Section XI imposed volumetric inservice inspection requirements on heat exchangers in nuclear plants after most of this equipment was designed and installed. Consequently the equipment was not designed for ultrasonic examination, and in some cases such volumetric examination is not justified. The man-rem dose received from the ultrasonic inspection of some of these components is very high, and there are no known mechanisms of degradation; thus, the volumetric inspection serves no useful purpose. With the use of the newly approved code case, N706, volumetric and surface inspection of the regenerative and residual heat exchangers in PWR plants may be replaced with a visual inspection. These two heat exchangers have high irradiation fields, and both have a number of complicated weld geometries that are difficult to inspect. The regenerative heat exchanger provides preheat for the normal charging water going into the reactor coolant system (RCS). The residual heat exchanger is designed to cool the RCS during plant shut down operations. The technical basis for changing these inspection requirements was derived from four fundamental arguments: 1. The heat exchangers were carefully constructed to nuclear quality requirements. 2. They were inspected during construction, and then during service, and there is no history of degradation. 3. The flaw tolerance of the components is very high, since their duty cycle is mild, and they are constructed of stainless steel. 4. The risk is not significantly changed by replacement of the examinations with visual examinations. This paper will describe in detail the technical arguments under each of these topics, which together form the basis for the code case.
Technical Basis for Revision of Inspection Requirements for Regenerative and Residual Heat Exchangers
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Bamford, W, Bishop, B, Haessler, R, & Bowler, M. "Technical Basis for Revision of Inspection Requirements for Regenerative and Residual Heat Exchangers." Proceedings of the ASME 2006 Pressure Vessels and Piping/ICPVT-11 Conference. Volume 1: Codes and Standards. Vancouver, BC, Canada. July 23–27, 2006. pp. 77-89. ASME. https://doi.org/10.1115/PVP2006-ICPVT-11-93892
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