Reactor Vessel Closure Head Drop Analysis: Sensitivity Study on the Effects of Representing Nonlinear Behavior in the Closure Head Assembly

Abstract

Nuclear power plants need to safely and efficiently remove their reactor vessel closure head assembly during plant outages. This is accomplished by lifting the closure head assembly out of the reactor vessel cavity and placing it on the closure head stand. In order for nuclear power plants to remove their closure head assembly, the United States Nuclear Regulatory Commission has mandated that nuclear power plants upgrade to a single failure-proof crane, show single failure-proof crane equivalence, or perform a head drop analysis [1]. The goal of head drop analyses is to qualify the maximum drop height in air per plant procedures. A significant percentage (greater than 30%) of the closure head assembly’s mass is comprised of components attached to the top of the head (such as: lifting fixtures, a missile shield, air cooling systems, and control rod drive housings). The analytical consideration of large deflection, plastic deformation, and local failure of these components can potentially change the energy imparted to the vessel during impact due to their energy-absorbing capacities during the drop event. This paper contains a sensitivity study to determine the benefits of modeling closure head assembly components, using nonlinear structural behavior. The guidelines of Nuclear Energy Institute Initiative NEI 08-05 [2] are followed for this study.