Influence of source elevation on a pool fire in a forced-ventilated enclosure

Keywords: Burning rate, compartments, elevated fires, pool fires

In nuclear facilities, fires can spread until reaching the upper part of a closed and mechanically-ventilated compartment. In this area of the room, the fire sources are under-oxygenated, but lie in an environment with significant radiative heat flux. Previous studies dealt with elevated fires, and results showed that fire pan elevation affects two phenomena: the combustion process and the flows inside the room. The present study aims to understand the effects of elevation on fire dynamics in a special ventilation configuration: air inlet in the lower part of the compartment, and vitiated air exhaust in the upper part. A series of tests was carried out in a reduced-scale apparatus, with a 0.09 m diameter pan filled with dodecane liquid fuel and placed at various heights. Two aspects were analyzed: the mass loss rate (MLR) and the gas temperature inside the room. First of all, the experimental results confirm the coupled and antagonistic effects of decreasing oxygen concentration and increasing temperature on the mass loss rate that were already highlighted in the literature. In addition, these results also show the role played by the flame height on the change in the combustion regime. Finally, vertical temperature stratification is much more pronounced when the fire is elevated until a certain height. Beyond this limit, stratification starts to disappear.

INTRODUCTION

In nuclear power plants, facilities often consist of closed, but mechanically-ventilated compartments. If a fire occurs, it can spread to the upper part of the room (e.g. cable tray fires). The fire is then evolving in an environment with a low oxygen concentration since smoke pollutes this area, and a high temperature, due to the radiation heat fluxes from smoke and the nearby ceiling. However, the increase in radiative heat flux contributes to the rise in fire power, while the decrease in oxygen concentration has the opposite effect. These two physical phenomena therefore have both combined and antagonistic effects. In this context, they compete with each other, and according to their respective contributions, combustion is driven by either under-oxygenation or external heat flux. Understanding elevated fire dynamics is thus of great interest in the field of nuclear safety.

In numerous research articles, elevated fires are only one series of tests among others, designed to study fairly broad topics, such as fires in mechanically-ventilated compartments for Backovsky et al. [START_REF] Backovsky | Temperature Profiles in Forced-Ventilation Enclosure Fires[END_REF] and Cetegen [START_REF] Cetegen | Entrainment and Flame Geometry of Fire Plumes[END_REF], or are only a means of achieving specific combustion regimes such as ghosting flames in Yang et al. [START_REF] Yang | Studies on ghosting fire from pool fire in closed compartments[END_REF] and Sugawa et al. [START_REF] Sugawa | Burning Behavior in a poorly-ventilated compartment fire -Ghosting Fire[END_REF]. More recently, studies have started to consider elevated fires as an issue in its own right.

When dealing with this issue, studies addressing this subject consider various ventilation configurations, depending on the applications intended for the work. Thus, the articles of Tiwari et al. [START_REF] Tiwari | Effects of elevated pool fire in a naturally ventilated compartment[END_REF], Polukhina [START_REF] Polukhina | Elevated Fires[END_REF], Mounaud [START_REF] Mounaud | A Parametric Study of the Effect of Fire Source Elevation in a Compartment[END_REF], Coutin et al. [START_REF] Coutin | Aerodynamic characterization of a compartment fire as a function of its behavior[END_REF], [START_REF] Coutin | Behavior of the combustion of a fuel material in the vitiated upper zone of an open compartment[END_REF] and Zhang et al. [START_REF] Zhang | Experimental study on elevated fires in a ceiling vented compartment[END_REF] dealt with elevated fires in a naturally-ventilated compartment, the works of Wang et al. [START_REF] Wang | Early Stage of Elevated Fires in an Aircraft Cargo Compartment: A Full Scale Experimental Investigation[END_REF] and Zhang et al. [START_REF] Zhang | Impacts of elevation on pool fire behavior in a closed compartment: A study based upon a distinct stratification phenomenon[END_REF], [START_REF] Zhang | Smoke filling in closed compartments with elevated fire sources[END_REF] a closed compartment and Pretrel a closed and mechanically-ventilated compartment [START_REF] Suard | Overview of the OECD PRISME 3 Project, 16th SMIRT Post-Conference Seminar on Fire Safety in Nuclear Power Plants and Installations[END_REF]. The conditions of each study differ substantially. Some use a gas fire source (propane) [START_REF] Mounaud | A Parametric Study of the Effect of Fire Source Elevation in a Compartment[END_REF], [START_REF] Coutin | Aerodynamic characterization of a compartment fire as a function of its behavior[END_REF], [START_REF] Coutin | Behavior of the combustion of a fuel material in the vitiated upper zone of an open compartment[END_REF], others liquids with various hydrocarbons, ranging from heptane [START_REF] Polukhina | Elevated Fires[END_REF], [START_REF] Zhang | Experimental study on elevated fires in a ceiling vented compartment[END_REF], [START_REF] Wang | Early Stage of Elevated Fires in an Aircraft Cargo Compartment: A Full Scale Experimental Investigation[END_REF], [START_REF] Zhang | Impacts of elevation on pool fire behavior in a closed compartment: A study based upon a distinct stratification phenomenon[END_REF], [START_REF] Zhang | Smoke filling in closed compartments with elevated fire sources[END_REF] to diesel [START_REF] Tiwari | Effects of elevated pool fire in a naturally ventilated compartment[END_REF] and dodecane [START_REF] Suard | Overview of the OECD PRISME 3 Project, 16th SMIRT Post-Conference Seminar on Fire Safety in Nuclear Power Plants and Installations[END_REF]. Some tests were carried out at a large scale, and others at a reduced one, with compartments of various sizes organized in different ways, especially for openings in naturallyventilated compartments. There can be door openings of different dimensions located in the side walls of the compartment [START_REF] Tiwari | Effects of elevated pool fire in a naturally ventilated compartment[END_REF], [START_REF] Polukhina | Elevated Fires[END_REF], [START_REF] Mounaud | A Parametric Study of the Effect of Fire Source Elevation in a Compartment[END_REF], as well as ceiling vents [START_REF] Zhang | Experimental study on elevated fires in a ceiling vented compartment[END_REF].

These various configurations lead to different results, especially for the mass loss rate (MLR). Figure 1 combines the results of the various articles on this subject, and presents a dimensionless mass loss rate, noted MLR*, which corresponds to the ratio between the mean MLR and the mean MLR at ground level, as a function of z*, the non-dimensional elevation (ratio between the pan elevation and the compartment height). Mean MLR was calculated for tests with a quasi-steady phase and computed over this period, which may vary from one test to another. Results show two opposite trends: an increase in MLR* with pan elevation for closed compartments, and a decrease in MLR* with elevation for naturally-and mechanically-ventilated rooms. These two trends illustrate the combined effects of under-oxygenation, which reduces elevated fire power, and the increase in radiative heat flux, which, on the contrary, enhances pyrolysis. These results therefore show that, under specific surrounding conditions, either radiative heat flux or oxygen concentration is predominant and drives the combustion process. However, the specific surrounding conditions for each regime are currently fairly misunderstood, and the relationship between burning rate, radiative heat fluxes and oxygen concentration is not yet exactly determined. It is interesting to note that closed compartments seem to generate an environment in which radiative heat flux prevails. Similarly, mechanically-and naturally-ventilated boxes create environmental conditions for which oxygen concentration drives combustion. Thus, the geometry of the device, as well as the ventilation configuration (type, positions, directions, flow rates and speeds) influence the emergence of these regime and are therefore crucial when studying elevated fires. In an attempt to explain these trends, the authors study combustion efficiency and combustion regimes. The results show that the pan elevation affects combustion quality: the higher the fire, the worse the combustion rate. Indeed, in a polluted atmosphere, the fire can access less oxygen and the combustion reaction is consequently less efficient. Moreover, elevated fire tests can sometimes be divided into two groups: those for which the flame does not impinge on the ceiling and the MLR has a quasi-steady phase, and those for which the flame impinges on the ceiling and the MLR is largely unsteady. We can infer that flame height is involved in combustion regime changes. The fire elevation therefore affects combustion, but results illustrate that it likewise has an effect on flows within the compartment, i.e. everything related to the room filling process by smoke, thermal stratification or concentration fields. The results indeed show that the interface between smoke and fresh air does not go down to the floor as it would for a ground test, but stops at the pan level. The lower part of the compartment will then continue to be filled through wall jets. After some time, the smoke accumulates on the ground and rises up until it fills the whole compartment. Finally, the results illustrate a more pronounced smoke stratification for elevated fires than for ground-level fires. We can infer a hot upper layer and a cold lower one. Moreover, the higher the fire, the more significant the break between these two layers.

Despite a growing interest in recent years, the understanding of elevated fires is still relatively limited. The present study aims to understand the effects of elevation on fire dynamics in a specific configuration with natural air inlet in the lower part of the room and mechanical vitiated air exhaust in the upper part. This configuration has not been investigated in the literature yet. The analysis is based on a series of reduced-scale tests, performed in the NYX facility of the Institut de Radioprotection et de Sûreté Nucléaire (IRSN). Two aspects are investigated: the burning rate and gas temperature in the compartment.

TOOLS AND METHODOLOGY

Experimental setup

The experimental device NYX has already been used for previous studies [START_REF] Prétrel | Multi-scale analysis of the under-ventilated combustion regime for the case of a fire event in a confined and mechanically ventilated compartment[END_REF], [START_REF] Mense | Experimental and numerical study on low-frequency oscillating behaviour of liquid pool fires in a small-scale mechanically-ventilated compartment[END_REF], [START_REF] Li | Experimental study on the effect of mechanical ventilation conditions and fire dynamics on the pressure evolution in an air-tight compartment[END_REF]. This fire test apparatus is a 1:4 reproduction of one room of IRSN's large scale DIVA facility, and therefore has the following dimensions: L = 1.5 m (length), W = 1.25 m (width) and H = 1 m (height) for a total volume of 1,875 m 3 . Its walls are made of a double layer: the first one is a 2 mm thick steel plate (ρ = 7817 kg/m 3 , k = 51.9 W/K/m, Cp = 446 J/kg/K, with ρ the density, k the conductivity and Cp the specific heat) and the second one is a 25 mm thick calcium silicate layer (ρ = 2700 kg/m 3 , k = 0.22 W/K/m, Cp = 920 J/kg/K). In order to see inside the compartment with cameras placed outside, one of the double-layer walls is replaced by a glass plate. Since the chosen ventilation configuration for this work comprises a natural fresh air inlet and a mechanical exhaust, openings are milled in the lower part, on the longest sides of the compartment. These openings are 0.75 m long, 0.10 m high and are centered. Smoke is extracted through two vents located in the upper part of the compartment. A picture and a brief description of NYX are given in Fig. 2. The weighing system used to measure fuel mass loss all along the test is a SARTORIUS scale with a precision of 0.01 g. The burning rate is then computed using the time derivative of the mass loss. Ktype thermocouples with a diameter of 0.5 mm are used to measure gas temperature in order to study thermal stratification. These thermocouples evaluate the temperature up to 1300 °C with a precision of 3 °C. They are distributed over 4 vertical trees located outside the plume region. Three lateral trees are placed in the compartment corners with 4 thermocouples. They are used to assess compartment gas temperature homogeneity. The fourth tree (x = -0.15 m , y = -0.35 m with the pan center as the origin) is composed of 12 thermocouples distributed every 0.1 m between 0.165 m and 0.665 m, and then every 0.05 m from 0.665 m to 0.965 m to improve discretization near the ceiling and catch the steep gradient. This mast will provide the data used for the analysis. Finally, a thermocouple is placed over the center of the pan, near the ceiling, at 0.95 m from the floor. Bidirectional probes placed in the exhaust ventilation ducts measure flow rates between 0 m 3 /h and 130 m 3 /h with an error not exceeding 4% of the reading data. All measurements are recorded by a National Instrument (NI) system at a frequency of 1 Hz.

Fire source

In order to simplify the study, the chosen fire source is a hydrocarbon pool fire, and more precisely dodecane (ΔH = 42 MJ/kg and Tb = 210 °C with ΔH the combustion enthalpy and Tb the boiling temperature). A volume of 0.134 L of fuel is poured into a borosilicate pan with an inner diameter of 0.09 m and a height of 0.028 m. This volume, poured for all tests, corresponds to an average fuel mass of 101.72 g ± 1.5 g.

Since the tests are carried out in a compartment, a reference in the open atmosphere is needed.

Correlations are commonly used such as Babrauskas' [START_REF] Babrauskas | Estimating large pool fire burning rates[END_REF], or Heskestad's [START_REF] Heskestad | Luminous heights of turbulent diffusion flames[END_REF]. However, they are only partially or not validated at all for such a small pan size. For this reason, the 0.09 m diameter pan was characterized in an open atmosphere in terms of flame height and MLR using a weighing system and a camera. With the mass loss, a mean burning rate of 0.06 g/s ± 6% was determined. In order to evaluate the flame height in a free atmosphere, image processing was based on a method from Audouin et al. [START_REF] Audouin | Average centreline temperatures of a buoyant pool fire obtained by image processing of video recordings[END_REF]. All the pictures are digitized: a threshold at which the flame is considered to be present in a pixel was chosen, which allowed each pixel to be assigned a value of 0 (flame is absent) or 1 (flame is present). The pictures were then computed with an arithmetic mean: the mean image obtained corresponds to the flame probability. Two heights are calculated for this image: a first one corresponding to a flame probability of 95% and the other a probability of 5%. The flame height is finally defined as the mean of these two values, with a correcting coefficient of 0.8. This method leads to a value of 0.167 m ± 2%.

Methodology

Eleven tests were carried out with the 0.09 m diameter pan. The initial extraction flow rate was set to 50 m 3 /h. This high value was chosen in a first approach in order to get rid of the smoke interface effect on the fire. The pan is then placed in the center of the compartment at a height zb. The different configuration settings are described in Table 1. The fuel is manually poured into the pan, and the fire is manually ignited with a propane burner. Finally, the glass wall is closed a few seconds after ignition. The mass loss, gas temperature, but also the flow rate which may slightly change after ignition, are measured throughout the test.

RESULTS

Elevation effects on the mass loss rate

The first variable of interest is the MLR. Variation in burning rate over time is given for various elevations in Fig. 3: for illustration purposes not all elevations are plotted on this graph. The curves are divided into two groups. Figure 3 a shows tests with a slightly decreasing MLR with pan elevation (group I), and Fig. 3 b tests with an increasing burning rate with elevation (group II). Moreover, the open atmosphere MLR for a 0.09 m diameter pan is added to each graph to facilitate comparison.

The group I graph shows three significant points. First, all the curves share the same behavior. During the ignition period, the increase in MLR is similar for all tests whether confined or not. Around 150 s, elevated fires carried out in NYX start to dissociate from the one performed in an open atmosphere and at ground level. Their burning rates decrease until reaching a quasi-steady phase. This phase is never completely steady because of the apparatus thermal inertia and the fixed fuel volume used. The MLR then increases slightly because of the borosilicate pan. At the end of the fire, only a small volume of fuel indeed remains: this small quantity is strongly heated by the pan and the flame. This is a common phenomenon for pool fires with variable fuel height during the combustion process. The last phase is the fire extinction by lack of fuel for all the tests performed. Secondly, fire duration increases with pan elevation: from around 2090 s for the ground-level fire to 3160 s for zb/H = 0.62. Since the fuel mass is constant for the whole campaign, fire duration quantifies the heat release rate of a fire, i.e. if the fire duration increases, it implies that the burning rate falls. Thirdly, the ground fire MLR is similar to the fire in a free atmosphere, whereas elevated fire burning rates are lower than for the ground-level test. Group II fire tests show contrasting results. The ignition and dissociation period are similar for all tests as previously described. From zb/H = 0.72, the steady value of the MLR is slightly greater than the value observed for group I. Beyond zb/H = 0.72, MLR keeps climbing until reaching almost twice the open atmosphere burning rate for the highest fire (zb/H = 0.87). Consequently, fire duration only reduces with pan elevation from 2840 s for zb/H = 0.72 to 1150 s for the highest test.

These two graphs clearly illustrate that elevation affects the MLR and fire duration. Furthermore, this effect is much more significant for group II fire tests. In order to focus on the effects of elevation on the fire, Fig. 4 shows the ratio between the mean MLR for tests carried out in NYX and mean MLR for the open atmosphere fire as a function of non-dimensional pan elevation. The mean mass loss rates are computed over the quasi-steady period defined by dashed lines for each test as shown in Fig. 3.

In Fig. 4, regions A and B combine tests from group I, and region C shows tests from group II. The ground-level test MLR is, as explained before, almost the same as in a free atmosphere. Then, between the floor and the first elevation (zb/H = 0.15), the burning rate shows a drastic decrease of around 40%. Actually, the inlet openings are located between these two elevations, i.e. the fire test performed below za (a value representing the opening upper limit) is in front of the air inlet. This position can explain the similar MLRs for open-atmosphere and ground-level tests. In region B, mean MLR ratio decreases slightly. Elevation does not have a significant influence. However, from a height zb/H = 0.72, a distinct behavior appears: in region C, the effect of elevation is much more visible with a rapidly-increasing mean MLR ratio.

This graph highlights two phenomena. First of all, the results underline a change in combustion regimes between regions B and C, symbolized by a continuous green line. From the height of zb/H = 0.72, the flame begins to impinge on the ceiling. The Fig. 5 displays flame pictures for region B and C tests. For zb/H = 0.62, the flame does not impinge on the ceiling, whereas for zb/H = 0.72, some parts of the flame start touching it. For zb/H = 0.81, the flame starts spreading over the ceiling and finally, for zb/H = 0.87, the flame impinges continuously on and spreads over the ceiling. When the flame reaches the ceiling, it is heated up as shown by the graph in Fig. 6. It displays the value of the nearby ceiling thermocouple as a function of non-dimensional pan elevation. At zb/H = 0.72, the temperature near the ceiling is greater than 300 °C. Beyond this elevation, this temperature is even above 600 °C, i.e. for these tests, the thermocouple close to the ceiling is inside the flame [START_REF] Cheung | A fully-coupled simulation of vortical structures in a large-scale buoyant pool fire[END_REF]. Since the ceiling is heated up, it produces additional heat fluxes which come back to the fuel surface. The flame, which spreads along the ceiling, also generates more radiative heat fluxes compared to a classical vertical flame (e. g. zb/H = 0.62). Radiative heat fluxes back to the fuel surface are therefore not only composed by vertical flame fluxes, but also retroactive fluxes from the ceiling and the spreading flame. This phenomenon enhances fuel pyrolysis, which is directly linked to heat fluxes received by the fuel surface. Since beyond zb/H = 0.72, the higher the fire, the flatter the flame, this change of shape explains the significant increase in the MLR at the greater elevation. The results show that the ceiling feedback begins earlier than this limit (near zb/H = 0.72): the ceiling effect is thus non-negligible even if the flame does not touch the ceiling continuously. Nevertheless, flame height seems to play a role in the change in combustion regime.

The second phenomenon highlighted by Fig. 4 is the competition between the decrease in oxygen concentration and the increase in radiative heat fluxes for the mass loss rate. In region B, the mean MLR ratio falls lightly. At these elevations, external heat fluxes have only a weak impact on the MLR, in which case, oxygen concentration drives the combustion process. On the contrary, in region C, radiative heat fluxes prevail and lead the combustion. This explains why the mass loss rate increases sharply beyond zb/H = 0.72. In the literature, these two tendencies were not obtained in the same configuration except for Polukhina, who only achieved results for two different heights.

Elevation effects on gas temperature

Figure 7 displays variation in temperature over time for tests at various heights. A typical temperature profile generally follows the same trends as the MLR. Thus, they all show a first stage with a rapid temperature increase, then for the group I test (zb/H = 0.31), temperatures decrease until a quasi-steady period. For group II pool fires, gas temperatures keep increasing slowly for zb/H = 0.72, and quickly for zb/H = 0.87. Despite these differences, all the curves presents a quasi-steady phase. Mean values can be defined over this period.

In a stratified compartment, temperature profiles are combined into two distinct groups: one group of thermocouples in the warm and dense smoke, where high temperatures are measured, and one group of thermocouples outside the smoke, where lower temperatures are measured. For zb/H = 0.06, no group of thermocouples is visible: thermal stratification is very weak. For zb/H = 0.31, two groups are discernible: the first with the two highest thermocouples, and the second with the other 4 thermocouples. However, the break between these two groups is not clear since the cold layer has temperatures ranging from 20 °C to 40 °C. For zb/H = 0.72, all thermocouples except the one at a height of 0.965 m measure rather cold temperatures. The temperature differences between these two groups are quite significant. The same stratification type is visible for zb/H = 0.87, but the colder thermocouple group shows larger temperature amplitudes: the stratification begins to disappear. This variation in temperature over time can also be used to compare compartment thermal trends for tests with seemingly similar MLRs. For example, the ratios between mean MLR and mean open atmosphere MLR for zb/H = 0.31 and zb/H = 0.72 are almost similar, but their temperature profiles show different thermal structures. For zb/H = 0.31, temperatures decrease just after the ignition phase until stabilization, while for zb/H = 0.72 they only increase. Thermal stratification is also much more pronounced for zb/H = 0.72: the top layer is warmer and the bottom one colder with very clustered thermocouples, which means that the break between them is largely pronouced. Figure 8 shows the average vertical temperature profiles (computed on the quasi-steady phase) for tests at different elevations. Since all the thermocouple trees show homogeneous results, data come from the one with the most thermocouples. In order to interpret the variation in thermal stratification, three quantities are considered : the lowest temperature (TL), the location of the highest gradient ZG (that can be considered as a possible interface between two layers) and the maximum temperature (TM). The lower and upper layers can be considered to be below and beyond the vertical position of the maximum gradient respectively. The variation in the three quantities versus the pool elevation (provided in Fig. 8) shows two typical regimes, one for low elevations and one for high elevations.

For the first regime for low fire elevations, the lowest temperature decreases with elevation due to the fact that the heating of the lower layer by flame radiation decreases as the fire is elevated. The maximum temperature does not increase significantly and follows the same trend as the MLR that remains constant for low elevations. The location of the interface climbs due to the increase in fire elevation. The second regime, for high elevations, shows different vertical temperature profiles. The lowest and the maximum temperatures increase because the MLR increases and the flame impinges on the ceiling. In addition, the interface position does not change substantially because it has already reached a high position close to the ceiling. The critical position that distinguishes the two regimes is located around 0.6-0.7 (in grey in Fig. 8). In order to quantify the thermal stratification in a tunnel, Newman [START_REF] Newman | Experimental evaluation of fire-induced stratification[END_REF] proposed a criterion, defined as follows:

𝑁𝐶 = Δ𝑇 𝑐𝑓 Δ𝑇 𝑎𝑣 = 𝑇 𝑐𝑒𝑖𝑙𝑖𝑛𝑔 -𝑇 𝑓𝑙𝑜𝑜𝑟 𝑇 𝑚𝑒𝑎𝑛 -𝑇 𝑓𝑙𝑜𝑜𝑟 (1)
where NC is the Newman's criterion, Tceiling the gas temperature near the ceiling, Tfloor the gas temperature near the floor, and Tmean the mean temperature in the region of interest. Even though the configuration studied is not a tunnel, this criterion can be a relevant indicator for variation in stratification with fire elevation. This criterion is plotted in Fig. 9 as a function of the non-dimensional height. It supports the findings explained earlier: the value of the criterion increases with elevation until a maximum at the height zb/H = 0.72, and then decreases. That is to say stratification is indeed clearer when the fire is elevated, but until a limit beyond which it starts to vanish. This limit seems to be related to flame impingement.

CONCLUSION

This work aims to study the effects of elevation on fire dynamics by focusing on the mass loss rate and gas temperature. The chosen ventilation configuration consists of natural air inlet in the lower part and vitiated air exhaust in the upper one. The analysis is based on a series of small-scale dodecane pool fire tests in an apparatus with a weighing system, to determine the mass loss rate, and thermocouples for measuring gas temperature. The results show that elevation affects the MLR in two ways. In a first range of heights, the influence of elevation is quite small, the mass loss rate decreases rather slowly: the reduction of available oxygen explains this decrease. In a second range, when the flame begins to impinge on the ceiling, the effects of elevation are more pronounced, and the MLR increases sharply until being well above its open atmosphere value. Since the pool fire is close to the ceiling and smoke, it lies in an environment with great radiative heat fluxes, which control the combustion and hence the increase in mass loss rate. The results also confirm the link between flame height and changes in the combustion regime for elevated fires. Finally, the study of the gas temperature in the compartment shows that stratification actually increases with pan elevation, but up to a maximum value, and then decreases. These findings, obtained in an original ventilation configuration, support and complete previous studies on elevated fires.

Fig. 1 .

 1 Fig. 1. MLR* as a function of the dimensionless elevation z*.

Fig. 2 .

 2 Fig. 2. A lateral view of the NYX apparatus is shown on the left. The triangles represent the thermocouples. The righthand image shows an example of a test for zb/H = 0.62.

Fig. 3 .

 3 Fig. 3. MLR as a function of time for various elevations. (a): decreasing burning rate with elevation tests (group I); (b) increasing MLR with pan elevation tests (group II).

Fig. 4 .

 4 Fig. 4. Ratio between mean MLR and mean open atmosphere MLR as a function of nondimensional pan elevation.

Fig. 5 .

 5 Fig. 5. Qualitative analysis of flame behavior for different pan elevations. In Fig. 4, the dashed blue line represents the region where the 0.09 m diameter pan flame should theoretically impinge on the ceiling. This limit position is based on the open atmosphere flame height. The results show that the ceiling feedback begins earlier than this limit (near zb/H = 0.72): the ceiling

Fig. 6 .

 6 Fig. 6. Temperature at a height of 0.95 m, centered on the fuel pan as a function of non-dimensional elevation.

Fig. 7 .

 7 Fig. 7. Variation in temperature over time. (a): zb/H = 0.06 ; (b): zb/H = 0.31 ; (c): zb/H = 0.72 ; (d): zb/H = 0.87.

Fig. 8 .

 8 Fig. 8. On the top, the mean vertical temperature profiles for various elevations. On the bottom, from left to right, the lowest temperature, the maximum gradient position and the maximum temperature extracted from the temperature profiles.

Fig. 9 .

 9 Fig. 9. Newman's criterion and temperature profile shapes for various elevations.

Table 1 . Test settings

 1

	zb/H	0.06	0.15	0.20	0.31	0.39	0.51	0.62	0.72	0.77	0.81	0.87
	Initial fuel mass (g)	100.2	102.2	101.2	101.5	101.8	101.8	101.4	102.1	102.3	100.9	101.6

ACKNOWLEDGMENTS

The authors would like to acknowledge Électricité de France (EDF) which supported this work.


Cecile Barron

Paul-Louis Lopez-Marnet

Kasper Van Der Cruijsen

Cyril Grandjean

Abdelkrim Sadoudi

Herman Höfte

Luisa Trindade

Mechanical properties of miscanthus rind : relation to tissue structure and composition

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.

Understanding the mechanical properties of lignocellulosic biomass is of great interest to improve their deconstruction (eg in the pre-grinding operation unit), and their functionality for different applications (eg biomaterials). Ten highly contrasted Miscanthus sinensis genotypes were selected based on their overall cell wall composition to cover the variability available within this species, and were harvested in two consecutive years. The rinds from the fifth internode were prepared and the shear force required to cut them was obtained using a dedicated prototype device. Based on the measured mechanical properties a subsample of rinds or associated internode were selected and characterized for histology (by light microscopy after FASGA staining or scanning electron microscopy) and cell wall biochemistry (neutral sugars, Klason lignin, hydroxycinnamid acids). A large variability was revealed for composition and structure (rind thickness and density) allowing to explain differences in the observed mechanical properties.

  Barron C. a , Lopez P.L. b , van der Cruijsen K. c , Grandjean C. b , Sadoudi A. a , Höfte H. b and Trindade L. M. c a IATE, Univ Montpellier, INRAE, Institut Agro, Montpellier, France b Institut Jean-Pierre Bourgin, INRAE , AgroParisTech, Université Paris-Saclay, Versailles, France c Wageningen University & Research, Plant Breeding, Wageningen, The Netherlands


Pascal Zavaleta

Sylvain Suard

Laurent Audouin

CABLE TRAY FIRE TESTS WITH HALOGENATED ELECTRIC CABLES IN A CONFINED AND MECHANICALLY-VENTILATED FACILITY

Keywords: cable tray fire, DIVA facility, explosion, ignition, PVC cable, unburnt hydrocarbons

Cable fires are one of the main fire hazards present in nuclear power plants (NPPs). Therefore, as part of the OECD PRISME-2 project, cable tray fire tests were performed both in open atmosphere conditions and in a confined and mechanically-ventilated facility, called DIVA. These tests aim at showing the effects of a confined and ventilated environment on fire characteristics and consequences. This study deals with five fire tests which used halogenated (poly(vinyl chloride) or PVC) cable types. Two tests were carried out in open atmosphere and three tests in the DIVA facility. The latter used a ventilation renewal rate (VRR) of either 4 or 15 h -1 . The confined and ventilated conditions reduced the mass loss rate and heat release rate compared with those obtained in open atmosphere. Furthermore, the three confined tests produced unburnt gases which ignited in the fire room. Two explosions were highlighted for the tests which used a VRR of 4 h -1 . These explosions indeed led to fast flame propagations over the entire upper part of the fire room and steep overpressures of almost 150 hPa. The low-qualified PVC cables and the ventilation set-up used in this study strongly contributed to the occurrence of these explosions.

𝑄 ̇

Heat release rate (kW) 𝑄 ̇𝐶𝐷𝐺 Heat release rate assessed from the CDG calorimetry method (kW) 𝑄 ̇𝑖𝑛𝑠𝑡 Energy variation per unit of time within the FR and AR (kW) 𝑄 ̇𝑜𝑢𝑡

Convective heat transfers through the inlet and outlet ducts of the ventilation network (kW)

𝑄 ̇𝑝

Power law equation that fits best with the HRR time evolution of the fire growth stage (kW) 𝑄 ̇𝑝𝑦𝑟 Energy per unit of time required for pyrolysis of fuel contained in the cables (kW) 𝑄 ̇𝑡ℎ𝑒𝑟𝑚𝑎𝑙

Heat release rate assessed from the thermal method (kW) 𝑄 ̇𝑤𝑎𝑙𝑙𝑠 Heat transfers through the walls of both the FR and AR (kW) PVC Poly(vinyl chloride) r

Stoichiometric mass fuel-to-air ratio (-) SE South-East 𝑡 [START_REF] Nrc | Verification and validation of selected fire models for nuclear power plant applications[END_REF] Fire growth characteristic time (s) VRR Ventilation renewal rate, i.e., number of changes of the room volume per hour (h -1 )

Greek characters Power cables are used for instance in the process rooms for supplying electricity to the pumps, turbines, transformers or heaters, or in the switchgear room that contains numerous electric cabinets connected to multiple cable trays [START_REF] Nrc | Verification and validation of selected fire models for nuclear power plant applications[END_REF]. Furthermore, many cable trays containing both instrumentation and control cables are also found in the cable spreading room and in the main control room. Instrumentation cables are used for digital or analogic transmission for various types of transducers, while control cables can operate valves, relays or contactors.

A serious cable tray fire occurred at the Browns Ferry NPP in 1975 [2] resulting in the loss of the Unit One emergency core cooling system. Since then, many efforts have been made to enhance the prevention of cable tray fires in the most recent NPPs. For instance flame retardant materials have been used in cables. Nevertheless, the OECD FIRE Database [START_REF]Committee on the Safety of Nuclear Installations (CSNI): "OECD FIRE Database, Version[END_REF] recorded nearly seventy fire events in NPPs involving electric cables between the late 1980s and the end of 2014. Cable tray fires and related consequences are therefore still a major issue for fire safety analyses in NPPs. Many cable tray fire experiments were carried out in the past and highlighted significant effect of the cable type [START_REF] Grayson | Assessing the Fire Performance of Electric Cables (FIPEC)[END_REF], cable tray geometry [START_REF] Mcgrattan | Cable Heat Release, Ignition, and Spread in Tray Installations During Fire (CHRISTIFIRE)[END_REF], nearby ceiling [START_REF] Mcgrattan | Cable Heat Release, Ignition, and Spread in Tray Installations During Fire (CHRISTIFIRE). Phase 2: Vertical Shafts and Corridors[END_REF] and support wall [START_REF] Passalacqua | Experimental characterisation of ITER electric cables in postulated fire scenarios[END_REF]- [START_REF] Zavaleta | Multiple Horizontal Cable Tray Fire in Open Atmosphere[END_REF]. However, all these experiments were conducted in open atmosphere conditions and very few studies were performed in confined and mechanically-ventilated rooms [START_REF] Keski-Rahkonen | Fire induced damage to electrical cables and fire growth on cables, EUROSAFE[END_REF], such as those found in NPPs. In contrast, many investigations, using hydrocarbon (Pretrel [10]- [START_REF] Pretrel | Experimental study of burning rate behaviour in confined and ventilated fire compartments[END_REF], Nasr [START_REF] Nasr | Fuel Mass-Loss Rate Determination in a Confined and Mechanically Ventilated Compartment Fire Using a Global Approach[END_REF], Melis [START_REF] Melis | Effects of vitiation on the heat release rate in mechanically-ventilated compartment fires[END_REF], Lassus [START_REF] Lassus | Ventilation effects in confined and mechanically ventilated fires[END_REF]), alcohol, polymethyl methacrylate (PMMA) or wood (Delichatsios [15]) as fuel, were conducted in mechanically-ventilated compartments. These fire tests showed a significant influence of the ventilation both on the combustion behaviour and species product formation. Compartment fires can indeed be classified as well-ventilated or under-ventilated according to the ventilation conditions. Well-ventilated enclosure fires (Tewarson [16]) have an excess air supply compared with the quantity of air needed by the fuel and so they lead to a very low generation of unburnt gases. In contrast, for under-ventilated fires (Gottuk and Lattimer [START_REF] Gottuk | Effect of Combustion Conditions on Species Production[END_REF]), the air supply is too low and unburnt gases, such as carbon monoxide (CO) and unburnt pyrolysis gases, are produced. Such gas species may ignite in a confined and mechanically-ventilated compartment as highlighted in previous study [START_REF] Magnognou | Risk analysis of unburnt gas ignition in an exhaust system connected to a confined and mechanically ventilated enclosure fire, IAFSS 12th Symposium[END_REF]. Consequences of these ignitions, such as an overpressure or the occurrence of a secondary fire, are major issues for fire safety analyses in NPPs.

Therefore, as part of the OECD PRISME-2 project [START_REF]Investigation Heat and Smoke Propagation Mechanisms in Multi-Compartment Fire Scenarios, Final Report of the PRISME project[END_REF], cable tray fire tests were performed both in open atmosphere conditions and in a confined and mechanically-ventilated facility, called DIVA. These tests aim at showing the effects of a confined and ventilated environment on fire characteristics and consequences. The fire characteristics discussed are the mass loss rate (MLR), heat release rate (HRR), effective heat of combustion (EHC) and combustion efficiency ratio. The fire consequences studied in the DIVA facility are the fire room temperature, smoke concentration (carbon dioxide, unburnt hydrocarbon gases and soot), pressure and ventilation flow rates in particular. Moreover, partners of the PRISME-2 project proposed to use two halogenated (poly(vinyl chloride) or PVC) cable types and two halogen-free flame retardant (HFFR) cable types which can be found in their NPPs. This study only considers the tests performed using the PVC cable types while previous works [START_REF] Zavaleta | Multiple Horizontal Cable Tray Fire in Open Atmosphere[END_REF], [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF] have discussed the tests involving the two HFFR cable types.

This paper first gives a detailed description of the cable trays, the DIVA facility and the related instrumentation. Then, the main fire consequences in the DIVA facility are discussed, which leads to highlight several ignitions of unburnt hydrocarbon gases for each confined test. Next, the primary fire characteristics of the cable tray fire tests and ignitions of unburnt gases are assessed. The last part first proposes to discuss the effects of the confined and ventilated environment on the cable tray fires. For that purpose, the fire characteristics assessed in both confined and open atmosphere environment are compared. This analysis is completed by the classification of the confined and ventilated fires according to the assessment of the combustion efficiency ratio. Finally, this last section addresses the conditions that led to ignite the unburnt hydrocarbon gases and also highlights the fact that two ignitions can be considered as explosions.

| DESCRIPTION OF THE FIRE TESTS

The five cable tray fire tests of the OECD PRISME-2 project considered in this study are indicated in Table 1. The CT1 and CT3 tests were carried out in open atmosphere conditions and the CT2, CT4 and CT5 tests in confined and mechanically-ventilated conditions (DIVA facility). The CT1 to CT5 tests used a vertical stack of five horizontal ladder cable trays containing PVC cable types (Fig. 1). The CT1 and CT2 tests used a first PVC cable type, called cable A, while the CT3 to CT5 tests used a second type, named cable B (Table 1). These two cable types only meet the requirements of the international standard IEC 60332-1-2 test [21] 1 and can be thus considered as low-qualified electric cables. Finally, the CT2, CT4 and CT5 tests used a ventilation renewal rate (i.e., number of changes of the room volume per hour, VRR) of 4, 15 and 4 h -1 , respectively.

| The cable trays

The cable trays were five horizontal ladder trays 3 m long and 0.45 m wide, with tray spacing of 0.3 m (Fig. 1). Each tray was filled with 49 cables A for the CT1 and CT2 tests and with 44 cables B for the CT3 to CT5 tests. The cables 2.4 m long were packed loosely along the five cable trays for all the tests, as illustrated in Fig. 2 for the CT2 test. This cable arrangement was also specified for nearly all the horizontal cable tray experiments conducted as part of the CHRISTIFIRE project [START_REF] Mcgrattan | Cable Heat Release, Ignition, and Spread in Tray Installations During Fire (CHRISTIFIRE)[END_REF]. A loose arrangement of the cables can be considered as realistic. The constraints of setting up in real facilities can indeed prevent from an ordered arrangement of the electric cables. The electric cables A and B are shown in Fig. 3. The non-metallic materials (outer sheath, internal sheath and insulation) mainly contain plasticized PVC and mineral loads such as calcium carbonate (CaCO3). Phthalates are used as plasticizers in these cables. Other features of the cables A and B are provided in Table 2. Thermophysical properties of the outer sheath at 20°C are available for the cable B and are given in Table 3. The five ladder cable trays were placed against an insulated side wall which was 3 m long, 2.4 m high and 0.05 m deep (Fig. 1). Properties of this wall are given in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF]. A square gas burner (300 x 300 mm 2 ), centred and located 0.2 m below the first tray (Fig. 1), was supplied with propane gas to ignite the cable trays. The burner provided a fire power of 80 kW for 1 minute 20 seconds during the CT1 and CT2 tests (cable A) and 2 minutes 26 seconds during the CT3 to CT5 tests (cable B). This power value of 80 kW was selected in order to ensure ignition of the cable trays regardless of the cable type and cable tray loading used in all the PRISME-2 cable tray fire tests [START_REF]Investigation Heat and Smoke Propagation Mechanisms in Multi-Compartment Fire Scenarios, Final Report of the PRISME project[END_REF]. The above durations were determined in open atmosphere conditions when the HRR of the cable tray fire reached 400 kW (without the gas burner contribution) [START_REF] Zavaleta | Multiple Horizontal Cable Tray Fire in Open Atmosphere[END_REF]. For such an HRR value five times higher than the burner fire power, the cable trays were undoubtedly ignited and the previous durations were therefore considered as the times to ignition.

| Set-up of the rooms (DIVA facility and ventilation used)

This section details the configuration of the DIVA facility and ventilation used for the three confined CT2, CT4 and CT5 tests. This facility contains five rooms in total (Fig. 4). Firstly, the ground floor consists of 3 rooms (rooms 1 to 3) of similar size and arranged in a row separated by fire doors. Moreover, each room also provides access to a corridor (room 0 in the background of Fig. 4) through fire doors. A fifth room is located on the first floor (room 4). The bearing walls of these rooms, made of reinforced concrete, are 0.3 m thick and 4 m high. The ventilation system of the DIVA facility consists of two separate inlet and outlet circuits (in blue and red in Fig. 4, respectively) which are connected to each room. The multi-rooms DIVA facility can be representative of room configuration found in real NPPs as described in [START_REF] Nrc | Verification and validation of selected fire models for nuclear power plant applications[END_REF].

The CT2, CT4 and CT5 tests used rooms 1 and 2 of the DIVA facility as seen in Fig. 5 and Fig. 6. Room 1 was called the fire room (FR) since it contained the five cable trays. These cable trays were centred against the west wall (Fig. 6). Room 2 was named the adjacent room (AR). The features (dimension, position…) of the rooms, walls, inlet duct, outlet duct and five cable trays are given in Fig. 5 and Fig. 6. The FR and AR were linked through an open doorway which was 0.79 m wide and 2.17 m high. The ceiling of the FR was thermally protected by a double layer of 30 mm thick Rockwool panels while the ceiling of the AR was only protected by a single layer of the same insulated materials of 30 mm in thickness. Only one inlet duct was installed in the upper part of the FR and one outlet duct was positioned in the upper part of the AR. Flow directions at both the inlet and outlet are also indicated in Fig. 6. Initial VRR (before ignition) for all the volume occupied by both the FR and AR (240 m 3 ) was 4, 15 and 4 h -1 for the CT2, CT4 and CT5 tests, respectively. The corresponding ventilation volume flow rates at the inlet and outlet were thus initially equal to about 960, 3600 and 960 m 3 /h for the CT2, CT4 and CT5 tests respectively.

| Instrumentation

This section presents the measurements discussed in this study. Nine thermocouples for measuring gas temperature were positioned in the North-West (NW), North-East (NE) and South-East (SE) corners of the FR along vertical trees (see the enlarged view in Fig. 7 (a) which shows, for instance, the thermocouples of the SE vertical tree). For the three trees, the spacing between the thermocouples was 0.5 m and the lowest and highest ones were located at 0.05 and 3.9 m from the floor, respectively. Only the temperature measurements at 1.05, 2.05, 3.05, 3.55 and 3.9 m are further shown in this work. Gas probes for continuous gas sampling were also located along the vertical tree in the SE corner at 0.7, 2.2 and 3.3 m from the floor (see the enlarged view in Fig. 7 (b) which only points out the gas probes). These probes were connected to gas analyzers (located outside the rooms) which measured the concentration of carbon dioxide (CO2), carbon monoxide (CO) and oxygen (O2) in the lower, mid and upper parts of the FR. An additional gas sampling probe was also fixed at the vertical tree in the SE corner at 3.2 m from the floor and was connected to a gas analyzer to measure the concentration of unburnt hydrocarbon gases (CnHm) in the upper part of the FR. In addition, concentration measurement of CnHm was also performed at the outlet of the AR. Furthermore, three smoke sampling probes were installed in the lower part of the FR and AR at 0.5 m from the floor and at the outlet duct of the AR. Each probe was connected to a bank of ten filters (BF) which measured the soot mass concentration in the three abovementioned locations. For each BF, ten smoke samplings for one minute duration were undertaken during different times of the test. These samplings led to soot deposits on each filters. The pressure in the FR, volume flow rate at the inlet and one heat flux measurement, positioned above the cable trays at 0.7 m from the ceiling of the FR, named HF (Fig. 8), are also discussed in this study. Finally, the mass loss was recorded from two electronic scales. These scales were placed in thermally insulated and metallic boxes and positioned below the supporting device of the cable trays (Fig. 8).

| RESULTS AND DISCUSSION

| Fire spread over the cable trays

An illustration of the fire spread over the five horizontal ladder cable trays is given in Fig. 9 for the CT2 test. The fire quickly spread vertically between the trays and the top tray was quickly involved in the fire 30 s after ignition (Fig. 9 (a)), and then fire spread horizontally (Fig. 9 (b) and (c)). At the same time, smoke also rapidly filled up the FR as seen in Fig. 9 (c) to (e) which indicate pictures of the cable trays at t = 2, 3 and 4 min, respectively. Moreover, Fig. 10 and Fig. 11 show the cable trays before and after the CT4 and CT5 tests, respectively. Fig. 9 (f) and Fig. 10 (b) first show that fire spread to the ends of the five cable trays for the CT2 and CT4 tests (VRR of 4 and 15 h -1 , respectively). Identical outcome was obtained for the CT1 and CT3 fire tests which were preliminarily conducted in open atmosphere conditions [START_REF] Zavaleta | Multiple Horizontal Cable Tray Fire in Open Atmosphere[END_REF]. However, for the CT5 test (VRR of 4 h -1 ), Fig. 11 (b) demonstrates that fire reached the five cable trays but it did not propagate up to the extremities of all the cable trays, especially for the lower cable trays. Therefore, a VRR of 4 h -1 limited the fire spread over the five cable trays containing the cable B (i.e., CT5 test) but not over those filled with the cable A (i.e., CT2 test).

| Fire consequences in the fire room (FR)

Fig. 12 to Fig. 14 point out the gas temperatures measured in the NW, NE and SE corners of the FR for the three CT2, CT4 and CT5 tests, respectively. These figures first show an increase of the gas temperatures for the three tests due to the growth of the cable tray fires. Then, during the decay fire stages of each test, two faster time evolutions of the temperatures are highlighted and lead to the temperature peaks (1) to ( 6) exceeding 600°C in the upper half-part of the FR. Flames are identified at location where temperatures are measured in excess of 600 °C. Drysdale previously used this as an indicator of flames [START_REF] Drysdale | An introduction to fire dynamics[END_REF]. In addition, Fig. 15 and Fig. 16 give the respective CO2 and O2 concentrations in the FR for the CT2, CT4 and CT5 tests. These figures indicate that the abovementioned temperature peaks (1) to ( 6) match with the sudden and significant variations (1) to ( 6) of both CO2 and O2 concentrations in the upper half-part of the FR. These simultaneous temperature and concentrations variations clearly reveal the fact that gas ignitions occurred twice for each test in the upper part of the FR during the decay fire stage. Moreover, Fig. 17 shows the concentration of the unburnt hydrocarbon gases (CnHm) measured in the upper part of the FR (i.e., 3.2 m high from the floor) as well as at the outlet of the AR for the CT2, CT4 and CT5 tests. This figure and Fig. 12 to Fig. 16 indicate that the decreases (1) to (6) of CnHm concentration 2 correspond with previous variations (1) to (6) of gas 6 temperature as well as CO2 and O2 concentrations. This undoubtedly showed that CnHm ignited twice in the upper half-part of the FR for each of the three tests.

Furthermore, Fig. 18 indicates that maximal soot mass concentrations were measured in the range 3-4 g/m 3 in both the FR and AR for the three CT2, CT4 and CT5 tests. In addition, Fig. 19 points out the same measurements carried out during two additional cable tray fire tests which were also conducted in the DIVA facility as part of the OECD PRISME-2 project [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF]. All the characteristics of these tests (cable tray geometry, room set-up, VRR…) were identical to those of the CT2, CT4 and CT5 tests, except for the cable type. They indeed used one HFFR cable type, as earlier mentioned in the introduction, and specified in detail in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF]. Fig. 18 and Fig. 19 indicate that the soot mass concentrations in the FR and AR are significantly lower for the tests which used the HFFR cable type compared with the CT2, CT4 and CT5 tests which involved the PVC cable types. This outcome was expected since the HFFR cable 3 [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF] satisfies the requirements of IEC 61034-2 [25] unlike the PVC cable types.

Finally, Fig. 20 and Fig. 21 point out the pressure in the FR and AR, respectively, and the volume flow rate at the inlet of the FR, for the three confined CT tests. These figures show that the higher pressure peaks (1), ( 2) and ( 6) led to strong reversals (1), ( 2) and ( 6) of the volume flow rates at the inlet of the FR for CT2 and CT5 tests. This last outcome thus indicates that smoke exited from the FR to the inlet ventilation circuit of the DIVA facility. Table 4 provides the main consequences of the unburnt gases ignitions (1) to ( 6) such as the pressure peaks, the related minimal flow rates at the inlet (as well as the nominal values) and the CnHm combustion zone. This combustion zone is the region involved by the combustion of the unburnt gases. It was estimated according to the locations of temperature measurements higher than 600°C in the NW, NE and SE corners at the peaks (1) to ( 6) (Fig. 12 to Fig. 14,respectively). Table 4 shows that the CnHm ignitions (1), ( 2) and ( 6) involved a large combustion zone in the upper part of the FR and led to steep pressure peaks in the range 70-150 hPa as well as important reversals of the volume flow rate at the inlet (in the range 3000-5000 m 3 /h from the FR to the inlet ventilation circuit). In contrast, the CnHm ignitions (3), ( 4) and ( 5) involved a limited combustion zone (in the NW corner of the FR, under the ceiling) and led to clearly lower consequences than the above ignitions. Consequently, the CnHm ignitions (1), ( 2) and ( 6) are considered as severe ignitions compared with the other CnHm ignitions (3), ( 4) and (5).

| Characteristics of the cable tray fires and CnHm ignitions

| Mass loss rate

Fig. 22 provides the mass loss rate (MLR or 𝑚̇) of the cable trays for the CT2, CT4 and CT5 tests, obtained from time derivation of the mass loss. Marks (1) and (2) in Fig. 22 (a) and mark [START_REF] Mcgrattan | Cable Heat Release, Ignition, and Spread in Tray Installations During Fire (CHRISTIFIRE). Phase 2: Vertical Shafts and Corridors[END_REF] in Fig. 22 (c) reveal sharp increases in MLR of the CT2 and CT5 tests, respectively. These increases were likely caused by the radiation effect related to the severe CnHm ignitions (1), ( 2) and ( 6) as seen in Fig. 23 (a) and(c). These figures indeed indicate corresponding heat flux peaks (1), ( 2) and ( 6) measured in the range 55-70 kW/m 2 at a height of 3.3 m (see HF in Fig. 8). A feed-back effect on the MLR is thus likely shown for the CT2 and CT5 tests. In contrast, for the CT4 test, this radiation effect is not visible for the MLR (Fig. 22 (b)) since the heat flux peaks (3) and ( 4), related to the two CnHm ignitions (3) and (4), were lower (maximal value of 40 kW/m 2 , Fig. 23 (b)). malfunctioned before (i.e., from t = 645 s as seen in Fig. 17 (a)). Nevertheless, since CnHm concentration at the outlet of the AR decreased at the event (2) (Fig. 17 (a)), it is thus supposed that CnHm concentration in the FR also dropped at the same time. The same analyzer also malfunctioned for the CT4 test from t = 1270 s (Fig. 17 (b)). These malfunctions would very likely caused by the overpressures which followed the CnHm ignitions, as further discussed. 3 In same way as for the two PVC cable types studied in this work, this HFFR cable, as specified in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF], was not classified according to the newly applicable EUROCLASS standard in Europe (EN 13501-6 [23]). Indeed, this cable was produced in Europe before 2012 when the EUROCLASS standard was not yet fully applicable.

| Heat release rate

Heat release rates (HRR or 𝑄 ̇) for the cable tray fire tests in the DIVA facility were determined from the thermal method and the carbon dioxide generation (CDG) calorimetry method. The former method was developed by Coutin et al. [START_REF] Coutin | Energy balance in a confined fire compartment to assess the heat release rate of an electrical cabinet fire[END_REF] for assessing the HRR of a cabinet fire, by establishing an energy balance in a single fire room. In this work, the energy balance was carried out inside both the FR and AR (Fig. 6) and the heat released by the fire was mainly distributed in four main parts 4 :

• energy per unit of time required for pyrolysis of fuel contained in the electric cables (𝑄 ̇𝑝𝑦𝑟 ),

• heat transfers through the walls of both the FR and AR (𝑄 ̇𝑤𝑎𝑙𝑙𝑠 ),

• energy variation per unit of time within the FR and AR (𝑄 ̇𝑖𝑛𝑠𝑡 ),

• convective heat transfers through inlet and outlet ducts of the ventilation network (𝑄 ̇𝑜𝑢𝑡 ).

The HRR from the thermal method ( 𝑄 ̇𝑡ℎ𝑒𝑟𝑚𝑎𝑙 ) was thus assessed using the following relationship:

𝑄 ̇𝑡ℎ𝑒𝑟𝑚𝑎𝑙 = 𝑄 ̇𝑝𝑦𝑟 + 𝑄 ̇𝑤𝑎𝑙𝑙𝑠 + 𝑄 ̇𝑖𝑛𝑠𝑡 + 𝑄 ̇𝑜𝑢𝑡 (1)
Moreover, the HRR of the fire can be also estimated with the CDG calorimetry method which was first proposed by Pretrel et al. [START_REF] Pretrel | Experimental determination of fire heat release rate with OC and CDG calorimetry for ventilated compartments fire scenario[END_REF] for fires in a multi-rooms facility. This approach also relies on mass balances of CO2, CO and soot carried out in the rooms of such a facility. This method was applied to the CT2, CT4 and CT5 tests, as was the case for previous cable tray fire tests [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF]. The HRR was thus assessed from the CDG method (Q ̇CDG ) and mass balances of gas species conducted in all the volumes involving the FR and AR. The HRR formulation is given as follows:

𝑄 ̇𝐶𝐷𝐺 = 𝐸 𝐶𝑂 2 [
Where 𝐸 𝑗 is the energy produced by mass unit of generated species j = CO2, CO or C (i.e., soot) (MJ/kg), 𝑚̇𝑗 𝑜𝑢𝑡 is the mass flow rate of species j = CO2, CO or C at the outlet of AR (g/s) and [START_REF] Pretrel | Experimental determination of fire heat release rate with OC and CDG calorimetry for ventilated compartments fire scenario[END_REF].

For each fire test, HRR evaluations were thus carried out from the previous thermal and CDG calorimetry methods, and they are shown in Fig. 24. Considering that the thermal method can underestimate the HRR by about 15% due to heat sinks not considered in the thermal method [START_REF] Coutin | Energy balance in a confined fire compartment to assess the heat release rate of an electrical cabinet fire[END_REF] and that the CDG method can generally overestimate the HRR, as already highlighted in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF] and [START_REF] Zavaleta | Fire spread from an open-doors electrical cabinet to neighboring targets in a confined and mechanically ventilated facility[END_REF], the two evaluations above can be considered to be rather consistent. These considerations also lead to assess the final HRR (𝑄 ̇) as the average of the two previous HRR assessments:

𝑄 ̇= 𝑄 ̇𝑡ℎ𝑒𝑟𝑚𝑎𝑙 + 𝑄 ̇𝐶𝐷𝐺 2
(3) 4 Coutin et al. [START_REF] Coutin | Energy balance in a confined fire compartment to assess the heat release rate of an electrical cabinet fire[END_REF] also considered the contribution of thermal inertia of the metallic structures of the cabinet in the HRR. For the five ladder-type trays considered in this study this contribution may be neglected compared with the other contributions discussed hereafter. | is actually the standard deviation or uncertainty 5 (𝑢) at every time of 𝑄 ̇ estimated from the two HRR measurements (i.e., 𝑄 ̇𝐶𝐷𝐺 and 𝑄 ̇𝑡ℎ𝑒𝑟𝑚𝑎𝑙 , see Fig. 24). The relative uncertainty 𝑢 ̃ of 𝑄 ̇ is then evaluated as follows: Fig. 25 first shows the growth of the cable tray fire when the fire spread over the five cable trays and then a rather steady fire stage for the CT2 and CT4 tests only. This figure finally shows the fire decay stage for the three tests which highlights the HRR peaks (1) to [START_REF] Mcgrattan | Cable Heat Release, Ignition, and Spread in Tray Installations During Fire (CHRISTIFIRE). Phase 2: Vertical Shafts and Corridors[END_REF]. These peaks, which are related with the CnHm ignitions (1) to ( 6), confirm that these ignitions occurred during the decay fire stage of the three tests.

𝑢 ̃= 𝑢 𝑄 ̇ (4)
Furthermore, Fig. 25 (a) and (c) also indicate that the HRR peak due to the cable tray fire is clearly higher for the CT2 test and the cable A (0.98 MW) than for the CT5 test and the cable B (0.57 MW). These two tests involved a VRR of 4 h -1 . This result corroborates the fact that higher MLR peak was measured for the CT2 test (100 g/s) than for the CT5 test (62 g/s), as previously seen in Fig. 22 (a) and (c). So regarding the MLR and HRR, the cable trays filled with the cable B have significantly better fire performances than those containing cable A, even if the two cables meet the same requirements (IEC 60332-1-2 test, see section 2).

| Growth rate

It is commonly assumed that the growth of a fire may be described by the following power law equation [START_REF]Fire safety engineering -Selection of design fire scenarios and design fires -Part 1: Selection of design fire scenarios[END_REF]:

𝑄 ̇𝑝(𝑡) =  • (𝑡 -𝑡 𝑜 ) 2 (5)
Where 𝑄 ̇𝑝(t) is given by the above power law equation that fits best with the HRR time evolution of the fire growth stage (kW),  is the fire growth rate (kW/s 2 ), 𝑡 is the time (s) and 𝑡 𝑜 is the starting time of 𝑄 ̇𝑝(t) (s). In addition, 𝑡 1 , the fire growth characteristic time (s), is defined as the time necessary after 𝑡 𝑜 to reach a HRR of 1000 kW (s). 𝑡 1 is estimated as follows:

𝑡 1 = √ 1000  (6)
It is also proposed to assess the growth rate of the HRR generated by the ignitions of the unburnt gases (1) to [START_REF] Mcgrattan | Cable Heat Release, Ignition, and Spread in Tray Installations During Fire (CHRISTIFIRE). Phase 2: Vertical Shafts and Corridors[END_REF] in the same way as for the cable tray fires, as discussed above. Fig. 26 to Fig. 28 give enlarged views of the growth stage of the HRR for both the cable tray fire and CnHm ignitions, for the CT2, CT4 and CT5 tests, respectively. Table 5 also reports the growth rates, , the growth characteristic times, 𝑡 1 , and the HRR peaks, for the three cable tray fires and CnHm ignitions (1) to ( 6) from these tests. This table indicates that the growth rates of the HRR produced by the severe CnHm ignitions (1), ( 2) and ( 6) ( in the range 5.5-35 kW/s 2 ) are significantly higher than those for both the CnHm ignitions (3), ( 4) and ( 5) and cable tray fires ( in the ranges 0.08-0.23 kW/s 2 and 0.022-0.19 kW/s 2 respectively). This outcome implies that flame propagation in the upper half-part of the FR for the severe CnHm ignitions (1), ( 2) and ( 6) was significantly faster than that for the other CnHm ignitions (3), ( 4) and ( 5) and also than the fire propagation over the five cable trays. Such outcomes are also illustrated by the assessment of 𝑡 1 (Table 5). Indeed, it took about 5 to 13 s for the severe CnHm ignitions (1), ( 2) and ( 6) to generate a HRR of 1 MW while the same value of HRR was reached from 66 to 112 s and 79 to 213 s for the other CnHm ignitions (3), ( 4) and ( 5) and cable tray fires, respectively. It should be noted that only the HRR determined from the thermal method (Fig. 24 (a) and (c)) was used for the assessment of  and 𝑡 1

for the severe CnHm ignitions (Fig. 26 (b) and (c) and Fig. 28 (c)). Indeed, the key sensors used for this method such as the heat flux sensors have faster response-time than gas analysers used for the CDG method. This implies that the thermal method better estimates the fast time evolution of the HRR for the severe ignitions than the CDG method. Finally, the HRR peaks produced by the severe CnHm ignitions (1), ( 2) and ( 6) were also deduced from the HRR determined from the thermal method as argued above. These last ones are thus in the range 0.5-1.75 MW while those produced by the other CnHm ignitions (3), ( 4) and ( 5) vary from 0.3 to 0.5 MW (Table 5).

| Effective heat of combustion

The combustion behaviour of the cable tray fires can be first characterized by the effective heat of combustion (𝐸𝐻𝐶 𝑜𝑟 ∆𝐻 𝑐,𝑒𝑓𝑓 ) defined as the ratio of HRR (𝑄 ̇) to MLR (𝑚̇) [START_REF] Tewarson | Generation of Heat and Chemical Compound in Fires[END_REF]:

∆𝐻 𝑐,𝑒𝑓𝑓 = 𝑄 ṁ ̇ (7)
EHC was evaluated from Eq. ( 7) for the CT2, CT4 and CT5 tests and then averaged over time intervals corresponding to MLR values higher than 20% of the peak MLR. Such a method, as done in [START_REF] Zavaleta | Effects of cable tray configuration on fire spread[END_REF], leads to minimization in EHC uncertainty. Indeed, this assessment therefore does not consider the low values of the MLR at the early and late fire stages, which can be affected by significant uncertainty.

| Effects of the confined and ventilated conditions

This section first proposes to discuss the effects of the confined and mechanically-ventilated conditions on the cable tray fires of the CT2, CT4 and CT5 tests. For this purpose, a comparison with the CT1 and CT3 fire tests performed in open atmosphere conditions [START_REF] Zavaleta | Multiple Horizontal Cable Tray Fire in Open Atmosphere[END_REF] is made; Table 6 summarizes the main fire characteristics. This comparative analysis is completed by the assessment of the combustion efficiency ratio between the confined and open fires, as presented hereafter. Finally, the last part of this section addresses the conditions that led to ignite the unburnt gases.

| Combustion efficiency ratio

To compare the combustion efficiency of the cable tray fires conducted in confined conditions and open atmosphere conditions, the combustion efficiency ratio () may be assessed according to [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF], [START_REF] Tewarson | Generation of Heat and Chemical Compound in Fires[END_REF] as follows:

 = ∆𝐻 𝑐,𝑒𝑓𝑓 𝑐𝑜 ̅̅̅̅̅̅̅̅̅̅ ∆𝐻 𝑐,𝑒𝑓𝑓 𝑜 ̅̅̅̅̅̅̅̅̅̅ (8)
Where ∆𝐻 𝑐,𝑒𝑓𝑓 𝑐𝑜 ̅̅̅̅̅̅̅̅̅̅ and ∆𝐻 𝑐,𝑒𝑓𝑓 𝑜 ̅̅̅̅̅̅̅̅̅̅ are the average EHC of the cable tray fires conducted in confined conditions and open atmosphere conditions respectively. It is stated from [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF], [START_REF] Tewarson | Generation of Heat and Chemical Compound in Fires[END_REF] that:

• for  < 1, the cable tray fire in confined conditions has a lower combustion efficiency than in open atmosphere conditions (i.e., in well-ventilated conditions) and can thus be classified as an underventilated fire or ventilation-controlled fire,

• for   1, the cable tray fire in confined conditions has similar combustion efficiency than in open atmosphere conditions and can be therefore considered as an well-ventilated fire or fuel-controlled fire.

However, it should be noted that the global equivalence ratio, , is commonly used to classify fire. For fire compartment, , can be assessed, according to Gottuk and Lattimer [START_REF] Gottuk | Effect of Combustion Conditions on Species Production[END_REF], as follows :

 = 𝑚̇/𝑚̇𝑎 𝑟 ( 9
)
Where 𝑚̇𝑎 is the air mass flow rate at the inlet of the fire compartment (kg/s) and r is the stoichiometric fuel-to-air ratio. It is widely agreed that fire is well-ventilated when the related  is lower than 1, otherwise the fire is under-ventilated [START_REF] Gottuk | Effect of Combustion Conditions on Species Production[END_REF]. Given the test set-up used for the CT2, CT4 and CT5 tests (Fig. 5), it is supposed that only a fraction of the inlet air mass flow rate (𝑚̇𝑎) reached the fuel (i.e., the cable trays). Other fraction of 𝑚̇𝑎 can indeed enter either in the AR through the doorway (Fig. 5) or can mix with the unburnt gases and combustion gas products in the upper part of the FR. Previous work [START_REF] Nrc | Verification and validation of selected fire models for nuclear power plant applications[END_REF] also mentioned that in fire compartment not all of the air supplied by the ventilation system does reach the fire, especially for ceiling-mounted supply ducts. So, for the CT2, CT4 and CT5 tests, considering the total inlet air mass flow rate (𝑚̇𝑎) in Eq. ( 7) can lead to assess  lower than 1 while the related cable tray fire can be actually under-ventilated. Additional illustration of above discussion is found in [START_REF] Magnognou | Risk analysis of unburnt gas ignition in an exhaust system connected to a confined and mechanically ventilated enclosure fire, IAFSS 12th Symposium[END_REF] that studied hydrocarbon fire (dodecane and heptane) in a reduced-scale confined and mechanicallyventilated enclosure. The authors of this study indeed stated under-ventilated fires that could be characterized by  lower than 1 (they specified under-ventilated fires for 0.7 ≤  < 1.5). For the current work, the use of the combustion efficiency ratio,  (Eq. (( 8)), to classify the fire was therefore preferred.

This last one was indeed assessed from fire characteristics such as the heat release rate which strongly depends on the air (i.e., oxygen) mass flow rate consumed by the fuel [START_REF] Pretrel | Experimental determination of fire heat release rate with OC and CDG calorimetry for ventilated compartments fire scenario[END_REF], [START_REF] Tewarson | Generation of Heat and Chemical Compound in Fires[END_REF]. This leads to a proper fire classification regardless of the test configuration. In contrast, as commented above, the global equivalence ratio, , can overestimate the amount of oxygen available for fuel when considering the inlet air mass flow rate, 𝑚̇𝑎, in Eq. ( 9), which can thus lead to underestimate its evaluation.

| Fire classification

Fig. 29 and Fig. 30 show the MLR and HRR for the CT1/CT2 tests (cable A) and the CT3 to CT5 tests (cable B) respectively. These figures first indicate that both the MLR and HRR of the confined and mechanically-ventilated tests initially grew in a similar way as the fires preliminarily conducted in open atmosphere conditions [START_REF] Zavaleta | Multiple Horizontal Cable Tray Fire in Open Atmosphere[END_REF]. Such an outcome thus reveals no oxygen limitation during the early fire stages. Then, both the MLR and HRR growths for the confined and ventilated cable tray fires suddenly stopped due to an oxygen concentration decrease in the FR (Fig. 16). Consequently, the peaks of both the MLR and HRR are reduced compared with those obtained for the related fire tests performed in open atmosphere conditions. The ratios of the MLR peaks between CT1 and CT2, CT3 and CT4 as well as CT3 and CT5 are about 1.6, 3.0 and 2.8 respectively (Table 6). The ratios of the HRR peaks between CT1 and CT2, CT3 and CT4 as well as CT3 and CT5 are about 3.3, 3.6 and 4.7 respectively (Table 6). These last outcomes indicate that for the three confined and ventilated cable tray fires, the HRR reduction was higher than for the MLR. This implies that a fraction of pyrolysis gases was not involved in the cable tray fire and thus increased the amount of unburnt gases accumulated under the ceiling of the FR or exited to the AR. This outcome is confirmed hereafter through the assessment of the combustion efficiency ratio.

The ratio of the combustion efficiencies of the CT2 and CT1 cable tray fires,  1 , is evaluated from Eq. ( 8) with the corresponding average EHC given in Table 6. So  1 gives 0.58.  2 of the CT4 and CT3 cable tray fires and  3 of the CT5 and CT3 cable tray fires are evaluated in the same way as for  1 and thus give 0.74 and 0.47, respectively. The confined and mechanically-ventilated cable tray fires of the CT2, CT4 and CT5 tests are therefore classified as under-ventilated fires since  < 1 (section 3.4.1).

| Conditions leading to ignite the unburnt hydrocarbon gases (CnHm)

This section proposes to discuss the conditions leading to the ignitions of the unburnt hydrocarbon gases for all the confined tests and also why the two ignition types (either severe or not) were observed. First, the three under-ventilated CT2, CT4 and CT5 cable tray fires ( < 1) released unburnt hydrocarbon gases which accumulated under the ceiling of the FR since there was no outlet duct in this room, as specified in section 2.2. In addition, CnHm mixed with the incoming air from the inlet located in the upper part of the FR (Fig. 5). During the formation of this premixed medium, the cable tray fire was still burning for the three tests (Fig. 25). So, once the premixed gas mixture reached the flammable range, it was very likely ignited by the burning cable trays (its top was located at about 2.2 m from the floor, see Fig. 5). Next, CnHm combustion involved either a limited or a large volume located in the upper part of the FR (Table 4). One of the plausible explanations of this difference would be due to the higher CnHm concentrations obtained before the severe ignitions ( 1), ( 2) and ( 6) than before the other ones ( 3), ( 4) and ( 5). This would indeed increase the premixed gas mixture zone where the flammable range was reached. This observation is notably confirmed when comparing CnHm concentrations for the CT4 and CT5 tests which used the same cable B but a distinct VRR of 15 and 4 h -1 , respectively. The CnHm concentrations were indeed higher before the severe ignition (6) (Fig. 17 (c)) than before the other ignitions (3), ( 4) and (5) (Fig. 17 (b) and (c), respectively). In summary, severe CnHm ignitions occurred for the lower VRR (4 h -1 ) (i.e., a lower  , as seen in Table 6) and especially during the fire decay stage.

Moreover, the ventilation set-up specified for these tests (no outlet duct in the FR and an air inlet located in the upper part of the FR) also contributed to the occurrence of the CnHm ignitions.

| Explosions

Croft [START_REF] Croft | Fires involving Explosions -A literature review[END_REF], Dobashi [START_REF] Dobashi | Studies on accidental gas and dust explosions, IAFSS 12 th Symposium[END_REF] and Zalosh [START_REF] Zalosh | Explosion Protection[END_REF] stated that a gas explosion can be characterized by a fast flame propagation through a premixed gas mixture of combustible gases and air. In addition, these authors also indicated that such explosion may result in a fast increase of the pressure when a confinement prevented the unrestrained expansion of the combustion products. Croft specified that the related overpressures could be in the range 50-100 hPa or higher and that the time to flashover could be of few seconds according to the enclosure size. Therefore, the severe ignitions ( 1) and ( 6) can be considered as explosions. These ignitions are indeed characterized by a fast flame propagation in the entire upper halfpart of the FR where the premixed medium was formed, HRR peaks exceeded 1 MW (i.e., flashover) in few seconds and steep overpressures of almost 150 hPa (see Table 4 and Table 5). Otherwise, for the third severe ignition (2), its classification as an explosion is not as obvious as previously. Indeed, even if the related overpressure was around 70 hPa, the fast flame propagation did not involve all the upper half-part of the FR and the HRR peak was limited to about 0.5 MW (see Table 4 and Table 5). So this study only considers the severe ignitions (1) and (6) as explosions.

12

As part of the OECD PRISME-2 project, cable tray fire tests were performed both in open atmosphere conditions and in a confined and mechanically-ventilated facility, called DIVA. These tests aim at showing the effects of a confined and ventilated environment on fire characteristics and consequences. This study deals with five cable tray (CT) fire tests which used two halogenated (poly(vinyl chloride) or PVC) cable types. Two CT tests, namely CT1 and CT3, were carried out in open atmosphere conditions and three tests, CT2, CT4 and CT5, were performed in the DIVA facility. The five CT1 to CT5 tests used a vertical stack of five horizontal ladder cable trays supported by a wall. The CT1 and CT2 tests used a first PVC cable type, called cable A, while the CT3 to CT5 tests used a second type, named cable B. These two cable types only meet the requirements of the international standard IEC 60332-1-2 test and can be thus considered as low-qualified electric cables. Moreover, the CT2, CT4 and CT5 tests used a ventilation renewal rate (i.e., the number of changes of the room volume per hour, VRR) of 4, 15 and 4 h -1 , respectively. These tests used two rooms of the DIVA facility; a fire room (FR) that contained the five cable trays and an adjacent room (AR). Only one inlet duct was installed in the upper part of the FR and one outlet duct was positioned in the upper part of the AR. Fire spread to the ends of the five cable trays for all the CT fire tests conducted in open and confined conditions, except for the confined CT5 test (VRR of 4 h -1 ). Therefore, the decrease of the VRR (from 15 to 4 h -1 ) can limit the fire spread over the cable trays in confined conditions.

The confined and mechanically-ventilated conditions reduced the mass loss rate (MLR) and heat release rate (HRR) compared with those obtained in open atmosphere conditions. Moreover, this reduction is higher for the HRR than for the MLR. The three confined cable tray fire tests are indeed under-ventilated fires, as confirmed by the assessment of the combustion efficiency ratio. They thus released unburnt hydrocarbon gases which accumulated in the FR and mixed with the incoming air from the inlet located in the upper part of the FR. The premixed medium then ignited twice in the upper halfpart of the FR for each fire test. The gas mixture was very likely ignited by the residual cable tray fire. Severe ignitions of unburnt gases were highlighted for the two less ventilated confined tests (VRR of 4 h -1 ). Two of these severe ignitions can be considered as explosions and occurred during the CT2 and CT5 tests. These explosions are indeed characterized by fast flame propagations over the entire upper half-part of the FR, HRR peaks exceeded 1 MW in few seconds, steep overpressures of almost 150 hPa. Therefore, the use of low-qualified PVC cables, a VRR of 4 h -1 and the set-up of both inlet and outlet as specified in this study, significantly contributed to the occurrence of these explosions. 5: HRR peaks and growth rates of both the cable tray fires and CnHm ignitions (1) to ( 6

List of tables

List of figures



This zone covers all the ceiling of the FR over a gas layer higher than 1 m depth. 20 ̅̅̅̅̅̅̅̅̅ , average effective heat of combustion; , combustion efficiency ratio.

 HRR peak of the cable tray fire.



The higher MLR peak for CT5 test (VRR = 4 h -1 ) than for CT4 test (VRR = 15 h -1 ) is explained by the feed-back effect on the MLR showed for the first test, as discussed in section 3.3.1.



  Fire growth rate (kW/s 2 ) Equivalence ratio (-)  Thermal conductivity (W.m -1 .K -1 )  Density (kg.m -3 )Several hundred kilometres of electric cables are present throughout nuclear power plants (NPPs).

Fig. 25

 25 Fig. 25 therefore gives the final HRR assessed from Eq. (3) for the CT2, CT4 and CT5 tests. Given that 𝑄 ̇𝑡ℎ𝑒𝑟𝑚𝑎𝑙 can generally underestimate 𝑄 ̇ while 𝑄 ̇𝐶𝐷𝐺 tends to over-estimate it, as above discussed, it is thus assumed that 𝑄 ̇ falls in the interval [𝑄 ̇-| 𝑄 ̇𝐶𝐷𝐺 -𝑄 ̇𝑡ℎ𝑒𝑟𝑚𝑎𝑙 2

  ) for the CT2, CT4 and CT5 tests......................................................................................................................... Table 6: Fire characteristics of cable tray fires of the CT1 to CT5 tests. ..............................................

Fig. 1 : 2 : 3 : 4 : 8 : 9 :

 123489 Fig. 1: Vertical stack of five horizontal ladder cable trays. ................................................................... Fig. 2: Loose arrangement of the electric cables (CT2 test). ................................................................. Fig. 3: Electric cables used for the PRISME-2 CT tests. (a). Cable A. (b). Cable B. ............................ Fig. 4: Perspective view of the DIVA facility........................................................................................ Fig. 5: Front view of the experimental facility: Room 1 as the fire room (FR) and Room 2 as the adjacent room (AR). ............................................................................................................................... Fig. 6: Top view of the experimental facility: Room 1 as the fire room (FR) and Room 2 as the adjacent room (AR). ............................................................................................................................... Fig. 7: Enlarged view of the SE corner (FR). (a) Vertical profile of nine thermocouples. (b) Gas sampling probes for the measurement of O2, CO2, CO and CnHm concentrations. ................................ Fig. 8: Two scales below the trays and the heat flux sensor (HF) in the upper part of the FR. ............. Fig. 9: Cable trays during and after the CT2 test. (a) to (e) during fire. (a) t = 30 s. (b) t = 1 min. (c) t = 2 min. (d) t = 3 min. (e) t = 4 min. (f) after fire........................................................................... Fig. 10: Cable trays for the CT4 test. (a) before fire. (b) after fire. ....................................................... Fig. 11: Cable trays for the CT5 test. (a) before fire. (b) after fire. ....................................................... Fig. 12: Gas temperatures in the NW corner of the FR. (a) CT2 test. (b) CT4 test. (c) CT5 test. ......... Fig. 13: Gas temperatures in the NE corner of the FR. (a) CT2 test. (b) CT4 test. (c) CT5 test. .......... Fig. 14: Gas temperatures in the SE corner of the FR. (a) CT2 test. (b) CT4 test. (c) CT5 test. ........... Fig. 15: CO2 concentration in the FR. (a) CT2 test. (b) CT4 test. (c) CT5 test. .................................... Fig. 16: O2 concentration in the FR. (a) CT2 test. (b) CT4 test. (c) CT5 test. ....................................... Fig. 17: CnHm concentration in the upper part of the FR and at the outlet of the AR. (a) CT2 test. (b) CT4 test. (c) CT5 test. ...................................................................................................................... Fig. 18: Soot mass concentrations in the FR and AR. (a) CT2 test. (b) CT4 test. (c) CT5 test. ............ Fig. 19: Soot mass concentrations in the FR and AR measured during additional cable tray fire tests of the OECD PRISME-2 project which used a HFFR cable type [20]. (a) VRR of 4 h -1 . (b) VRR of 15 h - 1 . ............................................................................................................................................................. Fig. 20: Pressure in the FR and AR. (a) CT2 test. (b) CT4 test. (c) CT5 test. ....................................... Fig. 21: Volume flow rates at the inlet of the FR. (a) CT2 test. (b) CT4 test. (c) CT5 test. .................. Fig. 22: Mass loss rate. (a) CT2 test. (b) CT4 test. (c) CT5 test. ........................................................... Fig. 23: Heat flux in the upper part of the FR. (a) CT2 test. (b) CT4 test. (c) CT5 test. ....................... Fig. 24: HRR assessments from the thermal and CDG methods. (a) CT2 test. (b) CT4 test. (c) CT5 test. ......................................................................................................................................................... Fig. 25: Final heat release rate. (a) CT2 test. (b) CT4 test. (c) CT5 test. ............................................... Fig. 26: Growth rates for CT2 test. (a) Cable tray fire. (b) CnHm ignition (1). (c) CnHm ignition (2). ... Fig. 27: Growth rates for CT4 test. (a) Cable tray fire. (b) CnHm ignition (3). (c) CnHm ignition (4). ... Fig. 28: Growth rates for CT5 test. (a) Cable tray fire. (b) CnHm ignition (5). (c) CnHm ignition (6). ... Fig. 29: CT1 and CT2 tests (cable A). (a) Mass loss rate. (b) Heat release rate. ................................... Fig. 30: CT3 to CT5 tests (cable B). (a) Mass loss rate. (b) Heat release rate. ......................................

Fig. 1 :Fig. 2 :

 12 Fig. 1: Vertical stack of five horizontal ladder cable trays.

Fig. 3 :Fig. 4 :

 34 Fig. 3: Electric cables used for the PRISME-2 CT tests. (a). Cable A. (b). Cable B.

Fig. 5 :

 5 Fig. 5: Front view of the experimental facility: Room 1 as the fire room (FR) and Room 2 as the adjacent room (AR).

Fig. 6 :

 6 Fig. 6: Top view of the experimental facility: Room 1 as the fire room (FR) and Room 2 as the adjacent room (AR).

Fig. 7 :Fig. 8 : 29 Fig. 9 :

 78299 Fig. 7: Enlarged view of the SE corner (FR). (a) Vertical profile of nine thermocouples. (b) Gas sampling probes for the measurement of O2, CO2, CO and CnHm concentrations.

Fig. 10 :

 10 Fig. 10: Cable trays for the CT4 test. (a) before fire. (b) after fire.

Fig. 11 :Fig. 12 :

 1112 Fig. 11: Cable trays for the CT5 test. (a) before fire. (b) after fire.

Fig. 13 :

 13 Fig. 13: Gas temperatures in the NE corner of the FR. (a) CT2 test. (b) CT4 test. (c) CT5 test.

Fig. 14 :

 14 Fig. 14: Gas temperatures in the SE corner of the FR. (a) CT2 test. (b) CT4 test. (c) CT5 test.

Fig. 15 :

 15 Fig. 15: CO2 concentration in the FR. (a) CT2 test. (b) CT4 test. (c) CT5 test.

Fig. 16 :

 16 Fig. 16: O2 concentration in the FR. (a) CT2 test. (b) CT4 test. (c) CT5 test.

Fig. 17 :

 17 Fig. 17: CnHm concentration in the upper part of the FR and at the outlet of the AR. (a) CT2 test. (b) CT4 test. (c) CT5 test.

Fig. 18 :

 18 Fig. 18: Soot mass concentrations in the FR and AR. (a) CT2 test. (b) CT4 test. (c) CT5 test.

Fig. 19 :Fig. 20 :

 1920 Fig. 19: Soot mass concentrations in the FR and AR measured during additional cable tray fire tests of the OECD PRISME-2 project which used a HFFR cable type. (a) VRR of 4 h -1 . (b) VRR of 15 h -1 .

Fig. 21 :

 21 Fig. 21: Volume flow rates at the inlet of the FR. (a) CT2 test. (b) CT4 test. (c) CT5 test.

Fig. 22 :

 22 Fig. 22: Mass loss rate. (a) CT2 test. (b) CT4 test. (c) CT5 test.

Fig. 23 :

 23 Fig. 23: Heat flux in the upper part of the FR. (a) CT2 test. (b) CT4 test. (c) CT5 test.

Fig. 24 :

 24 Fig. 24: HRR assessments from the thermal and CDG methods. (a) CT2 test. (b) CT4 test. (c) CT5 test.

Fig. 25 :

 25 Fig. 25: Final heat release rate. (a) CT2 test. (b) CT4 test. (c) CT5 test.

Fig. 26 :

 26 Fig. 26: Growth rates for CT2 test. (a) Cable tray fire. (b) CnHm ignition (1). (c) CnHm ignition (2).

Fig. 27 :

 27 Fig. 27: Growth rates for CT4 test. (a) Cable tray fire. (b) CnHm ignition (3). (c) CnHm ignition (4).

Fig. 28 :

 28 Fig. 28: Growth rates for CT5 test. (a) Cable tray fire. (b) CnHm ignition (5). (c) CnHm ignition (6).

Fig. 29 :

 29 Fig. 29: CT1 and CT2 tests (cable A). (a) Mass loss rate. (b) Heat release rate.

Fig. 30 :

 30 Fig. 30: CT3 to CT5 tests (cable B). (a) Mass loss rate. (b) Heat release rate.

  Average of 𝑢 ̃ is finally assessed over time intervals of [0 s; 1500 s] for CT2, [0 s; 2500 s] for CT4 and [0 s; 2000 s] for CT5, which gives 29, 15 and 27 %, respectively. In contrast, for time beyond previous time intervals, values 𝑄 ̇ are low (Fig.25), which leads to high values of 𝑢 ̃. In this case, it is more suitable to assess (absolute) uncertainty 𝑢 of 𝑄 ̇. Therefore, for time intervals of [1500 s; 2000 s] for CT2, [2500 s; 3000 s] for CT4 and [2000 s; 3000 s] for CT5, average of 𝑢 is assessed lower than 50 kW for all the tests.

Table 1 :

 1 Cable tray (CT) tests of the OECD PRISME-2 project considered in this study. ...................

Table 2 :

 2 Electric cables A and B used for the CT1/CT2 and the CT3/CT4/CT5 tests, respectively. ....

Table 3 :

 3 Thermophysical properties at ambient temperature (20 °C) of the outer sheath for the cable B. ................................................................................................................................................................

Table 4 :

 4 Consequences of the CnHm ignitions highlighted for the CT2, CT4 and CT5 tests. ................ Table

Table 1 :

 1 Cable tray (CT) tests of the OECD PRISME-2 project considered in this study.

	Test ID 6	Cable ID	Conditions
	CT1	Cable A	Open atmosphere
	CT2	Cable A	CV (VRR = 4 h -1 )
	CT3	Cable B	Open atmosphere
	CT4	Cable B	CV (VRR = 15 h -1 )
	CT5	Cable B	Confined (4 h -1 ) CV (VRR = 4 h -1 )
	Abbreviations: CT, cable trays; CV, confined and mechanically-ventilated; VRR, ventilation
	renewal rate.

Table 2 :

 2 Electric cables A and B used for the CT1/CT2 and the CT3/CT4/CT5 tests, respectively.

	Cable ID	Cable material	Outer diameter (mm)	Linear mass density (kg/m)	Supplier reference
	Cable A	Poly(vinyl chloride), phthalates and mineral	13	235	MCMK 3x2.5 mm 2
	Cable B	loads such as calcium carbonate (CaCO3)	14.5	330	SHCVV 8x2 mm 2

Table 3 :

 3 Thermophysical properties at ambient temperature (20 °C) of the outer sheath for the cable B.

	Thermophysical properties	Outer sheath
	Specific heat, c (J.kg -1 .K -1 )	1280
	Density, ρ (kg/m 3 )	1336
	Thermal conductivity, λ (W.m -1 .K -1 )	0.156
	(W/cm.K) (W/cm.K)

Table 4 :

 4 Consequences of the CnHm ignitions highlighted for the CT2, CT4 and CT5 tests.Values of measured flow rate at the inlet were negative (i.e., in the reversed direction than the nominal ventilation flow rate) when smoke exited from the FR caused by the overpressure.Nominal value of the ventilation flow rate at the inlet.

	CnHm ignition	Test ID	Pressure peak (hPa)	Minimal flow rate at the inlet (m 3 /h)	Combustion zone of the CnHm in the the FR
	1	CT2	145	-4810  (960  )	Entire upper part 
	2	CT2	72	-3010 (960  )	In the SE and NW corners, under the ceiling
	3	CT4	8	2720 (3600  )	In the NW corner, under the ceiling
	4	CT4	12	2290 (3600  )	In the NW corner, under the ceiling
	5	CT5	10	210 (960  )	In the NW corner, under the ceiling
	6	CT5	135	-4710 (960  )	Entire upper part 
	



Table 5 :

 5 HRR peaks and growth rates of both the cable tray fires and CnHm ignitions (1) to (6) for the CT2, CT4 and CT5 tests.

	Test ID	Growth rate,  (kW/s 2 )	Growth characteristic time, 𝐭 𝟏 (s)	HRR peak (MW)
	CT2
	Cable tray fire	0.16	79	0.98
	CnHm ignition (1)	5.6	13.4	1.34
	CnHm ignition (2)	5.5	13.5	0.54
	CT4
	Cable tray fire	0.022	213	0.75
	CnHm ignition (3)	0.22	67	0.31
	CnHm ignition (4)	0.23	66	0.50
	CT5
	Cable tray fire	0.037	164	0.57
	CnHm ignition (5)	0.08	112	0.45
	CnHm ignition (6)	35	5.35	1.75

Table 6 :

 6 Fire characteristics of cable tray fires of the CT1 to CT5 tests.

	Test ID	Cable ID Conditions	MLR peak (g/s)	HRR peak (MW) 	̅̅̅̅̅̅̅̅̅̅ ∆𝑯 𝒄,𝒆𝒇𝒇 (MJ/kg)	 (-)
	CT1	Cable A	Open	156	3.20	22.5	-
	CT2	Cable A	CV (4 h -1 )	100	0.98	13	0.58
	CT3	Cable B	Open	176	2.70	17	-
	CT4	Cable B	CV (15 h -1 )	58	0.75	12.5	0.74
	CT5	Cable B	Confined (4 h -1 ) CV (4 h -1 )	62 	0.57	8	0.47
	Abbreviations: CT, Cable trays; CV, Confined and mechanically-ventilated; MLR, mass loss rate; HRR, heat
	release rate; ∆𝐻 𝑐,𝑒𝑓𝑓

These two cable types do not meet the requirements of the C1 test of the French standard NF C 32-070[START_REF] Nf C | Classification tests for conductors and cables in terms of their fire behavior[END_REF] (they failed this test) and they are not ranking according to the newly EUROCLASS standard(EN 13501-6 [23]) applicable in Europe. Indeed, cable B was manufactured in Japan. Moreover, cable A was produced in Europe but before

when the EUROCLASS standard was not yet fully applicable.

For the CT2 test, the decrease (2) of C n H m concentration in the FR was not observed since the related analyzer

As specified in[30].

Initial names of the CT1 to CT5 tests, as specified as part of the OECD PRISME-2 project, were CFSS-1, CFS-1, CFSS-4, CFS-2 and CORE-7 respectively.

ACKNOWLEDGMENT

This work was performed by IRSN in the framework of the OECD PRISME-2 project with the support of the partners: ENGIE and Bel V (Belgium), CNSC (Canada), VTT (Finland), DGA and EDF (France), GRS (Germany), NRA and CRIEPI (Japan), CSN (Spain), SSM (Sweden) and ONR, an agency of the HSE (United Kingdom).