Rohit Rastogi

Probabilistic fracture analysis of a straight pipe with through-wall circumferential crack using R6 method

Abstract Deterministic fracture analysis of cracked structures has been successful in demonstrating the behavior of structures under observed or postulated flaws. The key ingredients in a deterministic fracture mechanics analysis are the initial crack size, crack-driving force, applied stress and material properties. However, crack size, postulated accidental loading, mechanical and fracture parameters are often subjected to considerable scatter or uncertainty. Hence, the result of fracture mechanics analysis must be viewed with some skepticism. Quite often, conservative bounds on inputs are employed on these by providing a conservative estimate to fracture. However, it may lead to predict the overly conservative and unrealistic results. These uncertainties encourage us to adopt a statistical or probabilistic approach to the structural integrity. R6 is a well-known engineering assessment procedure for the evaluation of the structural integrity of the flawed structure. This paper presents the determination of failure probabilities of a straight pipe with a through-wall crack in the circumferential direction, using the R6 method. The pipe is subjected to an in-plane bending moment. The scatter in the crack size, mechanical and fracture properties are considered. The probability of failure is studied for variation in applied loads. Monte Carlo simulation is used to find the failure probability. The study is performed by constructing failure assessment diagram (FAD) using all the three options of R6. The results are compared with the approximate analytical formulations and with those available in the literature.

Assessment of integrity of components in piping of 500 MWe PHWR: using R-6 method

Abstract The primary heat transport system of 500 MWe Indian PHWR comprises of straight pipes, elbows and headers. A study was conducted to qualify piping system for leak before break. R-6 method was used to assess integrity of system for leakage size crack (LSC), the margins on crack initiation load and unstable crack growth loads. Option 2 (material specific failure assessment diagram), Category 3 (ductile tearing) analysis was used for straight pipes, elbows and header. In order to enhance the confidence in the analytical results, detailed sensitivity analysis was also performed. For sensitivity analysis, variation in material properties, LSC was considered. The effect of variation in temperature on material properties was also considered. Tensile and fracture properties used for base and weld material data were generated from pipe material obtained from 220 MWe Nuclear Power Plant, under construction.

Radiological impact assessment of opencast uranium mining for large reservoirs-uncertainty analysis using Wilk's method

Opencast mining may lead to natural erosion of ore material due to overland flow of water accumulated from rainfall. The overland flow may ultimately reach the nearby surface water body. This process may lead to the release of U-238 and its daughter products into the surface water body. The present study is carried out to assess the radiological impact of the erosion in terms of dose due to U-238 and its progenies in the surface water body and to quantify the uncertainty associated with the dose due to consumption of the reservoir water. The in-growth of progenies is also taken into account. The properties like settling velocity, distribution coefficients etc. are having inherent uncertainty associated with their values. The uncertainty of various parameters involved is propagated to the model output. Hence, the uncertainty analysis becomes important to build confidence in the results. In this paper, Wilks method is used to calculate a value greater than 95th percentile value for the dose rate through drinking water pathway with 95 percent confidence level (95/95 value). In present study, the 95/95 value for the annual effective dose to the public due to U-238 and its progenies through drinking water pathway is found to be 12.5 times lower as compared to the WHO guidelines for drinking water.

Best-Estimate Evaluation of Large-Break Loss-of-Coolant Accident for Advanced Natural Circulation Nuclear Reactor

Abstract This paper presents the methodology, which will be adopted, for quantifying the effect of uncertainties on the peak clad temperature of an advanced natural circulation nuclear reactor. The method relies on probabilistic analysis, treating uncertain parameters as random variables. The paper will cover a case of a loss-of-coolant accident due to a 200% (that is, double ended) break of the largest pipe with partial unavailability of the low-pressure emergency core cooling system. The break has been postulated at the inlet header, which is the largest pipe in the main heat transport system. For this assessment a two-step procedure has been adopted. In the first step the probability of the peak clad temperature exceeding 800°C has been evaluated using the response surface, generated from the results of thermal-hydraulic analyses. One of the fuel failure criteria for this reactor is the peak clad temperature exceeding 800°C. Such a high temperature is expected during typical large-break loss-of-coolant accident conditions. The thermal-hydraulic analyses, using the computer code RELAP5/MOD3.2, were done for several cases involving different combinations of six selected uncertain parameters. The probabilistic analysis was carried out using Monte Carlo and first-order reliability methods. The first step results in conditional probability of the peak clad temperature exceeding the criteria subject to the condition of a 200% break in the inlet header. The probability of a 200% break is calculated in the second step. The probability of an inlet header pipe weld rupture has been evaluated based on probabilistic fracture assessment. The pipe break analysis considers the uncertainties in strength, fracture, and stress corrosion properties and initial crack/flaw sizes produced during fabrication or welding. It also accounts pre-service and in-service inspection, inspection quality, and different damage mechanisms such as fatigue and intergranular stress corrosion cracking. The combined results of both these steps give the overall probability of the peak clad temperature exceeding 800°C.