In-service Inspection Frequency Research Articles

Study on the Dynamic Inspection for Decreasing the Losses and Costs Caused by Accidents of Fixed Equipment in Petrochemical Plants

The risk- and condition-based dynamic inspection proposed to decrease losses and costs caused by accidents which is not only a decision-making and optimal method for inspection strategies, but is also a soft measurement system for fixed equipment safety status. In the risk- and condition-based dynamic inspection method, data collection is standardized to decrease the impact of personal factors on the data. Moreover, a procedure in which experts approve the data collected from plants is supplemented to amend error and supplement omission of data in time. A relevant inspection strategy is adopted based on the different damage types and its initiation and growth for the aspect of inspection decision-making. In the meantime, the cost-risk efficiency of the inspection is regarded as an objective function to optimize inspection strategies according to the risk status of equipment. Finally, a model is established to assess the frequency of in-service inspection and ensure equipment safety during the end of equipment life based on the fitted linear relationship between the corrosion rate and stress energy.

Balancing material selection and inspection requirements in structural design of fast breeder reactors based on “System Based Code” concept

The System Based Code concept proposed by Asada et al. [Asada, Y., Tashimo, M., Ueta, M., 2002a. System Based Code—Principal Concept. Proc. ICONE10, 22730; Asada, Y., Tashimo, M., Ueta, M., 2002b. System Based Code—Basic Structure. Proc. ICONE10, 22731] intends to realize margin exchange, in order to optimize design. This paper presents preliminary calculation of margin exchange between material strength and the accuracy and frequency of inservice inspection (ISI), taking a reactor vessel of a fast breeder reactor, of which dominant failure mode is creep-fatigue, as an example. The original design is a structure of forged rings of 316FR, material with superior creep strength. Alternative designs use either Type 304 stainless steel or welded structure of 316FR plates, leading to increased failure probability compared to the original design. The accuracy and frequency necessary to compensate this increase of failure probability was estimated. Results envisioned margin exchange between material strength and ISI under practical conditions. Sophistication of the procedure to calculate failure probability will ensure the application of the concept of margin exchange to practical design.

Risk-informed inservice inspection

The operational readiness and functional integrity of certain safety-related piping and associated structural elements such as piping supports are vital to the safety of operations in nuclear power plants. Inservice inspection (ISI) is one of the mechanisms used by the power plant owners to ensure piping integrity. Previously, the type and frequency of ISI have been based on the collective best judgment of the NRC and industry in a consensus code and rulemaking process. The ASME code-based ISI requirements and practices have not explicitly taken into consideration unique aspects of piping functions, piping degradation mechanisms, weld integrity, fabrication details, and the extent of the contribution to overall plant risk. Due to the general nature of the ASME code ISI requirements and non-reliance on quantification of risk estimates, current ISI requirements may unnecessarily emphasize inspection of less safety-significant piping segments, and thereby unnecessarily expose plant personnel to radiation exposure. Nuclear power plant owners are currently interested in optimizing inspection and testing by applying resources in more safety-significant areas. They are also interested in maintaining system availability and reducing overall maintenance costs which do not have any adverse effects on safety. The NRC has confirmed its intent of using probabilistic, as an adjunct to deterministic, techniques, to help define the scope, type, and frequency of ISI. The development of risk-informed inservice inspection programs (RI-ISI) has the potential to optimize the use of NRC and industry resources and to continue assuring adequate protection of the public's health and safety. Currently there are two methodologies being proposed by the industry for the implementation of the RI-ISI programs. One is being developed jointly by the ASME Research and Westinghouse Owners Group (WOG) and the other by the Electric Power Research Institute (EPRI). Both methodologies will be implemented for pilot plant applications. Based on discussions with the interested licensees, the NRC staff has tentatively accepted Surry, ANO-2, and Fitzpatrick as the RI-ISI pilot plants. The Surry pilot application is based on the WOG methodology, whilst ANO-2 and Fitzpatrick are based on the EPRI methodology.