Maintaining the mechanical strength and hermetic reliability of metal-to-glass-ceramics electrical penetration assembly (MtGc-epA) is a key concern for ensuring the pressure boundaries of nuclear power plants. the transient temperature change caused by power adjusting or accidents in High Temperature Reactor Pebble-bed Modules may affect the structural health of sealing glass-ceramics, even leading to radiation leakage. to evaluate whether the function could survive temperature variations during the service life, thermal cycling aging experiments were imposed to MtGc-epA. A grating length-mismatched sensing method to monitor the residual strain, an important factor of glass-ceramics structural health, was demonstrated in real-time by femto-laser inscribed fiber Bragg grating (fBG) sensor during the curing process and thermal cycling aging. Scanning electron microscope (SeM) and leakage rate tests were carried out to obtain the comparisons of microstructure and hermeticity before and after the thermal cycling. the residual strain showed a slight growth trend with thermal cycles repetition and it persisted a high value (~ 4,000 με) reflected by both Bragg wavelength shift and spectrum shape. the grating length mismatched single fBG embedded in glassceramics was feasible to demodulate the temperature and strain simultaneously, and the embedded fBG method achieved the structural health monitoring of MtGc-epA during thermal cycling aging with good accuracy and reliability. combining with the results of SeM and leakage rate detecting, the structural health of MtGc-epA was demonstrated to be capable to endure the severe thermal conditions in nuclear reactors. Electrical penetration assembly (EPA) is the essential component to keep integrality of pressure boundaries in nuclear reactors as shown in Fig. 1. However, long-term service at high temperature in HTR-PM may decrease the structural strength and hermeticity of EPA. The failure of pressure boundaries may lead to severe disasters such as radiation leakage of Fukushima Daiichi Nuclear Power Plant, in which the leakage of hydrogen occurred at the penetrations and the gasket seals of the flange 1. As a result, it is significant to ensure the hermetic reliability of EPA at high temperature environment. Metal-to-glass seal structure EPA was widely used in nuclear reactors. This structure, consisting of steel shell, sealing glass and electric conductor, was designed according to the mismatched coefficient of thermal expansion (CTE) 2. The difference between the CTE of steel shell and sealing material generated the residual strain in sealing material after the curing process, which maintained good hermetic reliability of EPA 3. The similar CTE of sealing glass and conductor guaranteed the sealing glass will not be destroyed by the tensile stress resulted from the expansion of central conductor.

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