Analysis and repair of cracking in partition plate of RHR heat exchanger at Brunsbüttel nuclear power station Original Research Article

According to Germanyʹs pressure vessel code, the residual heat removal (RHR) heat exchangers of System TH installed at Brunsbüttel nuclear power station must be subjected to an internal examination every 4 years. This inspection was carried out for heat exchanger TH35 B101 on October 21, 1994. In the course of the examination, the sealing strips at the channel cover were found to be slightly deformed. Replacement of the sealing strips required removal not only of these strips but also of another strip which holds the sealing strips in place and is bolted to the channel partition plate. After all of the strips had been removed, a crack was discovered at the weld joining the partition plate to the channel shell. The metallographic and fractographic examinations as well as the mathematical root-cause analyses showed that the fracture was caused by alternating bending stresses. At the top of the partition plate, a maximum stress intensity of 932 MPa was found in the region of the incipient crack. A new adequately sized partition plate with a wall thickness of 20 mm was installed to support the old plate, which is only required in the future to serve as a leak-tight barrier. The new partition plate is held in place by eccentric-head bolts at the tubesheet and metal strips at the channel cover. This concept enabled the partition plates in the TH heat exchangers to be quickly repaired and restored the full functional capability of these heat exchangers.