Finite Element Analysis of A Dissimilar Material Junction Of A Class-1 Nuclear Pressure Equipment Steam Outlet

Abstract

A nuclear power plant is a power generation station using the heat source from a nuclear reactor. This reactor is the heart of the nuclear power plant wherein the nuclear reaction takes place using nuclear fuel. Apart from the reactor there are many other equipment’s required for the operation & intended purpose. All equipment’s close/attached to the reactor are categorized as the Nuclear Class-1. The one considered in the present thesis is also a Nuclear Class-1 Pressure Equipment (heat exchanger) which is a shell and tube heat exchanger used for generation of steam in nuclear power plant to generate electricity. The equipment is directly connected to the reactor core where in the heated heavy water is used a primary source for energy. pressurized heavy water reactor is used to generate steam at high temperature and high pressure. Steam generated at high temperature and pressure is used to run turbine. Turbine turns generator to produce electricity. In Class-1 pressure equipment, at the exit of the steam outlet nozzle has a tri material junction. i.e., Austenitic Stainless-steel Venturi, Inconel cladded nozzle having base material as Low alloy steel. Venturi, cladding and the nozzle form a trimetallic junction which is subjected to high temperatures of 2560 C and higher. Being a different material, the thermal property of each material is different. However, the thermal expansion of the cladding and the nozzle were in close match and similarly, other thermal properties of both materials have parity. Whereas comparing the properties of venturi the same is not observed and a very highly stressed weld junction is reported. The vessel\heat exchanger being intended for 40 years of operation and the plant getting subjected to high nos. of cyclic loading it is a serious concern to qualify for the intended purpose. Thus, the transients are used to check for the fatigue and ratchetting qualification for cyclic loading. The thesis/report covers the identification of the issue due to thermal at high temperatures check with manual calculation and linear analysis for integral behaviour by ASME Section III NB. Further the nonlinear material & geometrical behaviour have also been utilized as tool for checking & qualification of more realistic cyclic loading. The ASME Section VIII Div.2 has been used as a guideline for the nonlinear analysis purpose. Also, a change in material is proposed for the intended purpose with lesser secondary stresses under cyclic loading. CAD & CAE tools used for the purpose is NX for Conceptual modelling, ANSYS for FEA and ASME codes for the guidelines of failures to be qualified.