Computational and experimental studies of the causes of crack network formation in the area of the heat exchanger tube sheet in the BN'600 reactor

In order to examine the condition of intermediate sodiumtosodium heat exchangers (IHX) and to substantiate their operation life extension to 45 years at the BN600 reactor plant at Beloyarsk NPP Unit 3, one of the six heat exchangers was removed from the reactor in April 2006. Inspection revealed cracks with 7 maximum depth of mm on the outer surface of the upper tubesheet (UTS) and adjacent shell.To predict durability of substantially fatigued metal, verification of the existing relationship for the threshold stress intensity factor range for the 10Cr18Ni9 steel was needed. To this end, specimens of two structural elements in the IHX – upper tube sheet and protection block were tested.To identify failure mechanisms, fractographic studies were performed on the surface of cracks detected in the tube sheet and produced in the specimens. The studies led to the conclusion that the failure mechanism for cracks detected on the tube sheet was identical to the mechanism generated in the test specimens. In both cases the intergranular failure prevailed that is typical for the material stress level indicative of crack growth termination. This result makes it possible to speak about crack initiation and propagation in the IHX tube sheet in the high-cycle fatigue region at low-level strain ranges and stress intensity factor ranges.An analysis of causes of crack formation showed that the cracks could have formed as a result of the temperature pulsation effect produced by mixing of sodium flows having different temperatures – sodium entering the IHX inlet and sodium coming from the reactor vessel cooling system.Computational analysis results showed that for all thermal pulsation conditions and crack propagation cross-sections under consideration, the leak-tightness condition is met for the upper tube-sheet and IHX outlet chamber shell that divide the primary and secondary coolant circuits.The computational and experimental studies have proved that presence of these cracks does not limit the potential service life extension to 45 years for the IHX in the BN600 reactor plant.