Vincent Le Cam

Louis-Marie Cottineau

Laurent Lemarchand

Frédéric Bourquin

More than technological questions, the generic potentiality of a wireless sensors platform drives the way engineers could design the system. A complex equilibrium must be found between the desire to implement generic (at least reusable) functionalities embedded in each wireless sensor of the network and the necessity of electronic optimizations in view of reducing energy consumption and dimensions. Nevertheless, nearly each specific application that has to be implemented on a wireless sensors network can be approach through a generic way of thinking.

As an example, a relevant solution consists in designing sensor that is the compilation of heterogeneous electronic boards plugged one onto the others; one is the mother board that ensures fundamental functions from signal processing to wireless communications and some are pluggable boards each dedicated to a specific sub-function (data acquisition, conversion from a sensing component, filtering,...). But the wireless constraint and the wish of generic designs ask the questions of time synchronization, energy consumption... thus emerging technologies and protocol standardization (Zigbee, Wi-Fi) could provide immediate answers.

French Public Works Laboratory (LCPC) has recently developed a generic wireless platform that fits many applications needs such as U.A.V. instrumentation, bridge or quarries monitoring... By including the TCP/IP stack and an embedded ucLinux O.S. at each sensor level, LCPC could propose a generic and reusable platform on which many applications could be implemented. This article aims at presenting the originalities of that wireless sensors platform and the first positive results and dissemination it recorded when applied to real structures.

INTRODUCTION

Most of the scientific community and particularly the Wireless Sensors Network (WSN) community know how long and how expensive could be the works associated to small electronic designs that include from radio-frequency communications, power consumption optimizations, or embedded software developments. Embedded works for wireless applications are synonymous of rigorous works. Where simple software and a plugged data-acquisition board is adapted for a traditional PC directly connected to wired sensors, wireless applications must ensure reliable and continuous measurements from months and months without any human interaction or maintenance.

When thinking to a new design in Wireless Sensors Networks domain in view of research such as in view of satisfying the specific needs of an application, genericity and reusability of the works must question engineers and searchers. As evidence, thinking as soon as possible how the same platform could be re-used for other applications or different contexts could lead to positive and profitable results. At this step, a first conclusion could be "it's easier and less expensive to adapt than it is to create from scratch".

After few years of specific developments for various WSN applications, LCPC took the decision to develop its own generic wireless platform. LCPC goal is to develop a WSN platform for which simplest (or none) hardware and software modifications can cover a large range of applications. By its identity, today's LCPC platform essentially fits civil engineering applications.

Global design and functionalities of LCPC wireless platform

In most of the cases, as the above figure 1 shows, a typical wireless sensor can be seen as the addition of functional layouts.

Physical sensor :

accelerometer, PT100, etc.

Conditioning & digital conversion

Computational Core (DSP, µP)

FLASH / RAM memory

Wireless Communication

Bluetooth, Zigbee, Wifi, etc.

Optionnal :

-Energy management -Sync. Mechanism -Other GPIO, SPI ports… After a long study of applications where specific layouts have been distinguished from generic ones, LCPC decided to make its WSN platform modular. This modularity takes form in a sensor made of a "mother board" that integrates redundant needs (computation, communication, data storage) and a pluggable "daughter board" that integrates application specific needs such as the sensitive element (accelerometer, temperature, pressure,…) or its specific conditioning design including hardware filters, analog to digital converters. The mother board is presumed designed for good -hopefully for few years-and provides the platform cost advantage. Its study and tests took a long time to LCPC; now this product is proposed to any company or laboratory interested in, by the way of LCPC third party partners. Thanks to its exact reproducibility when produced to customers, its cost rapidly decreases.

The daughter board design is the only entity that must be rethought according to the application specifics needs. But, as this design is the simplest of the system (compared to microprocessor implementation, memory addressing through buses and electronic layers), its cost is much cheaper than the one a customer would pay if redesigning his WSN entirely and ex nihilo. Moreover, this combination allows user to make low cost and heterogeneous networks by designing as many various daughter boards as required by the application.

BENEFITS AND INTEGRATION OF EMERGING TECHNOLOGIES

LCPC wireless generic platform is the fruit of years of specifics developments and technological survey in WSN domain that cover many fields such as micro-electronic designs, wireless communications or smart embedded algorithms. Thus, the platform is, first of all, the "best of the technology" integration that fits LCPC and civil engineering needs.

The standard TCP/IP as the WSN communication protocol

Ten years ago, at the beginning of first TCP/IP wireless implementations via Wi-Fi 802.11b devices, the IP solution was generally precluded for WSN due to the "heavyweight" of that originally wired protocol and due to battery limitations [START_REF] Marcy | Wireless Sensor Networks for Area Monitoring and Integrated Vehicle Health Management Applications[END_REF]. During years, many teams and companies worked hard to make the TCP/IP protocol viable for wireless sensors [START_REF] Dunkels | Making TCP/IP Viable for Wireless Sensor Networks[END_REF] and [START_REF] Scottsdale | Wi-Fi Moves into the Sensor Networking[END_REF]. In parallel, it is notable that 802.11 hotspots deployments have increased from 53000 in January 2005 to 155000 on July 2007, and, more than 50% of US companies have deployed 802.11 networks as of 2006 [START_REF] Adams | Capitalizing on 802.11 for sensor networks[END_REF].

The ongoing social and economic evolution of mobility, portable connectivity, and the need of heterogeneous media convergence that emerges those last years elect TCP/IP and especially Wi-Fi ubiquitous. By choosing the IP protocol, LCPC WSN platform gets the same benefits as from a traditional PC network [START_REF] Shih-Haohung | Overview of Embedded OS and Linux[END_REF]:

-Network reliability: TCP/IP stack ensures reliable transfers of information between sensors, -Low cost and faster deployment: no need of specific devices, Wi-Fi devices are proposed to all public and requires only traditional computer competences, -Scalability: IP network can address thousands of sensors up to 100 Mbps. TCP/IP stacks are now available in integrated electronic devices easy to include in developer's designs. The figure 3 above shows one of the first self-sufficing Wi-Fi devices qualified to make an accelerometer sensor wireless. That kind of devices is addressed by the sensor processor via standard serial RS or PCMCIA ports.

Embedding "ucLinux" as sensor Operating System

Implementation of Operating System in embedded world is a quite difficult task that requires high level knowledge in industrial data processing. Choice of an OS for WSN is crucial; adding such a wide software layout between the processor hardware capacities and application algorithms may increase the risk of power overconsumption, of bugs, and requires more RAM and FLASH memories.

Nevertheless, wireless sensor platforms embedding an OS already exist: TinyOS that uses the wireless 802.15.4 standard, BlueCat OS based on Linux, VxWorkx … [START_REF] Shih-Haohung | Overview of Embedded OS and Linux[END_REF].

LCPC WSN platform uses a Digital Signal Processor (DSP) as the computational core; this specific processor -an Analog Device BlackFin BF537-has been designed able to run an OS. The "ucLinux" is one of those derived from the Linux family that fits the embedded world. It can be configured to deal with exactly the needed peripherals: SPI bus and/or RS232 port and/or PWM port and/or CAN bus, etc. Thus, it can size from 50 KB only to 4 MB. This OS implementation offers two main advantages.

First, a very high level of abstraction between hardware ports and source code is obtained. As any OS does, ucLinux OS includes many drivers that allow searchers and engineers to program wireless sensors in the ANSI C [START_REF]American National Standards Institute (ANSI) : C programming language[END_REF] language using its traditional use; in other words, the OS integrate all specific access to private DSP registries. This reduces consequently the code size and, first of all, makes sensor's algorithms fully independent from hardware. Thus, the developed code that implements algorithms becomes completely portable and reusable from an application to another. If necessary, ucLinux allow user to write its own drivers.

Secondly, as the majority of OS, ucLinux fully deals with the IP protocol. Thus, porting ucLinux on LCPC mother board where Wi-Fi is the implementation of IP makes sensors fully accessible from any remote PC linked to a Wi-Fi access point. When their Wi-Fi stack is turned on, sensors are directly addressable by means of the universal "Telnet" service. Combining embedded OS to Wi-Fi implementation delivers a real breakthrough in structural health monitoring by means of wireless sensors. LCPC wireless sensors can be completely and remotely updated and (re)programmed without any expensive uninstallation from the structure. Moreover, the wireless communication hardware only requires non proprietary devices but typical Wi-Fi access points.

Sensor self and absolute synchronisation by means of a GPS receiver

Time synchronization for wireless sensors networks is often a real need. Typically, algorithms that ensure the structure monitoring at Supervisor level, manage heterogeneous and numerous samples collected by sensors. Those data are used to localize a default, a dangerous tendency or to fill statistical databases.

Thus, most of instrumentation configurations require data able to be compared and time correlated. This need is much more obvious in cases where instrumentation is used for distributed tasks amongst sensor nodes or for control algorithms. Wave propagation in concrete or real-time suspended cables monitoring are applications where sensors time-synchronization requires high accuracy that can reach up to 1 µS.

Because a sensor is an electronic entity, its natural time-base varies due to quartzes aging and temperature. Thus, time synchronization is an essential sub-function to solve in a WSN. Many solutions [START_REF] Elson | Time Synchronization for Wireless Sensor Networks[END_REF] exist and can be classified in three categories: distributed synchronization, "Post-facto" synchronization and sensor self -synchronization. Few of them are really able to provide accurate and deterministic dating [START_REF] Le | Synchronization of wireless sensors: review of methodologies[END_REF].

LCPC wireless platform provides an accurate and absolute time-synchronisation mechanism by integrating a very small GPS receiver on the mother board. This solution is based on the property of each GPS receiver to output two numerical signals: a sequence of bytes containing absolute time and localization, and, a synchronization binary signal named PPS (Pulse Per Second) that has a period of 1s with a guarantee of +/-20 nanoseconds. The solution consists in routing the PPS signal to a reset input of one of the processor timers. Then, on PPS rising edge, microseconds timer counter is reset, and, hours, minutes and seconds are updated. An organized test campaign has been carried out and results are very satisfactory. As every PPS signals synchronously rise up with the same period of 1S +/-20nS, the only drift between two sensors is due to ucLinux OS latency (time to raise the appropriate interrupt and to reset timer counter). As the figure 10 show, same accurate and synchronous Dirac signals of 1 Hz frequency were injected in two wireless sensors. Figure 11 is the result of 20 measurements of sensor 1 and sensor 2 time counter variation on Dirac rise. Absolute time stamping between two LCPC wireless sensors never exceeds 5 µS.

APPLICATIONS OF LCPC WIRELESS GENERIC PLATFORM

First applications of LCPC WSN cover internal projects whose needs paved the ground of the wireless platform consequent development.

Unmanned aerial vehicle (U.A.V.) instrumentation

Car traffic is nearly uninterruptable, and France counts approximately 50000 railway bridges and 200000 road bridges that need periodical inspections. Those two constraints lead to the conclusion that structure inspection and integrity checking by means of an instrumented unmanned aerial vehicle (U.A.V.) could reduce the cost of those expensive operations [START_REF] Derkx | Laboratoire Central des Ponts et Chaussée France[END_REF].

One of LCPC major instrumentation project refers to the evaluation of a military U.A.V. bought able to fly according to security rules and for turnkey flights (by means of an integrated autopilot). The challenge consists in customizing U.A.V. (figure 12) by embedding the wireless platform and to make it able to control each onboard instrument. In this case, wireless system is customized to allow real time control of many devices onboard: a camera monitored on a 3D axial robot (figure 13), a laser-meter, GPS receivers, anticollision system, etc. Embedded ucLinux and the connected point to point IP protocol fit the needs of security and accuracy.

Cable health monitoring

In view of safety, structure durability and cost efficiency, detection and localization of wire breaks in cables is a strategic component in cable monitoring. Knowing the number of breaks and their locations contributes to understand and to supervise the health state of the structure [START_REF] Brevet | [END_REF].

Acoustic emission emerges as one of the favourite non-destructive evaluation method. In this case, LCPC wireless sensors detect acoustic waves due to wire breaks (figure 14) and accurately date its propagation all along the cable. On such an event, sensors upload their information to a remote Supervisor. Thus, by knowing sensor's location, wave form and its occurred time, Supervisor is able to localize the wire break (figure 16). After large scale campaigns (figure 15) including real structures, LCPC "wire breaks in cable detection system" is actually subject to industrial transfers.

CONCLUSION

LCPC has developed an efficient wireless sensor network platform customizable to a diversity of applications, especially for civil engineering contexts. Its genericity is based on its mother/daughter pluggable boards that respectively integrate recurrent SHM needs and application specific sensing. Integration of a small, reliable and real-time ucLinux OS also furnish gages of the platform perenniality. The small GPS receiver and the PPS based synchronization mechanism ensures absolute and very accurate time stamping even if this property limits the use to outdoor instrumentations (thus GPS indoor re-emission could be studied). IP native ucLinux protocol such as any supervision PC and the well known costs associated to Wi-Fi technology contribute to delivers low cost deployments of this platform by excluding any proprietary materials, protocols or antennas.

Energy is a notable aspect that has not been related in this article. The reasons are the following: sensor power consumption is always very application-dependent and LCPC has many unexplored ways to consequently decrease power consumption (that actually concerns few mW): use of DSP sobriety modes, run GPS on cyclic period only, turning on the Wi-Fi module just when necessary, development of daughter board to recover various energies from vibrations, wind or solar cells. Other efforts will be leaded soon: more massive use of DSP capacities or development of a set of turnkey daughter boards. Nevertheless, LCPC wireless platform is already subject to industrial transfers and the goal clearly consists in making it as a reference sensor network platform in civil engineering applications. Thus, any kind of contact or cooperation would be appreciated.

Figure 1 :

 1 Figure 1: Typical wireless sensor functional layouts.

Figure 2 :

 2 Figure 2: mother board, daughter board, and the both plugged one to the other.

Figure 3 :

 3 Figure 3: TCP/IP stack available in small device easy to integrate.

Figure 5 :

 5 Figure 5: Sensors addressable via a simple Wi-Fi Access Point and the Telnet service.

Figure 4 :

 4 Figure 4: One-shot expensive sensors installation on a structure.

Figure 7 :

 7 Figure 7: Obvious source code reduction and independence from hardware.

Figure 6 :

 6 Figure 6: ucLinux as the embedded operating system.

Figure 9 :

 9 Figure 9: Embedded GPS synchronization principle.Figure 8: GPS receiver on the mother board.

Figure 8 :

 8 Figure 9: Embedded GPS synchronization principle.Figure 8: GPS receiver on the mother board.

Figure 11 :

 11 Figure 11: Sensor µS counter variation just before PPS.Figure 10: Synchronization test-bench for two wireless sensors.

Figure 10 :

 10 Figure 11: Sensor µS counter variation just before PPS.Figure 10: Synchronization test-bench for two wireless sensors.

Figure 13 :

 13 Figure 13: Embedded instrumentation Figure 12: LCPC U.A.V. for structure inspection

Figure 15 :

 15 Figure 15: Large scale test under real conditions.

Figure 16 :

 16 Figure 16: Supervisor able to localize wire breaks.

Figure 14 :

 14 Figure 14: LCPC wireless sensor for cable monitoring.


Sofia Arris-Roucan
email: sofia.roucan@univ-pau.fr

Céline Perlot

Antonin Fabbri

Fionn Mcgregor

Assessing the indoor CO 2 adsorption capacity of earthen materials: A novel experimental methodology

Keywords: Indoor air quality, clay-based plasters, CO2 adsorption, Hygrothermal properties

This study examines the effectiveness of natural plasters, and in particular clay-based plasters, in improving indoor air quality by measuring their CO2 adsorption capacity. The experimental approach consists, at first, to characterise the interactions between the material and CO2 to better predict its behaviour under real conditions. The results for a clay-based plaster show that it has the potential to buffer CO2 through physisorption of CO2 on clay minerals. This research provides the first protocol for determining the adsorption capacity of indoor CO2 concentrations of coating materials already known for their good hygrothermal properties, which is the case of clay-based or bio-based materials.

Introduction

Indoor air quality (IAQ) is a growing concern in modern society, with people spending more time indoors than ever before [START_REF] Awada | Occupant health in buildings: Impact of the COVID-19 pandemic on the opinions of building professionals and implications on research[END_REF] [START_REF] Jones | Indoor air quality and health[END_REF]. Poor IAQ can have significant health effects, including respiratory problems, headaches and allergies. The use of bio-and geo-based materials in construction has been proposed as a solution to improve IAQ, as they are known to have lower emissions of volatile organic compounds than conventionnal building materials [START_REF] Florbela | Influence of eco-materials on indoor air quality[END_REF] [START_REF] Mhuireach | Differing effects of four building materials on viable bacterial communities and VOCs[END_REF]. They are also known for their good hygrothermal properties, helping to regulate indoor humidity [START_REF] Mcgregor | The moisture buffering capacity of unfired clay masonry[END_REF]. The microstucutre and surface activity of the constituents favouring the moisture adsorption may also have the potential to regulate indoor pollutant concentrations. However, there is currently no protocol to measure this potential, making it difficult to evaluate its effectiveness.

This research aims to help fill this gap by developing an experimental methodology to characterise the CO 2 adsorption capacity of building materials at the microscale. This methodology will be of particular interest for bio-and geo-based materials, which are known for their hygroscopic properties. CO 2 was chosen as the study gas to develop the experimental protocol because it is present in significant amounts in indoor environments and is a common indicator of IAQ [START_REF] Jacobson | Direct human health risks of increased atmospheric carbon dioxide[END_REF]. However, it is important to note that the experimental protocol developed in this study could potentially be applied to other gas compounds present in indoor air, opening the way to a more comprehensive evaluation of the ability of bio-and geo-based materials to improve indoor air quality.

This presentation discusses the experimental approach of measuring the CO 2 adsorption capacity and provides a case study of a clay-based plaster.

Experimental Approach

The approach involves characterising the interactions between the material and the gaseous pollutant of interest, in this case CO 2 . The nature of these interactions can help to determine whether the pollutant can be retained and later released. The experiment uses a TG-DSC apparatus capable of measuring mass and heat flux changes associated with one or more reactions. Several devices are used upstream of the apparatus to create an atmosphere similar to indoor air, including a gas mixer to control the CO 2 concentration and a wet gas generator to control humidity and temperature. The gas mixture is introduced into the TG-DSC apparatus for simultaneous thermogravimetric and differential scanning calorimetric measurements.

The test procedure for measuring CO 2 adsorption capacity consists of fixing external parameters such as temperature and humidity, while varying the CO 2 concentration. The material's response to changing CO 2 concentrations is then studied under these controlled conditions. By applying successive concentration steps followed by a return to zero, the effects of varying CO 2 concentrations on the material can be analyzed systematically.

The results of the experiment are analysed as follows:

1. The shape of the TG curve indicates whether the process is reversible, partially reversible, or irreversible. This allows for an initial distinction between physical interactions, which are more reversible, and chemical interactions, which are more irreversible. 2. The equilibrium mass determination provides the maximum retention capacity of the material for the given conditions. 3. The direction of the heat flow peak indicates whether the reaction is endothermic or exothermic, and by combining mass measurement and heat release values, the associated enthalpy can be calculated. 4. To calculate the enthalpy, we integrate the 95% of the signal and divide by the corresponding mass. The comparison of the calculated value with known values for the assumed process allows for the validation of the assumptions regarding the nature of interactions.

Case study on clay-based plaster

For this study, we decided to focus on clay-based plasters. The material introduced in the TG-DSC apparatus is approximately 60 mg of natural earth sieved at 80 µm. After a stabilisation phase, concentration steps are applied for a humidity set at 0%RH and temperature of 35°C. The protocol consist of 5 hours of exposition to 5000 ppm of CO 2 and 5 hours of exposition to a free-CO 2 air. Figure 1 shows the results of the experiment:

1. Firstly, in part (a) we can see the mass gain associated with the concentration step from 0 ppm to 5000 ppm and then back to 0 ppm. The reversibility of the reaction is indicative of physical interactions such as physisorption. 2. The CO 2 retention capacity Q (in mg of CO 2 captured within the material per kg of material) of the sample is calculated from the relative mass variation of the sample after and before the exposure step. According to the curves, Q = 142 mg/kg at 5000 ppm 35°C and 0%RH. 3. Then in part (b) we can see the associated heat flow peak. Here the flow is exothermic as it is directed upwards, confirming the possibility of adsorption, which is known to be exothermic. 4. Finally, part (c) shows the integration zone from which the enthalpy is calculated. The result of 60 kJ/mol for adsorption enthalpy is quite large compared to the usual values for physisorption (between 0 and 20 kJ/mol and up to 40 kJ/mol in some case), but too small to be chemisorption (more than 100 kJ/mol). It can therefore be concluded that some adsorption sites are very energetic and contribute to increasing the adsorption enthalpy value.

There are therefore sites with a strong affinity for CO 2 in the material.

Conclusion

In conclusion, this study has demonstrated the potential of bio-based materials, particularly clay-based plasters, to adsorb CO 2 . Through a novel experimental approach, we have characterized the interactions between a raw earth and CO 2 , providing insight into its behavior under real-world conditions. Our findings suggest that clay-based plasters are able to effectively buffer CO 2 through the physisorption of the gas on their clay minerals surfaces. Importantly, this research has established a protocol for measuring the CO 2 adsorption capacity of other bio and geo-based coating materials, such as raw earth or fibers. This work underline the need to consider the impact of building materials on indoor air quality and highlights the potential for sustainable, bio-based materials to support healthy indoor environments. Future studies should explore the longterm effects of these materials on indoor air quality and human health and the impact of external condition such as humidity and temperature.

  (a) TG signal and associated CO2 concentration of the experiment (b) Adsorption phase only: heat flow and associated TG signals (c) Extracted heat flow and associated TG signals between the limits of integration for the enthalpy calculation

Fig. 1 :

 1 Fig. 1: Results of the experiments and analysis methodology


Georges Repetto
email: georges.repetto@irsn.fr

Benoit Bruyère

Stephane Eymery

Tony Glantz

Thermal hydraulics behavior of a rod bundle with partially blocked area during the reflooding phase of the Loss Of Cooling Accident

Keywords: Coolability, blockage, LOCA, Fuel relocation, DRACCAR

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

During an accident causing the loss of the primary coolant (LOCA), partial or even complete drying of the fuel assemblies may occur. In these conditions, the fuel temperature increase and the coolant pressure drop in the reactor core may lead to significant deformation and rupture of the fuel rod cladding. The results (Fig. 1), which illustrate this phenomenon, come from a LOCA experimental program performed, in the 1980's by the Institut de Protection et de Sûreté Nucléaire now IRSN [START_REF] Bourdon | Analyses of the Phebus-LOCA tests 215-R and 2018 using FRETA-F V1.1 code[END_REF], in 5x5 bundle including fresh fuel rods. In case of irradiated fuel rods, fuel fragmentation may occur, leading to fuel relocation within the ballooned area. Depending on the size and the distribution of the ballooned areas within the fuel bundle, the cooling flow might be impaired. All the LOCA experiments performed with irradiated fuel rods (in particular 3 examples, at 35 GWd/t for the FR2 experiments (Germany) in 1983 (Fig. 2a) [START_REF] Karb | LWR fuel rod behaviour in the FR2 in-pile tests simulating the heat up phase of a LOCA[END_REF], in the ORNL Tests (USA) (Fig. 2b) [START_REF]NUREG-2121, Fuel fragmentation, relocation and dispersal during the loss-of coolant-accident , US-NRC[END_REF] for various burnups and, more recently, in Halden (Norway) [START_REF] Kekkonen | LOCA testing at Halden, the fourth experiment IFA-650.4[END_REF] at 90 GWd/t (Fig. 2c) in 2006, have shown such an accumulation of the fragmented fuel pellets in the ballooned region. The resulting increase of the power density in the ballooned area may challenge the fuel assembly coolability and, by extension, the core coolability. Depending on the burnup of the fuel rod, extensive fragmentation may also lead to fuel particle dispersal into the coolant through the ruptured cladding. So, the consequences of both deformations and fuel relocation on the cooling efficiency when reflooding the core by the ECCS (emergency core cooling systems) are important safety issues, and have never been experimented. The knowledge generated by these new experiments will answer long shared concerns by the international community (such as the effect of fuel relocation in blocked areas in the core), and will allow to validate the DRACCAR code [START_REF] Repetto | DRACCAR, a new 3D-thermal mechanical computer code to simulate LOCA transient on Nuclear Power Plants -Status of the development and the validation, proccedings of ICAPP 2009[END_REF] developed and used by IRSN since 2006 to simulate any prototypical LOCA transient. As the previous experiments (as FEBA, SEFLEX [START_REF] Ihle | FEBA Flooding Experiments with Blocked Arrays[END_REF][START_REF] Ihle | SEFLEX -Fuel Rod Simulator Effects in Flooding Experiments[END_REF] and others) were not representative enough (rod thermal inertia and fuel relocation was not taken into account), the need of such complementary experiments has been proposed in 2005 by IRSN [START_REF] Grandjean | Coolability of blocked regions in a rod bundle after ballooning under LOCA conditions, Main findings from a review of the past experimental programmes[END_REF][START_REF] Grandjean | Coolability of blocked regions in a rod bundle after ballooning under LOCA conditions -Main findings from a review of past experimental programmes[END_REF].

THE COAL EXPERIMENT

The COAL experiments, in the frame of the PERFROI project [START_REF] Repetto | Core coolability in loss of coolant accident: the PERFROI project[END_REF][START_REF] Repetto | The R&D PERFROI project on Thermal mechanical and thermal hydraulics behaviors of a fuel rod assembly during a Loss Of Coolant Accident[END_REF] will concern thermal hydraulics experiments on electrically heated rods, with simulation of large blockage and fuel relocation.

The test section and the bundle geometry

The COAL bundle [START_REF] Repetto | The core coolability in a Loss Of Coolant Accident: the COAL experiments[END_REF] will consist of rods (electrically heated), some with a pre-deformed zone with local overheating representing fuel relocation. The bundle will be made of 49 (7 x 7) rods (including 16 deformed rods and 30 non-deformed rods -Fig. 3a), similar to that of the THETIS [START_REF] Pearson | Reflooding Experiments on a 49-Rod Cluster Containing a Long 90% Blockage[END_REF][START_REF] Cooper | Jowitt -The THETIS 80% Blocked Cluster Experiment[END_REF] tests performed in the past by AEA Winfrith UK in 1983-1984, which will also facilitate experimental comparisons. The presence of 3 guide tubes will increase the representativity of the thermal hydraulics behavior for the reactor case. The test section will also include 6 spacer grids each 500 mm apart.

3/15

For these experiments, the cladding of the electrical rods was proposed to be constructed with Hastalloy material to prevent oxidation. To avoid any chemical interaction, the grids should be in Inconel alloy. Inconel grids for the COAL bundle have been built with a design including mixing vanes that would have an effect of thermal hydraulic behavior. The presence of guide tubes is also necessary for instrumentation and of course mechanical reason. The heating zone is about 3 m high (close to the real PWR geometry). The bundle will be inserted in 94mm square shroud (Fig. 3b) with roughly 5 mm thickness to be representative of the surrounding fuel rods regarding their thermal inertia. The test section will be included in a pressure vessel, (220 mm diameter -Fig. 3c) with two lower and upper plenums for the electrical and instrumentation connections and the fluid supply. The COAL program will be conducted in an existing ad -hoc thermal hydraulics facility (cf section 2.2).

The thermal hydraulic loop

COAL experiments will be performed in the AREVA BENSON facility (Fig. 4a) at Erlangen (Germany) which allows running high pressure tests to 3 MPa (30 bar). This facility has operating for roughly 40 years for different kinds of experiments in the thermal hydraulic field [START_REF] Schmidt | High-Pressure Test Facility -25 Years of Operation VGB Power Tech[END_REF]. Figure 4b) describes the circuit of the BENSON facility able to carry out reflooding experiments without important modification, including the installed power needed for heating the electrical rods. a) b) Fig. 4 The BENSON Test facility at AREVA Erlangen The experimental work will proceed in two steps: 1. Qualification tests will be performed by the end of 2017 on a bundle with non-deformed heated rods. These tests will be used as a reference tests in order to qualify all the experimental process and to compare with already performed experiments such as PERICLES  (cf section 3.3), 2. Then tests with heated pre-deformed simulators, including local overheating of the balloon regions, will be performed from 2018 up to 2019. Three series of tests are planned with different flow blockage ratios or ballooning lengths (cf. Table 1). In each series, the effects of thermal hydraulic parameters (cf. section 3.3) will be carefully investigated. A third campaign is optional (for 2020) and depends on the results of the previous campaigns. 3. Finally, all the tests will be analyzed and interpreted. The objective is to validate a heat transfer model adapted to bundle conditions, to be implemented in the DRACCAR code [START_REF] Bascou | Computational Analysis of Multi-pin Ballooning during LOCA and Post LOCA Transient using the Multi-physics Code DRACCAR[END_REF][START_REF] Ricaud | Multi-pin Ballooning during LOCA transient: A Three Dimensional Analysis[END_REF].

The experimental parameters of the test campaigns will be defined in order to investigate the bundle geometry related phenomena (blockage ratio, balloons lengths) and the thermal hydraulic related The choice of geometries and the associated axial power distributions were guided to obtain a gradual evolution of the pressure drop of the ballooned area and a gradual increase of the maximum linear power in the ballooned area of the fuel rods which constitute the different bundle (B0, B1 and B2).

The axial power distributions were obtained by DRACCAR calculations which can simulate with some parameters the fuel relocation in the balloon (Fuel rating in the balloon according the cladding deformation -Fig. 5) and the corresponding power profile deformation (Fig. 6).

The instrumentation plan

For these kinds of experiments, the major physical variables for model qualification are as follows:  Injected water and steam mass flow rate generated during reflooding,  Temperature of the steam at different points (type K thermocouples),  Temperature inside the fuel rods (thermocouples at different elevations and different radial positions in the bundle) in an adequate distribution,  Total pressure and pressure drop sensors at the periphery of the rod bundle.

Up to 184 thermocouples (4 each 46 rods) will be available every 10 cm to follow the quench front propagation. Additional thermocouples are foreseen, in some fluid channels, on the 6 grids and in the 3 guide tubes. The shroud will be also equipped with thermocouples on each faces in order to accurately set the boundary thermal conditions during the reflooding process. The outlet mass flow rate of steam and water is foreseen to be evaluated by using a T junction as a separator (Fig. 3 c). The collected water will be measured by the increase of the differential pressure (ΔP as function of time) inside a water collector tank while the outlet steam flow will be measured by a "venturi" technique. A total of roughly 340 measurements is foreseen to be recorded by the data acquisition system.

PRECALCULATION USING THE DRACCAR CODE

The simulation tool DRACCAR [START_REF] Bascou | Computational Analysis of Multi-pin Ballooning during LOCA and Post LOCA Transient using the Multi-physics Code DRACCAR[END_REF][START_REF] Ricaud | Multi-pin Ballooning during LOCA transient: A Three Dimensional Analysis[END_REF], developed by IRSN, with the support of EDF, is at this stage, used to define some parameters of the experiments such as the blocked bundles geometry, the deformed axial powe rprofile, the heat-up scenario of the bundles or the test matrix. Then DRACCAR will be used to perform the pre-calculation of all the tests foreseen in the COAL program.

Description of the DRACCAR code

DRACCAR is a simulation tool for fuel assembly mechanical behavior and coolability assessment during a LOCA transient. Its aim is to simulate the 3D thermomechanical deformation and reflooding of a fuel rod assembly including its coolability as well as structure embrittlement.

The DRACCAR code is based on a 3D non-structured meshing able to model a simple fuel rod, a partial or a full assembly, as well as a surrounding shroud. It is based on an axial discretization of the rod which leads to analyze quasi-independent 2D thermal mechanical problems. Important modeling such as pellet eccentricity, heat transfers (within the solid and through the fluid) or material properties evolutions (oxidation layer, phase changing,...) can thus be taken into account and the cladding integrity during a LOCA transient can be addressed even in case of contact between the structural elements. In that case, the geometry is strongly changed (flattened zone contact) as well as the loading nature (mixed stress-displacement loading) and so the rupture is more difficult to model than a threshold criteria used in most of the multi-rod codes: with DRACCAR, non-linear geometrical effects are added to non-linear behaviour laws in the modeling. Also important is the possibility to get a better knowledge on the system's capability to cool structures whatever are the evolutions of the deformation of the rods and the blockage of the sub-channels. Obviously these two critical issues which are essential to treat in modeling LOCA transient effects, can only be dealt with in a realistic manner with a multi-pin code coupled to an efficient 3D thermal hydraulics code, and that's why DRACCAR V2.3 is currently coupled to the two phase flow module CESAR of the ASTEC code [START_REF] Trégoures | Reactor Cooling Systems Assessment of the ASTEC V1.3 code in Support to the French IRSN PSA-2 on the 1300 MWe PWR[END_REF], able to compute deformed geometry evolutions thus actualizing the coolant flow passage within the different sub-channels. In this case DRACCAR is able to calculate the heat exchange between the wall and the fluid which have been validated against PERICLES and ACHILES tests . The code has capability to deal with dispersion of flow, but droplet behavior will be developped later.

DRACCAR modeling

The bundle is simulated on a height equivalent to the rods heating length, with the square housing (metallic shroud in a similar material as the cladding). One boundary condition at the inlet and one at the outlet of this portion close the system. DRACCAR is able to model each 46 electrical rods of the bundle and 3 guide tubes (Fig. 7a). Taking into account the symmetry of the test section, half a bundle is modeled (Fig. 7b). The ballooned bundles included 16 rods with a local ballooning (Fig. 7c). In the balloon, the fuel fragment is simulated by a porous material which has a density and conductivity close to those of the UO 2 pellet. The one presented here, for the first series of calculations, is composed of balloons that have a height of 100 mm, the same elevation at the middle of the bundle, leading to a 81 or 90% blockage of the test section. The axial power profile imposed in the intact rods has a cosine shape (Fig. 6). The relocation in the balloon has been simulated by setting a specific axial deformation profile in the ballooned rods (Fig. 6). This deformed axial power profile has been determined by a DRACCAR calculation after fuel relocation in a pre-deformed rod. The meshes height never exceeds 100mm (smaller in the ballooned area). Six grids are taken into account by setting a singular pressure loss at their positions.

Test scenario and preliminary test matrix

The scenario is divided in two phases as for classical "Reflooding experiments" performed in the 80's:

1. Heat up phase in dry steam atmosphere (the depressurization is not simulated), 2. Reflooding by water injection with different thermal hydraulic conditions (see test matrix).

The bundle will be preheated up to 300°C by hot steam injection (step 1). As the shroud temperature has to be as close as possible to the rods temperature at the end of the first phase in order to ovoid perturbation during the reflooding, the rods and the shroud will be heated up to 600°C (step 2 and 3) before the water injection phase (step 4).

The main thermal hydraulics parameters foreseen are in the range below:  inlet water flow velocity : 1,5 to 8 cm/s (15 to 80 kg/s/m 2 ),  sub-cooling water temperature (-60°C to saturation conditions),  coolant pressure : 0.1 to 3 MPa (30 bar),  power 3.3 kW  , respectively 2 kW, per rod for Large Break (low pressure) scenario, respectively for Intermediate Break (high pressure) scenario.

There T/H conditions are coming from recent NPP CATHARE calculations (Table 1 of [START_REF] Repetto | The core coolability in a Loss Of Coolant Accident: the COAL experiments[END_REF]) with different break sizes (1 to 40 inches) and conditions that were used for previous programs such as PERICLES experiments. The COAL test matrix referring to (Q, P) diagram where Q is the inlet water flow rate in kg/s/m 2 and P, the system pressure in bar (0,1 MPa) is given in the Fig. 8 below.  in order to simplify the test procedure, the power will be keep constant during the transient, nevertheless some tests will be performed with a reduction of power representative of the decrease of the residual power such as ACHILLES Tests [START_REF] Denham | Pearson -ACHILLES Un ballooned Cluster Experiments -Part 1 : Description of the ACHILLES Rig, Test Tection and Experimental Procedures[END_REF]. One of their conditions will be reproduced with COAL Test 14 (2 bar, 20 kg/s/m 2 ). 7/15 30 thermohydraulics conditions were proposed to develop a test matrix for the reference bundle B0 (46 intact rods + 3 guide tubes). Some thermal hydraulics conditions (circle in figure 8) are similar to those of PERICLES experiments for comparison tests for past reference cases. About 20 experiments would be sufficient to test the effect of the pressure, the effect of coolant temperature, the injection water flow rate with the bundles including the deformed rods (B1 and B2 bundles). As the targeted number of experiments seems too ambitious, an optimization of the test matrix, in progress, is needed. The total number of tests will depend on the reliability of the rods.

PERICLES conditions (circle)

Preliminary results of the DRACCAR simulation

The results will concern the main thermal hydraulics parameters governing the reflooding such as:  The temperature of the rods during the transient,  The outlet flow rate (water and steam production),  And the quench front propagation.

Temperature of the rods

The COAL tests matrix ( §3.3) will cover the thermal-hydraulic conditions characteristics of a large break LOCA and those of an intermediate break LOCA. As far as possible, the same test conditions will be applied on the intact bundle and on the ballooned bundles, to be able to compare the impact of the blocked area with large ballooning of the fuel cladding. Figure 9 give typical DRACCAR results for rods temperature at different radial and axial locations in the bundle. This example corresponds to a rather representative TH conditions for a Large break scenario (P=2bar, Q=20kg/s/m 2 , water temperature 20°C below saturation) with power reduction during the transient. These conditions conrrepond to that of ACHILLES A2R038 [START_REF] Denham | Pearson -ACHILLES Un ballooned Cluster Experiments -Part 1 : Description of the ACHILLES Rig, Test Tection and Experimental Procedures[END_REF]. Reflooding was launched for temperature around 700°C for total duration of about 10 mn leading to a complete refreezing of the rods. For this case, maximun temperature reached 860°C (1000°C for deformed rods). For the cases with the non-deformed bundle, all the maximum clad temperature ranged between 650 and 800°C during the transient, which lasted from 2 mm to 8 mm, before refreezing according to the injected water flow rate (Fig. 19a). For the lowest value of water flow rate (15kg/s/m 2 ) corresponding to a flow velocity of 1.5 cm/s, the maximum reached a peak temperature around 1000°C at level 1.85m with a hottest point near 1070°C at the 1.95 m elevation (Fig. 10): we will see later ( § 3.4.3) that this case (Test 7) was not completely coolable. The Test 19, which correspond to the same conditions except the pressure (30 b instead of 3 bar), peak temperatures were similar but the behavior of the upper part was different showing the coolability, excepted the level 2.9 (Fig 11). According to DRACCAR pre-calculations, those cases, showing too high temperatures (Figs. [START_REF] Repetto | Core coolability in loss of coolant accident: the PERFROI project[END_REF][START_REF] Repetto | The R&D PERFROI project on Thermal mechanical and thermal hydraulics behaviors of a fuel rod assembly during a Loss Of Coolant Accident[END_REF], could be excluded from the test matrix: at least, depending on preliminary tests results, those conditions will be tested at the end of the each experimental campaigns or performed with power reduction. The maximum temperature (excepted some cases, low flow rate for deformed bundle, ex Fig 10) were observed to be lower than 1000°C, situation which seemed suitable according to the technological possibilities of the electrical rods. This should be taken into account for the final test matrix.

Outlet flow rate during reflooding

The results obtained by the simulation regarding the steam and the water generated at the outlet of the bundle during the reflooding of the rods are illustrated in the figures of appendix C (Fig. 20). Those results are given for extreme thermal hydraulics conditions at low (0.3 MPa) and high (3 MPa) pressure and for different inlet water flow velocity from 1,5 to 8 cm/s.

The main observations are the following:

 For low pressure conditions, large amount of droplets is remaining at the outlet of the fuel test assembly; for that pressure condition (0.3 MPa) steam is only produce when low water flow rate is injected (0.105 kg/s which correspond to the lowest velocity);  The higher the inlet water flow is, the higher the % of water at the outlet is (Fig. 13);  For high pressure conditions most of the injected water is vaporized due to the higher efficiency of the reflooding. The production of steam during the transient, predicted by the DRACCAR code, to be not very much affected by the bundle geometry, as it is a macroscopic information representing overall behavior of the reflooding process.

Front propagation and reflooding duration

The numerous thermocouples at different axial elevations in the bundle will allow the determination of the quench front propagation. Temperature evolution, given by DRACCAR at the position of the thermocouples for different injected water flow velocities is illustrated in fig. 15 For Test 7 (very low flow velocity), the upper part of the bundle was observed to be not coolable (Fig. 5b) as the quench front does not exceed the level 2.0 m over the total length of the rods (3.0 m). So, Fig. a) illustrates the quenching time as function of the water flow velocity for levels where complete reflooding was achieved (example for level 1.85 m). The results of the simulations of the two tests (Test 14 "grey curve" and test 20 "brown curve" at the same inlet flow velocity : 2 cm/s) illustrate the effect of the pressure respectively at 2 and 30 bar. At high pressure, the reflooding process is more efficiency due to the higher water density.

On the base of those calculations, Table 2 summarize the duration of the complete (and partial) reflooding for different tests as function of T/H conditions (pressure and water flow rate). The time for a complete reflooding is decreasing as function of the increase of the inlet water flow velocity. Even for intact geometry, the configuration was not completely coolable for very low flow rate and low pressure (1,5cm/s-3bar). As the quench front propagation stopped at level 1.85m, the red curve indicates the duration of the partial reflooding (just above the blockage area) which showed a similar trend as that was observed for debris bed reflooding (Fig. 16 of [START_REF] Repetto | Main outcomes on debris bed cooling from PRELUDE experiments -6th European Review Meeting on Severe Accident Research[END_REF]). The reflooding duration is slightly increased for the bundle including a partially blocked area at least for the partial reflooding. The total reflooding is not affected by blocked area and depends only on the inlet water flow velocity. The quench propagates slowly when the water arrived in front of the blocked area due to the increase of the local power and the flow diversion, nevertheless above this region the quench front propagates faster for deformed bundle due to the reduction of the residual power in the second part of the bundle resulting to a quasi-same time for a complete reflooding for B0 and B1.

For the same flow velocity, the pressure of the system above 10 bar (Test 23, 20 and 21) seems to have weak impact on the efficiency of the reflooding. The impact occurred mainly between 3 and 10 bar.

Regarding those preliminary simulations the largest effect of the increase of the water supply occurred for flow velocities between 1 and 4 cm/s whereas the benefit of the increase of water supply seems to be weak between 4 and 8 cm/s, situation which was experimentally observed for debris bed reflooding experiment such as PRELUDE [START_REF] Repetto | Main outcomes on debris bed cooling from PRELUDE experiments -6th European Review Meeting on Severe Accident Research[END_REF] or PEARL experiments.

The critical point for the coolability of the fuel assembly seems to be ranged for inlet flow velocity between 1 and 2 cm/s (≈ 10 and 20 kg/m 2 /s). At the moment, the results giving by the code have shown the non-coolability of the blocked region for such low flow rate at low pressure, at least for the upper part of the bundle. The results of the COAL experiments will bring important information to confirm the amount of water supply needed for such a configuration during a LOCA transient in a Pressurized Water Reactor.

CONCLUSION

This paper presents the new COAL experiments on the reflooding of a deformed fuel bundle with simulation of the fuel fragments relocation inside the balloons. We have described the test section, the thermal hydraulics parameters for the experiments and some pre-calculations (B0 and B1 bundle) using DRACCAR code developed by IRSN, in order to prepare the experiments planned from 2018 to 2020.

The preliminary calculations have shown the limit of the coolability of the bundle for a flow velocity ranging from 1,5 to 2 cm/s, at least for the first bundle configuration (moderate bundle deformation). The pre-calculation of the largest blocked bundle (90% and long ballooning with the B2 bundle) will be performed in the next step. The thermal hydraulics behavior (temperature evolution, quench front propagation..) observed with the numerical simulation has to be compared with the future experimental COAL results. The results of these experiments will help to improve and validate the heat exchange models implemented in the code studying the reflooding process and the behavior of the code in such a configuration to be used to nuclear power plants for safety applications.

In addition to the COAL experiments, it is foreseen to perform specific separate-effect-tests at the subchannel scale [START_REF] Pena Carrillo | Experimental study on heat transfer inside a blocked region of a rod bundle during a LOCA , NUTHOS-11[END_REF] in order to improve our understanding on different processes involved in the reflooding of a deformed fuel assembly, which is not possible, at least more difficult to get from an integral experiment.

NUTHOS- 11 :Fig. 1 .

 111 Fig.1. Bundle of deformed rod Fig. 2. Fuel relocation in the ballooned area Phebus LOCA experiments (1983) a) FR2 fresh or irrad. Fuel rods. b) ORNL c ) Halden test

NUTHOS- 11 :

 11 The 11th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Operation and Safety Gyeongju, Korea, October 9-13, 2016. N11P0194

Fig. 5 : 6 :

 56 Fig. 5 : Fuel relocation rate in the balloon Fig. 6 : Axial power profile for inner rods

11 :

 11 Fig. 7 : DRACCAR nodalization of the bundle

Fig. 8 :

 8 Fig. 8 : Preliminary COAL test matrix in a (Q, P) diagram.

NUTHOS- 11 :

 11 The 11th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Operation and Safety Gyeongju, Korea, October 9-13, 2016. N11P0194

Fig. 9 :

 9 Fig.9: Typical DRACCAR results for intact geometry Several reflooding pre-calculations have been performed with the DRACCAR code. The overall results regarding temperatures evolution for different water injection flow rate (from 25 to 80 kg/s/m 2 ) at low pressure (0.3 MPa) and high pressure (1 to 3 MPa) at low flow rate (20 kg/s/m 2 ) are illustrated in appendix B, for the two geometries (B0 and B1 bundle with partially blocked area). For the cases with the non-deformed bundle, all the maximum clad temperature ranged between 650 and 800°C during the transient, which lasted from 2 mm to 8 mm, before refreezing according to the injected water flow rate (Fig.19a). For the lowest value of water flow rate (15kg/s/m 2 ) corresponding to a flow velocity of 1.5 cm/s, the maximum reached a peak temperature around 1000°C at level 1.85m with a hottest point near 1070°C at the 1.95 m elevation (Fig.10): we will see later ( § 3.4.3) that this case (Test 7) was not completely coolable. The Test 19, which correspond to the same conditions except the pressure (30 b instead of 3 bar), peak temperatures were similar but the behavior of the upper part was different showing the coolability, excepted the level 2.9(Fig 11). According to DRACCAR pre-calculations, those cases, showing too high temperatures (Figs.[START_REF] Repetto | Core coolability in loss of coolant accident: the PERFROI project[END_REF][START_REF] Repetto | The R&D PERFROI project on Thermal mechanical and thermal hydraulics behaviors of a fuel rod assembly during a Loss Of Coolant Accident[END_REF], could be excluded from the test matrix: at least, depending on preliminary tests results, those conditions will be tested at the end of the each experimental campaigns or performed with power reduction.

Fig. 10 :Fig. 11 :

 1011 Fig. 10 : Rod temperatures (15kg/s/m 2 at 3 bar) Fig. 11 : Rod temperatures (15kg/s/m 2 at 30 bar)

Figure 12 Fig. 12 :

 1212 Fig. 12: Temperature evolution at the mid plan (Level 1.5 m) : Test 2

Fig. 13 :Fig. 14 :

 1314 Fig. 13: Pourcentage of remaining water as function of injected water .

V ~ 2

 2

  temperature Level 1.85m b) Rod temperature Level 2.9m Fig. 15 : Temperature evolution given by DRACCAR simulation (at low pressure).

Fig. 16 :

 16 Fig. 16: Duration of reflooding as function of water flow rate

NOMENCLATURECOAL:NUTHOS- 11 :Outlet gas and water flowrate -test 21 Fig. 20 :

 112120 Figure 17 b) gives the pressure evolution during a LOCA transient for different break sizes: with the maximum pressure (at 30 bar) foreseen for the COAL experiments, we cover most of the sequence (except for those at very small break size, lower than 5 inches).

NUTHOS-11: The 11th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Operation and Safety Gyeongju, Korea, October 9-13, 2016. N11P0194 4/15 phenomena

  (local overheating of the balloon, injected water temperature, coolant flow rate and system pressure to simulate various break sizes).

Table 1 . Geometry and parameters of the blockage zone

 1

	B0	B1	B2

Table 2 . Time for complete reflooding of the bundle versus water flow rate and pressure

 2

				Test 4	Test 3	Test 1	Test 2	Test 14 Test 23 Test 20 Test 21
	Pressure (bar)			3			2	10	25	30
	Water flow ( kg/s/m 2 )	80	50	36	25		20	20	20
	Bundle B0 (/partial)	112/77 183/109 254/138 440/189 606/249 242/131 238/130 238/130
	Bundle B1 (/partial)	150	176/120 256/155 434/224 587/300 248/129 235/117 235/114
		700
					Partial Reflooding -B0
		600			Complete Reflooding -B0
	(s) duration	400 500			Partial Reflooding -B1 Complete Reflooding -B1
	Reflooding	200 300
		100
					Water flow velocity (cm/s)
		0
		0	2	4	6	8	10

ACKNOWLEDGMENTS

This work is completed within the framework of RSNR Project from a French State aid managed by the National Agency of Research under the program of Investments for the Future carrying the reference n° ANR-11-RSNR-0017. Particular acknowledgments are also given to EDF and more recently, US-NRC for their financial support.