Research On Optical Fiber Sensors For Measuring The Temperature And Mechanics Parameters Of Nuclear Power Unit

Accurate measurement of process parameters of nuclear power plants(NPPs)is extremely important to the safety and reliability of its operation.Optical fiber sensors(OFS)have unique advantages such as resistance to harsh environments and anti-electromagnetic interference in the measurement of process parameters of NPPs.However,there are three key problems that need to be solved urgently to accurately measure these parameters of the NPPs.(1)Temperature-strain coupling deteriorates the measurement accuracy of OFS.(2)Radiation causes the sensing performance drift of the OFS,which makes it difficult to achieve accurate measurement of the parameters.(3)Radiation-induced loss(RIL)degrades the sensing performance of OFS.For the above-mentioned key issues,this dissertation focused on the design and implementation of optical fiber Fabry Perot and grating microstructure array sensors suitable for nuclear radiation environments,and has carried out related research works.The main achievements are as follows:(1)Optimizing thermal annealing process to reduce the RIL of the fibers.Aiming at the technical problem of radiation attenuation optical power in the fibers,the optimization of thermal annealing process was explored by studying the effects of preheating,self-repair at room temperature,stepped heating and cooling,and other processes on the recovery effect of the loss of irradiated fibers.The results show that:during stepped annealing process,there were optimal annealing temperatures for Ge-doped single-mode fiber(SMF),Ge-doped polarization-maintaining SMF,pure silica SMF,and F-doped SMF,their values were 350 ?,400 ?,300 ?,and 450 ?,respectively.Compared with F-doped SMF sample without pre-annealing,pre-annealing at 100 ? can reduce the radiation sensitivity and increase the rate of RIL reduction of this fiber in stepped annealing process.This work provided support for the development of OFS suitable for radiative environments.(2)Multi-functional all-fiber sensors with high strain sensitivity and capable of measuring strain/pressure/temperature were designed and implemented.Aiming at the technical problem of temperature-strain coupling introducing measurement error,using femtosecond laser inscribed fiber Bragg gratings(FBG)to compensate thermal drift of the Fabry Perot interferometers(FPIs)or the complementary of thermal expansion of the FP cavity structure,to realize simultaneous measurement of temperature and strain.The results show that: 1)Two sensing structures,in which FBG was respectively cascaded with microhole FPI and sensitivity-enhanced FPI,have excellent high temperature strain and high pressure sensing performances.Compared with FBG written on SMF(10 ??/°C),the cross-sensitivity of strain-temperature of microhole FPI(0.1??/°C)and sensitivity-enhanced FPI(0.139 ??/°C)was reduced by nearly 2 orders of magnitude.2)Compared with the various extrinsic FPIs with low temperature coefficients reported so far,the sensitivity-enhanced FPI with length-matched cavity exhibited the lowest temperature sensitivity(?0.1 pm/?).And compared with FBG written on SMF,its cross-sensitivity(0.015 ??/°C)of strain-temperature was reduced by about 667 times.This work proposes a method to control temperature sensitivity of the FPI sensor by controlling the ratio of the cavity length to the length of the truncated cone in FP cavity,which provides a new solution for reducing the temperature sensitivity of the FPI sensor.(3)A high-performance all-fiber FPI strain sensor suitable for high radiation environment was designed and implemented.When applying FPI sensor in radiative environments,it faces the bottleneck of measurement error caused by sensing performance drift of the FPI sensor induced by radiation.Using the complementarity of radiation-induced compaction effect of the cladding and the truncated cone of the FP cavity,an FPI sensor design method for the length-matched FP cavity was proposed,to realize an FPI strain sensor that works stably in radiative environments.The results show that: after steady-state gamma irradiation(279.483 k Gy,70 ?),the FPI sensor with length-matched FP cavity has the smallest cavity length drift of-0.037 ?m and the smallest strain sensitivity deviation of 0.52%,and the microhole FPI produced a cavity length drift of 1 ?m.Compared with the microhole FPI,the FPI with length-matched cavity reduced the strain measurement error caused by the cavity length drift by 30 times.(4)An array design method of FBG-FP microstructure composed of two identical FBGs,which is suitable for multi-point measurement in radiative environments,was proposed.For multi-point measurement requirements in radiative environments,the FBG-FP array was used as the sensing unit,and the reflectance arrangement of the FBG in the array was designed,to solve the technical problem of RIL degrading the sensing performance of the FBG-FP array.The design criterion for the reflectivity arrangement of the FBG in array was that the reflected power of the first FBG in each FBG-FP received by the detector was equal in extreme radiation environments,and the reflected power of the FBG-FP microstructure was within the dynamic range of the system.A design method of FBG-FP array was proposed in which the arrangement of the first FBG reflectivity in the array was enhanced according to a specific rule.Simulation results show that: compared with the FBG array with identical reflectivity or Rayleigh scattering system,FBG-FP array designed by this method can equalize optical power,and reduce the requirements for the dynamic range of the system by 7.5 d B under strong radiation,which was the difference in power between the strongest point and the weakest point of the optical power.This work provides a new technical approach for the application of FBG-FP array sensors in radiation environments.In summary,the research objects of this dissertation were all-fiber extrinsic FPI and FBG.The goal was to achieve temperature and mechanics parameters measurement of NPPs.The corresponding technology was proposed to solve the technical problems faced by extrinsic FPI and FBG in the measurement of temperature and mechanics parameters of NPPs.Dual parameter sensing or the complementarity of thermal expansion of FP cavity structure was used to solve the problem of temperature-strain coupling.The complementarity of radiation-induced compaction of the FP cavity structure and enhanced arrangement of reflectivity of the grating in FBG-FP array,were used to solve the problem of radiation-induced drift in sensing performance and RIL degraded the sensing performance,respectively.The research work of this dissertation lays the technical foundation for the application of OFS in radiative environments.Therefore,the research work of this dissertation has important scientific research significance and practical engineering application value.