crack growth. A theoretical analysis has also shown that the non-tunnelling material at the specimen side surfaces exerts very little restraint on the cumulative (tunnelling) mode of crack propagation, a prediction that is consistent with the experimental finding that Stage 1 crack growth in irradiated material is associated with an essentially flat J(R) curve.