Alberto Fernandez

High total dose radiation effects on temperature sensing fiber Bragg gratings

Fiber Bragg gratings (FBGs) written in a 10 mol.% Ge-doped core silica fiber using a phase mask were exposed to /spl gamma/-radiation. The transmission and reflection spectra were recorded during irradiation up to doses in excess of 1 MGy. There was no detectable change of the Bragg peak amplitude and the grating temperature sensitivity. The radiation-induced shift of the Bragg wavelength saturated at a dose of 0.1 MGy at a level less than 25 pm, which could still be decreased by optimization of the grating parameters. Our results confirm that FBGs are good candidates for sensing applications in radiation 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.

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 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.

Radiation-resistant WDM optical link for thermonuclear fusion reactor instrumentation

The future International Thermonuclear Experi- mental Reactor (ITER) is a complex installation that will require permanent monitoring and frequent maintenance operations. The high-gamma dose rates, the high neutron fluence, and other radiological hazards call for the use of remote-handled equipment. The management of heavy umbilicals connecting the control systems with the remote tools is therefore a key issue. Multiplexing signals can relieve the cable-handling difficulties. In this respect, the intrinsic wavelength division multiplexing (WDM) capabilities of fiber-optic technology make it a very promising candidate for integration in ITER instrumentation links. However, the radiation hardness of a complete WDM optical link still needs to be assessed. In this paper, as a first step toward the development of a rad-hard WDM optical link, we report on irradiations of different parts of a typical WDM optical link. We present our irradiation results on COTS fiber-optic devices, including WDM single-mode couplers, which remain operational up to MGy dose levels while the channel drift observed in narrow-band couplers compromises their use in WDM multiplexers. The intrinsic wavelength encoding of fiber Bragg grating (FBG) sensors makes them ideal candidates for WDM fiber-optic sensor networks. Therefore, we also investigated the -radiation response of FBGs written in germanosilicate fibers. We irradiated such sensors up to MGy dose levels. At a total dose of 0.1 MGy, saturation of the radiation-induced Bragg peak shift has been observed, evidencing the potential radiation hardness of FBG-based devices in highly radioactive environments. To illustrate wavelength multiplexing in sensing, we discuss our preliminary results on a new multicomponent force sensor design based on eight multiplexed FBG sensors intended for use at the end effector wrist of remote-controlled robots. Finally, we present the in-reactor irradiation results of standard Corning Ge-doped fiber up to GGy dose levels. Index Terms—Fiber Bragg grating (FBG) sensor, gamma radia- tion, International Thermonuclear Experimental Reactor (ITER), nuclear robotics, optical fiber sensor, radiation effects, wavelength division multiplexing (WDM).

Fibre-optic gamma-flux monitoring in a fission reactor by means of Cerenkov radiation

We demonstrate the possibility of using Cerenkov radiation to monitor the reactor power and the high energy gamma-ray flux in a high neutron flux reactor. The system employs a radiation-resistant pure silica glass fibre to measure the Cerenkov radiation in the infrared region (800–1100 nm). A model is proposed to determine the order of magnitude of the gamma-ray flux from the measurement. The method and concept can be extended to the monitoring of low reactor powers if Cerenkov radiation is measured in the 450–500 nm region by means of hydrogen-treated fibres.

The effects of gamma-radiation on the properties of Brillouin scattering in standard Ge-doped optical fibres

We have experimentally studied the effects of gamma-radiation up to very high total doses on the physical properties of Brillouin scattering in standard commercially available optical fibres. A frequency variation of about 5 MHz for both Brillouin frequency and linewidth has been measured at the total dose of about 10 MGy. The radiation-induced shift has a negligible practical impact and makes Brillouin scattering very immune to radiation, so that distributed sensors based on this interaction exhibit an interesting potential for use in nuclear facilities.

Radiation effects in optical communication devices

We measure the ionizing radiation induced optical loss in different fiber-optic communication devices and evaluate the total optical loss in a multimode fiber link for application in high total dose nuclear environments.