Mechanical Collateral Damage Assessment of Reactor Vessel Bottom Mounted Nozzles: Part I—Requirements and Methodology

Abstract

A comprehensive work scope including the engineering safety assessments, Non-Destructive Examination (NDE) and repair design, is developed by AREVA NP Inc. for the Reactor Vessel (RV) Incore Monitoring Instrument (IMI) nozzles. The joint Bottom Mounted Nozzle (BMN) Assessment Plan is coordinated under the Electric Power Research Institute (EPRI) Materials Reliability Program (MRP). The purpose of such coordination is to produce a safety assessment of consistent scope and methodology to address the different IMI nozzle designs in all U.S. Pressurized Water Reactors (PWRs). The IMI nozzles, which are also referred to as the BMNs are installed in the bottom of the reactor vessel RV. For the Babcock & Wilcox (B&W) designed plants the nozzles consist of the original Alloy 600 nozzle material attached to the reactor vessel by a partial penetration Alloy 182 weld. To increase the resistance of the nozzles against flow induced vibration (FIV), the nozzles were modified, which consisted of a thicker, more rigid Alloy 600 nozzle welded to the RV inside radius surface. Recent industry experience indicates that the Alloy 600 BMNs and their Alloy 82/182 weld metal may be more susceptible to primary water stress corrosion cracking (PWSCC) than previously thought. Although the BMNs have been ranked low in susceptibility to PWSCC, they are ranked as having the most severe consequences of failure. Failure of BMNs represents a scenario that would result in a leak or loss of coolant accident (LOCA). Failure of a BMN was not included in the original design basis for the B&W designed plants. This paper describes the mechanical collateral damage analysis of the BMN engineering safety assessment project performed under the sponsorship of PWR Owner’s Group (PWROG) for the seven operating B&W 177-FA PWR units. Failure of a BMN could potentially lead to pipe whip that could impact other IMI pipes. The goal of the mechanical collateral damage assessment is to determine the potential loads on adjacent IMI pipes. First, the IMI piping configurations for all B&W plants were determined. Based on the piping configurations, potential pipe whip pairs were identified and several representative finite element models of the IMI piping were developed. Using the results of the nonlinear transient dynamic pipe whip analyses, response surfaces were developed, which provided the basis for determining loads due to pipe whip at several different locations. The conservative ultimate capacity analysis corresponding to 50% ultimate strain of the materials showed that the maximum ultimate stress ratio of the intact nozzle cross section at the RV outside radius was acceptable. In addition, the fracture mechanics evaluation of the flawed nozzles, at the RV inside radius, showed that the maximum critical half flaw angle was large enough that early detection of leaking BMNs is possible. For other possible failure modes of the piping, such as the jet impingement, asymmetric cavity pressure effects and insulation frame movement, it was shown that the loads obtained from the pipe whip analyses envelop those loads. The description of this work has been divided into two papers. Part I, detailed in this paper, describes the development of the comprehensive collateral damage assessment methodology. Part II, [1], to be also presented at PVP-2011, presents illustrative examples of the pipe whip analyses and application of response surfaces.