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Light Water Reactors
Overview
Ferritic steels commonly used for pressure vessels and reactor supports in light water reactors exhibit dynamic strain aging resulting in decreased ductility and toughness. In addition, recent work indicated decreased toughness during reverse-cyclic loading that has implications on reliability of these structures under seismic loading conditions. This paper summarizes the authors' recent work on these aspects, along with synergistic effects of interstitial impurity atoms and radiation-induced point defects, which result in interesting beneficial effects of radiation exposure at appropriate temperature and strain-rate conditions. While the cyclic loading effects on toughness are studied in A516 steel, the dynamic strain aging and radiation-defect interactions were investigated on pure iron as well as several
ferritic steels. In addition, studies on fast vs. total (thermal+fast) neutron spectra revealed unexpected results due to the influence of radiation exposure on source hardening component of the yield stress; grain size of pure iron plays a significant role in these effects. The paper concludes with future research needed to address these concerns.
INTRODUCTION
Ferritic steels have many applications in fission reactors, such as in the construction of pressure vessels in light water reactors, reactor support structures, and steam-generator housings in liquid metal fast-breeder reactors. Radiation embrittlement of the ferritic steels used for pressure-- boundary applications is usually monitored by Charpy impact tests on specimens fabricated from base, weld, and heat-affected zone materials irradiated in surveillance programs in operating power reactors,' This embrittlement is characterized in terms of decreased upper-shelf energy accompanied by increased ductile-to-brittle transition temperature (DBTT).
The extensive database on various reactor vessel surveillance capsule programs revealed the influence of alloying elements such as copper, nickel, and phosphorus on the changes in DBTT and upper-shelf energy along with the superimposed effects of radiation fluence and irradiation temperature.2,3 The effects of alloying and impurity elements such as copper, nickel, and phosphorus have been well characterized, resulting in modern vessel materials and welding techniques with reduced amounts of these trace elements. In addition, the effects of interstitials such as carbon and nitrogen on the mechanical and fracture behaviors of these materials through strain aging have been shown to result in dips in the shelf energies and ductilities when plotted as a function of the test temperature.4,5 While...