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We have reached a point in the nuclear power industry where we need to develop adaptive solutions that will fix plant aging problems, thereby extending the overall lives of reactors and increasing power production efficiency for many years to come. In Sweden, as in other parts of the world, substantial time is spent during every maintenance outage to analyze and fix such problems.
Recently, inspectors at the Ringhals Plant on the west coast of Sweden found a pipe crack in the vicinity of a Control Rod Drive Mechanism (CRDM) housing nozzle at Unit 1. The crack was approximately 20 mm from the welded joint between the attached SCRAM pipe and the nozzle. The SCRAM system is part of the safety system that activates in case of an emergency.
A decision was made to install a plug in the nozzle, block the control rod in fully inserted position and operate for one cycle. This temporary fix to the problem may have somewhat reduced plant capacity, but it allowed time to develop a permanent repair procedure. For help, Ringhals called its Europeanbased service partner Uddcomb Engineering who looked to additional partners, including U.S.-based Climax Portable Machine Tools, Inc., experts in developing on-site machining solutions.
Search for a solution
The ideal solution would enable full capacity in the short term and be adaptable for other similar cracks in the future. It had to apply to the least accessible nozzle position to avoid a complex and risky dismantling of the CRDM housing. Additionally, it needed to meet the ALARA (As Low As Reasonably Achievable) principle for radioactive contamination. Finally, to help extend the lives of the reactor, our solution needed to last for 20 years or longer.
Uddcomb Engineering evaluated the cutting and machining aspects of the operation, which were the most challenging. Since the crack was near the end of the nozzle, that section and the end of the SCRAM pipe had to be removed and replaced by a new piece of pipe. Unfortunately, the area is humanly inaccessible. The pipe is one of 157. which are spaced approximately 10 centimeters apart and run floor to ceiling beneath the reactor core. The largest area near the damaged pipe was 8.9 cm by 80 cm.
One option was to...