Failure analysis of a Monel-400 heat exchanger tube from a nuclear power plant

Failure of a heat exchanger was reported in one of the nuclear power plants; The plant has eight boilers and each boiler has eleven heat exchanger bundles. The Monel-400 heat exchanger bundles are of U-Tube construction consisting of 192 tubes having 12 mm outer diameter and 1mm wall thickness. The tube sheet of the heat exchanger is made of 65mm thick stainless steel with monel overlay on the outer face. The heat exchanger tubes were fully expanded in the tube sheet and welded on to the outer face of the tube sheet having the monel overlay. The tubeside fluid is heavy water and the shell side fluid is feed water. The failure in the form of a leak was noticed in the particular heat exchanger when routine inspection of the boiler feed water was analyzed. The heat exchanger bundle was cutout of the boiler and the leaky tube was identified by conducting pressurization tests. The failure location was close to the tube sheet at the heavy water inlet end where large amount of sludge deposit was noticed. Detailed failure investigation revealed extensive wall thinning in the failed region. The failure was found to be intergranular in nature and leaching of nickel from the grain boundary region was also observed. The cause of the failure was found to be 'underdeposit' corrosion. Details of the failure investigation carried out are presented in the paper.