IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52, NO. 6, DECEMBER 2005 2689 Radiation-Tolerant Raman Distributed Temperature Monitoring System for Large Nuclear Infrastructures A. Fernandez Fernandez, P. Rodeghiero, B. Brichard, F. Berghmans, A. H. Hartog, P. Hughes, K. Williams, and A. P. Leach Abstract—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. Index Terms—Gamma radiation, optical fiber sensing, radiation effects, Raman scattering, temperature nuclear power plant. I. INTRODUCTION T HE range of applications of optical fiber distributed tem- perature sensors is rapidly expanding in the industry [1], [2]. Nowadays, this fiber optic sensing technology is mature for industrial applications such as fire detection inside buildings and tunnels, process vessel monitoring, leak detection in cryo- genic storage vessels (liquid natural gas, ammonia, ethylene) or oil wells [3] and the measurement of energy cable thermal distribution for the power supply industry [4], [5]. These appli- cations rely on the well-known immunity of fiber optic sensors to electro-magnetic interference and on the ability of fiber sen- sors to be operated in hazardous environments. Moreover, the use of optical fiber distributed sensors for temperature sensing is a powerful way of monitoring, quasisimultaneously, thousands of points avoiding the requirement of optimum positioning of discrete temperature sensors. Most commercially-available distributed temperature sensors derive the temperature profile from the measurement of the Raman backscattered light inten- sity along the fiber, using optical time domain reflectometry techniques. The Raman signal comprises two elements: the Stokes and anti-Stokes lines. The longer-wavelength Stokes line is only weakly temperature sensitive but the intensity of Manuscript received July 8, 2005; revised August 26, 2005. This work was supported by the European Commission under the Contract of Association be- tween Euratom and the Belgian State. It was carried out within the framework of the European Fusion Development Agreement (EFDA). The views and opinions expressed herein do not necessarily reflect those of the European Commission. A. Fernandez Fernandez is with SCK CEN, Belgian Nuclear Research Centre, B-2400 Mol, Belgium and also with Université libre de Bruxelles, B-1050 Brussels, Belgium (e-mail: afernand@sckcen.be). P. Rodeghiero was with SCK CEN, Belgian Nuclear Research Centre, B-2400 Mol, Belgium. He is now with the Institute of Physics, Université catholique de Louvain, B-1348 Louvain la Neuve, Belgium B. Brichard and F. Berghmans are with SCK CEN, Belgian Nuclear Research Centre, B-2400 Mol, Belgium. A. H. Hartog, P. Hughes, K. Williams, and A. P. Leach are with Sensa (a Schlumberger Company), Chilworth Science Park, Southampton SO16 7NS, U.K. Digital Object Identifier 10.1109/TNS.2005.860736 the backscattered light at the shorter anti-Stokes wavelength increases with an increase in temperature. The nuclear industry shows a growing interest for the pos- sibilities offered for temperature sensing applications [6], [7]. Fiber optic sensing technology could be considered as an alter- native to classical measurements techniques in a wide range of applications. The potential of distributed temperature measure- ments for the monitoring of large nuclear infrastructures such as reactor containment buildings, nuclear waste repositories and reactor primary circuitry have already been shown [8]–[11]. However, a major problem in the application of optical fibers in nuclear environments is the presence of ionizing radiation fields that induce an increase of the optical fiber attenuation [12], [13]. Since the Raman systems rely on optical intensity measurements, differential radiation-induced attenuation (RIA) for the Stokes and anti-Stokes lines causes incorrect temper- ature measurements. Two special correction techniques of ra- diation-induced losses have been proposed for RDTS systems using open-ended arrangements. The first correction technique requires two calibrated thermocouples located in a well-known position next to the fiber but it is limited to nuclear areas with an almost uniform dose rate and temperature [8]. The second technique, proposed for nuclear environments with an unknown ionizing field distribution, consists of making a loop with the sensing fiber [10]. The fiber loop makes points along adjacent fiber sections effectively equivalent in terms of temperature and dose rates. The Raman ratio is then calculated as the geometric mean of the backscattered Raman intensities [14]. The main drawback of this technique is the significant increase of the com- plexity and therefore the cost of fiber installation in hazardous areas. Radiation-tolerant pure silica has been proposed as sensing fiber to keep RIA minimal [9]. We propose to rely on com- mercial-grade multimode fibers using the double-ended Raman detection scheme to cope with radiation-induced losses, without any specific calibration technique. In such a configura- tion, the sensing fiber is probed from each end. A double-ended measurement has two major advantages over single-ended measurements. The first is the accuracy. Since the effect of optical losses is eliminated from the temperature measurement, the system becomes not only insensitive to bending or con- nector loss but also to radiation-induced loss as we will show in this paper. The second advantage is its intrinsic robustness that makes this system very suitable for nuclear environments. Temperature profile measurements can continue along the total length of the cable in the event of a fracture of the fiber since the fiber will continue to be probed from each end regardless. 0018-9499/$20.00 © 2005 IEEE