Among the various degradation modes that cause pipe wall thinning in the secondary system of Nuclear Power Plants (Corrosion, Galvanic Corrosion, Environmental Corrosion, Flow Accelerated Corrosion, Cavitation, Droplet impingement, Erosion and Abrasion), FAC is the one that is the more widespread in the installation and that requires constant efforts to fight. The wall thickness loss due to FAC can effectively occur in any carbon steel piping containing hot water or wet steam and can lead to pipe ruptures with dramatic consequences.

Electricité de France operates 58 Pressurized Water Reactors that were put in service between 1977 and 1997. The degradations and leaks due to FAC that EDF has experienced and the numerous accidents that have been reported abroad, lead EDF to develop a global strategy to control FAC on piping and to prevent any dramatic rupture. This global strategy is based on a National Maintenance Rule which has been written by the corporate engineering level of EDF called “RNM” and applied by every Nuclear Power Plant operator of the EDF fleet. The RNM is mainly based on the use of the FAC-prediction software "BRT-CICERO™" as well as on specific actions for the lines or elements that are not modeled in the software.

In summer 2004, a large-bore carbon steel pipe of a PWR secondary system ruptured at the Mihama Unit 3, Japan, with some fatalities. The Main Committee on Power Generation Facility Codes (MC-PGFC) of the Japan Society of Mechanical Engineers (JSME) established technical rules for PWRs and BWRs, which were requested by the nuclear power industry as well as the regulatory bodies. In Japan, different wall-thinning management rules have been established for PWR plants and BWR plants because of the difference in their management, such as water chemistry and inspection points. For example, in BWR plants, the wall-thinning rate is relatively small at the condensation system and the feed water system after oxygen injection point. The rules for each type of plant are based on data obtained by wall-thinning measurement. The endorsement of these rules by Nuclear and Industry Safety Agency (NISA) was completed in summer 2008. A special feature of the management of pipe wall thinning in Japan is that Japanese utilities have a large amount of field data for wall thinning. Regarding BWR plants, there are data for 26,000 locations obtained from above two inspections of 29 plants, and for PWR plants, there are data for 20,000 locations obtained from above three inspections of 23 plants.

Moreover, MC-PGFC have produced a roadmap for R&D studies to support and enhance the rules, and many subjects of R&D studies are contained in the roadmap on methods of predicting pipe wall thinning, phenomenological screening procedures for parts whose management is necessary, new methods of measuring pipe wall thickness, improved methods for evaluating the wall-thinning rate, and repair methods. In this paper, the wall-thinning management rules, summary of the roadmap, and trends in recent R&D studies in Japan are outlined.

Electromagnetic acoustic transducer (EMAT) provides non-contacting measurements and is often applied to monitoring in high temperature environment. One of drawbacks of EMAT is its low sensitivity and S/N ratio. One way to overcome the drawback is to introduce an Electromagnetic acoustic resonance method (EMAR); it has a high capability of evaluating thickness or ultrasonic velocity, and it is applied to the measurement of thickness, elastic constants, damping properties and so on. Since the principle of the method is based on the through-thickness resonances of bulk waves, that is, thickness oscillations, targets are usually limited to ones with simple geometry such as plates. In this study, pipe wall thinning, where the thickness changes on the curved surface, is evaluated by EMAR. For the purpose, several data processing methods are applied to extract thickness information from spectral responses. Measured spectra are obtained by an experiment using a carbon steel pipe with two-dimensional thinning machined by milling, and results of the data processing are compared in view of accuracy and stability. Finally, the data processing method is applied to the EMAR spectra of a pipe specimen cut from a mock-up test loop.

This paper demonstrates the effectiveness and feasibility of monitoring system for pipe wall thinning　using electromagnetic acoustic transducer (EMAT). The proposed EMAT system provides no contact, high resolution measurement, and remote capability of inspection procedures. First, a simulation based analysis is performed to study the measurement sensitivities with respect to pipe wall thinning. Secondly, taking into account that FAC damage is extended into the downstream of orifice, verification tests are implemented based on thickness measurements at grid points on the outer surface of pipe. Various type of test pipes with FAC damage model were provided for our laboratory experiments. Finally, the validity and the feasibility of our proposed method are demonstrated through the practical test at the real plant.