## 4 Conclusions  
In this paper, the PCCV monitoring project by using FBG sensors provided us much information about the PCCV structure during the ILRT and showed good potential. Because the monitoring results of each strain FBG had the same trend compared with pressure variation, the feasibility of FBG had been proven. The results showed FBG could be widely used in PCCV structure health monitoring. During the data process stage, the FBG strain caused by temperature change could be eliminated by temperature-compensating FBG. However, the strain caused by concrete thermal expansion or contraction could not be eliminated by using temperature-compensating FBG. According to the monitoring results, the vertical direction strain changed about \(-100\)\(\mu\epsilon\) due to the abrupt temperature drop of \(11\)\({}^{\circ}\)C. Nevertheless, the circumferential direction strain was not sensitive to abrupt temperature drop compared to vertical direction strain.  
The monitoring results of each measuring points indicated that the circumferential direction strain was higher than vertical direction strain during ILRT. However, strain magnitude differences existed between four measuring points. This might be caused by the direction and the concrete material property.  
There were two cracks monitored by four FBGs. The results showed that crack No. 1 had the same trend compared with general measuring points while crack No. 2 had a sudden increase about 130 \(\mu\epsilon\) at 0.483 MPa. Attention should be paid to the sudden increase of crack No. 2 to prevent further cracking.  
In addition, a FEM model was built to simulate the ILRT process. Because the maximum inner pressure was 0.483 MPa and this magnitude was far from the ultimate inner pressure, the elastic  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Phase** & **Measure Point** & **Vertical** & **Circumferential** \\ \hline pressurization & MP 1 & 0.5372 & 0.0193 \\ pressurization & MP 2 & 0.0785 & 0.0106 \\ pressurization & MP 3 & 0.0516 & 0.0071 \\ pressurization & MP 4 & 0.1244 & 0.0324 \\ depressurization & MP 1 & 0.2718 & 0.0033 \\ depressurization & MP 2 & 0.0388 & 0.0026 \\ depressurization & MP 3 & 0.0083 & 0.011 \\ depressurization & MP 4 & 0.0954 & 0.0046 \\ \hline \hline \end{tabular}
\end{table}
Table 3: The ratio of residual sum of squares (RSS) to total sum of squares (SYY).  
material property was chosen for simplification. The calculation load step history in proportion to real ILRT time was imposed on the FEM model. The results showed that there was little difference on the strain magnitude of each direction due to the homogeneous material property. Besides, the calculation results were very close to the monitoring results, so the calculation results were proven. In addition, the strain distribution contour in two directions could give advice about where strain sensors should be installed. According to the results, more FBGs should be installed at the junction of the dome and cylinder. Moreover, the concrete strain-inner pressure relationship was compared between monitoring results and calculation results. Apparently, the relationship was almost linear. There were more fluctuations in the vertical direction monitoring results which might be caused by temperature change. Because the concrete strain-inner pressure relationship was almost linear, the material elastic hypothesis of FEM model mentioned above was proven.  
The research was supported by the Research Project of Suzhou Nuclear Power Research Institute (SNPI) under Grant No. R-2014 SZLM11 TM, and the National Natural Science Foundation of China under Grant No. 51278085 and No. 51479031.  
[PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] conceived and designed the experiments; [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] performed the experiments and collected the monitoring data; [PERSON], [PERSON], [PERSON] and [PERSON] analyzed the data; [PERSON] built the FEM model; [PERSON] and [PERSON] wrote the paper.  
The authors declare no conflict of interest.