Radiation resistant fiber Bragg grating in random air-line fibers for sensing applications in nuclear reactor cores.

This paper reports the testing results of radiation resistant fiber Bragg grating (FBG) in random air-line (RAL) fibers in comparison with FBGs in other radiation-hardened fibers. FBGs in RAL fibers were fabricated by 80 fs ultrafast laser pulse using a phase mask approach. The fiber Bragg gratings tests were carried out in the core region of a 6 MW MIT research reactor (MITR) at a steady temperature above 600°C and an average fast neutron (>1 MeV) flux >1.2 × 1014 n/cm2/s. Fifty five-day tests of FBG sensors showed less than 5 dB reduction in FBG peak strength after over 1 × 1020 n/cm2 of accumulated fast neutron dose. The radiation-induced compaction of FBG sensors produced less than 5.5 nm FBG wavelength shift toward shorter wavelength. To test temporal responses of FBG sensors, a number of reactor anomaly events were artificially created to abruptly change reactor power, temperature, and neutron flux over short periods of time. The thermal sensitivity and temporal responses of FBGs were determined at different accumulated doses of neutron flux. Results presented in this paper reveal that temperature-stable Type-II FBGs fabricated in radiation-hardened fibers can survive harsh in-pile conditions. Despite large parameter drift induced by strong nuclear radiation, further engineering and innovation on both optical fibers and fiber devices could lead to useful fiber sensors for various in-pile measurements to improve safety and efficiency of existing and next generation nuclear reactors.

Discrimination of Temperature and Strain in Brillouin Optical Time Domain Analysis Using a Multicore Optical Fiber.

Brillouin optical time domain analysis is the sensing of temperature and strain changes along an optical fiber by measuring the frequency shift changes of Brillouin backscattering. Because frequency shift changes are a linear combination of temperature and strain changes, their discrimination is a challenge. Here, a multicore optical fiber that has two cores is fabricated. The differences between the cores' temperature and strain coefficients are such that temperature (strain) changes can be discriminated with error amplification factors of 4.57 °C/MHz (69.11 µ ϵ /MHz), which is 2.63 (3.67) times lower than previously demonstrated. As proof of principle, using the multicore optical fiber and a commercial Brillouin optical time domain analyzer, the temperature (strain) changes of a thermally expanding metal cylinder are discriminated with an error of 0.24% (3.7%).

High resolution monitoring of strain fields in concrete during hydraulic fracturing processes.

We present a distributed fiber optic sensing scheme to image 3D strain fields inside concrete blocks during laboratory-scale hydraulic fracturing. Strain fields were measured by optical fibers embedded during casting of the concrete blocks. The axial strain profile along the optical fiber was interrogated by the in-fiber Rayleigh backscattering with 1-cm spatial resolution using optical frequency domain reflectometry (OFDR). The 3D strain fields inside the cubes under various driving pressures and pumping schedules were measured and used to characterize the location, shape, and growth rate of the hydraulic fractures. The fiber optic sensor detection method presented in this paper provides scientists and engineers an unique laboratory tool to understand the hydraulic fracturing processes via internal, 3D strain measurements with the potential to ascertain mechanisms related to crack growth and its associated damage of the surrounding material as well as poromechanically-coupled mechanisms driven by fluid diffusion from the crack into the permeable matrix of concrete specimens.

High pressure optical cell for synthesis and in situ Raman spectroscopy of hydrogen clathrate hydrates.

We report the design, realization, and test of a high-pressure optical cell that we have used to measure the Raman spectra of hydrogen clathrate hydrates, synthesized in situ by the application of 200-300 MPa of gas pressure on solid water. The optical apparatus is mounted on a cryogenic system so to allow measurements and sample treatment at any temperature between 300 and 20 K. A capillary pipe is connected to the inside of the cell to allow the gas flow into and out of the cell, and to regulate the internal pressure at any value from 0 to 300 MPa. In the experimental test described in this paper, the cell has been partly filled, at room temperature, with a small amount of water, then frozen at 263 K before injecting hydrogen gas, at pressure of 150 MPa, into the cell. This procedure has permitted to study hydrogen clathrate formation, by measuring Raman spectra as a function of time.

High pressure synthesis and in situ Raman spectroscopy of H2 and HD clathrate hydrates.

By means of a newly constructed high pressure and low temperature optical apparatus we have measured the Raman spectra of H(2) and HD simple clathrate hydrates, synthesized in situ by the application of more than 2500 bar gas pressure on solid water. High resolution spectra of the molecular vibration have been measured at low temperature (about 20 K). In the case of HD this band is simpler than in the case of H(2), where the presence of the ortho- and para-species complicated the interpretation of the spectrum. We have determined frequency positions of the bands arising from multiple occupancy of the large cages of the sII clathrate, some of which are almost superimposed. The intensity of the bands gives information on the average and distribution of cage occupation, and of the ortho-para (o-p) ratio of H(2) molecules. Hydrogen o-p conversion rate is measured, for molecules in the small cages and in the large cages, and it is observed that these are different. A model considering both intrinsic and extrinsic conversion processes is applied to the measured data. The intrinsic conversion rate so derived is compared favorably to that measured for pure hydrogen in different situations.