A novel method was proposed to monitor the working state (temperature and stress) of sealing glass in electrical penetration assemblies, which was used for electrical connection in containment structures or pressure vessels of nuclear power plants, based on femto-laser inscribed fiber Bragg grating (FBG) sensors. Aging tests under thermal (~200°C) and radiation (~3.5MGy) conditions were carried out to demonstrate the feasibility of FBG in harsh environment. On-line state monitoring experiments were performed under high temperature 100~400°C and high pressure 7 MPa, referring to real conditions in the nuclear reactor. During monitoring, one FBG was embedded in sealing glass and the other was set outside the glass. Experimental and numerical results showed that the femto-laser inscribed FBG sensors could achieve simultaneous temperature and stress monitoring with good accuracy (monitoring deviation less than 10%) and transient response under harsh environment. This work set a base for the long-term real-time diagnosis of electrical penetration assembly in nuclear power plant.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Electric penetration assembly (EPA), as shown in Fig. 1, is an assembly of insulated electric conductors, conductor seals, etc. It provides the passage of the electric conductors through a single aperture in the nuclear containment structure or reactor pressure vessel, while constructing a pressure barrier between the inside and the outside . Metal-to-glass sealing EPA has unique advantages of higher temperature and pressure endurance than organic material sealing EPA, and has been applied in the pressure vessel of High Temperature Reactor Pebble-bed Modules (HTR-PM) at the Shidao Bay Nuclear Power Plant in China. The residual stress in sealing glass keeping the hermeticity of EPA  may release as a result of being affected by external loads such as high temperature 150°C and high pressure 7 MPa in HTR-PM, which is the main failure mechanism of sealing materials. To ensure the reliability of EPA during whole life cycle of nuclear reactor, the on-line state monitoring of sealing glass in harsh environment should be achieved.
In latest years, optical fiber sensing technique has been applied in nuclear facilities and arouses great interests of researchers. Rizzolo applied the Rayleigh-based optical fiber sensor to monitor water level and temperature inside the nuclear fuel pools ; Phéron proposed the distributed strain and temperature monitoring method of nuclear waste repository based on the Brillouin scattering sensing technology ; and Cangialosi developed the Raman distributed temperature sensors for nuclear wastes  and so on. The fiber Bragg grating sensor (FBG) has unique advantages among these optical fiber sensors, and has been reported to be used in many fields such as structure health monitoring [6–8], power industry [9–11], and the harsh environment of nuclear power plant [12–15]. However, on-line state monitoring of sealing glass in EPA of nuclear reactor has never been reported before. Fiber Bragg grating sensors inscribed by the femtosecond infrared laser point-by-point method were chosen as the probes to monitor multiple state parameters of EPA simultaneously. The femtosecond pulse duration infrared (fs-IR) FBG with type II gratings has excellent thermal stability up to 1000°C  and resistance to radiation , and the chemical component of FBG and sealing glass in EPA was similar. These advantages promised femto-laser inscribed FBG sensors a better performance for monitoring of sealing glass of EPA than conventional strain gauges.
Thermal and radiation aging tests were carried out first to examine the feasibility of FBGs in harsh environment. Then the simultaneous temperature and stress monitoring experiment of sealing glass under varying thermal load was achieved by femto-laser inscribed FBG sensors, with one FBG embedded in sealing glass and the other set outside sealing glass as a temperature compensation, which could separate the temperature effect on FBG embedded in sealing glass and obtained the independent temperature and stress changing results. Experimental results showed that femto-laser inscribed FBG was able to achieve simultaneous temperature and stress monitoring. In simulation part, the sensitivity analysis of parameters including temperature and pressure was carried out to study changes of stress in sealing glass caused by working environment parameters, and numerical results were consistent with experimental results monitored by FBGs. Moreover, this research made an initial attempt for realizing an on-line real-time long-term state monitoring and set a base for the life cycle diagnostics of EPA in nuclear reactors.
2. Sensing principle and experimental setup
2.1 FBG sensing principle
The femto-laser inscribed FBG sensors used in this study were provided by FemtoFiberTec. FBGs were inscribed on the Corning SMF-28 fiber. During measurement, the Bragg wavelength λB of FBG would be affected by temperature change ΔT or strain ε of the object, which was given by :
where ΔλB was the Bragg wavelength shift, ζ was thermo-optic coefficient, α was thermal expansion coefficients, pij was strain-optic coefficients, ε1 was the strain along fiber axis, εt was the strain transverse to fiber axis, and n was the effective refractive index.
The FBG embedded in sealing glass experienced a homogeneous transverse strain distribution of compression direction at the middle position of sealing glass, as a result, the transverse strain wouldn’t induce birefringence and the effects on monitoring were neglected [18,19], and the formula (2) could be simplified as formula (3). Combined the monitored ΔλB-ε during experiment and formula (3), the axial strain of could be obtained. Since the fiber was well-bonded with sealing glass, the axial strain of sealing glass was equal to that of FBG.
2.2 Ageing tests of FBG sensors
The Bragg wavelength might generate tiny drift in long term operation, so the Bragg wavelength drift caused by long-term thermal or radioactive impact should not be ignored , which would lead to a monitoring deviation. To prove the feasibility of FBG sensors in harsh environment, it was necessary to evaluate the wavelength drift under high temperature and radiation.
Researchers have studied the radiation and temperature responses of FBGs. Gusarov summarized radiation effects on various types of fiber gratings , and they mentioned femto-laser inscribed FBGs had a better performance compared with UV laser inscribed FBGs under radiation environment. This technique also made it easily to inscribe FBGs on radiation hardened fibers such as pure silica fibers .Many radiation resistant FBGs for applications in nuclear power plant were reported. Morana proposed a fabricating method which could limit radiation induced Bragg wavelength shift (RI-BWS) below 10 pm under 3 MGy gamma radiation [23,24]. Zaghloul developed FBGs in random air-line fibers, which had resistance to temperature and neutron flux in nuclear reactors . The combined thermal-radiation experiments for FBG were carried out to evaluate the possible measuring deviation under harsh environments, and results showed that thermal treatments during radiation would reduce the RI-BWS, which could be considered for FBGs’ applications in nuclear reactors [23,25].
In this research, thermal aging test was performed in a high temperature test chamber, and the aging temperature was 200°C, lasting for 1000 hours. The radiation aging test was carried out by a 60Co radiation source with a dose rate of 10 kGy/h, the total dose of which was 3.5 MGy, same as that in HTR-PM for 40-years life cycle, to evaluate the largest RI-BWS of FBGs and verify the possibility of FBGs applications in HTR-PM. Femto-laser inscribed FBGs and UV laser inscribed FBGs were tested together to investigate the resistance to harsh environment of different FBGs.
2.3 On-line working state monitoring of sealing glass in EPA
The on-line state monitoring experiment under thermal load was performed at different temperatures 100°C, 200°C, 300°C and 400°C, and each measurement was kept for 100 minutes. One key technique to realize state monitoring under thermal load was the cross sensitivity of temperature and strain, because changes of temperature and stress both affected the Bragg wavelength shifting . Some practical methods had been demonstrated to demodulate the simultaneous monitoring of temperature and strain, such as using a superstructure FBG , through the bandwidth and the Bragg wavelength of a single FBG , and setting temperature compensations  and so on. A novel sensing arrangement for EPA was developed in this study based on femto-laser inscribed FBG sensors. One of the FBG was embedded in sealing glass and the other one was set nearby sealing glass as a temperature compensation as shown in Fig. 2(b), the initial wavelength of which was respectively 1545.6 nm and 1555.0 nm. The FBG embedded in sealing glass measured the change of stress and temperature simultaneously expressed as the Bragg wavelength λμ + T, and the FBG outside sealing glass only measured the temperature by λT. The change of stress monitored by Δλμ could be separated through subtracting Δλμ + T from ΔλT. This was a precise demodulation method for simultaneous temperature and stress monitoring of sealing glass during EPA in service.
The on-line state monitoring of sealing glass under pressure load was carried out with high pressure helium, which was also the coolant medium in the HTR-PM. The high-pressure helium was passed into the EPA module and FBG could obtain wavelength data with pressure changing. Then the wavelength shift Δλμ was obtained and the effect on the stress caused by pressure changing could be studied. The FBG was located in the middle of sealing glass, shown in Fig. 2(e), with initial wavelength for about 1549.8 nm.
3. Finite element analysis
The finite element model of metal-to-glass sealing EPA was the same as the real model in on-line monitoring experiments. In order to simulate working state of EPA, the thermal load with temperature changing from 100°C to 400°C and the pressure load varying from 1 MPa to 7 MPa were respectively imposed on the EPA model, which was similar to external loads of on-line monitoring experiments to provide the theoretical support and completion for experimental results. The external loads and calculating paths of the EPA were shown in Fig. 3, and the axial path was exactly the monitoring position of the femto-laser inscribed FBG sensors in sealing glass (Table 1).
4. Results and discussion
4.1 Results of FBG’s ageing test
The Bragg wavelength drift Δλdrift generated under high temperature and radiation was the evaluation criteria for resistance to harsh environment of FBG. The smaller Δλdrift was, the better resistance FBG had to the harsh environment. The aging results was shown in Table 2.
The Δλdrift caused by radiation and thermal aging of UV laser inscribed FBG was respectively 0.251 nm and 0.230 nm, and the results of femto-laser inscribed FBG were 0.073nm and 0.008nm. The Δλdrift of femto-laser inscribed FBG would lead to deviation of temperature measuring about 6°C after 40-years life cycle in HTR-PM, and the effect on strain measuring caused by Δλdrift could be minimized by setting a compensation. The spectra of FBGs before and after radiation were shown in Fig. 4(a) and 4(b). The spectrum of femto-laser inscribed FBG almost remained unchanged, but the amplitude and full width at half maximum (FWHM) of UV laser inscribed FBG decreased obviously. Thus femto-laser inscribed FBG had the potential to be applied in nuclear power plant.
The algorithm dealing with spectra in this research was the Gaussian fitting, which could fit the reflection spectrum accurately with a Gaussian curve as explained in formula (2).
where fi was the reflectivity of FBG, λi was wavelength, λB was central wavelength of FBG, ai and bi were coefficients make fi match the best Gaussian curve. The algorithm searched Gaussian curve by minimizing Min in formula (3) .
To make a detailed comparison of the thermal stability of FBG sensors, the relationship between Bragg wavelength and temperature was calibrated before and after the thermal aging test, shown in Fig. 5(a) and 5(b). It showed that the thermal sensitivity coefficient (dλ/dT) of UV laser inscribed FBG changed from about 0.013 to 0.0118, and the value of femto-laser inscribed FBG was constant, 0.008. The results showed good thermal stability of femto-laser inscribed FBG and also a calibrated base for the on-line monitoring experiment under thermal load.
4.2 Results of on-line working state monitoring under thermal load
The Bragg wavelength curves of FBG sensors inside and outside sealing glass were presented as λμ + T and λT respectively. Results of the on-line monitoring were shown in Fig. 6(a)–6(d), respectively monitored by femto-laser inscribed FBG sensors and UV laser inscribed FBG sensors. The same thermal load was imposed on these two cases.
As shown in these middle pictures of Fig. 6(a) and 6(b), the temperature monitoring curves λT were almost the same, and the accurate temperature of sealing glass could be obtained by ΔλT. However, the monitoring curves λμ + T of temperature and stress showed differences in signal accuracy. In the bottom pictures of Fig. 6(a) and 6(b), the Bragg wavelength shift Δλμ caused by the stress was demodulated by subtracting Δλμ + T from ΔλT. Based on the fundamentals of FBGs, the specific values of changing stress and temperature monitored by femto-laser inscribed FBGs were obtained shown in Fig. 6(c) and 6(d). For the result monitored by femto-laser inscribed FBG, the average stress change of 25~100°C, 100~200°C, 200~300°C and 300~400°C was about −12MPa, −14MPa, −16MPa and −20MPa based on formula (3), and the negative sign before each result represented for stress release in sealing glass which was caused by the steel shell expanding under thermal load. Total stress release with temperature varying from 25°C to 100°C was 62MPa. Since the residual stress in sealing glass monitored by femto-laser inscribed FBG after the EPA annealing was 56MPa, the stress in sealing glass at 400°C had turned from compressive stress into tensile, and this would lead to the failure of EPA because the tensile strength of glass materials was much less than the compressive strength.
The stress change monitored by UV laser inscribed FBG showed larger deviation than the result of femto-laser inscribed FBG, which showed better performance of femto-laser inscribed FBG sensors to monitor temperature and stress simultaneously under thermal load. Based on this phenomenon, the spectra analysis of FBGs at different temperatures was carried out, and results were shown in Fig. 7(a)–7(c).
Spectrum of femto-laser inscribed FBG showed good shapes and distinct peaks from 25°C to 400°C. Spectra of UV laser inscribed FBG showed multiple peaks especially at low temperature shown in Fig. 7(a), which would lead to deviation of data reading by the interrogator, and this was the reason why the λμ + T curve of UV laser inscribed FBG showed considerable fluctuation. The Bragg wavelength generated red shift with temperature rising, as the sensing principle of temperature, at the same time, FWHM of the stress sensing FBG (FWHMμ + T) decreased as stress released, with FWHM of the temperature sensing FBG FWHMT keeping constant. There were similar phenomena reported by Chang  and Morey . After the non-uniform axial stress generating in sealing glass, the grating was chirped and FWHM would be broadened as the axial stress in EPA increased. Based on this principle, the FWHMμ + T could be an effective method to demodulate the stress change by a single FBG.
The relationship between FWHM and environment parameters was shown in Fig. 8(a). It showed that FWHMμ + T and stress was almost linear, so another possible method to demodulate stress and temperature could be achieved by the FWHMμ + T change and Bragg wavelength shift of a single femto-laser inscribed FBG embedded in sealing glass. The stress demodulation results by Δλμ + T − ΔλT and FWHMμ + T in sealing glass were compared in Fig. 8(b). The monitoring deviation between these two methods was about 6%, which indicated the FWHM method was reliable and had potential to simplified the arrangement of FBGs in EPA.
4.3 Results of on-line working state monitoring under pressure load
The wavelength gradually decreased with rising pressure as shown in Fig. 9, indicating that axial stress in sealing glass was increasing. However, for every 1 MPa rising in helium pressure, the wavelength decreased by only 0.015 nm on average, which represented about 0.39 MPa increasing of the stress in sealing glass. Furthermore, after unloading the pressure, Bragg wavelength returned to the original value again. The results showed the working pressure generated a small effect on axial stress in sealing glass, and the pressure could be monitored in real time by the embedded femto-laser inscribed FBG sensor.
4.4 Results of finite element analysis
Figure 10(a) showed the axial stress change in sealing glass at different temperatures. The axial stress released rapidly as temperature rising. When the working temperature was 400°C, stress in sealing glass turned from compressive stress into tensile stress, which was dangerous for the hermeticity of sealing glass. The result of numerical simulation was compared with the results of on-line monitoring experiment under thermal load, as shown in Fig. 10(b). The stress monitored by femto-laser inscribed FBG was close to the numerical result, with the deviation less than 8%, but the deviation between simulation and experimental result monitored by UV laser inscribed FBG was more considerable. The numerical result was consist with experimental results, and proved the femto-laser inscribed FBG could be reliable for on-line state monitoring under high temperature environment.
The numerical result of working pressure analysis was shown in Fig. 12(a). The axial stress increased with imposed pressure rising. However, the axial stress was larger in lower part of sealing glass and also showed a more significant change than the higher part. This was because that the position of imposed pressure was on the bottom surface of sealing glass as shown in Fig. 3, and it was the evidence for pressure durability of glass materials. The numerical result of stress change in sealing glass agreed with experimental result monitored by FBG shown in Fig. 11(b), with the deviation less than 10%. The pressure change of helium could be monitored by femto-laser inscribed FBG, although the varying pressure was less effective on stress in sealing glass compared with thermal load.
The comparative analysis of on-line monitoring experiment and numerical simulation under thermal and pressure load showed the feasibility and accuracy of femto-laser inscribed FBG for EPA state monitoring.
On-line state monitoring experiments of the sealing glass in EPA based on the femto-laser inscribed FBG sensors were demonstrated in this study.
- (1) Thermal and irradiation aging tests of FBGs were carried out and the results showed that femto-laser inscribed FBG generated less Bragg wavelength drift than UV laser inscribed FBG. Thermal sensitivity coefficient of femto-laser inscribed FBG was almost constant after thermal aging test, which showed good resistance to harsh environment especially under thermal load.
- (2) Simultaneous temperature and stress monitoring experiments under thermal and pressure load were performed by setting femto-laser inscribed FBG sensors in EPA, during which stress changes in sealing glass were obtained.
- (3) Two approaches to demodulate stress were presented. One was the temperature compensation method by femto-laser inscribed FBG sensors and the other one was the FHWM-wavelength method by one single FBG embedded in sealing glass. The monitoring deviation between the two methods was less than 6%.
- (4) Theoretical stress change of sealing glass under different variable parameters was obtained by finite element method, and the results showed good consistency to the experimental results with deviation less than 10%.
The presented results broadened the applications of FBGs and laid a foundation for the on-line monitoring of sensitive equipment based on FBGs in nuclear power plant.
National S&T Major Project of China (ZX06901); Tsinghua University Initiative Scientific Research Program (2014z21024).
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