Role of Irradiation Embrittlement in RPV Lifetime Assessment

Integrity and lifetime of reactor pressure vessels are practically determined by their material resistance against fast/non-ductile failure and consequently by their radiation damage resulting in irradiation embrittlement and hardening. Mechanism of irradiation embrittlement of RPV materials depends on selected materials and operation conditions but their values depend in a large extent on reactor design, i.e. on neutron flux/fluence depending on RPV wall. Generally, new RPV design allows smaller neutron fluences but absolute value of irradiation embrittlement still depends on a choice of RPV material. Even though radiation damage (especially irradiation embrittlement) is important for RPV behavior, integrity and lifetime depends, in principle, on final value of applied fracture mechanics parameter — transition temperature. Thus, its initial value as well as its shift due to irradiation embrittlement is of interest but only the embrittlement can be affected during operation. The paper discusses absolute and relative effect of both these parameters for integrity and lifetime assessment for different type of reactors — of old and new as well as the newest design. Then, role of surveillance specimen programs for the evaluation of radiation embrittlement of RPV materials is discussed, first of all from point of view of existing codes and standards. Additionally, some recommendations are given for decreasing the effect of irradiation embrittlement on final integrity and lifetime evaluation.