Julien Banchet

Laser ultrasonics detection of an embedded crack in a composite spherical particle

Laser ultrasonics was applied to the manufacturing control of the integrity (no failure) of coated spherical particles designed for High Temperature Reactors (HTR). This control is of major importance, since the coating of the nuclear fuel kernel is designed to prevent from the diffusion of fission products outside the particle during reactor operation. The SiC layer composing the coating is particularly important, since this layer must be an impenetrable barrier for fission products. The integrity of the SiC shell (no crack within the shell) can be assessed by the ultrasonic vibration spectrum of the HTR particle, which is significantly changed, compared to the reference spectrum of a defect-free particle. Spheroidal vibration modes of defect-free dummy particles with a zirconium dioxide (ZrO(2)) core were observed in the 2-5MHz range. A theoretical analysis is presented to account for the observed vibration spectra of defect-free or cracked HTR particles.

X-Ray Radiography: An Alternative Method for Determining HTR Fuel Particle Coating Layer Thickness and Density

The HTR TRISO particle consists of a fissile kernel and surrounding layers, whose density and thickness are among the key fuel parameters. Destructive methods i.e. the sink float method and image analysis of ceramography, were developed in the past and are still used to evaluate these particle parameters. Although exhibiting great accuracy, these methods generate effluents and wastes, and are extremely cost/time consuming. In the framework of the AREVA NP HTR R&D program, the development of nondestructive evaluation methods as alternatives to destructive methods is carried out and aims at a new HTR Fuel QC strategy. In this scope, an innovative method was developed to automatically measure particle layer density and thickness from X-Ray Phase Contrast Imaging (PCI). First tested at the European Synchroton Radiation Facility (ESRF), this method was then applied to a custom built industrial demonstrator. Comparisons between the density and thickness values obtained by the developed method and their corresponding values obtained with destructive methods justify progressing to the validation phase. Particle samples were selected among the particle batches that were characterized by destructive methods. Layer density and thickness were determined by the X-Ray based technique on the industrial demonstrator as well as at the ESRF. Correlation levels obtained from this benchmark demonstrated that both parameters can be confidently measured by the developed method. Additionally, it is important to stress that this technique provides the opportunity to directly determine buffer density on finite particles as opposed to the sink float method. Thanks to its accuracy, its rapidity and its absence of waste generation, it is planned to implement the X-Ray thickness and density measurement method on the French lab scale fuel line. It was also decided to enter the characterization work package of the IAEA Coordinated Research Project 6 in order to benchmark the AREVA NP method with foreign techniques and materials.Copyright

Laser ultrasonics evaluation and testing of coated HTR nuclear fuel

Laser ultrasonics was applied to the manufacturing control of the quality and integrity (no failure) of coated spherical particles designed for High Temperature Reactors (HTR). The coating of the nuclear fuel kernel is designed to prevent from the diffusion of fission products outside the particle during reactor operation. The quality assessment of the coating layers is of major importance. Using laser ultrasonics, we determined the vibration eigenmodes of dummy HTR particles. The vibration spectrum of a HTR particle provides a non-destructive method of evaluating some important mechanical parameters of the coating. Moreover, without damaging the particle, the laser ultrasonics technique allows to test the presence of a crack in the SiC layer, through the observation of the particle vibration spectrum, which is significantly changed, compared to that of a defect-free particle. We applied nanosecond acoustic pulses, i.e., high frequency laser-generated ultrasound, to measure the acoustic velocity of longitudinal waves the SiC layer. This technique provides an alternative method of evaluation of the Young modulus of the SiC layer. We measure the velocity of surface acoustic waves (SAW) on a pyrocarbon layer cross-section and we demonstrated that the anisotropy of the internal pyrocarbon layer can be evaluated by laser ultrasonics.

Image Quality Assessment of Four Different Computed Radiography Systems for NDT Applications

Computed radiography (CR) is a digital radiographic technique, which uses very similar equipment to conventional radiography except that in place of a film to create the latent image, an imaging plate (IP) made of a photostimulable phosphor is used [1].CR systems are commonly used in medical applications since they have proven reliability over more than two decades. Conversely, the NDT community has discussed the efficacy of film replacement by CR for more than 15 years. Though some standards were introduced in 2005 (ASTM E 2033, CEN EN 14784-2) and others are on the way (PR ISO 17636-2), CR is actually not included within the French RCCM, while the technique is commonly used in US for nuclear applications according to ASME (Section V, article 2).Since 2006, AREVA has been evaluating the performance of CR in comparison to conventional RT in the framework of EN 14784 for the digital part and the RCCM for the conventional part. The objective was to build a technical justification report to eventually support introduction of CR into the RCCM.In 2009 the subject gave rise to collaboration between AREVA NP – NETEC and EDF-CEIDRE, for a joint project to establish performance limits of CR towards EN 14784 specifications and RCCM image quality indicator (IQI) requirements [2]. In this paper, we present performance comparison results of four different CR systems. The measurements were conducted in 2012 and they demonstrate the current state of achievable image quality in CR. The performance has been evaluated for steel with a thickness range of 20÷60 mm using an Iridium 192 gamma source. Image quality has been assessed in terms of EN 462 and ASTM (E 747, E 1742) IQI. The results have been scored considering the PR ISO 17636-2, RCCM 2007, and ASME V-2010. This also permitted comparison among the different standard requirements.Copyright

Time of Flight Diffraction Technique (TOFD-T) as an Alternative to X-Ray Examination for Thick Welds: A Literature Review

In the nuclear industry, and in particular, regarding large steel components including the nuclear steam supply system, weld integrity must be assessed and confirmed during the fabrication process, for the initial field-welds and periodically during in-service follow-up of some critical assembly welds. In France, this quality control is prescriptively carried out via nondestructive inspections in accordance with the RCCM code primarily via X-Ray Radiography or Gammagraphy (RT) coupled to conventional ultrasound (C-UT). These two techniques are integrated into code inspection requirements since the industry has good evidence that RT and C-UT are able to detect and characterize defects, and are well suited to the large weld thickness of the reactor pressure vessel, pressurizer or stream generators. C-UT is frequently used where it may be shown equivalent to RT. Use of RT however becomes more and more problematic because of the trend of regulatory restrictions to limit radiological source transport, extension of radiological exclusion zones to limit the dose to which workers are exposed, and pressure to increase production for new component fabrication and on-site assembly to support aggressive new-build schedules. Replacement of RT by another dose free technique such as the Time of Flight Diffraction Technique (TOFD-T) would be desirable. In this context, AREVA conducted a study of other industries and other countries management of RT particularly focusing on replacement of radiography by TOFD-T. Interviews were conducted surveying industries manufacturing pressure vessels and making similar welds to those within the nuclear industries, i.e. the oil and gas and the submarine industry. In addition, a literature review on the TOFD-T performances, existing codes and standards, and past approaches to justify the replacement of radiography by the TOFD-T was performed. For all this study, European, American and Japanese industries were surveyed or considered. This study showed that TOFD-T is widely used in the US oil and gas industry thanks to ASME code case, but the global nuclear industry has been reluctant to accept TOFD-T due to the lack of specific acceptance criteria. Follow-on work must be performed for TOFD-T to be proposed and accepted as an alternative to RT in France.Copyright

HTR Fuel Integrity With Electromagnetic, Vision and Radiographic Nondestructive Evaluation Methods

In order to ensure HTR fuel qualification, as well as reactor safety, particles need to satisfy a set of specifications including particle integrity. To achieve this goal, AREVA NP has been engaged for several years in a R&D program aiming at the development of innovative industrial non destructive evaluation methods for HTR fuel as alternatives to destructive methods. After investigating a number of potential techniques, development has been focused on vision and eddy currents, both aiming at crack detection. High resolution Phase Contrast X-Ray imaging was also studied for structural defects characterization. For all these techniques, besides the development of HTR fuel dedicated control methods, equipment and probes were specifically designed, tested and optimized thanks to experiments conducted on real and artificial flaws, yielding for some of the methods to potential industrialization and quality control performed over 100% of the fuel production.Copyright