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This paper is an examination of the effect of an axially varying fluid temperature on the measurement of $\Delta {T}({z})$ , the difference in the temperature between the optical fiber within the thermal mass and the optical fiber within the capillary tube that is attached to the outer sheath of an Optical Fiber Based Gamma Calorimeter (OFBGC). Our analysis indicates that for an OFBGC with the general design of the OFBGC that was built and tested in the Ohio State University Research Reactor (OSURR), a sinusoidal variation with spatial frequency ${k}_{B}$ in the bulk fluid temperature ${T}_{B}$ along the length of the OFBGC will create a sinusoidal variation in $\Delta {T}({z})$ , which could be misinterpreted as a sinusoidal variation in the linear energy deposition rate within the OFBGC’s thermal mass. However, for the specific design of the OFBGC that was built and tested in the OSURR, for the slowly varying spatial variation of ${T}_{B}({z})$ along the length of the OSURR’s Water Irradiation Facility, within which the OFBGC was tested, the perturbation in $\Delta {T}({z})$ is negligibly small. [ABSTRACT FROM AUTHOR]

Optical frequency domain reflectometry (OFDR) is a family of optical techniques which can be used to produce distributed temperature measurements from the spectral shift of an interference pattern based on the Rayleigh backscatter signature of an optical fiber. Adaptive signal processing techniques have recently been used with OFDR to record meaningful spectral shift data from commercially available SMF-28 optical fibers heated beyond 950 °C. However, a correlation between the measured spectral shift and temperature has not yet been developed at these high temperatures. To extend the measurable temperature range of OFDR in SMF-28, this work describes the development of such a correlation from room temperature (22 °C) to 1000 °C. The relationship between spectral shift and temperature change over this range was found to be best characterized by the fourth-order polynomial $\Delta {T}=(-4.241\ast 10^{-11})\text {S}^{4} +(-2.017\ast 10^{-7})\text {S}^{3} +(-3.677\ast 10^{-4})\text {S}^{2}+(-0.8057)\text{S}$ , where $\Delta {T}$ represents the temperature difference compared to the reference temperature, and S represents the spectral shift measured by the fibers. The calibration developed in this work assumes that the fiber has been fully annealed by heating the fiber to 1000 °C for a few hours. This paper is the first to demonstrate the calibration and use of SMF-28 distributed optical fiber sensors up to 1000 °C, enabled using adaptive OFDR-based signal processing. [ABSTRACT FROM AUTHOR]

Optical frequency domain reflectometry measurements in internally clad single crystal sapphire fiber have received attention in recent years due to their high temperature distributed sensing potential. As work with these fibers has proceeded, there have been some inconsistencies in the results. Deeper investigation and testing has identified two critical considerations for the proper functioning of these fibers. First, users must address the propagation of multimode light along the outer surface of the fiber. By observing the far-field image of an internally clad sapphire fiber when adding index matching fluid to the outer fiber surface, we demonstrate the effects of removing the higher order modes from these fibers. The addition of index matching fluid resulted in nearly single mode performance where multimode performance was previously observed. Second, users must address the effect that coupling the fiber to the interrogator via silica based fiber has on the internally clad sapphire fiber's performance. Direct fusion splicing of silica to sapphire, as has been used in the recent work with these fibers, has a mode filtering effect which can be beneficial towards the modal behavior of the fibers. However, in this paper we demonstrate that the splicing can cause a sensing failure due to little or no low order mode light, that is useful for sensing, returning to the detector. The positive results from recent years have demonstrated that optical frequency domain reflectometry sensing performance will be successful in clad sapphire fiber; but only when the considerations described herein are addressed properly. [ABSTRACT FROM AUTHOR]

This paper describes and validates, by comparison with numerical modeling results, an analytical model of thermal transport in an optical fiber based gamma thermometer (OFBGT) that is appropriate for use in university research reactors. The maximum temperature difference between the thermal mass and the outer sheath ($\Delta {T}$) for the OFBGT design that we have considered is approximately ${50}^{\circ }C$ , for the OFBGT in the Central Irradiation Facility of the Ohio State University Research Reactor (OSURR) with the reactor operating at full power (450 kW). The maximum value of $\Delta {T}$ that is predicted by the analytic model for the OFBGT design is smaller by approximately 1.1 °C than the value of $\Delta {T}$ which is predicted by the numerical model, for the same OFBGT design, but including all the details of the design. We have used the analytical model of thermal transport in an OFBGT to determine a normalized Modulation Transfer Function ($MTF'(k))$ for the OFBGT. We conclude that $\textit {MTF}^{\prime }\left ({{k}_{\textit {OSU}} }\right)>{0.99}$ , where ${k}_{\textit {OSU}}$ is the spatial frequency for the axial dependence of the reactor power distribution in the OSURR. [ABSTRACT FROM AUTHOR]

Type I and type II Bragg grating arrays in silica fiber have been irradiated at the Ohio State University Nuclear Reactor Laboratory in the gamma facility and the pool-type reactor, to receive a relatively small gamma dose (2.82 Mrad) and a relatively large gamma–neutron dose (638 Mrad, ${6\times }{10}^{17}\rm {({n}/{cm^{2}})}$), respectively. The Bragg grating fibers were first irradiated in the gamma facility and those same fibers were then subsequently irradiated in the reactor, while being monitored in situ by optical frequency domain reflectometry. The response of the Bragg gratings, in terms of reflected amplitude, was analyzed, both in the frequency and spatial domains. A Bragg grating modeling code, written in MATLAB, was then used to generate modeled fiber Bragg grating (FBG) responses to match the experimental responses, based on some fundamental assumptions. With the modeled responses, we were able to deduce the alterations to the refractive index profile of the Bragg grating inscribed fibers as a result of the irradiation. [ABSTRACT FROM AUTHOR]

Experiments were performed to test fiber optic temperature sensors under thermal conditions similar to those that would be experienced in the transient reactor test facility (TREAT) at the Idaho National Laboratory. Specifically, four single-mode optical fiber sensors were dropped into a pre-heated furnace to simulate a step temperature transient, for steps from room temperature to indicated furnace temperatures, ranging from 700 °C to 1100 °C. The four fibers were a standard SMF-28 fiber, a fiber with inscribed type-I fiber Bragg gratings (FBGs), and two fibers with inscribed type-II FBGs. The two type-II FBG fibers were different in the initial backscatter signal of the gratings; one fiber had gratings with greater backscatter reflection than the other. The type-I FBG sensor and standard SMF-28 sensor measured temperature for temperature transients with a final temperature of 700 °C. The low reflection type-II FBG sensor measured temperature transients with final temperatures up to 950 °C. The high-reflection type-II FBG sensor successfully measured temperature for short times with final temperatures up to 1050 °C. For long times at 1050 °C, there was a drift in the temperature sensing. We conclude that the high-reflection type-II FBG sensor would perform acceptably for application in the thermal environment of TREAT, or for other step temperature transients from room temperature to final temperatures up to 1000 °C. [ABSTRACT FROM AUTHOR]

Multimode and single-mode silica fibers have been tested in a high-temperature setting, while simultaneously experiencing a bend. The range of temperature tested was 500 °C–1000 °C and the bend radii considered were 1.27, 2.54, and 3.81 cm (0.5, 1.0, and 1.5 in). Multimode fiber (MMF) was analyzed for transmission, while single-mode fiber (SMF) was both analyzed for transmission and interpreted by optical backscatter reflectometry. Attenuation was observed to increase in MMF with increasingly high temperatures and tighter bend radii. SMF was far more resilient to transmission loss than MMF, under similar conditions of temperature and curvature. Optical backscatter reflectometry was performed for bent SMF up to 1100 °C and revealed evidence that, at high temperatures, bends in fiber can enhance devitrification that can inhibit light transmission through the fiber. This paper suggests that the devitrification process begins at the surface of the fiber and moves inward. Moreover, this paper suggests that bend stress applied to surface flaws of the fiber leads to significantly higher crystallization rates at temperatures at which silica does not typically devitrify so rapidly. [ABSTRACT FROM AUTHOR]

The use of single-crystal sapphire optical fibers has been considered to extend fiber-optic sensing to the extreme temperature (>1000 °C) environments encountered in nuclear applications. However, before these sapphire fiber–based sensors can be deployed, their optical transmission and dimensional stability (which impacts drift of some sensors) must be characterized under representative testing conditions. Data regarding the optical transmission of sapphire following high-dose neutron irradiation at temperatures >100 °C is extremely limited. This work provides measurements of optical density (i.e., attenuation) and directional dimensional changes in bulk single-crystal sapphire materials irradiated to a fast neutron fluence of 2.4 × 1021 n/cm2 (3.5 displacements per atom) at temperatures ranging from 95 to 688 °C. Optical density measured after irradiation at 95 and 298 °C showed ultraviolet and visible absorption bands corresponding to known defect centers and temperature trends that were generally consistent with previous ex situ and in situ measurements made at much lower neutron fluence. However, optical density measured after irradiation at 688 °C was as much as two orders of magnitude higher, indicating that the fundamental mechanism for radiation-induced attenuation changes at this irradiation temperature. Additional analysis and comparison with previous works suggest that the attenuation may result from void formation, leading to increased Rayleigh scattering losses in the material and increased swelling that would also result in drift of Bragg grating-based sensors in sapphire fibers. These results pose serious questions regarding the feasibility of sapphire fiber–based sensors for high-temperature nuclear applications. [ABSTRACT FROM AUTHOR]

A 4-step data analytic methodology has been devised for the purpose of inferring the distribution of power in a reactor core, based on the response of an array of optical fiber based gamma thermometers (OFBGTs). This data analytic methodology is crucial for the development of a system of OFBGTs for the purpose of calibrating local power range monitors in boiling water reactors. Such a system would be an improvement to the present calibration system in boiling water reactors, in terms of safety, efficiency, and permanence. The first step of this methodology is to establish an energy balance method. In this method, one uses MCNP to determine response functions, which allow one to convert from gamma thermometer response to power. The gamma thermometers and the reactor core are segmented, such that each gamma thermometer segment provides an estimate of power for each reactor core segment. The estimates of power for the reactor core segments (hereafter referred to as fuel assembly segments) are calculated based on the measured response of the gamma thermometers. The second step of the methodology is to employ a weighting scheme to combine the estimates of the power of the various fuel assembly segments, based on the response of the various OFGBT segments and the incremental dose rates of the various fuel assembly segments to the OFBGT segments. The third step is to iteratively calculate the estimates of the power of the fuel assembly segments, until a convergence criterion is met, which indicates that the calculation has converged. The fourth and final step of the methodology is to estimate the uncertainty of the power for each fuel assembly segment. The mathematical basis for this step is not the focus of this paper. We have used a 3-D homogeneous reactor model to demonstrate the data analytic methodology; and have found that the data analytic methodology operates as intended. • A data analytic methodology was developed to infer the power distribution in a reactor, based on optical fiber-based gamma thermometer responses. • The data analytic methodology is based on energy balance, and the weighting of the data is based on incremental dose rates. • A 3-D model was used to provide an example of how the methodology works, when the modeled data and "measured" data disagree. [ABSTRACT FROM AUTHOR]