Manabu Arakawa

Probabilistic fracture mechanics analysis for pipe considering dispersion of seismic loading

Studies on probabilistic fracture mechanics (PFM) have been performed for the quantitative integrity assessment of the pipes, which are used in nuclear power plants. The seismic load is one of the dominant loads for the failure assessment of the pipes. Its probabilistic dispersion, however, was not taken into account in the past PFM analysis. Authors have developed PFM analysis code, which accommodates the dispersion of the seismic load and performed parametric PFM analyses using it. The seismic stress has more effect on the break probability, but not for the leak probability. The earthquake, whose occurrence probability is less than 10 -5 /year has little effect on the break probability. The break probability is affected by the dispersion of the stress due to earthquake rather than the seismic hazard curve.

Safety Factor for Flaw Evaluation of a Pipe Having a Circumferential Surface Flaw

This paper describes the safety factors used for fracture assessments of pipes having circumferential surface flaws. The “Fitness-For-Services Codes (the FFS Codes)” of the Japan Society of Mechanical Engineers (JSME) restrict the depths of flaws according to their angles to prevent pipes with flaws from being fracture. Past restrictions were determined based on deterministic evaluations. In fracture assessments of pipes having flaws, however, the effects of measurement errors in flaw dimensions and of variations on material strength must be taken into account. Thus, we evaluated the effects of such variations on fracture assessments of pipes having flaws, and examined safety factors for giving failure probability (or reliability) equal, irrespective of the cracking angles. We found out that failure probability is heavily dependent on the measurement accuracy of flaw depths and material strength (flow stress). In view of this finding, we examined and proposed a simple approach which meets the target reliability without conducting complex evaluations by the Monte Carlo method and reliability evaluation methods (e.g., first-order second-moment method (FOSM)).Copyright

Benchmark Analysis on the Failure Probability Assessment of Piping With Stress Corrosion Cracks

This paper describes the benchmark analysis of two probabilistic fracture mechanics (PFM) codes developed for structural reliability evaluation of primary loop recirculation system (PLR) piping of a boiling water reactor (BWR) in which the stress corrosion cracking (SCC) has been observed frequently in recent years. The two codes PASCAL-SP and PEPPER-M used for benchmark analysis have been independently developed by Japan Atomic Energy Agency (JAEA) and TEPCO systems Corporation (TEPSYS), respectively. After the validity of the procedure of analysis in these codes was conformed, the reliability assessment of PLR piping containing cracks due to stress corrosion cracking (SCC) was carried out according to some regulatory requirements and the rules on fitness-for-service in Japan. The analytical results from both codes were well in agreement. Through comparison of the results, influences of some items and input parameters on failure probabilities were identified.© 2011 ASME

Partial Safety Factors Assessment of Pipes With a Circumferential Surface Flaw

This paper describes the evaluation of partial safety factors (PSF’s) for parameters related to flaw evaluation of pipes which have a circumferential surface flaw, and proposes the important matter which should be pay attention in the setup of the safety factors used in flaw evaluation. PSF’s were evaluated considering randomness of flaw size, a fracture resistance curve (J-R curve) and applied loads using load and resistance factor design method (LRFD). The limit state function is expressed by fracture resistance (resistance-related parameter) and applied J integral (load-related parameter). The measure parameters in the reliability assessment are the flaw size and the J-R curve, and PSF’s of them are larger than those of applied loads. Since the material properties used in the flaw evaluation are generally set to the engineering lower limit of their variation (e.g., 95% lower confidence limit), variation of the flaw size is considered to have important role on flaw evaluation. Therefore, when setting up the safely factors used in Rules on Fitness-for-Service (FFS), it is necessary to take into consideration not only the influence of variation of loads or material strength but the influence of variation of flaw size.Copyright

Development of LBB Assessment Method for Japanese Sodium Cooled Fast Reactor (JSFR) Pipes: 2—Crack Opening Displacement Assessment of Thin Wall Pipes Made of Modified 9Cr-1Mo Steel

This paper presents crack opening displacement (COD) evaluation methods used in leak before break (LBB) assessment of sodium piping system in the Japanese sodium cooled fast reactor (JSFR). Thin wall and low work hardening material are features of piping of JSFR. Detectability of coolant sodium is very high, the target leak detection performance of leak detector of JSFR is 100g/hr. Internal pressure of JSFR piping is very low, and large through wall crack is allowable to satisfy LBB condition. The existing COD evaluation method cannot be applied to the piping of JSFR which has above-mentioned features. Finite element analysis (FEA) were performed to evaluate COD for thin wall and low work hardening material piping with a circumferential through wall crack. These analytical results were arranged into engineering evaluation formulae. Proposed method gives high accuracy COD estimation, and is applicable to LBB assessment of JSFR piping.Copyright

Study on Probabilistic Fracture Mechanics Analysis Codes for Pipes With Stress Corrosion Cracks

Risk-Informed integrity management methodologies have been developed in Japanese nuclear power plants. One of the issues of concern is the reliability assessment of piping with flaws due to stress corrosion cracking (SCC). Therefore, the probabilistic fracture mechanics analysis code have been developed, which can perform the reliability assessment for the austenitic stainless steel piping with flaws due to SCC. This paper describes technical basis of this code. This method is based on Monte-Carlo technique considering many sample cases in a piping section, where the initiation and growth of cracks are calculated and piping failures, including leaks and rapture, are evaluated. A notable feature is that multiple cracks can be treated, consequently, assessment of coalescence of cracks and intricate break evaluation of piping section have been included. Moreover, the in-service inspection (ISI) and integrity evaluation by Fitness-for-Service (FFS) code are integrated into the analysis, and the contribution to failure probability decrease can be assessed. Key parameters are determined on a probability basis with the designated probability type throughout the procedure. Size, location and time of crack initiation, coefficients of crack growth due to SCC and factors for piping failure are included in those parameters. With this method the reliability level of the piping through the operation periods can be estimated and the contribution of various parameters including ISI can be quantitatively evaluated.Copyright