The first fiber Bragg gratings were accidentally written in a Ge-doped silica fiber using a high power argon-ion laser [Hill 1978]. Following this first evidence of photosensitivity in optical fibers, a huge effort was put into fiber gratings: improving their fabrication (they are now all externally inscribed), obtaining complex profiles, optimizing their performance, and incorporating them into devices and systems. On this line, the last decade was characterized by the birth of an emerging class of fiber gratings which can be defined as ldquomicrostructured fiber Bragg gratings.rdquo They refer to two main categories: one relies on short period gratings writing in microstructured optical fibers, whereas the second deals with standard short period gratings where structural defects at microscale are properly created within the hosting fiber by postprocessing techniques. This paper reviews the fabrication processes at the basis of this new technology as well as its properties and applications. Emphasis will be placed on principles of operation, technological developments and applications discussing perspectives, and challenges that lie ahea

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Spargers are mutli-hole injection pipes used in Boiling Water Reactors (BWR) and Advanced Pressurized (AP) reactors to condense steam in large water pools. A steam injection induces heat, momentum and mass sources that depend on the steam injection conditions and can result in thermal stratification or mixing of the pool. Thermal stratification reduces the steam condensation capacity of the pool, increases the pool surface temperature and thus the containment pressure. Development of models with predictive capabilities requires the understanding of basic phenomena that govern the behavior of the complex multi-scale system. The goals of this work are (i) to analyze and interpret the experiments on steam injection into a pool through spargers performed in the large-scale facilities of PPOOLEX and PANDA, and (ii) to discuss possible modelling approaches for the observed phenomena. A scaling approach was developed to address the most important physical phenomena and regimes relevant to prototypic plant conditions. The focus of the tests was on the low steam mass flux and oscillatory bubble condensation regimes, which are expected during a long-term steam injection transient, e.g. in the case of a Station Black Out (SBO). Exploratory tests were also done for chugging and stable jet conditions. The results showed a similar behavior in PPOOLEX and PANDA in terms of jet induced by steam condensation, pool stratification, and development of hot layer and erosion of the cold one. A correlation using the Richardson number is proposed to model the erosion rate of the cold layer as a function of the pool dimensions and steam injection conditions.

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In the frame of the OECD/SETH-2 project, an experimental programme is being carried out in parallel in the PANDA facility at the Paul Scherrer Institute and in the MISTRA facility at the Commissariat à l'Energie Atomique. Part of the program focuses on gas stratification break-up induced by mass sources and similar tests have been performed in both facilities. Indeed, the scaling effect of the phenomena involved in the erosion of a gas stratified layer can be assessed. Depending on the interaction Froude number, different regimes have been identified including pure diffusive mixing, global dilution or slow erosion processes. These phenomena are driven by different time scales. Small value of the non-dimensional number leads to mixing process driven by molecular diffusion. When the interaction Froude number is increased to large values, the dilution process can be described by a global time scale based on volumetric mixing provided that the air entrainment by the jet is taken into account. The intermediate case with two layers is more complicated and a single time scale cannot be derived. These test results with high-quality measurements can be regarded as a good basis for CFD models verification.

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