Determining the admissibility of the metal corrosion wear and estimating the residual life of corroded pipelines at NPP.

Abstract

Since in the field of nuclear energy, there is a need to assess the risk of erosion-corrosion wear (ECW) in pipelines, the key task of the article is to solve specific issues related to determining the admissibility of the depth caused by ECW defects in the metal walls of pipelines at nuclear power plants and estimate their residual life. A method for calculating the threshold values of wall thickness reduction depths in pipelines caused by corrosion has been processed in accordance with the established methodology adopted by the National Nuclear Energy Generating Company «Energoatom» and additionally, by incorporating regulatory and recommended documentation for pipeline design. The use of the methodology demonstrates certain advantages in terms of assessing erosion-corrosion wear and the fitness of the object for operation, specifically by reducing the requirements for the minimum permissible wall thickness of pipelines with a corrosion defect, especially for small-sized thinning, allowing for a reduction in the planned repair work volumes. In practice, the implementation of the methodology can pose difficulties due to its complexity and time-consuming nature of the procedures for calculating limit values. The paper provides an example of calculating the residual resource of defective pipeline systems at nuclear power plants using a probabilistic-physical approach to assess the technical condition of these objects. For this purpose, a probabilistic-physical reliability model was employed in the form of a diffusion monotonic distribution (DM-distribution) of failures. Its parameters are interpreted physically as the coefficient of variation for the generalized degradation process and the mean rate of the defining parameter. The defining parameter is considered to be the minimum allowable wall thickness of the pipeline under the influence of erosion-corrosion wear, which was determined as a result of calculations using the above-mentioned methodology and additionally by incorporating accompanying regulatory documents.