Improvement of the Damaged Thermowell of the Monju Fast-Breeder Reactor Based on Vibration Analysis

Abstract

In December 1995, the thermowell in the secondary cooling system of the Japanese fast-breeder reactor Monju was damaged, resulting in a sodium leak. Sodium flow was thought to have produced a vibration in the thermowell that then caused high-cycle fatigue that eventually destroyed the thermowell. Using the finite element method, the natural frequency of the thermowell was found to be approximately 230 Hz, which is in the region of resonance of the fluid vibration. Next, in a theoretical study, the authors altered the position at which the diameter of the thermowell changes and examined the resulting natural frequencies. They also studied the effects of thickness and diameter on the natural frequency of the thermowell and examined the case for a truncated conical thermowell. The results indicate that production of a thermowell that is not affected by vibration due to resonance is possible. This paper recommends improved shapes for the thermowell.