Pascal Zavaleta

Sophie Bascou

Sylvain Suard

EFFECTS OF CABLE TRAY CONFIGURATIOI\ ON FIRE SPREAD

Keywords: Protected cable tray, Slanted ladder cable tray, Ignition, Fire spread

Fires involving electrical cables are one of the main fire hazards in Nuclear Power Plants (NPPs). The aim of this work is to study the impact of cable tray configuration on fire spread over multiple cable trays. Two real-scale cable tray fire tests were thus carried out as part of the OECD PRISME-2 Project (CORE campaign) for studying the effect of a protected cable tray (CORE-2) and slanted ladder cable trays on the main fire characteristics. By considering new configurations, these tests completed the preliminary CFSS tests of the PRISME-2 project which involved five horizontal ladder cable trays. The CORE-2 test implemented the same horizontal trays set-up except that the lower tray was a protected cable tray. Compared with the five ladder cable trays set-up, the protected cable tray delayed the ignition of about l6 min and led to decrease the total mass loss (TML).

Furthermore, the fire growth rates and the heat release rate (HRR) were slightly higher for the CORE-2 test. The CORE-3 test used five horizontal ladder cable trays I m long followed by five slanted ladder cable trays 2 m long with a 30 degree angle. This configuration shortened the ignition of about 4 min, led to similar TML and increased the fire spread over the cable trays meaningfully and the HRR peak compared with the horizontal ladder trays configuration. Finally, the other fire characteristics such as the average effective heat of combustion or the gas and soot yields were not affected by the presence of the lower protected cable tray or the ladder cable trays orientation (horizontal or slanted).

Ideal gas constant (8.314 J/mole/K) Starting time of the power law equation describing the growth of the fire (s) Fire growth characteristic time (s) Gas temperature (K) Total mass loss (kg) Molar fraction of gas species i: Oz,COz, CO and HzO (mole/mole) Greek characters Fire growth rate (kWs2)

INTRODUCTION

Several hundred kilometers of electrical cables are present throughout nuclear pov/er plants (NPPs). 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 electrical cabinets connected to multiple cable traysr. 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 oftransducers, while control cables serve for example for controlling valves or operating relays and contactors.

A serious cable fire occurred at the Browns Ferry NPP in 19752 resulting in the loss of the emergency core cooling system of unit l. Ever since many efforts have been made on the most recent nuclear installations, all over the world, to enhance the prevention of cable fires, for instance by using flame retardant materials in the cable composition. Nevertheless, nearly seventy fire events from NPPs involving electrical cables as fuel were recorded in the current OECD FIRE Database3 between the late 1980s and the end of 2014. The study of cable fire is therefore a major concern for fire safety analyses in NPPs. Many cable tray fire programmes were carried out over the last decadesa '5'6'7'8'e and also more recentlyl0'll,t2'13'14'ts. All these last ones implemented ladder cable trays either horizontally or vertically oriented. However, the NPP tray installations may also feature slanted ladder cable trays or protected cable trays. These last ones are used for the fire protection of electrical cables involved in safety trains circuits. Nevertheless, no available programme has yet studied the impact of such cable tray configurations on fire spread.

Two real-scale cable tray fire tests were thus carried out as part of the OECD PRISME-2 Projectr6 (CORE campaign) for studying the effect of a protected cable tray (CORE -2) and slanted ladder cable trays (CORE-3) on the main fire characteristics. By considering new configurations, these tests completed the CFSS-I to CFSS-4 testsr3 (PzuSME-2) project which investigated fire spread over five horizontal ladder cable trays according to various cable-types. The CORE-2 test implemented the same horizontal trays set-up except that the lower tray was a protected cable tray. This last one was a metallic non-perforated and covered cable tray. Furthermore, the CORE-3 test involved five horizontal ladder cable trays I m long followed by five slanted ladder cable trays 2 m long with a 30 degree angle. The c 0 0 n EX G in CORE-2 and CORE-3 tests used the same cable-type, containing a halogen free flame retardant (HFFR), as the CFSS-2 test and they were conducted in open atmosphere under a large-scale calorimeter.

Firstly, this paper gives a description of the cable tray fire sources, the large-scale calorimeter of the SATURNE facility and the instrumentation used for the tests. Then, the assessments of the main fire characteristics of the CORE-2 and CORE-3 tests, such as the mass loss rate (MLR), the heat release rate (Iil,R) and the effective heat of combustion (EHC) are showed in detail. Furthermore, the evaluations of additional fire characteristics such as the ignition delay, the fire growth rate and the fire duration, all derived from the HRR, are also proposed in this section.In the last part of this paper, the effects of the cable tray configuration are highlighted from comparative analysis of the previous fire properties with the ones obtained for the horizontal ladder trays set-up of the CFSS-2 test. Finally, the evaluation of the gas and soot yields, proposed in this last part, complete the above comparative analysis.

DESCRIPTION OF'THE X'IRE TESTS

Cable tray fire sources

The three cable hay fire sources used forthe preliminary CFSS-2 test, the CORE-2 and CORE-3 tests had the same following characteristics: o Five ladder cable trays 3 m long, 0.45 m wide, spaced from each other by 0.3 m and set up against an insulated side wall (Figure 1), . Every tray was filled with 32 samples of 2.4 m long of the same cable-type (its mass per unit length is about 570 kg/km). This cable-type contains Poly(ethylene/vinyl acetate) (EVA) and Polyethylene (PE) as polymeric materials and aluminium hydroxide (ATH) as halogen free flame retardant (HFFR), o The cable samples were packed loosely along the five trays.

The differences between the three fire sources are listed hereafter: r For the CORE-2 test, the lower cable tray was a metallic non-perforated and covered cable tray (Figure 2), o For the CORE-3 test, the fire source was composed of five horizontal ladder cable trays 1 m long, followed by five slanted ladder cable trays 2 m long, with a 30 degree angle (Figure 3). The cable samples length was 0.8 m along the first part while the one along the slanted trays was 1.6 m. for the CORE-2 test.

2:

The cable set Ignition source A square sand burner (300 x 300 mm2), located 0.2 m below the lower tray, was used as an ignition source (Figure 2). This gas burner was supplied with propane gas and its fire power was 80 kW for all the tests. Furthermore, the gas burner was centred under the first cable tray for both the CFSS-2 and CORE-2 tests while it was centred beneath the horizontal part of the lower tray for the CORE-3 test (Figure 3). The ignition source was stopped when the HRR exceeded 400 kW (without taking into account the contribution of the gas bumer). Given that, this value is five times higher than the fire po\ryer of the gas burner (80 kW), the cable tray fire source is thus considered as ignited and the fire as self- sustained. The ignition delay (tig) of the cable tray fire source is thus defined as the time to reach a HRR of 400 kW.

Large-scale calorimeter and instrumentation

The tests were carried out under a large-scale calorimeter (Figure 4) located in a facility, named SATURNE. Numerous openings are located at the top of this facilþ over each wall. They provide enough air to maintain the oxygen concentration at2l %o around the fire source, as in open atmosphere.

The fire source is centred under the large-scale hood (4.5 x 4.5 m2 side by side with a 3 m inner diameter, Figure 4). This last one is connected to an exhaust ventilation network which collects all the combustion products released by the fire. Gas species concentrations (O2, COz and CO) as well as soot mass concentrations, gas temperature, pressure and volume flow rate were measured in the exhaust network.

Two weighing devices were located below the cable trays (Figure 4) and allowed measuring the mass loss of materials contained in cables during the fire. For all the tests, five thermocouples were positioned along each of the hays directly above the cables (or just above the metallic cover of the lower tray for the CORE-2 test). All the thermocouples were spaced from each other of 0.5 m. In addition, for the CORE-2 test, three thermocouples were also located inside the protected cable trayjust above the cables.

They were positioned at the centre and at 0.5 m from the centre in both South and North directions.

for the CORE-3 test. The mass loss (ML) is deduced from the mass measurement carried out with the weighing devices. In addition, the mass loss rate (MLR) is assessed from time derivation of the ML. Both the ML and the MLR are shown in Figure 5 for the CORE-2 and CORE-3 tests. This figure also indicates the total mass losses (TML) of 73 kg and93 kg as well as the MLR peaks of 78 and 92 {s, for the CORE-2 and CORE-3 tests respectively. The fire heat release rate (HRR) can be assessed with fwo chemical methods, which are the carbon dioxide generation (CDG) and the oxygen consumption (OC) calorimetry methods.

For the CORE-2 and CORE-3 tests, conducted in open atmosphere, negligible fraction of carbon in the fuel is converted to CO or soot instead of COz, as confirmed by their yields assessed in a further section.

Consequently, the HRR may be assessed from the CDG calorimetry method (Qcoò as followsrT:

tll Qcoc = Erg,ñ\o,

. Ecor: Energy produced by mass unit of generated COz (13.3 MJ/kg);

. th\or: Mass flow rate of generated COz (kg/s).

In addition, thfo, is evaluated from: . Tex: Gas temperature in the exhaust network (K);

. tf*: Volumetric flow rate in the exhaust network (m3/s).

Moreover, the HRR is also evaluated from the OC calorimetry method (Qoò.For negligible fraction of carbon in the fuel converted to CO or soot instead of COz, the HRR may be assessed from the OC calorimetry method (Qoc) as followsr8: . th"x: mass flow rate in the exhaust network (kds);

t4l Qoc = ro, ffi ^*H (-x#,o -xib.) x'ü
. Moz: Molecular mass of Oz (0.032 kg/mole);

. Ma:Molecularmass ofthe ambient air (0.029 kg/mole); . X!": Molar fraction of gas species (i: COz, HzO and Oz) in the incoming air (mole/mole);

. Xf' : Molar fraction of gas species (i : COz and Oz) in the exhaust network (mole/mole).

Heat release rate for the CORE-2 test

The HRR assessments from the CDG (QcDc) and OC (Qoò calorimetry methods are pointed out in Figure 6 (including the gas burner contribution to the HRR) and show very good agreement. So, the final CORE-2 HRR (0, Figure 7) is evaluated from the average values of Qcoe and Qor, minus the fire po\¡ier of the gas burner (Qaurn"r), as follows:

t6l

Q=(Qro"+ Qoò/T-Quu,,",
The main stages of the CORE -2 ftre test are the ignition stage, the fire growth stage, the fully developed fïre stage and the fire decay stage. They are described hereafter based on the HRR assessment (Figure 7).

Ignition stage (27 min 34 s)

For the CORE-2 test, the gas burner provided a fire power of 80 kW for l72l s (28 min 41 s) and led to ignite the cable tray fire source at27 min 34 s (ttg) when the HRR reached 400 kW (Figure 8).

Fire growth stage (from 27 min 34 s lo 35 mìn 54 s)

It is assumed that the following power law equation can describe the growth of the . to,starting time of the power law equation describing the growth of the fire (s); ¡ tr: fire growth characteristic time (s). tr is defined as the time necessary after to to reach a HRR of 1000 kW (s).

Figure 8 points out that the best power law equation that fits with the HRR growth of the CORE -2 fire is y -0.0032 . (t -7320)'. So according to equations l7)-I91, the fire growth rate (a) is 0.0032 kWs2, tr: 558 s (9 min l8 s) and t,: 1320 s (22 min). It is considered for the CORE-2 test that the fully developed fire stage started when the HRR exceeded the value of 2 MW. Indeed the fire nearly involved all the cable tray surface area for such value of the HRR (Figure 9). Moreover, it is supposed that this stage ended when the HRR reached for the last time the previous value. According to the HRR of the CORE-2 test (Figure 7), the fully developed fire stage thus started at 35 min 54 s (2154 s) and ended at 38 min 55 s (2335 s). The HRR peak (2380 kW) was reached during this stage at37 min22 s.

Fire decay stage (from 38 min 55 s to 97 min 22 s)

The fire decay stage for the CORE-2 fire started after the fully developed fire stage at 38 min 55 s (Figure 7). Furthermore, the fire is supposed to be extinguished once its fire power is lower than25 kW. This value is indeed similar to the uncertainty of the HRR assessed from the chemical methods considered in this study.

The CORE-2 fire is therefore extinguished at97 min22 s since this time matches with a HRR of 25 kW (Figure 7). As it is considered that the CORE-2 fire starts with the gas burner operation (t: 0 s), then its fire duration is therefore of 97 min 22 s.

Heat release rate for the CO -3 test For the CORE-3 test, the HRR evaluations from the CDG (Qcrò and OC (Qoò calorimetry methods are also very relevant until t: 2000 s (Figure 10). Beyond this time, the two calorimetry methods show different values. In particular, the OC method gives unrealistic values, greater than zero for t > 4077 s, which is the time when the fire is over according to the CDG method (see further for more details). Analysis of the experimental results indicates a slight drift of the oxygen analyzer occurring in the last stage of the CORE-3 test. Indeed, this analyzer pointed out after the test an oxygen concentration in the exhaust duct (20.78o/ovol) slightly lower than the one measured before the test (20.91 %ovol). This lower oxygen content is therefore interpreted as a continuing fire by the OC method, thus associating it with a fictive heat release.

Thus, it is proposed to evaluate the CORE-3 HRR (0, Figure l1) only from the CDG method, minus the fire power of the gas burner:

Q=Qcne-Qburn", t10l

The same fire stages as for the CORE-2 are now described hereafter for the CORE-3 test (Figure l1)

Ignition stage [START_REF] Mangs | Full-scale fire experiments on vertical and horizontal cable trays[END_REF] min 45 s)

For the CORE-3 test, the gas burner provided a fire power of 80 kW for 468 s [START_REF] Mangs | Full-scale fire experiments on vertical and horizontal cable trays[END_REF] min 48 s) and allowed the ignition of the cable tray fire source at7 min 45 s (trg) when the HRR reached 400 kW (Figure l2).

Fire growth stage (from 7 min 45 s to 13 min 38 s)

Figure 12 also exhibits that the best power law equation which fits with the HRR growth of the CORE-3 fire test is y = 0.0073 ' (t -255)2. So according to equations l7l-191, the fire growth rate (ø) is 0.0073 kWs2, tt:371 s (6 min I I s) and t,: 255 s (4 min 15 s).

Fully developedlire støge (from 13 min 38 s to 20 min 53 s)

As for the CORE-2 test, it is assumed that the fully developed fire stage started when the HRR exceeded the value of 2 MW. Indeed the fire spread almost over all the cable tray surface area for such value of the HRR (Figure l3). This stage is also supposed ended when the HRR was lower than 2 MW. Given the HRR of the CORE-3 test (Figure 1l), this stage therefore started at 13 min 38 s (818 s) and finished at 20 min 53 s (1253 s). The HRR peak (2550 kW) was achieved during this stage at 17 min23 s.

Fire decøy stage (from 20 min 53 s to 66 min 57 s)

The fire decay stage for the CORE-3 fire test started after the fully developed fire stage at 20 min 53 s and ended at 66 min 57 s (Figure I l). The fire is assumed to be extinguished once its fire power is lower than 25 kW (as for the CORE-2 test). The CORE-3 test is thus over at 66 min 57 s since this time matches with a HRR of 25 kW (Figure I l). As it is considered that the CORE-3 fire starts with the gas burner operation (t : 0 s), then its fire duration is therefore of 66 min 57 s. The combustion behaviour of a fire may be characterized by the effective heat of combustion (EHC or Aí'"rf) defined as the ratio of HRR (0) to MLR (m) '8: tl ll aír,"r¡ -* Figure 14 therefore points out the EHC evaluated from equation I l] and also the MLR assessed above for the CORE-2 and CORE-3 tests. First, this figure highlights that the EHC is quite low in the early stage of these two tests. Indeed, the highly endothermic dehydration of the flame retardant ATHIe'20, contained in the cable-type studied, delays the burning of the polymeric materials (EVA and PE blend) in particular during the first stage of the tests. Then the EHC continuously increases given the fire spread over all the cable trays area which leads to the progressive consumption of the ATH flame retardant. The EHC after the fire peak (ranging from 30 to about a0 MJ/kg) is indeed consistent with the EHC of the EVA (about 35 MJ/kg2t '22) andthe PE (43 MJ/kg23).

Figure 14 indicates that the EHC shows fast and steep fluctuations for the CORE-2 test when t > 3000 s and for the CORE-3 test when t> 1920 s. These variations are caused by non-negligible fluctuations of the MLR as well as rather low values of this last one (lower than 15 g/s and l8 g/s for the CORE-2 and CORE-3 tests, respectively). Indeed, the EHC is evaluated according to the inverse of the MLR (equation [11]). So it is suggested to assess the average E}{C (LHc,e1Ð for 1600 s < t < 3000 s when MLR > l5 g/s for the CORE-2 test and for 425 s < t < 1920 s when MLR > 18 g/s for the CORE-3 test (Figure 14). These MLR values are higher than 20 Yo of the MLR peak for the two tests since these last ones are of 7 8 g/s for the CORE-2 test and of 92 gls for the CORE-3 test (Figure 14). The average EHC is thus assessed at28.6 MJ/kg for the former test and at29.2 MJ/kg for the latter test. The effect of the cable tray configuration on the fire characteristics previously determined are highlighted from comparison with the ones obtained for the CFSS-2 testr3 (five horizontal ladder cable trays). These last ones are summarized in Table 1. This table firstly indicates similar TML for the CORE-3 (93 kg) and CFSS-2 (97 kg) tests. Indeed, for these two tests the fire involved entirely the five ladder cable trays which were filled with the same cable loading (32 cable samples per tray). By contrast, the TML is lower for CORE-2 (73 kg). For this test, the three gas temperatures measured inside the lower protected cable tray are clearly lower (Figure l5) than the maximal temperatures measured along the burning upper trays (as seen further in Figure 19). Indeed, the cable samples contained inside the lower protected tray showed damages only over a limited portion (Figure l6). These outcomes suggest that the mass loss for this first hay is significantly lower than for the first ladder cable tray of the CORE-3 and CFSS-2 tests, thus leading to the lower TML pointed out above for the CORE- 2 test.

The HRR as well as the ignitions delays of CORE-2, CORE-3 and of CFSS-2 tests are summarized in Figure 17. This figure and Table I firstly highlight that the protected cable tray delays of about l6 min the ignition of the CORE-2 ftre source compared with the five ladder cable trays (CFSS-2). The non-perforated and covered lower tray, located just 0.2 m above the gas burner (Figure 2), acted as a protection screen for the upper ladder cable trays. Compared with the CFSS-2 test (five horizontal ladder cable trays), the average gas temperatures measured along each of the four upper cable trays were indeed significantly lower before the ignition of the CORE-2 cable tray fire source occurring at27 min 34 s (Figure l9). These temperatures are evaluated as the average of the measurements carried out by the five thermocouples located along each cable tray.

By contrast, the slanted cable trays configuration shortened by about 4 min the ignition of the CORE-3 fire source (Figure l7) compared with the five horizontal ladder cable trays (CFSS-2). The total heat required for the ignition of the cable tray fire source is obtained from the product of the fire duration of the ignition source by its fire power (80 kW). A total heat of 37 MJ was therefore required to ignite the CORE-3 fire source vs 60 MJ for the CFSS-2 test (about one and a half times higher) and 138 MJ for the CORE-2 test (nearly four times larger). In addition, the best power law equations fitting with the HRR of these three tests are also summarized in Figure 18. This figure and Table I indicate that the fire growth rate of the CO -2 test (0.0032 kV//s2) is lightly higher than for the CFSS-2 test (0.0022 kWs2). It means that the fire spreads slightly faster over the four upper ladder cable trays for the CORE-2 test than over the five ladder cable trays for the CFSS-2 test. Furthermore, Figure 18 points out that the slanted cable tray configuration leads to clearly increase the fire growth rate. This last one is indeed more than three times higher for the CORE-3 test (0.0073 kWs2) than for the CFSS-2 test (0.0022 kV//s2). This outcome is consistent with the faster increase of the average gas temperatures for the CORE-3 test compared with the CFSS-2 test, except for the lower cable tray (Figure 20). Compared with the horizontal cable tray configuration (CFSS-2 test), the slanted tray orientation (30" angle) of the second part of the CORE-3 fire source could indeed favors the pre-heating of the cables by the plume. This effect is indeed supposed to increase the spread rate of the fire as pointed out by previous cable tray fire experimentsr2. ln addition, compared with the CFSS-2 test, the slight increase of the fire growth rate for the CORE-2 test could also be due to a higher pre-heating of the cables before they ignite, as a consequence of the much longer operation of the gas burner for the latter test (28 min 4l s vs l2 min 24 s for the former test). In addition, the HRR peak f'or the CORE-2 test (2.38 MrW, Table l) is slightly higher than tor the CFSS-2 test (2.20 MW, Table l) and the one obtained for the CORE-3 test (2.55 MW, Table 1) is the highest compared with the former tests. This last outcome suggests for these tests that the HRR peak may increase slightly when the fire growth rate increases (Table 1). Moreover, the fire duration, that includes the gas burner operation time, is equivalent for the CORE-3 hre (about 67 min, Table 1) and the CFSS-2 fire (about 72 min, Table l). Otherwise the fire duration is significantly higher for the CORE-2 fire (about 97 min) due to the much higher time needed by the gas burner for its ignition.

Furthermore, the gas and soot yields (Y¿) are given in Table 2 for the CFSS-2, CORE-2 and CORE-3 tests. I/¿ is defined as the ratio of mf to TML: ll2l

u. -^f L--r 'írut mf : Total mass of gas or soot species (i : COz, CO and soot) generated by the cable tray fire (kg). For COz, m\o, is evaluated according to: t13l m\o" : I!:;"t""0n3o, -ff)at

Where itfo,isobtained from equation [3]. The mass flow rate of COz generated by the gas burner (.

"

) is removed from rhfs, for proper evaluation of the total mass of COz only generated by the cable tray fire (m\o).

Furthermore, for CO and soot, as their generation by the gas burner is supposed negligible, mf is therefore assessed as follows:

tl4l mf = I::iot*" nf at for i: Co and soot

Moreover, these species are also assumed negligible in the incoming air (rh| = 0, for i: CO and soot). So, ncro is obtained from equation [5]:

th\o = rir"{o = X¿5ffi f' [15]
andmfoo¡ from equation [6] t16l with:

ñFot-ñ3lor=cllotf' Table 2 first points out that the COz yields are much higher than the CO and soot yields for the three tests.

This outcome validates the assumptions considered for the HRR assessment from the CDG and OC calorimetry methods, as previously discussed. In addition, the COz, CO and soot yields are similar for the three tests (Table 2). Finally, the evaluations of the avetage EHC for the CFSS-2, CORE-2 and CORE-3 fires give close values (from 28 to 29 MJlkg, Table l). The COz, CO and soot yields as well as the average EHC are thus not affected by the presence of the lower protected cable tray or the ladder cable trays orientation (horizontal or slanted). The CORE-2 test implemented the same horizontal trays set-up as for the CFSS-2 test except that the lower tray was a metallic protected cable tray. This last one \¡/as a non-perforated and covered cable tray which delayed the ignition of about 16 min compared with the CFSS-2 test. This lower cable tray indeed acted as a protection screen for the upper ladder cable trays. In addition, the total mass loss (TML) was lower for the CORE-2 test (73 kg) than for the CFSS-2 test (97 kg). Indeed, the cable samples contained inside the lower protected cable tray showed damages only over a limited portion. Furthermore, the fire growth rate (0.0032 kWs2) for the CORE-2 test was slightly higher than for the CFSS-2 test (0.0022 kWs2). The longer gas burner operation (about 29 min for the CORE-2 test vs about 12 min for the CFSS-2 test) could indeed lead to a greater pre-heating ofthe electrical cables before they ignite. This effect is indeed supposed to increase the spread rate ofthe fire.

The CORE-3 test involved the same horizontal cable trays set-up I m long as for the CFSS-2 test, followed by five slanted ladder cable trays 2 mlong, with a 30 degree angle. Compared with the horizontal trays setup (CFSS-2 test), the slanted trays configuration shortened by about 4 min the ignition of the multiple cable trays and also clearly increased the fire growth rate. This last one is indeed assessed as more than three times higher for the CORE-3 test (0.0073 kWs2) than for the CFSS-2 test (0.0022 kWs2). The slanted cable trays orientation (30' angle) of the second part of the CORE-3 fire source could indeed favor the pre-heating of the cables by the plume and therefore the spread rate of the fire. The heat release rate (HRR) peak was also shown higher for this test (2.55 MW) compared with the CFSS-2 test (2.2 MW). This outcome, also observed for the CORE-2 test, suggests for these tests that the HRR peak may increase when the fire growth rate increases. Furthermore similar TML was obtained for the CORE-3 test (93 kg) and the CFSS-2 test (97 kg). Indeed, for these two tests, the fire involved entirely the five ladder cable trays which were filled with the same cable loading (32 cable samples per tray).

Finally, the other fire characteristics such as the average effective heat of combustion or the gas and soot yields were not affected by the presence of the lower protected cable tray or the ladder cable tra¡rs orientation (horizontal or slanted).

Further experiments involving similar cable tray fire sources will be characterized in a confined and mechanically-ventilated facility. These investigations will allow for highlighting the effect of confined and ventilated conditions on fire spread over these fire sources.

Figure 1 :

 1 Figure 1: The five horizontal ladder cable

  Figure 4: The large-scale calorimeter of the SATURNE

Figure 5

 5 Figure 5: ML and MLR.

  l2l,it\o,: rh"{o, -rhfo, Mass flow rate of COz in the exhaust network (kds); Mass flow rate of COz in the incoming air (kds).

  is shown thatm[6.may be assessed as follows t3l th1o, = (x36,-xib)% f. . X"c6.: Molar fraction of COz in the exhaust network (mole/mole); . Xlo": Molar fraction of COz in the incoming air (mole/mole); o M cort Molecular mass of CO z (0.044 kglmole); o Pex: Pressure in the exhaust network (Pa); o R: Ideal gas constant (8.314 J/mole/K);

  with

  t Energy produced by mass unit of consumed Oz (13.1 MJ/kg); . 0: Oxygen depletion factor; o 0: volumetric expansion factor ( I . 105);

Figure 6 :

 6 Figure 6: HRR assessed from the CDG and OC calorimetry methods (CORE-2 test).

Figure 7 :

 7 Figure 7: HRR and the main fire stages of the CORE-2 test.

Figure 8 :

 8 Figure 8: Ignition and fire growth stages of the CORE-2 test.

Figure 9 :

 9 Figure 9: Start of the fully developed fire stage at about 36 min (CORE-2 test).

Figure l0 :

 l0 Figure l0: HRR assessed from the CDG and OC calorimetry methods (CORE-3 test).

Figure

  Figure I l: HRR and the main fire stages of the CORE-3 test.

Figure 12 :

 12 Figure 12: Ignition and fire growth stages of the CORE-3 test.

Figure l3 :

 l3 Figure l3: Starting of the fully developed fire stage at about 13.5 min (CORE-3 test).

  and MLR EF'FECT OF THE CABLE TRAY CONF'IGURATION

Figure 15 :

 15 Figure 15: Gas temperature inside the lower protected trav (CORE-2 test).

Figure 16 :

 16 Figure 16: cable samples contained in the protected lower cable tray after the CORE-2 fire.

  Figure l7: Ignition delays (tig) for the CFSS-2, CORE-2 and CORE-3 tests.

Figure l8 : 2 CORE

 l82 Figure l8: Fire growth rates for the CFSS-2 CORE-2 and CORE-3 tests.

  Figure 19: Average gas temperatures along the cable

.

  XÊ6: Molar fraction of CO in the exhaust network (mole/mole); o Mcoi Molecular mass of CO (0.028 kglmole); . C1lot: Mass concentration of soot in the exhaust network (kg/m').

  scale cable tray fire tests were thus carried out as part of the OECD for studying the effect of a protected cable tray (CORE-2 test) and slanted ladder cable trays on the main fire characteristics. These tests completed the preliminary CFSS-I to CFSS-4 tests of the PRISME-2 project which investigated fire spread over five horizontal ladder cable trays according to various cable-types. The CORE-2 and CORE-3 tests used the same cable-type, containing a halogen free flame retardant (HFFR), as the CFSS-2 test and they were conducted in open atmosphere under a large-scale calorimeter.

Table l :

 l Main fire characteristics of the CFSS-2, CORE-2 and CORE-3 tests.

	Test	CFSS-2	CORE-2	CORE-3
		(horizontal ladder	(protected cable tray)	(slanted ladder cable
		cable trays)		trays)
	rML (kg)	97	t5	93
	Peak of MLR (dr)	78	78	92
	Ignition delay	llmin5Ts	27 min34 s	7min45s
	Fire growth rate (kWs2)	2.2. l0-3	3.2. t0-3	7.3 ' t0-3
	Peak of HRR (MW)	2.20	2.38	2.55
	Fire duration (min)	72	97	67
	W @J/kg)	28	28.6	29.2

Table 2 :

 2 COz, CO and soot yields for the CFSS-2, CORE-2 and CORE-3 tests.

	CFSS-2	2.23
	CORE-2	2.17
	CORE.3

Test

Yco,(g/g) Yco (me/e) Yroot

(múE)

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 HSE (United Kingdom).


Pascal Zavaleta

Laurent Audouin

FIRE SPREADING FROM A REAL OPEN-DOORS ELECTRICAL CABINET TO OVERIIEAD MULTIPLE CABLE TRAYS INTO A COI\FINED ANI) ME CHANICALLY-VENTILATED FACILITY

Electrical cabinet fire is one of the main fire hazards in Nuclear Power Plants (NPPs). The ability of the fire to spread beyond the burning cabinet is therefore a major concem for fire safety analyses in NPPs. In the framework of the OECD PRISME-2 programme led by the French "Institut de Radioprotection et de Sûreté Nucléaire" (IRSN), three fire tests, named CFS-5 to CFS-7, involving a real open-doors electrical cabinet and three overhead cable trays, are performed. In this study, the main objectives are to investigate the effects of ventilation and types of electrical cable on the fire spread from the electrical open-doors cabinet to the three overhead cable trays. The fire tests are carried out into a confined and mechanically ventilated facility, named DIVA. Both CFS-5 and CFS-6 tests involve a ventilation renewal rate (VRR, defined as the room volume number renewed per hour) of 15 h-r, while the third CFS-7 test early implements a significantly lower VRR of about t h-r. For this last one, fire dampers are indeed shutdown 2 min 30 s afterthe starting of the cabinet ignition. In addition, the overhead trays are filled with two types of electrical cable used in NPPs, named cable C for the CFS-5 test and cable A for both CFS-6 and CFS-7 tests. These tests highlight that fire spreads from the real open-doors cabinet to the overhead cable trays for a VRR of 15 h-t and whatever the type of cable. However, this last one influences the fire behaviour of the upper cable trays. The cable A trays indeed ignite once and later than the three ignitions highlighted for the cable C trays. Moreover, the cable A tray fire is longer than the three cable C tray fires. The former mainly grows after the cabinet fire while the latter match with the cabinet fire. Finally, for the CFS-7 test, the fire dampers shutdown shortens the cabinet fire and prevents the upper cable trays ignition.

INTRODUCTION

Nearly 400 fire events in Nuclear Power Plants (NPPs) were recorded since the 1980's to the end of 2010 in the current OECD FIRE Databaser. From this last one, WerneÉ points out that half of these fires are electrical fires since they are ignited from electrical sources. Wernet' also indicates that one of the main contributions to the fire hazard is attributed to electrical cabinets, representing 12 Yo of all fire events (i.e. 48 cabinet fires). Furthermore, Chaveza mentions that some of these cabinet fires lead to severe damage due to heat, smoke and corrosion. Consequently, the study of electrical cabinet fires and its consequences especially in room for electrical control equipment or in a switchgear-type room of NPPs is a major concern for fire safety analyses. WerneÉ indeed points out that a cabinet fire in such room could lead to failure of one NPPs safety train or more, if fire spreads beyond the burning cabinet. Previous investigations focused particularly with:

. Conditions for cabinet to ignite, . Cabinet fire spread, . Ability of the fire to spread beyond the burning cabinet, . Cabinet fire consequences into both fire and adjacent rooms.

Only the main results dealing with the second and third items are discussed in this paper.

During the 1980's at Sandia National Laboratories, Chavez4's performed fire tests involving several full-scale cabinets. These tests were conducted either in a well-ventilated room of about 300 m3 in volume or in a room with a larger volume of 1400 m3. Only one real (vertical or benchboard) cabinet was internally ignited by fuel or electrical ignition source. The cabinets were most often without door and loaded with IEEE-383 qualified or un-qualified cables. In addition, the cabinets were separated by a double wall (a wall for each cabinet and an air gap of about 2.54 cm between). All these tests highlight that the fire can quickly propagate throughout the internally ignited cabinet without door whatever its type. Moreover, maximal heat release rate (HRR) \À/as measured higher than 1200 kW for vertical open-doors cabinet and for the two cable-types (vs 185 kW for the closed-doors cabinet).

However, despite such HRR for the open-doors burning cabinet, fire did not spread to adjacent cabinets. The air gap between the cabinets did not allow the wall temperature of adjacent cabinet to be enough high to lead to auto-ignition of cables fixed to the adjacent cabinet along the wall.

At VTT, Mangs6 conducted during the 1990's nine full-scale experiments involving two types of real vertical closed-doors electronic cabinets. All experiments were carried out beneath a large hood which collected all combustion products. They contained either relays or circuit boards but the two types of cabinet also included electrical cables. Finally, an adjacent cabinet was fastened to the burning cabinet. This last one contained Poly(vinyl chloride) (PVC) cables attached to the wall against the burning cabinet. The fire cabinet was ignited in all experiments with a propane gas burner. The higher HRR peaks were measured in the 100-350 kW range for all the experiments leading to flashover inside the cabinet. The highest HRR peak was obtained when the door locking mechanism failed during one of the real cabinet fires. Finally, the adjacent cabinet ignited caused by self-ignition of PVC cables contained inside it.

In the 2000's, CoutinT'8 performed eleven real cabinet fire tests in the framework of the PICSEL research program led by IRSN in collaboration with AREVA NC. Four cabinet fires were carried out in open atmosphere (under a large-scale calorimeter) while the seven other were performed in a confined and mechanically ventilated facility. All the fire tests involved the same real twin modules for the electro-technical cabinet. However, two of the cabinets tested in open atmosphere had closed- doors while the nine other cabinets tested had no door. A linear propane burner ignited the fuel load at the base of the cabinet, along its entire width. All the open-doors cabinet fires showed, after the incubation stage, fast fire propagations and significant HRR peaks although they reach higher values in open atmosphere conditions (1200 and 1600 kW) than in the confined conditions (from 400 to 950 kW). Otherwise, the two closed-doors cabinet fires gave HRR lower than 50 kW.

McGrattane recently performed one hundred and twelve full-scale experiments involving eight electrical cabinets under a medium-scale calorimeter. These cabinets were acquired from a NPP built :-+L^ l^+^ I O?flr-L,,+ -^,,^-^^^-^+^l 'l-L^,, ..,^-^ -^^^-+i-"-^,{ ..,i+h .,^.¡^"o amnrrnfc .-'l t ,^^" ^f ¡rr Lrrv t4L9 a ,, w J uut ¡r9v!r wPwrctvu. r r¡wJ vv!r! ¡gvvr¡¡róurwu electrical cable to represent many kinds of cabinet. The main goals were to provide the HRR data for fire models used to assess fire consequences in NPPs. Eleven of the cabinet fires had HRR peaks higher than 100 kW and ten of these last ones had a HRR peak lower than 300 kW. Such HRR peaks were obtained for both open-doors and closed-doors configurations and whatever the cable-type.

However, the highest peak HRR measured at nearly 600 kW was obtained for an open-doors cabinet loaded with un-qualified cables and loosely arranged.

All the previous programmes highlight significant HRR for cabinet fire, in particular for open-doors cabinet configuration. NPPs typically contain hundreds of kilometers of electrical cables which run along cable trays and may be connected to numerous electrical and electronic cabinets. However, no study deals with propagation from a burning cabinet to cable trays located above or below. Only the ability of the cabinet fire to spread to adjacent cabinet was previously investigated and discussed before (Chaveza'5 and Mangs6). In the framework of the OECD PRISME-2 programme (CFS campaign) led by IRSN (Audouinrr), three fire tests, named CFS-5 to CFS-7, involving a real open-doors electrical cabinet and three overhead cable trays, are performed. The goal of this study is to investigate the effects of ventilation and types of electrical cable on fire spreading from the electrical cabinet to the three overhead cable trays. The fire tests are conducted into a confined and mechanically ventilated facility, named DIVA. Both CFS-5 and CFS-6 tests involve a ventilation renewal rate (VRR, defined as the room volume number renewed per hour) of 15 h-r, while the third CFS-7 test early implements a significantly lower VRR of about I h-r. For this last one, ftre dampers are indeed shutdown 2 min 30 s after the starting of the cabinet ignition. In addition, the overhead trays are filled with two types of electrical cable used in NPPs, named cable C for the CFS-5 test and cable A for both CFS-6 and CFS-7 tests. Polymeric material contained in these cables is Poly(vinyl chloride) (PVC) for the former and mainly Poly(ethylene/vinyl acetate) (EVA) for the latter. The cable A also includes aluminium tri-hydroxide (ATH) as halogen free flame retardant while the halogenated flame retardant for the cable C is the chlorine included in the PVC polymeric material.

Firstly, this paper gives a description of the experimental set-up as the electrical cabinet fire source, the overhead cable trays and the DIVA facility. Then, analysis of the gas temperature measurements performed along the overhead cable trays gives first results dealing with both cable ignition and fire spreading. Next this paper focuses on HRR assessment based on both thermal and chemical methods. Finally, the last part deals with analyses of both HRR and mass loss rate (MLR) for the three CFS-5 to CFS-7 tests. These last ones highlight the effects of both ventilation and type of cable on the fire spread from the electrical cabinet to the three overhead cable trays.

DESCRIPTION OF'THE F'IRE TESTS

The real open-doors electrical cabinet and the three overhead cable trays involved for the three The open-doors electrical cabinet

The electrical cabinet used as fire source is a real twin module electro-technical cabinet, as previously used by CoutinT'8. As for this last one, cabinets are without door since this configuration, on one hand, leads to the most critical fire scenario, on the other hand, is taken into account in the NPPs fire safety analyses. The cabinet width, depth and height are respectively of 1.2, 0.6 and 2.2 m. The main components inside the cabinet (Figure 2) are transformers (1), terminal blocks (2), motor circuit breakers and contactors [START_REF] Werner | Enhancements in the OECD fire database -Fire frequencies and severity of events[END_REF], relays (4), circuit breakers (5), vertical trunkings (6), horizontal trunkings [START_REF] Coutin | Phenomenological description of actual electrical cabinet fires in a free atmosphere[END_REF] and cables. Figure 2 also points out a single pipe [START_REF] Coutin | Characterization of open-doors electrical cabinet fires in compartments[END_REF] with small holes, located at the bottom of cabinet, and used as a propane burner. This last one provides a fire power of about 10 kW for a duration between 4 and 5 minutes (depending on the test). Because the ignition due to hot shorts is not the purpose of this study, the electrical cabinet is not electrically powered. Finally, the test starts (t: 0 s) when the gas burner begins to operate. The starting of the test also matches with the beginning of the cabinet ignition.

The three overhead horizontal cable trays Three horizontal cable trays of 5 m in length are installed 30 cm above the electrical cabinet and deported beyond its front panel (Figure 1). These trays are ladder-type trays. Spacing between trays is of 30 cm and nearly of 45 cm between the upper tray and the ceiling. Table I summarizes the main characteristics of the electrical cables A and C, packed loosely over all the trays. This table also gives the tray filling (i.e. number of cable samples per tray) applied for the CFS-5 to CFS-7 tests. This last one is determined in order to ensure the same cable total surface per tray (i.e. l0 m2) for the three tests.

Table I : Characteristics of the cables and the tray filling used for the CFS-5 to CFS-7 tests.

Cable Polymeric material

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

The fire tests involve two rooms (rooms I and 2 shown in Figure 3) of a large-scale and mechanically ventilated facility, named DIVA, which contains five rooms in total. The electrical cabinet and the three overhead cable trays are located into the room I (fire room) at the centre of the west wall (Figure 3). In addition, rooms I and 2 communicating through a doorway are only equipped by one inlet, located at the upper part of room 1, and one outlet, settled at the upper part of room2 (adjacent room). Open sides of both inlet and outlet are also indicated in Figure 3. Fire dampers are implemented at both inlet and outlet only for the CFS-7 test. For the three tests, the initial VRR (i.e. before the starting of cabinet ignition) involving both rooms I and 2 is of 1 5 h-1. Volume flow rates at both inlet and outlet are therefore initially equal to 3600 m3/h. Furthermore, for the CFS-7 test, these last ones strongly decrease from 3600 to about 200 m3lh (i.e. a VRR of about t h -t), after the fire dampers shutdown monitored at 150 s after the starting of cabinet ignition. Ten thermocouples for measuring gas temperature are located along each of the three cable trays (Figure 4). They measure the gas temperature just above the upper layer of the cable samples and they are separated from each other of 50 cm. Figure 5 shows these measurements for CFS-6, the legends indicating the thermocouple positions from the tray centre (in cm). Fire firstly spreads throughout the cabinet (as illustrated in Figure 6) and leads to a first increase of the gas temperature along the three cable trays up to maximal values of about 500"C (Figure 5). Successive and larger increases of the gas temperature (up to values in the 800-860 oC range, Table 2) are next observed only on the right-hand sides of the cable trays. These results highlight the ignition of the three cable trays. Indeed, as assumed by Drysdale12, gas temperature higher than 600'C shows the presence of flame. Moreover, this assumption is also used to determine ignition time for each of the three cable trays. Ignition time indeed matches with gas temperature reaching 600'C for the first time along each of the cable trays. Fire thus ignites first at the lower tray (1225 s), then at the mid tray (1725 s) and finally at the upper tray (1850 s), as seen in Figure 5. In the reverse way, it is assumed that cable tray fire is extinguished when gas temperature is 600"C for the last time. So, the cable A tray fire stops to burn at t: 6000 s (Figure 5). As this last one starts at 1225 s, fire durationis 4775 s (about 80 min). In addition, Figure 7 also illustrates that fire propagates only over the right-hand side of the three trays. High air volume flow rate (initially of 3600 m3/h), even though being directed towards the opposite wall of the fire source (Figure 3), could influence the fire spreading over the upper cable trays.

For the CFS-7 test, maximal gas temperatures are lower than 600oC (Table 2). So no cable trays ignition is considered.

Figure 8 shows the gas temperature measured along the three cable trays for the CFS-5 test. As for the CFS-6 test, fire hrstly spreads throughout the cabinet (as illustrated in Figure 9) and leads to a first increase of the gas temperature up to maximal values of about 400 oC (Figure 8). Sharp peaks of gas temperature, exceeding 600 'C (Table 2), are then observed and therefore show ignitions of the cable trays. The two first ones only involve the upper tray and occur at respectively 837 s and 949 s (times corresponding to a gas temperature of 600'C for the first time, Figure 8). On the other hand, the third ignition involves all the trays since it occurs in nearly same time in the lower (1086 s), the mid (1096 s) and the upper (1097 s) trays (Figure 8). Three short cable tray fires then followed these ignitions. They indeed extinguished at respectively 854, 982 and 1156 s (times corresponding to a gas temperature of 600'C for the last time, Figure 8), showing fire duration around one minute (respectively of 17,33 and 70 s). Furtherrnore, Figure 8 also points out that fire mainly involves the -i^L+ L^-l ^:J^^ ^f +1^^ +l^-^^ k^.,^ LI^,,,^,,^-f^-+L^ '.-^^-+-^., [i-^ -1.^ .^--.,{.

^.'o. i+o l-fr h.-,{ r rË¡tL-rr4ttu ÐtuwÐ v¡ tltw LllIwv LI4Jù. r rvvvwvwr, ¡v¡ Lrrw wyÍtwt LtøJt ¡rrv c¡rv oPrwsur side. This last outcome is confirmed by Figure 10. Methods used for assessment of heat release rate

Heat release rates (HRR or Q¡ for the CFS-5 to CFS-7 tests in the DIVA facility are determined from a thermal method and the carbon dioxide generation (CDG) calorimetry method. The former was developed by Coutinr3 for assessing HRR of a cabinet fire by establishing an energy balance inside a single fire room. the three overhead cable trays (CFS-5).

Centre and right-hand side of the lower tray.

I

This method may be extended to test configuration involving several rooms and to other complex fire sources as for the CFS-5 to CFS-7 tests. In this case, the energy balance is carried out inside both fire and adjacent rooms (i.e. rooms I and 2, see Figure 3). The heat released by the fire is mainly distributed in four main parts:

o heat transfers to steel structures ofthe electrical cabinet (for the three ladder-type cable trays, this term is assumed to be negligible compared to the one for the cabinet) (Q¡n"rt¡.o), ¡ snergy per unit of time required for pyrolysis of fuel materials contained in the electrical cabinet and the three cable trays (Qrrr), o heat transfers through the walls of both fire and adjacent rooms (Q*o¿¿r), ¡ convective heat transfers through inlet (fire room) and outlet (adjacent room) ducts of the ventilation network (Q ou).

The fire HRR from the thermal method ( Qtn"rrnoù is therefore assessed thanks to the following relationshipt othertnar = Qínertia * Qon, * Qwou, * Qout tl]

Alternatively, the fire HRR can also be evaluated from the carbon dioxide generation (CDG) calorimetry method. For fires with large amount of CO and soot (considered as pure carbon), Tewarsonra and Brohezl5 proposed to assess the HRR from the CDG calorimetry method (Q6¡6) as follows:

Qcne = Ero,h\o, + Econi¿\o + EcTirE l2l . Ecor,Eç6,86: energy produced by mass unit of respectively COz, CO and soot generated (kJ/g);

. ,h\or,ñtfo andmf : Mass flow rate of respectively COz, CO and soot generated (g/s).

Where the average values of E¿s,:13.3 kJ/g according Tewarsonra and Eco: ll.l kJ/kg and ãc : 15.8 kJ/kg according Brohezr5. Recently, Pretrelró proposed evaluations of HRR for hres in multi-rooms facility based on the previous equation 12] and mass conservation of COz, CO and soot in such facility. This evaluation applied for the CFS-5 to CFS-7 fire tests is therefore based on the mass conservation of these species performed in both fire room (Rl) and adjacent room (R2) and is written as follows:

Qcoe = sro,lrh'rb' *#^Eb,***[fi,]

+EcolTh"cbt * *^Eb * *r^|¿'"1+ Ecllir"cut * *^E' * *^E'l t3l Where:

. ùtfut: Mass flow rate of species j : COz, CO or soot at the outlet of R2 (g/s);

. ftmf¿, Mass variation rate of species j : COz, CO or soot in the room i, i : I or 2 (g/s).

Figure l1 and Figure 12 show respectively for the CFS-6 and CFS-5 tests that the HRR assessments from both CDG and thermal methods are consistent. The final HRR for these tests is thus obtained from average of the two previous HRR evaluations (Figure 13 for CFS-6 and Figure l7 for CFS-5).

The mass variation rates of the species (see the *^i' terms in equation [START_REF] Werner | Enhancements in the OECD fire database -Fire frequencies and severity of events[END_REF]) show significant fluctuations when the masses of the species in the rooms quickly change. If the former are larger than the mass flow rates of the species at the outlet of the room 2 (see the ñtfutterms in equation [3]) then the assessed HRR from the CDG method (see equation [3]) gives large uncertainty. In this case this method cannot be considered. The fire dampers shutdown for the CFS-7 test leads to nearly nil mass flow rates of the species atthe outlet of the Room 2. Moreover, it also gives quicker variations of the masses of the species in particular in the fire room than the ones obtained for both CFS-5 and CFS-6 tests. Consequently, HRR evaluation from the CDG method for the CFS-7 gives too large uncertainty. For this test, HRR assessment is only based on the thermal method (Figure 14).

tr'ire spread for CFS-6

Two weighing devices are used to measure mass loss of the three cable trays and one for the cabinet.

Mass loss rate (MLR) is obtained from time derivation of mass loss. Figure l3 indicates for the CFS-6

test MLR for both cabinet and three cable trays and also HRR. The first and higher HRR peak at 560 s (735 kW) only arises from the cabinet fire since MLR for the cable trays is nil at the same time. Ignition of the cable A trays, previously identified at about 20 min, therefore occurs at about I I min after the cabinet hre peak (about 9 min). Furthermore, the last HRR peak at2400 s (440 kW) is mainly caused by the cable A trays fire. Indeed, at the same time, MLR for the cabinet is nearly nil while MLR for the cable A trays reaches its maximal value (19 g/s). So, the peak of the cable A tray fire occurs about 30 min after the peak of the cabinet fire. The cable A tray fire therefore mainly grows after the cabinet fire. Ventilation effects on fire spread (CFS-6 vs CFS-7)

Figure 14 points out that the fire dampers shutdown leads to shorter fire for the CFS-7 test than for the CFS-6 test (VRR of 15 h-r). Furthermore, Figure 15 and Figure 16 show oxygen (Oz) concentration in the upper part of the fire room and MLR for the cable trays respectively for the CFS-6 and CFS-7

tests. Vertical location of this 02 concentration (3.3 m) is close to the height of the mid tray. The lower cable tray ignites at t : 1225 s for the CFS-6 test when 02 concentration in the upper part of the fire room is of l6 %Vol and MLR for cable trays is of 6 g/s (Figure l5). Otherwise, for the CFS-7 test, Oz concentration in the upper part of the fire room ranges from 5 to 7 YoYol while MLR for the cable trays is higher than 6 g/s (Figure 16). Oz concentration is thus too low for allowing ignition of the cable trays for the CFS-7 test. Consequently, MLR for the CFS-7 cable trays quickly decreases to zero after its peak (9 g/s), as seen in Figure 16. The fire dampers shutdown prevents ignition of the three cable trays for the CFS-7 test. Otherwise, the high VRR of l5 h-t used for the CFS-6 test leads, on one hand, the open-doors cabinet fire to ignite the lower cable tray, on the other hand, the cable tray fire growth. Total mass loss (TML) of the CFS-7 cable trays is therefore very low (lower than 2 kg) compared with the TML (39 kg) of the CFS-6 cable trays. Time (sl Time (s) Cable-type effects on fire spread (CFS-6 vs CFS-S) Figure 17 points out, for the CFS-5 test, MLR for both cabinet and three cable trays and also HRR. The first HRR peak at 560 s (700 kW) only arises from the cabinet fire since MLR for the CFS-5 trays is nil at the same time. Three successive HRR peaks match with the three peaks of MLR for the trays at845,975 and ll47 s (Figure l7), showing the effect of the three cable Ctray fires on HRR. Their contributions to the total HRR peaks are respectively of about 150, 200 and 450 kW and are reached quickly after the cable ignitions. Indeed, these last ones were previously identified at 837, 949 and 1097 s (i.e. at about 5, 7 and 9 minutes after the cabinet fire peak). The third HRR peak is the higher since the corresponding cable tray fire involves the three trays. FurtherTnore, when the cable C trays ignites three times (between 837 and 1097 s), MLR of the cabinet is in the 15-25 g/s range, showing that the three fast cable C trays fires match with the cabinet fire (Figure l7).

As the CFS-6 and CFS-5 cabinet fires are similar (Figure l8), the differences between the CFS-5 and CFS-6 fires (Figure 19) confirm the cable-type (cable A or cable C) effects on the fire behaviour of the three cable trays. The long cable A tray fire mainly grows after the cabinet fire while the three short cable C tray fires match with the cabinet fire. Indeed, as the cable A trays ignite later than the cable C trays, the air renewal inside the fire room (VRR of 15 h-t) leads to higher Oz concentration before the ignition of the forme r (16 %oY ol, Figure I 5) than for the latter ( 14 YoY ol, Figure 20). In addition, after the ignition of the CFS-6 lower tray, 02 concentration still increases up to 17 Yol%o for about 10 min (Figure l5), favoring the cable A tray fire growth. On the contrary, the three CFS-5 cable tray fires lead to the quick decreases of the Oz concentration below 13 YolYo (Figure 20). Such 02 concentration level is supposed to extinguish the cable fire. The ends ofthese fires are taken at respectively 863,995 and lI72 s when 02 concentration is minimal (Figure 20). Considering the times of ignition for the CFS-5 cable trays, fire durations are thus of about I min. These last ones are consistent with the previous evaluation based on the analysis of the gas temperatures measurements carried out along the trays.

CONCLUSION

Electrical cabinet fire is one of the main fire hazards in Nuclear Power Plants (NPPs). The ability of the fire to spread beyond the burning cabinet is therefore a major concern for fire safety analyses in NPPs. In the framework of the OECD PRISME-2 programme led by the French "Institut de Radioprotection et de Sûreté Nucléaire" (IRSN), three fire tests, named CFS-5 to CFS-7, involving a real open-doors electrical cabinet and three overhead cable trays, are performed. In this study, the main objectives are to investigate the effects of ventilation and types of electrical cable on the fire spread from the electrical open-doors cabinet to the three overhead cable hays. The fire tests are carried out into a confined and mechanically ventilated facility, named DIVA. Both CFS-5 and CFS-6 tests involve a ventilation renewal rate (VRR, defined as the room volume number renewed per hour) of 1 5 h-r, while the third CFS-7 test early implements a significantly lower VRR of about I h-r. For this last one, fire dampers are indeed shutdown 2 min 30 s after the starting of the cabinet ignition. In addition, the overhead trays are filled with two types of electrical cable used in NPPs, named cable C for the CFS-5 test and cable A for both CFS-6 and CFS-7 tests.

These tests highlight that fire spreads trom the real open-doors cabinet to the overhead three cable trays for a VRR of 15 h-' (i.e. for both CFS-5 and CFS-6 tests) and whatever the type of cable (cable A or cable C). However, this last one influences the fire behaviour of the cable trays. Indeed, the cable C trays (CFS-5) ignite three times and earlier than the single ignition of the cable A trays . The former actually ignite at about 5, 7 and 9 min after the peak of the cabinet fire while the latter ignites 11 min after this peak. Moreover, the cable A tray fire is longer (fire duration of about 80 min) than the three cable C tray fires (fire duration of about 1 min for each one). The former mainly grows after the cabinet fire thanks to oxygen concentration higher than the one available for the latter which match with the cabinet fire. Total mass loss (TML) is higher for the CFS-6 cable trays (39 kg) than for the CFS-5 cable trays (16 kg). Furthermore, the two first CFS-5 cable trays ignitions only occur in the upper tray while the third ignition involves at nearly same time the three trays. Otherwise, for the CFS-6 test, the lower cable tray first ignites and then the fire reaches the mid tray before spreading over the upper tray. In addition, for both CFS-5 and CFS-6 tests, fire mainly involves the right-hand sides of the three cable trays. High air volume flow rate (initially of 3600 m3lh), even though being directed towards the opposite wall of the fire source could influence the fire spreading over the cable trays. Finally, for the CFS-7 test, the fire dampers shutdown shortens the cabinet fire and prevents the upper cable trays ignition since 02 concentration is too low.

  tests ar:e shown in Figure l. The CFS-5 to CFS-7 tests are carried out in the confined, mechanically ventilated and multi-rooms DIVA facility.

Figure l :Figure 2 :

 l2 Figure l: The open-doors electrical cabinet and

Figure 3 :

 3 Figure 3: The rooms I and2 of the DIVA facility used for the CFS-5 to CFS-7 tests.

Figure

  Figure 4: Thermocouples located along the three trays.

Table 2 :

 2 Maximal gas temperature measured along the three cable trays, just above the cables.

Figure 5 :Figure 8 :

 58 Figure 5: Gas temperature measured along the three overhead cable trays (CFS-6).Left-hand side of the lower tray.

  Figure

Figure

  Figure I l: HRR assessments from the thermal and CDG methods (CFS-6).

Figure 13 :

 13 Figure 13: MLR for both cabinet and cable trays and HRR (CFS-6).

Figure 12 :

 12 Figure 12: HRR assessments from the thermal and CDG methods (CFS-5).L200 1000

Figure 14 :

 14 Figure 14: HRR for both CFS-6 and CFS-7 tests.

Figure 15 :

 15 Figure 15: Upper 02 concentration and MLR for Figure 16: Upper 02 concentration and MLR for the CFS-6 cable trays.the CFS-7 cable trays.

Figure 19 :

 19 Figure 19: HRR for both CFS-6 and CFS-5 tests.

Figure 20 :

 20 Figure 20: Upper 02 concentration and MLR for the CFS-5 cable trays.

ACKNOWLEDGEMENTS

The authors are grateful for the financial support of the participating OECDNEA member countries (Belgium, Canada, Finland, France, Germany, Japan, Spain, Sweden, United Kingdom) to the joint OECD PRISME 2 Project.


Pascal Zavaleta

Olivier Bouygues

Céline Lapuerta

Ignition of electrical components contained in a closed enclosure adjacent to a controlled fire source

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

INTRODUCTION

About 400 fire events occurred in OECD nuclear power plants (NPPs) between 1975 and 2010 (OECD FIRE database [START_REF]OECD FIRE Database[END_REF]) About 50 fire events involved electrical cabinets [START_REF]OECD FIRE Database[END_REF] which are commonly arranged in rows of adjacent cabinets in NPPs

The ability of a cabinet fire to spread to adjacent cabinets is a major concern for fire safety in NPPs How can ignition occur in the adjacent cabinets? What is the impact on ignition of the electrical component type contained in the cabinet? What are the effects on ignition of the air gap (between the cabinets) and the cabinet ventilation mode ?

Row of adjacent cabinets in a NPP Real-scale adjacent cabinets studied [START_REF] Zavaleta | Fire spread from an open-doors electrical cabinet to neighboring targets in a confined and mechanically ventilated facility[END_REF] O n e -q u a r t e r s c a l e reduction:

EXPERIMENTAL SET-UP

Fire test matrix

o Cabinet fire [START_REF] Zavaleta | Fire spread from an open-doors electrical cabinet to neighboring targets in a confined and mechanically ventilated facility[END_REF] Average FW temperature calculated with the CALIF 3 S/ISIS software [START_REF]CALIF3S/ISIS software[END_REF] Fire enclosure Adjacent enclosure

  the fire and adjacent walls Six 0.5 mm type K Thermocouples brazed on each wall Heat flux in the adjacent enclosure Two Heat Flux sensors (Gardon type) close to the adjacent wall Instrumentation Inside the adjacent enclosure (wall removed) Adjacent wall

  Half-part of real-scale a d j a c e n t c a b i n e t s was first considered o T h e n , a h e i g h t reduction of test device PVC, poly(vinyl chloride), HFFR, halogen-free flame retardant; TB, terminal block One-quarter reduction of the maximal fire power (1200 kW) of the real-scale cabinet fire [2] Fire power of 300 kW for the gas burner of the reduced-scale device Design study Insulated wall Numerical simulations with the CALIF 3 S/ISIS fire field model confirm that the reduced-scale fire enclosure design with a fire power of 3 0 0 k W a l l o w s t o r e a c h f i r e w a l l temperature ≥ 600°C (as measured for real cabinet fire [2])Fire enclosure modelling with the CALIF 3 S/ISIS software[START_REF]CALIF3S/ISIS software[END_REF]

  Av. temperatures of the FW and AW Av. temperatures of the FW and AW Terminal blocks Heat flux in the AE close to the target PVC cables Terminal blocks