B. Brichard

Radiation Effects on Silica-Based Optical Fibers: Recent Advances and Future Challenges

In this review paper, we present radiation effects on silica-based optical fibers. We first describe the mechanisms inducing microscopic and macroscopic changes under irradiation: radiation-induced attenuation, radiation-induced emission and compaction. We then discuss the influence of various parameters related to the optical fiber, to the harsh environments and to the fiber-based applications on the amplitudes and kinetics of these changes. Then, we focus on advances obtained over the last years. We summarize the main results regarding the fiber vulnerability and hardening to radiative constraints associated with several facilities such as Megajoule class lasers, ITER, LHC, nuclear power plants or with space applications. Based on the experience gained during these projects, we suggest some of the challenges that will have to be overcome in the near future to allow a deeper integration of fibers and fiber-based sensors in radiative environments.

Radiation-tolerant Raman distributed temperature monitoring system for large nuclear infrastructures

Raman Distributed Temperature Sensors (RDTS) are attractive for the monitoring of large structures in nuclear power plants such as containment structures and coolant loop systems. We demonstrate the high radiation tolerance of a Raman distributed fiber optic temperature sensor, up to total gamma doses in excess of 300 kGy, using a double-ended configuration and commercially-available optical fibers.

Temperature monitoring of nuclear reactor cores with multiplexed fiber Bragg grating sensors

In-core temperature measurement is a critical issue for the safe operation of nuclear reactors. Classical thermocouples require shielded connections and are known to drift under high neutron fluence. As an alternative, we propose to take advantage of the multiplexing ca- pabilities of fiber Bragg grating (FBG) temperature sensors. Our experi- ments show that sensitivity to radiation depends on both the radiation field and the grating characteristics. For some FBGs installed in an air- cooled graphite-moderated nuclear reactor the difference between the measurements and the readings of calibrated backup thermocouples was within the measurement uncertainty. In the worst case, the differ- ence saturated after 30 h of reactor operation at about 5°C. To reach megagray per hour level gamma-dose rates and 10 19 neutron/cm 2 flu- ences, we irradiated multiplexed FBG sensors in a material testing nuclear reactor. At room temperature, FBG temperature sensors can sur- vive in such radiation conditions, but at 90°C a severe degradation is observed. We evidence the possibility to use FBG sensing technology for in-core monitoring of nuclear reactors with specific care under well- specified conditions.

Dose-rate dependencies in gamma-irradiated fiber Bragg grating filters

Wavelength-division multiplexed fiber-optic links could find applications in various radiation environments. The space industry, for example, could benefit from radiation-tolerant high-speed optical links for the next generation of observation satellites. For applications in the nuclear energy industry, the use of passive optical devices also needs to be evaluated at high dose rates and total doses. Our previous results have shown that in-fiber Bragg grating filters are key components in radiation-resistant passive optical links, as they showed a remarkably high radiation-acceptance level in terms of total absorbed dose. The dose-rate dependencies also need to be evaluated. We therefore studied gamma-irradiated fiber Bragg grating filters at several dose rates ranging from 1 to 25 kGy/h. We show, for the first time, that the dose rate does not affect the full-width at half-maximum of the Bragg peak and that the saturating Bragg peak shift remains.

Radiation effect in silica optical fiber exposed to intense mixed neutron-gamma radiation field

We measured in situ the radiation-induced absorption of pure silica core fibers exposed to a fission nuclear reactor. We observed the growth of the 1.39-/spl mu/m. OH vibration band in polymer coated fiber. Three mechanisms are responsible for this effect: recoil protons, hydrogen diffusion, and a probable compaction effect. Based on this experiment, a fiber-optic neutron monitor prototype is proposed.

An Introduction to Radiation Effects on Optical Components and Fiber Optic Sensors

We review the effects of ionizing radiation on various types of optical components including optical fiber sensors and summarize some of their applications in particular environments where the presence of energetic radiation is a concern.

Round-robin irradiation test of radiation resistant optical fibers for ITER diagnostic application

Fused silica core optical fibers are expected to play crucial roles especially in the size-reduced International Thermonuclear Experimental Reactor (ITER-FEAT). Several radiation resistant optical fibers have been developed in Japan and the Russian Federation. The task force on radiation effects in diagnostic components in the ITER-EDA (engineering and design activity) promoted international round-robin irradiation experiments on the developed optical fibers. Ten different optical fibers were tested in a cobalt-60 gamma-ray irradiation facility and in the Japan Materials Testing Reactor. The paper reports results obtained on five different optical fibers, which include purified, hydrogen loaded, and fluorine doped ones. Results show that the developed optical fibers could be deployed in remote handling and out-of-vessel applications. But, for the in-vessel diagnostics in the visible range optical spectroscopy, further improvement of the radiation resistance of optical fibers will be needed.

Round-robin evaluation of optical fibres for plasma diagnostics

Optical plasma diagnostic systems often require a complex optical path involving mirrors, lenses and windows. The use of optical fibres could significantly simplify the design of such a system. Radiation-induced effects, namely induced absorption and luminescence, may compromise the integration of optical fibres in the diagnostic system. However, recent developments of new fused silica glass types open perspectives to use optical fibres as a wave-guide device. In this paper, we present the common European, Japanese and Russian efforts in setting up a standardized round-robin procedure to test the optical fibres under relevant ITER conditions. The round-robin procedure should allow to select the most appropriate optical fibre. We briefly illustrate the discussion with results of an in-core reactor irradiation of pure silica optical fibres in the BR2 material testing reactor of the Belgian Nuclear Energy Research Centre.

Gamma radiation effects in Er-doped silica fibers

The radiation behavior of a commercially available Er-doped fiber is evaluated under varying gamma dose rates with in-situ spectral loss measurements. Complementary post-irradiation photoluminescence measurements allow us to better understand the radiation effects. Our results suggest that the microscopic environment of the Er/sup 3+/ ions is not much affected by the gamma irradiation, unlike the host matrix. We discuss the impact for potential applications of these commercially available fibers in radiation environments.

Real-time gamma dosimetry using PMMA optical fibres for applications in the sterilization industry

The use of polymethylmethacrylate (PMMA)-based plastic optical fibre as an intrinsic real-time gamma dosimeter is investigated. The radiation-induced attenuation of the fibres is monitored in situ during the course of irradiation. The PMMA fibre exhibited a linear radiation- induced attenuation response at various wavelengths for a dose range of 50 Gy to 50 kGy. The sensitivity, ranging from 0.4 dBm−1 kGy−1 to 0.03 dBm−1 kGy−1, is wavelength dependent, with high sensitivity at the lower wavelengths.