We have studied the microstructural evolution of a pressure vessel steel (C shell base metal) irradiated in CHOOZ A reactor surveillance capsules. This material has been irradiated at 265°C with fluences ranging from 2.2 × 10¹⁹ to 14 × 10¹⁹ n ∙ cm^-2 (E > 1 MeV); the longest irradiation has lasted about 13 years. The maximum ΔRT_NDT measured is 145°C for a fluence of 9 × 10¹⁹ n ∙ cm^-2 (E > 1 MeV). The microstructural studies have been carried out by transmission electron microscopy (TEM) and small angle neutron scattering (SANS).

The unirradiated material has a ferritobainitic structure that contains carbides and a high density of dislocations partially organized in subboundaries. After irradiation, careful conventional TEM examinations did not reveal any evolution of this structure other than a decrease in dislocation density. We did not observe microvoids or dislocation loops such as those already noticed in materials irradiated in test reactors with similar fluences but with higher dose rates. The point defects created by irradiation probably enhance dislocation mobility, thus leading to the observed decrease in dislocation density. This mechanism therefore results in the trapping of individual point defects, which prevents their condensation into large defect clusters. High resolution electron microscopy (HREM) experiments have revealed small defects in the body centered cubic (BCC) lattice of the irradiated material.

SANS experiments have confirmed the formation of small scattering centers during irradiation. It has been established that the radius of these defects (r ≈ 1.3 nm) is nearly independent of the fluence and that their volume fraction increases with it.