Flow and Modal Analysis for the Investigation of PWR Safety Injection Line-Installed Thermal Sleeve Separation Failure Mechanism

Abstract

In conventional pressurized water reactors, a thermal sleeve (named simply ‘sleeve’ hereafter) is seated inside the nozzle part of each safety injection (SI) branch pipe to prevent and relieve potential excessive transient thermal stress in the nozzle wall when cold water is injected during the safety injection mode. Recently, mechanical failures that the sleeves are separated from the SI branch pipe and fall into the connected cold leg main pipe occurred in sequence at some typical PWR plants in Korea. This paper investigates the flow field in the pipe junction through a numerical simulation and vibration characteristics of thermal sleeves through a modal analysis to analyze the root cause of sleeve separation mechanism. By performing both flow simulation in the SI pipe junction and modal analysis of thermal sleeve, the fluid force and modal characteristics have been identified to be able to lead or contribute to separate thermal sleeves inside safety injection branch pipes in PWR plants from their original seating locations.