Combustion Characterization according to Accelerated Deterioration Temperature of a Non-class 1E Cable

Seok Hui Lee; Min Ho Kim; Seung Yeon Jeong; Sang Kyu Lee; Ju Eun Lee; Min Chul Lee

doi:10.7731/KIFSE.d86e802e

Int J Fire Sci Eng > Volume 36(1); 2022 > Article

Lee, Kim, Jeong, Lee, Lee, and Lee: Combustion Characterization according to Accelerated Deterioration Temperature of a Non-class 1E Cable

Research Paper

International Journal of Fire Science and Engineering 2022;36(1):19-29.

Published online: March 31, 2022

DOI: https://doi.org/10.7731/KIFSE.d86e802e

Combustion Characterization according to Accelerated Deterioration Temperature of a Non-class 1E Cable

Seok Hui Lee¹, Min Ho Kim¹, Seung Yeon Jeong¹, Sang Kyu Lee², Ju Eun Lee², Min Chul Lee^1,^3,^†

¹Department of Safety Engineering, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 22012, Republic of Korea

²Department of Reactor System Evaluation, Korean Institute of Nuclear Safety, 62 Gwahak-ro, Yuseong-gu, Daejeon 34142, Republic of Korea

³Fire Disaster Prevention Research Center, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 22012, Republic of Korea

^†Corresponding Author, TEL: +82-32-835-8295, FAX: +82-32-835-0779, E-Mail: LMC@inu.ac.kr

Received March 2, 2022 Revised March 16, 2022 Accepted March 16, 2022

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

In this study, the temperature effect of accelerated deterioration on combustion characteristics was investigated when accelerating aging a non-class 1E cable for nuclear power plants. The accelerated aging of 40 years was conducted under five temperature conditions of 90 °C, 100 °C, 110 °C, 130 °C and 150 °C. In the early period of combustion when the first peak of the heat release rate emerged, the heat release of the non-aged cable exhibited the largest peak value of 225 kW/m² while the heat release of the aged cables exhibited a comparatively low first peak value. The first peaks of aged cables decreased as the temperature condition increased. This tendency is considered to emerge from the thermal decomposition and destabilization of long-chained polymer structure when the sheath and insulation of cables are exposed to a thermal degradation environment. Hence, this loosening of the chemical bond and its decomposition severely affected the degradation of the flame retardant performance. In particular, in the combustion characteristics for the aged cables under temperature conditions of 100 °C or higher, the first peak value of the heat release rate did not exceed 200 kW/m². In the middle period of combustion, the heat release rates of both the non-aged and the aged cables were kept constant at approximately 25-30 kW/m² without significant change. In the later period of combustion, the second heat release peaks emerged for only non-aged, 90 °C and 100 °C cables because the cables aged under low temperature conditions (90 °C and 100 °C) maintained a certain level of flame retardant performance while those aged above 100 °C did not. Therefore, it can be considered that the higher accelerated deterioration temperature triggers the higher degradation of the flame retardant performance, and 100 °C is a critical temperature that involves the significant degradation effect.

Keywords: Non-class 1E cable; Accelerated deterioration; aging temperature conditions; Combustion characteristic; Nuclear Power Plants (NPPs)

1. Introduction

Domestic nuclear power plants (NPPs) are designed with safety equipment for safe operations and accident mitigation during their design life. Furthermore, class 1E, non-class 1E, and optical cables are installed throughout the NPP to transmit the power and signals of safety equipment [1]. The sheath and insulation of these cables are made of organic polymers that have the advantages of being lightweight and exhibiting excellent insulation performance. However, if these cables are exposed to various external environments such as heat and radiation, they may trigger changes in the unique properties of the polymers, such as heat resistance and wear resistance. Consequently, a possibility of losing the original function of the cable or increasing the risk of fire exists, due to the deterioration of flame retardant performance [2]. In NPPs, there are various aging conditions, such as temperature, humidity, and radiation, and their impacts on the cables inevitably increase because these conditions occur in combination. In particular, thermal deterioration by temperature is considered as the main factor of deterioration for cables used in indoor spaces such as NPPs. For example, performance deterioration tests are performed in which the temperature is applied and accelerated as an external stress factor, to verify the life span of rubber materials such as organic polymer. In this case, accelerated deterioration at a very high temperature can trigger changes in the deterioration mechanism of the rubber itself [3]. Moreover, according to Zhang B et al. who analyzed the effect of aging conditions on cables for four aging conditions (thermal, xenon arc, ozone, and hydrothermal aging) in NPPs, the most significant factor was thermal deterioration by medium temperatures [4].

Various studies on the deterioration of cables used in NPPs have been continuously conducted. Zhang et al.[5] researched on the deterioration of cables, considering the chemical impact of oxygen diffusion. Kang et al.[6] investigated the insulation characteristics of aged cable via thermal and electrical factors. Although prior studies primarily considered thermal conditions, only a few studies have researched on the flame retardant performance of cables. It is considered that when analyzing fire risk, in addition to the cables’ loss of functions, the combustion aspects in terms of cable damage due to external factors should also be analyzed. Lee et al. [7] studied the combustion characteristics via the aging deterioration of class 1E cable for NPPs. Seo et al. [8] analyzed the combustion characteristics and toxicity that emerge during the combustion of non-class 1E cable. However, both studies focused on the aging level, and calculated the deterioration time by assuming the same temperature condition for aging deterioration; in addition, these studies did not include the characteristics of exposure to various temperatures.

Therefore, this study considered temperature as the most universal and major deterioration factor, and selected a non-class 1E cable-type exposed to various temperature environments throughout the NPPs without being directly exposed to radiation, to focus on analyzing the effects of thermal factors. In addition, by ascertaining that it is necessary to analyze the effects of temperature conditions, to consider appropriate temperatures on the accelerated deterioration of cables, the accelerated deterioration temperature conditions were subdivided into five (90 °C, 100 °C, 110 °C, 130 °C, 150 °C). Subsequently, 40 years of aging deterioration, which is the NPP design life, were reproduced in the cable by calculating the accelerated deterioration time according to each temperature condition. To compare the degree of the cable’s loss of function due to external factors such as fire, the combustion characteristics of cables for thermal deterioration were analyzed by performing cone-calorimeter tests.

2. Testing Material and Method

2.1 Testing material

A non-class 1E cable-type was selected among flame- retardant cables used in NPPs. This cable passed the flame test set by the IEEE 383-1974, "IEEE Standard for Type Test of Class 1E Electric Cables, Field Splices, and Connections for Nuclear Power Generating Stations" in accordance with the NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plant." The criterion for this flame test, which measures the flame retardant performance via a vertical tray flame test, must verify that there is no spread of flame even if the insulation or cable sheath is damaged at the flame contact for a test period of 20 min. The non-class 1E cable selected in this study is composed of polychloroprene rubber (CR) and ethylene propylene rubber (EPR). The detailed specifications are presented in Table 1 and the structure is illustrated in Figure 1.

2.2 Test method

2.2.1 Accelerated deterioration test

In this section, the accelerated deterioration method adopted in this accelerated deterioration study is described.

To reproduce the accelerated deterioration cable, a cable accelerated deterioration test was performed after estimating the required accelerated deterioration time according to the equivalent life at a specific temperature, by applying the thermal deterioration model that solely considers time and temperature. The accelerated deterioration test was performed in a temperature chamber (ESPEC Corp., Osaka). In addition, this cable was fabricated by drying a non-aged cable inside a chamber in which hot air of a certain temperature was circulated. To calculate the accelerated deterioration time, we adopted the Arrhenius transformation equation expressed as:

(1)

k1=k2exp[EaKB(1T1-1T2)]

where k₁, k₂, E_a, K_B, T₁, and T₂ denote the required accelerated deterioration time (h) for equivalent life, equivalent life (h), activation energy (eV), Boltzmann constant (8.617 × 10-5 eV/K), used temperature (K) of the cable, and accelerated deterioration temperature (temperature of chamber, K), respectively.

In this study, an aged cable with an equivalent life of 40 years was reproduced, and the accelerated deterioration temperature conditions were set to 90 °C, 100 °C, 110 °C, 130 °C, and 150 °C. For T₁, we conservatively applied 333 K (= 60 °C), 11 K higher than 120 °F (322.04 K), which is the normal condition of the environmental resistance verification test for the 600-V class control cable in the NPP. The activation energy was estimated using a thermogravimetric (TG) analyzer.

The TG analyzer is a useful device for investigating the thermal decomposition behavior of materials according to the heating rate (β) [9,10]. In this study, TG analysis of the sheath and insulation was performed at the same weight ratio, to simultaneously consider the properties of the two polymer materials that comprise the non-class 1E cable. The TG analysis test was conducted under the heating rate conditions of 5 °C/min, 10 °C/min, 15 °C/min, and 20 °C/min from 25 °C to 500 °C. Among the thermal decomposition properties investigated using the TG analyzer, the initial exothermic peak temperature (T_m) according to each heating rate was measured and applied to the Kissinger method, to determine E_a. The basic equation of the Kissinger method is expressed as [11]:

(2)

lnβTm2=lnαREa-(EaRTm)

where α and R represent the Arrhenius pre-exponential factor and ideal gas constant (8.314 J/mol K), respectively. The Kissinger method calculates E_a by grouping the terms that contain the pre-exponential factor as the intercepts of a linear equation. The activation energy is calculated using the slope of Eq. (2) [12]. Therefore, E_a is determined from the slope of a straight line obtained using the plots of ln (β /T_m²) and -1/T_m on the left and right sides of Eq. (2), respectively.

The weight changes and change rate according to the temperature of the non-class 1E cable are presented in Figure 2. The weight reduction according to thermal decomposition starts from approximately 300 °C. The weight reduction occurs at a higher temperature range as the heating rate increases. This is because an increase in the heating rate tends to delay the thermal decomposition process at a higher temperature, and the thermal delay increases because the material reaches the corresponding temperature within a short time at a limited temperature [13]. However, because the shape of the thermogravimetric change curve is expressed similarly at each heating rate, the TG analysis according to the heating rate exhibits valid results.

T_m values according to each heating rate condition of the non-class 1E cable are presented in Table 2. Two T_m values were measured at the heating rate condition of 5 °C/min. Thermal decomposition progressed at different temperatures for sheath and insulation, respectively; consequently, two peak values were derived. In this study, the average of the thermal decomposition temperature of sheath and insulation was adopted to consider the overall properties of the cable. The range of T_m of the non-class 1E cable applying the average value was 344.15-367.96 °C.

The maximum thermal decomposition temperature according to each heating rate obtained from the TG analysis was applied to Eq. (2), and the plots of -ln (β /T_m²) and 1/T_m are presented in Figure 3. The obtained plot was expressed as y = ax + b, and the coefficient of determination (R²) in this plot was 0.9656, y = 21.212x-23.074. The slope of the plot implies that E_a/R; hence, the activation energy of the non-class 1E cable was calculated by multiplying the slope by the ideal gas constant (R). The calculated activation energy was 176.38 kJ/mol. By applying the activation energy to Eq. (1), the time required to reproduce the aged cable for 40 years, according to the accelerated deterioration temperature condition, is presented in Table 3.

2.2.2 Cone calorimeter test

To analyze the combustion characteristics of the cable, a cone calorimeter test (Fire Testing Technology, UK) was performed according to ISO 5660-1. This test measures the combustion factors such as the time to ignition (TTI), heat release rate (HRR), and mass loss (ML). The specimens were prepared by installing four cables with lengths of 100 mm and outer diameters of 25 mm in parallel inside a 100 × 100-mm frame. Each specimen was placed in a horizontal position below the cone heater and exposed to an external heat flux set in the cone heater. Then a flame was ignited on the surface of the specimen using an igniter. This test was performed thrice for 20 min or longer for every specimen. The external heat flux was set to 50 kW/m². Figure 4 presents a schematic diagram of the cone calorimeter test and specimen.

3. Test Result and Discussion

Each test was performed thrice for non-aged cable and aged cable for 40 years, respectively, according to five accelerated deterioration temperature conditions, and the results of combustion characteristics are presented in Table 4 and Figure 5. The peak of heat release rate (PHRR), which occurs during the cable combustion was lower for the aged cable than that of the non-aged cable. The time to PHRR (tPHRR) tended to be shorter as the deterioration temperature condition increased. These factors are adopted to analyze the initial combustion characteristics of cables, and the analysis of these results are described in Section 3.1. The TTI, which represents the time until the specimen is ignited during the cone calorimeter test was approximately 40 s for both non-aged and aged cables, with negligible differences. The ML, which is a factor that indicates whether the specimen is fully combusted after the test is completed, represents the weight change of the specimen during the test. The ML of the non-aged cable was 7.21%; however, the MLs of all the aged cables were approximately 6% with negligible differences.

3.1 Initial combustion characterstics

The measured PHRR was within 200 s after the test commenced for both non-aged and aged cables. Hence, this section was defined as the initial combustion characteristic section, and the test results for each cable are presented in Figure 6. The average initial PHRR of the non-aged cable was the largest at 225 kW/m². The measured PHRRs of aged cable at 90 °C and 100 °C were approximately 209 kW/m² a nd 2 08 kW/m², respectively. The peak values of the aged cables at 110 °C or higher conditions were 180-195 kW/m², and did not exceed 200 kW/m². The TTI values were approximately 40 s for both non-aged and aged cables with negligible differences. However, the tPHRR tended to decrease as the degeneration temperature increased. As illustrated in Figures 5(a) and 5(b), the sheath (CR) and insulation (EPR) of the non-class 1E cable are polymer materials composed of C, H, O, and Cl. These components are volatile elements and exhibit the property of volatilizing in molecular forms such as HCl, CO₂, and H₂O in the process of generating cracks in the organic polymer during thermal decomposition [16]. Consequently, the thermal deterioration of the non-class 1E cable triggers instability in the structure of the polymer material by inducing the thermal decomposition of the sheath and insulation, which are polymer materials. In general, a certain level of flame retardant is added to the polymer materials of the sheath and insulation, to improve their flame retardant performance during cable manufacturing. However, for the non-class 1E cable adopted in this study, we did not obtain the accurate composition of the cable and flame retardant-type because they are confidential information of the manufacturer. However, halogen-based flame retardants were mainly used, which can severely impart the excellent flame retardant performance, regardless of the type of polymer material in most PVC cables. Among the halogen-based components, F (fluorine) exhibits the lowest flame retardation effect because the C-F bonding energy is too strong, while I (Iodine) is inefficient for maintaining flame retardant performance because its bonding energy is excessively weak. Hence, additives containing Cl and Br are mainly used in polymer materials [17]. When the cable is exposed to a high-temperature environment, the dichlorination reaction of the C-Cl bond occurs. This reaction is more active at high temperatures of 120-160 °C. It appears that as the deterioration temperature increases, the initial PHRR tends to decrease because such an effect on thermal deterioration also occurs in the additive of the Cl component used as flame retardant, including the Cl added to the composition of polymer materials [18].

However, the measured initial HRR of the 150 °C-aged cable is larger than that of the 110 and 130 °C-aged cables, although the temperature is higher. This appears to be because the effect of thermal deterioration was not obtained sufficiently, as the estimated time of accelerated deterioration is only 0.46 h, which is too short.

3.2 Middle and late combustion characteristics

In this study, the section from the initial combustion characteristic section (0-200 s) to 1200 s, where the HRR is maintained without significant changes, was defined as the middle combustion characteristic section. In this section, the HRRs of all cables, including the non-aged cable, were approximately 25-30 kW/m², and no significant increase was observed. The section after the middle combustion characteristic section (200-1200 s) to the end of the test (2000s) was defined as the late combustion characteristic section. In Figures 6(a) and 6(b) and 6(c), the second HRR exhibited a peak value in the section after 200 s, at least once during the three repetition tests in the non-aged cable and in the 90 and 100 °C-aged cables. Regarding this characteristic, previous studies [7,19] have reported that after the first PHRR, the flame retardant added to the cable sheath and insulation after the first PHRR forms a char layer on the cable surface and blocks heat penetration. They suggest that if the flame retardant performance decreases, the formation of the char layer becomes unstabe, which weakens its intensity, and after the flame retardant performance is maintained for a certain period, the second PHRR emerges, owing to the occurrence of cracks on the surface. The fact that the 90 and 100 °C-aged cables exhibit the second PHRR, even though they were exposed to thermal aging condition, implies that a certain level of flame retardant performance is maintained, regardless of the flame retardant performance compared to the non-aged cable. This suggests that this level of thermal deterioration cannot degrade the cable performance and functionality. The second PHRR is a characteristic related to the maintenance of the cable’s flame retardant performance against thermal deterioration. Hence, regarding the fact that the second PHRR was measured only in the first test among the three repeated tests of the non-aged cable, and that the second PHRR was measured in the second test result of the 90 and 100 °C-aged cables, it is more appropriate to interpret it as an attribute of the inhomogeneity of the specimen and the error between the results that may be triggered by the nature of the fire test, rather than the position that a higher flame retardant performance was maintained for the 90 and 100 °C-aged cables than that of the non- aged cable. The fact that the second PHRR was not measured in all the three repeated tests in the 110 °C, 130 °C, and 150 °C-aged cables as illustrated in Figures 6(d) and 6(e) and 6(f) implies that the cable completely lost flame retardant performance due to thermal deterioration, and that the char layer was not formed. Furthermore, the fact that no additional change in HRR occurred even though the external heat flux was continuously examined using the cone heater implies that the sheath and insulation, which are combustible materials, were completely burned in the initial combustion characteristic section. It appeared that the effect of thermal deterioration was not large for the 150 °C-aged cable in particular because the accelerated deterioration time is too short to trigger structural changes and decrease the flame retardant performance of polymer materials in the initial combustion characteristic section. However, the fact that the second heat release characteristics did not appear indicates complete combustion in the initial combustion characteristic section like the 110 and 130 °C-aged cables. This implies that even if the effect of the short-term deterioration time is insufficient, the change in the high-temperature thermal deterioration clearly exists from the long-term perspective.

4. Conclusions

In this study, 40 years of aging deterioration was reproduced via accelerated deterioration for five temperature conditions of the non-class 1E cable used in NPPs. By analyzing the combustion characteristics accordingly, the following conclusions were obtained.

(1) In the initial combustion characteristic section corresponding to 0-200 s, the TTIs of both non-aged and aged cables were approximately 40 s, exhibiting negligible a difference. The first PHRR of the non-aged cable was measured as the largest at 225 kW/m². The aged cable exhibited a decreasing trend of the first PHRR at higher temperatures. This appears to be because the thermal deterioration of cable triggers the thermal decomposition of the sheath and insulation (polymer materials), which leads to the instability of the components; consequently, the flame retardant performance decreased. In particular, the first PHRRs of the aged cables for the 110 °C or higher conditions did not exceed 200 kW/m². This is considered to be caused by the fact that the dichlorination reaction according to the thermal decomposition of polymers was the most active at approximately 120-160 °C. The first PHRR of the aged cable for the 150 °C temperature condition was measured to be smaller than that of the 110 and 130 °C-aged cables, regardless of the highest deterioration temperatures, because the accelerated deterioration time was excessively short and insufficient in receiving the effect of thermal deterioration.

(2) In the middle combustion characteristic section from 200 to 1200 s, the HRR values of both non-aged and aged cables were approximately 25-30 kW/m², which remained constant without significant changes.

(3) In the late combustion characteristic section after 1200 s, the second PHRR was measured in a non-aged cable and 90 and 100 °C-aged cables. The second PHRR originated from the cracking of the char layer on the cable surface by the flame retardant. The fact that the second PHRR was measured in the 90 and 100 °C-aged cables implies that a certain level of flame retardant performance is maintained, even though these cables were exposed to thermal aging condition. The aged cables after 110 °C did not exhibit the second PHRR, as the first PHRR verified the significant effect of deterioration. This suggests that the combustible materials completely burned because the cable lost its flame retardant performance in the initial combustion characteristic section. Owing to the short-accelerated deterioration time, the 150 °C-aged cable exerted less effects on the thermal deterioration than the 110 and 130 °C-aged cables. However, the existence of these effect on thermal deterioration was validated by the fact that the second PHRR was not observed like the 110 and 130 °C-aged cables.

The findings of this study verify that the exposure of a cable to high temperatures above 100 °C significantly impacts its flame retardant performance. Therefore, 90-100 °C is estimated to be the most appropriate range fir the accelerated deterioration temperature of the cables in NPPs.

Notes

Author Contributions

Conceptualization, Seok Hui Lee. and Min Chul Lee.; methodology, Min Ho Kim; validation, Seung Yeon Jeong; formal analysis, Seok Hui Lee; investigation, Min Ho Kim. and Sang Kyu Lee. and Ju Eun Lee; data curation, Seung Yeon Jeong; writing―original draft preparation, Seok Hui Lee; writing―review and editing, Min Chul Lee; visualization, Min Ho Kim; supervision, Min Chul Lee; All authors have read and agreed to the published version of the manuscript.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgments

This work was supported by the Nuclear Safety Research Program through the Korea Foundation of Nuclear Safety (KOFONS), using the financial resource granted by the Nuclear Safety and Security Commission (NSSC) of the Republic of Korea (No. 1705002).

Figure 1.

Structure and cross-sectional view of a non-class 1E cable.

Figure 2.

TG and differential TG(DTG) curves of the non-class 1E cable at different heating rates.

Figure 3.

Kissinger plot of the non-class 1E cable.

Figure 4.

Schematic diagram of cone calorimeter test and specimen.

Figure 5.

Construction of CR, EPR.

Figure 6.

Heat release rates of non-class 1E cable according to aging temperature conditions.

Table 1.

Specifications of Experimental Cable

Division		Detail
Application		Power and control
Voltage [V]		600
Outer diameter [mm]		25
Material properties	Sheath	Polychloroprene rubber (CR)
	Insulation	Ethylene propylene rubber (EPR)
	Conductor	Copper

Table 2.

Specifications of Experimental Cable

Heating rate [β, °C/min]	First exothermic peak temperature [T_m, °C]
5	337.30 / 351.00 (Avg.: 344.15)
10	359.72
15	363.80
20	367.96

Table 3.

Specifications of Experimental Cable

Accelerated aging temp. [°C]	Required time [h]
90	1820.20
100	380.42
110	86.28
130	5.53
150	0.46

Table 4.

Results of Combustion Characteristics Obtained via Cone Calorimeter

Accelerated aging temperature [°C]	PHRR [kW/m²]	tPHRR [s]	TTI [s]	ML [%]
Non-aged	225.31	73	44	7.21
90	209.52	82	45	6.77
100	208.32	73	45	6.87
110	191.51	67	43	6.30
130	181.87	70	44	6.80
150	194.42	63	40	6.46

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