Sodium-Free Level Fluctuations: Concern on Structural Integrity for the Pool-Type Sodium-Cooled Fast Breeder Reactor

Abstract

In the case of sodium-cooled pool-type fast breeder reactor, the weight of the entire reactor assembly along with sodium coolant is carried by a large-sized cylindrical vessel called the main vessel. Since main vessel is one of the primary boundaries from the radiation shielding point of view, the structural integrity of the main vessel is an essential safety feature to be ensured under all operating conditions. The inner surface of the main vessel is partly filled with high-temperature sodium coolant, and the remaining part is in contact with relatively low-temperature argon cover gas. Due to large surface area of the main vessel pool, the free surface level of the sodium oscillates. This environment creates drastic temperature cycling in the main vessel wall at the sodium-free level interface. This transient temperature cycling imposed on the main vessel with negligible attenuation can lead to high-cycle thermal fatigue damage in the vicinity of sodium-free level. This high-cycle thermal fatigue can be detrimental in ensuring the structural integrity of the main vessel considering the number of cycles (approximately 9.5 × 10⁸ cycles) applied in the plant’s lifetime. This paper presents the numerical studies carried out towards assessing the structural integrity of the main vessel by considering the effect of sodium-free level fluctuations. The magnitudes of level fluctuations of 100 and 30 mm with a frequency range of 0.1–10 Hz with different increment of frequencies are considered as a parametric study. In this study, the critical portion due to level fluctuation has been identified from the thermal stress cycling point of view, and the effect of level fluctuation frequency on the structural integrity of the main vessel has been quantified.