Computational and experimental studies on the causes of crack network formation in the area of the heat exchanger tube sheet in the BN-600 reactor
UDC: 621.039.5:539.4 БН
In order to examine the condition of intermediate sodium-to-sodium heat exchangers (IHX) and to substantiate their operation life extension to 45 years in the BN-600 reactor plant at Beloyarsk NPP Unit 3, one of the six heat exchangers was removed from the reactor in April 2006. Results of inspection showed that there were cracks 7 mm deep maximum on the outer surface of the upper tubesheet and adjacent shell. To predict durability of substantially fatigued metal, a need arose to verify the existing relationship for the threshold stress intensity factor range for the 10Cr18Ni9 steel. To this end, respective testing was conducted on specimens of two structural elements in the IHX – upper tubesheet and protection block. To identify failure mechanisms, fractographic studies were performed on the surface of cracks detected in the tubesheet and made in the specimens. Based upon the studies, a conclusion was made that the failure mechanism for cracks detected on the tubesheet was identical to the mechanism generated in the test specimens. In both the cases, the intergranular failure prevailed that is typical for the material loading level testifying to termination of the crack growth. This result makes it possible to speak about crack initiation and propagation in the IHX tubesheet in the high-cycle fatigue region at a 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. Based upon computational analysis results, for all thermal pulsation conditions and crack propagation cross-sections under consideration, the leak-tightness condition is met for the upper tubesheet 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 possibility of service life extension to 45 years for the IHX in the BN-600 reactor plant.
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