Microstructural development of ferritic steels during service is driven by the interaction of neutrons and metal atoms. Collisions between the incident neutron and constituent atoms result in momentum transfer to the lattice atoms. If the transfer is above ^40 eV, atoms can be permanently displaced from their lattice sites resulting in vacancy-interstitial pairs. Indeed, lattice atoms with tens of kiloelectron volts may be created and a branching, tree-like distribution of displaced atoms formed, termed a displacement cas­cade. Vacancy and interstitial clustering may occur within the cascade. Vacancies and interstitials escap­ing from the cascade give rise to concentrations of vacancy and interstitial point defects throughout the material. The fate of the point defects formed in the irradiation depends most sensitively on irradia­tion dose rate and temperature, and also material factors such as composition.

This chapter focuses on the microstructural development of RPV steels. These steels experience a relatively low dose rate (and thus lifetime dose) at a relatively low temperature. The microstructure developed under these conditions is very different to that developed at high doses and high tempera­tures in the operating regime typical of fusion or fast reactors. The critical features are that at the temper­ature range of most operating pressure vessels both vacancy and interstitial point defects are mobile. Freely migrating defects (vacancies or interstitials) and mobile interstitial clusters escaping from the initial damage event may interact with point defect sinks, such as preexisting dislocations, recombine with each other, either directly or at solute traps, or cluster to form vacancy or interstitial clusters. The end result of the interactions described above is a microstructure comprising of a high density of small clusters in the matrix. These clusters may be point defect solute complexes, and, depending on steel composition, solute clusters formed from radiation-enhanced precipitation. At the dose, dose rate range, and temperature of interest to operating reactors, the clusters formed are usually thermally stable. Lastly, segregation of solutes to grain bound­aries or other sinks may occur.