Feed Water Line Cracking in Pressurized Water Reactor Plants

Abstract

As early as 1979, a through wall crack was detected in a pressurized water reactor (PWR) plant. This crack initiated at the counter bore region of the pipe, adjacent to the weld joint attaching the pipe to the steam generator feed water nozzle. Subsequent inspection of the remaining feed water piping revealed cracking in the same vicinity but these were limited to partial wall penetration. As a result of this incident, the US Nuclear Regulatory Commission issued a directive to all operating plants requiring them to perform inspection of their feed water lines. The cracks were subsequently detected in the immediate vicinity of the steam generator nozzles in a number of plants. An exhaustive investigation was undertaken subsequently and this revealed that the primary cause of cracking was due to a fatigue loading mechanism induced by thermal stratification and high cycle thermal oscillations (striping) during low flow conditions. Thermal stratification phenomenon results from a temperature differential across the pipe cross section with the top fluid stream hot and bottom stream relatively cold. During normal plant operations at low flow conditions, when the feed water nozzle is not completely full, hot water from the steam generator remains in the nozzle to fill up the rest of the volume. The difference in buoyancy between the hot and cold fluids inhibits their mixing so that the feed water becomes and remains thermally stratified. Separation of these two flow regions is due to the density difference in the hot and cold streams. The stratified temperature conditions can produce very high stresses, and can occur may times during normal low power operations; therefore this has the potential to initiate cracks in a relatively short period of time. Thermal striping is a local phenomenon that occurs at the interface between hot and cold flowing fluids. The interface level oscillates with periods ranging from 0.1 to 10 seconds. The oscillating fluid temperature gives rise to fluctuating stresses. The magnitudes of the striping stresses are not as high as those due to stratification itself, but the number of cycles is so large that they contribute significantly to fatigue crack initiation. During normal plant operation, a series of temperature measurements has been taken around the pipe circumference at the vicinity of the of the feed water nozzle/pipe weld. Analysis of the data indicates that the stratified temperature distributions may be grouped into a handful of basic profiles corresponding to different levels of the interface between the hot and cold fluids. For analysis purposes these profiles could be assumed to be at steady state conditions because of their long duration observed during the tests. Nuclear piping systems (Class 1) are designed according to the rules of NB 3600 of the ASME Boiler and