Pressure tubes in CANDU reactor are the most important components that contain fuel. The operating experiences show leaks and burst in pressure tubes over the past two decades. The integrity of pressure tubes is, therefore, key concern in CANDU reactor. Once a CANDU reactor put into operation, their integrity could be checked by in-service-inspection. However, comparing to the total number of pressure tubes in a CANDU reactor, only a small number of pressure tubes are selected for inspection, since there is no weld. The inspection scope and results have been treated so far using a deterministic approach. Taking into account the difficulty in inspection sampling and in extrapolating the results to the entire core, a probabilistic approach is necessary. In this study, probabilistic integrity assessments are carried out considering key factors, such as initial hydrogen concentration, defect shape, delayed hydride cracking (DHC) velocity and fracture toughness. The leak and failure probabilities are calculated as a function of time by applying Monte Carlo simulation.
Since several incidents of leaking in the pressure tube have been experienced, significant efforts have been made to improve the pressure tube integrity design, material and fabrication upgrades during the last decade. As a result of the research, the fitness for service guideline (Hopkins et al., 1998) provides the flaw acceptance criteria for pressure tubes. FFSG is developed under the basis of ASME Code Sec. XI Code (1996), where FFSG uses conventional deterministic fracture mechanics approaches are used.
Due to the uncertainty in examination, conservative data are used for the integrity evaluation. For example, lower bound in fracture toughness and upper bound in stress data, crack growth rate are used. It results in, therefore, difficulty in estimating lifetime. The 53 tubes at Wolsong unit 1 are inspected, the number is far beyond the code requirement (CSA, 1994). Statistical analysis of inspection results by Park et al. (2002) shows that about 45% of pressure tube has defects regardless of their significance. Considering the inspection coverage is limited to 15% of pressure tubes, it is necessary to develop a tool for evaluating pressure tube integrity accounting for un-inspected portion. The probabilistic approach would be a suitable way for the evaluation in taking into account the uncertainties associated with important integrity parameters, such as in-service-inspection sampling sizes, flaws distribution, initial hydrogen concentration and material properties (Bloom, 1984). Many outstanding probabilistic research works have been conducted, especially for reactor pressure vessel in the event of pressurized thermal shock (Jhung et al., 2003, OECD/NEA, 1999, Shibata, 1999 and USNRC, 1896) and also for nuclear piping (Dillstrom, 2000; Harris, 1992). However, since the probabilistic integrity assessment of pressure tubes requires more complicated input data, only limited studies were so far performed.
In this paper, the evaluation of failure probabilities of pressure tubes considering delayed hydride cracking (DHC) mechanism was investigated using Monte Carlo simulation (Rubinstein, 1981). PROBie-PT (probabilistic integrity evaluation code for CANDU pressure tube) was developed. Sensitivity analysis for each input variables were conducted using PROBie-PT.
Sensitivity analyses are conducted for evaluating CANDU pressure tube integrity using probabilistic fracture mechanics. For more realistic failure estimation, failure assessment diagram are proposed instead of current failure criteria.
Among the parameters selected for the probabilistic failure analysis, the dimensional change and number of cool-down cycle have high sensitivity for failure probability, whereas fracture toughness, flow stress and flaw generation time have low sensitivity.
In all cases, integrity is assured during 30 years of design life, but careful analysis is required after design the life.
