Risk reassessment of thermal striping due to a macro-crack pre-existent in nuclear reactors at high temperature

Abstract

Thermal striping phenomenon is a random temperature fluctuation that may cause thermal fatigue damage of components until their break. These situations can cause the failure of important mechanical systems. For nuclear industry, it is a serious safety concern; however, the severity of the consequences depends on technology. Indeed, in the case of nuclear reactors cooled by liquid metal, the reactor core components integrity can be threatened.The pre-existence of a macro-crack upon structures, which are already suffering from thermal striping, at high temperature, can complicate further the situation. The initiation and propagation of crack can lead to the failure, before the thermal fatigue does. The component damage is then accelerated until its rapid breakdown. The nuclear incident becomes unpredictable.Thanks to engineering industry standards and new computation software, which give efficient tools to study this phenomenon and help to find suitable solutions.To provide for such contingencies, especially for liquid metal reactors, the proposed study focuses on the use of the RCC-MRx code to estimate the lifetime drop factor due to the presence of macro-crack upon structures suffering of thermal striping.The preliminary results of macro-crack initiation and propagation, under thermal fatigue, show that macro-crack presence shifts the lifetime curve at left. This modification is more likely at low than at high thermal load, at this level of bulk temperature. When, the lifetime drop is even more significant that the Biot number and bulk temperature are higher.