Materials and Corrosion Issues in the Primary Coolant Circuit of Boiling Water Reactors

Currently, about 450 civil commercial nuclear reactors are in operation world-wide, mainly Generation II reactors that were built after the first prototype power reactors from the 1960s until the end of the 1990s, and about 80 % of them are light water reactors (LWR) that use water as reactor coolant and moderator. About ~ 75 % and 25 % of the LWR fleet worldwide are pressurized water reactors (PWR, e.g., all the French reactors) and boiling water reactors (BWR), respectively. The LWRs provide a small, but important contribution to a reliable electric power supply with little CO ₂ emissions. The average age of the current LWR fleet is high, about 60 % have an age above 30 years and about 20 % are older than 40 years, which is the typical original design or license lifetime of LWR. The original lifetime was rather legally/regulatory motivated than technically justified and many reactors have received lifetime extensions and license renewals for 60 years. Very recently, the first US reactor has received a second license renewal with a lifetime extension to 80 years at the end of 2019.

Due to their high age, operation-induced material ageing and degradation phenomena such as stress corrosion cracking (SCC), flow-accelerated corrosion (FAC), fatigue or irradiation embrittlement are important concerns for the safe long-term operation of the LWRs. Damage (e.g., the formation of cracks) directly impacts on plant availability and economics, e.g., due to stillstand periods (1 day stillstand causes costs and losses of ~ 1 million Euro) and augmented periodic in-service inspections, component repairs and replacements, but may also affect plant safety and lifetime or lifetime extensions. All the utilities thus have implemented suitable ageing and lifetime management programs in the last decades to assure the safe and economic long-term operation of their plants and huge research efforts were performed to understand the ageing mechanism and to develop potential mitigation actions and to predict and assess their potential impact. Due to continuous large investments in safety measures and maintenance, many operating reactors have reached a significantly higher level of safety than when they were originally commissioned decades ago.

Safety of LWRs is assured by controlling the reactivity of the fission reaction in the core, ensuring the cooling of the fuel and containing the radioactive fission and activation products. The safe containment of radioactive species is assured through the integrity of three distinct safety barriers between the fission products and the environment. Firstly, the fuel element cladding contains and confines the nuclear reaction products. Secondly, the reactor pressure vessel (RPV) and the reactor coolant system...