Welding Engineering
Appropriate weld design, procedures, and qualification are vitally important at nuclear plants. Structural Integrity Associates, Inc. (SI) offers comprehensive welding engineering services, including development and review of welding programs; selection and development of welding processes for critical applications; review and evaluation of procedure qualification records and weld procedure specifications; welder qualification; review and evaluation of weld failures; development of corrective actions; and welding code review and reconciliation. SI is also available to develop innovative weld and repair techniques for specific situations. In fact, SI pioneered development and application of weld overlay and temperbead welding for nuclear plant repairs.
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Structural & Stress Analysis
The performance of complex structures under thermal, static, and dynamic loading is a crucial question at nuclear plants. Armed with an array of state-of-the-art computer codes and analytical techniques, Structural Integrity Associates, Inc. (SI) can provide linear-elastic and non-linear structural and stress analyses for nuclear plant components and systems, including pressure vessels, tanks, piping systems, pumps, tubing, headers, heat exchangers, valves, reactor internals, and turbines. Examples of stress-analysis codes employed by SI range from ANSYS, Inc.'s ANSYS® and Livermore Software Technology Corp.'s LS-DYNA® finite element software, to DST Computer Services, SA's PIPESTRESS for piping system analysis, to SI's own specialized programs, including TOPBOT™, used to determine stress distributions in stratified piping systems, and PIPE-TS2™, used to calculate through-wall thermal gradient stresses for piping thermal transients.
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Fracture Mechanics Analysis & Risk Assessment
When a flaw is found in a reactor pressure vessel, piping, or other nuclear plant component, fracture mechanics calculations are used to analyze and predict flaw behavior, including crack growth rates and critical crack sizes. Structural Integrity Associates, Inc. (SI) uses deterministic and probabilistic fracture mechanics techniques, including finite element analysis, to disposition flaws in a variety of materials, geometries, and applied stress fields. Specialized fracture mechanics software developed and used by SI includes pc-CRACK™, often employed for ASME Code Section XI flaw evaluations and weld overlay design; APPENDA™/MAPPA™, used for ASME Code Section XI vessel flaw evaluations; and ANSC™, used for net section collapse analysis for arbitrarily flawed sections in ductile materials. SI also uses Viper™/Viper-Noz™ — developed for EPRI® by SI — to determine probability of failure for reactor pressure vessel welds and nozzles.
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Probabilistic Fracture Mechanics Analysis & Risk Assessment
Probabilistic fracture mechanics (PFM) analysis takes conventional deterministic fracture mechanics analysis a step further, using selected random variable inputs for situations that involve a significant degree of uncertainty — such as in crack size, or scatter in test results — and PFM analyses usually yield a failure probability, rather than a specific calculation of crack size or lifetime. Structural Integrity Associates, Inc. (SI) has applied PFM analysis to piping, reactor-head penetration nozzles, and other nuclear plant components. Many of SI's PFM analyses employ WinPRAISE™, originally developed under NRC sponsorship: SI's latest version of the software, WinPRAISE 07, can analyze crack initiation and growth due to fatigue, IGSCC, PWSCC, and flow-accelerated corrosion. WinPRAISE is also available for purchase from SI in easy-to-use Microsoft® Windows® format.
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Wall Thinning Evaluations
Even when local wall thinning in nuclear pressure vessels, piping, valves, or other components is well below the general minimum wall thickness required by ASME Code, continued operation may be justified by ASME-approved evaluation methodologies. Structural Integrity Associates, Inc. (SI) is a leader in wall-thinning evaluations for nuclear plants, including evaluation of mechanisms and rate of thinning, as well as familiarity with applicable ASME Code criteria. In fact, the evaluation methods in ASME Code Section XI Code Cases N-480, N-513, and N-597 were based on a SI report prepared for EPRI® (N-480) or developed by the Code committees using significant SI contributions. SI uses either the simplified analytical methods found in those three ASME Code cases, or more complex finite element models that determine local stress distributions in thinned components. In situations where there is local through-wall leakage, SI conducts fracture mechanics analyses to demonstrate sufficient margin against fracture. With its unique industry expertise, SI can complete wall thinning evaluations quickly, enabling rapid run/repair/replace decisions.
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Piping Analysis
Sophisticated piping analysis — and the customized software for conducting it — are Structural Integrity Associates, Inc. (SI) specialties. Some examples of complex SI piping analyses used to resolve nuclear plant design and operational issues include ASME Class 1 design transient evaluations for extended license operation; thermal stratification analyses in pressurizer surge, feedwater, and spray piping; piping vibration evaluations involving socket weld cracking; analyses of flow-induced vibration in piping systems; evaluations of flow-accelerated corrosion-induced wall thinning; analyses of power uprate changes; and seismic analyses. Proprietary SI-developed software is used for analyzing thermal stratification, vessel fatigue, wall thinning, thermal shock at transitions, vibration loading, and other specific conditions. SI also uses DST Computer Services, SA's PIPESTRESS for ASME Class 1,2, 3, B31.1, and B31.3 piping analyses, and ANSYS, Inc.'s ANSYS® for ASME Code, Section III, NB-3200 finite element analyses.
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Weld Residual Stress Analysis
Certain welding processes — pipe-to-pipe butt welds, small-bore penetration J-groove welds, and weld overlay repairs, for example — leave residual stresses that can affect the strength of welded nuclear plant components. Structural Integrity Associates, Inc. (SI) offers a full range of temperature-controlled, non-linear, path-dependent weld residual stress analyses, using ANSYS, Inc.'s ANSYS® finite element analysis software to simulate various welding processes. These analyses simulate a thermal pass, complete with appropriate weld heat input, heat efficiency, and appropriate cooling time, to determine temperature distributions due to welding processes; a simulated stress pass in turn calculates residual stresses due to non-linear, elastic-plastic load/unload stress-reversal cycles from the heating and cooling of the weld elements throughout thermal transient history. For especially complicated weld geometries, SI can perform three-dimensional moving heat source residual stress analyses.
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Root-Cause Failure Analysis
At the bottom of every plant component failure is a root cause, and root-cause failure analyses are a proven way to avoid repeat or related failures. Structural Integrity Associates, Inc. (SI) offers a complete range of root-cause failure analyses, varying in complexity from one or two specialists focused on material fractography to a multi-discipline team performing a full-scope investigation. To support its investigations, SI maintains both a metallurgical laboratory and an advanced nondestructive testing center. Additionally, SI performs human-error analyses and organizational/programmatic evaluations to uncover possible contributing factors. SI is also available to perform third-party reviews of root-cause failure analyses, and evaluations of corrective-action programs.
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Thermal-Hydraulic Analysis
Thermal-hydraulic analyses of nuclear plant components and structures are used to determine loadings on related components. Structural Integrity Associates, Inc. (SI) conducts a full array of steady-state and transient thermal-hydraulic analyses, using either standard textbook methods or specialized software, including proprietary SI software. Examples of complex problems that SI has solved via thermal-hydraulic analyses: evaluation of severe wall thinning in piping beyond a feedwater heater drain tank found that insufficient back-pressure and two-phase flow conditions were the culprits; analysis of reactor containment pressure-temperature response justified conducting BWR reactor pressure tests at elevated temperatures with open containment; PWR thermocouple test data were evaluated to determine the internal piping flow and stratification conditions that would exist during various operation modes; and analyses have determined stratified flow levels and heat transfer coefficients in piping systems subject to such stratified flow. SI also offers computational fluid dynamics analyses using ANSYS® CFX® software.
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Materials/Metallurgical/Corrosion Engineering
Accurate materials, metallurgical, and corrosion evaluations are essential to the appropriate disposition of degraded plant components. Such evaluations also help identify improved materials and fabrication processes to prevent future degradation. Structural Integrity Associates, Inc. (SI) offers a complete range of materials engineering services, supported by SI's full-service metallurgical laboratory in Austin, Texas. Materials engineering specialties include welding engineering; corrosion and stress corrosion cracking evaluations; materials selection; and corrosion-fatigue and creep assessments. Low-temperature and high-temperature corrosion phenomena, including microbiologically influenced corrosion, are a long-established area of SI expertise.
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Metal Fatigue Evaluations
Metal fatigue from nuclear plant cyclic stresses and strains is a major contributor to degradation of aging components — and a specialty at Structural Integrity Associates, Inc. (SI). In fact, EPRI® relied on SI's metal fatigue expertise to develop the EPRI Fatigue Management Handbook as well as FatiguePro™, a widely used, real-time thermal fatigue monitoring system that automatically tracks fatigue usage and transients for critical nuclear components. SI's metal fatigue services and products include FatiguePro evaluations; fatigue management handbooks that help identify potential damage locations; fatigue repair support; design of modifications to extend component fatigue life; analysis of reactor vessels and piping to determine effects of reactor water environment on fatigue life; determination of fatigue impact of cyclic thermal stratification in reactor piping; and vibration fatigue management.
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