Complex sentence: During the course of the repair, the turbine blades were welded, because, after a long period of operation, cracks and cavities had appeared; however, the turbine wheel was not properly rebalanced after these repairs had been completed.
Split into simpler sentences: During the course of the repair, the turbine blades were welded, because, after a long period of operation, cracks and cavities had appeared. However, the turbine wheel was not properly rebalanced after these repairs had been completed.