Factors Involved in Stress Corrosion Cracking of Tubes from a Nuclear Power Plant Feedwater Heater

Manuela Fulger,Dumitra Lucan,Lucian Velciu,Maria Mihalache

doi:10.37358/rc.19.11.7674

Abstract

This study has been carried out to identify the operating factors involved in the failure of two tubes made of 304 stainless steel removed from a high pressure feedwater heater working in a nuclear power plant. The samples cut from tubes have been analyzed by different methods: visual examination, optical (metallographic) microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Following analyses, on the surface of tubes was highlighted the presence of many pits in which cracks have started. Inside the pits, impurities of the type S, Cl, K, Ca were detected too. The branched cracks most likely have occurred as a result of precipitation of small amounts of chlorides deposited and concentrated on the surface of the tubes over a long period of operation. The stresses that favored this type of corrosion cracking were both residual stresses and stresses occurring at the torsion and bending of the tubes, while the high water temperature from feed heater was also a favorable factor for corrosion. The final conclusion was that the tubes failed by a chloride induced stress corrosion cracking mechanism.

Highlights

This study has been carried out to identify the operating factors involved in the failure of two tubes made of 304 stainless steel removed from a high pressure feedwater heater working in a nuclear power plant
Experimental part The both analyzed tubes removed from high pressure (HP) feedwater heater are made of SA 213 TP 304 stainless steel
The chlorides were detected by SEM/energy dispersive X – ray spectroscopy (EDS) on the surface of tubes analyzed, so the conclusion is that tube degradation can be related to chloride stress corrosion cracking (CSCC)

Summary

Introduction

This study has been carried out to identify the operating factors involved in the failure of two tubes made of 304 stainless steel removed from a high pressure feedwater heater working in a nuclear power plant. The most common configuration of feedwater heater is a shell and tubing with the water flowing inside the tubes and steam condensing outside. Literature [3-8] shows that the susceptibility of austenitic stainless steels to this kind of corrosion, especially chloride stress corrosion cracking (CSCC), depends on a combination of the following factors: environmental variables (chlorides concentration, temperature, pH, oxygen), stress level (residual stresses or service stresses), surface finishing and the metallurgical condition of the steel.

Results

Conclusion