To ensure their safe operation and predict remaining life, environmental monitoring is necessary. In particular, temperature and radiation dose are considered to ...
Distributed optical fibre temperature measurements in a low dose rate radiation environment based on Rayleigh backscattering A. Faustov1,2,3, A. Gusarov, Member IEEE1, M. Wuilpart, Member IEEE2, A.A. Fotiadi2,4,5 L.B. Liokumovich3, O.I. Kotov3, I. O. Zolotovskiy5, A. L. Tomashuk5,6, T. Deschoutheete7, P. Mégret, Member IEEE2 1
SCK•CEN, Boeretang 200, 2400 Mol, Belgium University of Mons, place du Parc 20, 7000 Mons, Belgium 3 St. Petersburg State Polytechnical University, 195251 St. Petersburg, Russia 4 Ioffe Physical-Technical Institute of RAS, 194021 St. Petersburg, Russia 5 Ulyanovsk State University, 432970 Ulyanovsk, Russia 6 Fiber Optic Research Center of RAS, 119333 Moscow, Russia 7 Laborelec, Rodestraat 125, 1630 Linkebeek, Belgium 2
ABSTRACT On-line monitoring of environmental conditions in nuclear facilities is becoming a more and more important problem. Standard electronic sensors are not the ideal solution due to radiation sensitivity and difficulties in installation of multiple sensors. In contrast, radiation-hard optical fibres can sustain very high radiation doses and also naturally offer multi-point or distributed monitoring of external perturbations. Multiple local electro-mechanical sensors can be replaced by just one measuring fibre. At present, there are over four hundred operational nuclear power plants (NPPs) in the world 1. Operating experience has shown that ineffective control of the ageing degradation of major NPP components can threaten plant safety and also plant life. Among those elements, cables are vital components of I&C systems in NPPs. To ensure their safe operation and predict remaining life, environmental monitoring is necessary. In particular, temperature and radiation dose are considered to be the two most important parameters. The aim of this paper is to assess experimentally the feasibility of optical fibre temperature measurements in a low doserate radiation environment, using a commercially available reflectometer based on Rayleigh backscattering. Four different fibres were installed in the Sub-Pile Room of the BR2 Material testing nuclear reactor in Mol, Belgium. This place is man-accessible during the reactor shut-down, allowing easy fibre installation. When the reactor operates, the dose-rates in the room are in a range 0.005-5 Gy/h with temperatures of 40-60 °C, depending on the location. Such a surrounding is not much different to some "hot" environments in NPPs, where I&C cables are located. Keywords: Radiation environments, nuclear power plants, distributed optical fibre temperature measurements, optical frequency domain reflectometry
1. INTRODUCTION Although many European countries have recently chosen to decrease the impact of nuclear energy, there are more than 400 nuclear power reactors in the world. This situation cannot be changed rapidly, thus it will persist in future especially while other countries keep on building new NPPs. However, the safety requirements become stricter which demands constant monitoring and awareness of all processes and condition changes in the controlled area. One of the reasons for this is ageing of in-containment instrumentation and control cables. Cables are inherent components of any NPPs as they are used to link power instrumentation end equipment utilised for monitoring and controlling the plant. There are thousands of kilometres of electrical cables and wires of several hundred different types and sizes. They are supposed to
withstand normal operation conditions as well as design basis events (DBE) and post-DBE conditions occurring at any time during its service life 1. Also economic reasons bring about the need of plant service life extension. This means that all its components including cabling should endure significantly longer operational life. The biggest concern for ageing of the cables is related to organic polymer materials which are used as electrical insulators, dielectrics, and jacket materials. Most important factors speeding up ageing degradation of the polymers are temperature and ionising radiation. The hazard arising due to ageing degradation is mainly caused by mechanical properties changes, cracking of the insulator which might end up with a n electrical failure and with ignition as the worst1. At present, most of qualification tests of the cables utilised at NPPs are based on accelerated ageing. The main idea of it is to pre-expose cables to radiation doses and temperatures much higher than those which the cables are planned to be exposed to. Although for most of the materials such a test allows to estimate ageing close enough to a real environment long term exposure, there are still some materials which failed the DBE test at doses much lower than predicted 1. It occurs due to the difference of the mechanisms of the accelerated ageing and those during real operation. For instance, for most of the insulation materials the ageing degradation depends on the dose rate which is much higher in case of accelerated ageing. There are many kilometres of cables at NPPs and, of course, it is impossible to monitor every one of them. Therefore there should be a defined list of the cables which are of a higher concern than the others due to being exposed to severer environment. Since a cable is in the list it should be constantly inspected. The inspection frequency is usually based on the predicted parameters of the environment. Apparently, the operation environment conditions can not only be severer then predicted, which reduces the service, but also milder. If the latter case, it is economically unreasonable to replace the cables before they have reached a certain weary state. Thus, the decision of cabling replacement related with ageing processes becomes a trade-off between the economic reasoning and growing risks of failure. However, the best solution to have an adequate estimation of cable degradation is to perform constant measurements of temperature and radiation dose at the cable location through its whole operational life. This may significantly simplify the problem of mis-predicting of environment condition and help to re-estimate the qualified cable life. To perform the constant monitoring of temperature and radiation dose electromechanical sensors are used nowadays. However, optical fibres become more and more popular in this area thanks to their advantages over the conventional sensor technologies. One of the main advantages is performing distributed measurements. There are several approaches to measure temperature distribution along a fibre. They are based on different scattering effects inside the fibre core, such as Raman, Brillouin and Rayleigh scattering. Also systems based on multiple FBG sensors can be referred to distributed temperature sensing. Besides temperature measurements optical fibres might be also used for radiation dose estimation. It is commonly known that ionising radiation generate point defects in optical fibres 2. It results in Radiation Induced Attenuation (RIA) which is considered as unfavourable effect for the telecommunication purposes and it is constantly searched for the fibre type with the highest radiation resistivity. However, RIA can be also considered for the exposed radiation dose estimation 3. The feasibility of optical fibre dosimeters was demonstrated by Henschel et al. in 4. Although it confirmed that optical fibres can be used for dosimetry, it was also mentioned that not every fibre can satisfy required radiation sensitivity. Apart from sensitivity there are two other drawbacks of using optical fibres as a dose sensing medium. A first one is saturation. With dose accumulation optical fibres become less sensitive to radiation. Secondly, environment temperature influences RIA. At high temperatures thermo annealing increases. Therefore with the same exposed doses at different temperatures RIA magnitude might significantly differ. To avoid it environment temperature should be tracked constantly and included into the dose estimation model. Temperature and radiation have to be measured as close as possible to the cable location to avoid wrong estimation of influence of surrounding heating or radiation sources 1. For these purposes optical fibres suit the best thanks to their small dimensions. In principle, use of optical fibres should reduce redundancy of local monitoring systems and sensors by installing it along the cables of concern. In this paper we assess experimentally the feasibility of use of the Rayleigh scattering based Optical Frequency Domain Reflectometry (OFDR) for in-situ under radiation temperature measurements. We describe with the attention to details preparation steps for the OFDR temperature measurements starting from a controlled laboratory conditions, where
temperature is stabilized and vibrational perturbations are negligible, to a quasi-industrial environment with exposure to low-dose gamma-rays and other perturbing factors.
2. TEMPERATURE MEASUREMENT THEORY Temperature monitoring is an important issue for every installation where long term temperature exposure might cause an operational failure due to thermo ageing degradation of vital components of the system. It becomes even more significant with the presence of ionising radiation. The monitoring is usually performed locally at the pre-selected locations. However, for cable ageing monitoring this approach is quite often insufficient. Due to the possibility of hot spots appearance at random places a full temperature distribution picture changing in time along the monitored component is desirable. To perform distributed temperature monitoring a relatively recent technique based on Rayleigh scattering in optical fibres was chosen. It is a commercially available Optical Backscattering Reflectometry (OBR) technique using OFDR 5. It allows for a high spatial resolution down to several micrometres for backscattering profile measurements and down to two centimetres for temperature distribution 6. The basic principle of the OFDR temperature measuring technique consists in finding correlation between reference profile, the environment temperature distribution of which is supposed to be known, and heated one, which temperature distribution was changed. The Fourier transform of the intensity fringe at the photodetector can be scaled as a distribution of Rayleigh scattering along the fibre length. Then, to assess the spatial distribution temperature changes one choose the same segments in these two profiles and transfer their spectra back into the optical frequency domain. The cross-correlation between these two inverse-Fourier transformed segments is taken. The shift of the cross-correlation peak relative to reference segment auto-correlation peak is linearly proportional to the temperature change of the segment Δ
∆ ,
(1)
– temperature sensitivity coefficient. More detail where Δ – temperature change, ∆ – correlation peak shift, information about temperature measurements using OFDR can be found in 7. It follows from this description that for making OFDR temperature measurements with optical fibres an initial calibration of the temperature sensitivity must be performed. In our work, in order to define the temperature coefficient, several Rayleigh distribution profiles were taken at known on fibres heated up to temperatures , . . , . The spatial distributions of the spectral shift relative to the reference temperature for every were calculated. In practice, one always operates with the discrete data. Therefore these distribution should be represented as ∆ , where subscript 1 corresponds to different temperatures and superscript 1 corresponds to the position of the spectral shift on the length scale, – total number of points of the spectral shift distribution. For each position (index l – is fixed) we calculate using linear least square method for the set of spectral shifts measured at different temperature sensitivity coefficient temperatures (index j – is fixed) and take the average of them . The standard deviation was estimated for the full set . of calculated coefficients along the fibre. This procedure was repeated five times. The final coefficient The experimental results in the next section are organised as follows. First, we describe in details the calibration experiment; section 3.1. Our intention is to perform temperature measurements under radiation. It is not obvious that the calibration characteristics are stable in such environment. Therefore, we repeated the calibration under a high-dose rate gamma-radiation to check possible influence of ionizing radiation on temperature coefficient. The results are described in section 3.2. Finally, experimental results obtained during exposure in the quasi-industrial environment (the BR2 SubPile Room) are explained in details in the section 3.3.
3. EXPERIMENTS AND RESULTS 3.1 Optical fibre temperature coefficient calibration As it was explained earlier, to perform temperature measurements the temperature sensitivity coefficient , eq. (1), is required. This coefficient depends on fiber chemical composition, drawing conditions, and possibly on some other factors and can vary in the 10% range among standard telecom fibres. In our experiments we plan to use special fibres dedicated for the use in radiation environments and the actual variations can be significantly higher.
A scheme of the measurement set-up is given in Fig. 1. To measure the temperature sensitivity coils of optical fibres were prepared. To avoid thermally-induced stresses the coils were left without support when installed into a furnace.
Fig. 1 Temperature coefficient calibration setup.
At the beginning of the measurements the temperature in the furnace was stabilized at 35°C and the reference backscattering profile was taken. Then the temperature in the furnace was increased with 10°C steps up to 75°C. At every step the backscattering profile was taken after the temperature stabilisation was achieved. Temperature stabilisation and increase was done by means of a PID-controller switched by the LabVIEW program. When temperature stabilisation conditions were met backscattering profile measurement was taken. The stabilisation criterion was temperature changes below 0.20°C during 20 min. The total time needed to make measurements of the backscattered profiles at all 5 temperatures was about 3 hours. Uniform temperature distribution inside the furnace is important for – coefficient accurate calculation. Temperature gradient in furnace manifests itself as periodical oscillations in the spectral shift distribution and high standard deviation. 1°C difference inside the furnace can result in a high standard deviation up to 10% of the coefficient. For acquired five backscattering profiles four temperature distributions were calculated using OBR software. One profile was used as a reference. It is shown n Fig. 2a as the line at the zero level. The temperature distributions at different temperatures for a Pure Silica fibre SMPS 1550-125 P from Oxford Electronics are shown in Fig. 2a.
Fig. 2 Spectrum shift trace segments measured using OBR along 5 meter segment of the Oxford Pure Silica fibre: a) normal traces, b) high noisy traces.
In the subsequent Sub-Pile Room exposure the following fibres were used: Oxford SMPS 1550-125 P (PS), Corning Gedoped SMF-28 (GD), Draka Radiation Hard (RH), Draka Al-doped (AL). The -coefficient for those fibres are listed in Table 1.
Table 1 Temperature sensitivity coefficients
Optical fibre type , °C/GHz
Pure Silica (PS) -0.798±0.010
Corning Ge-doped SMF-28 (GD) -0.764±0.004
Radiation Hard (RH)
Al-doped fibre (AL)
-0.787±0.006
-0.756±0.01
3.2 Temperature measurements under high dose rate gamma radiation There are two main effects which might influence the OFDR temperature measurements based on Rayleigh backscattering in radiation environment. These are radiation-induced attenuation (RIA) and radiation-induced refractive index change. RIA might lead to a change of the Rayleigh backscattering profile or at least to an increase of the noise level. The last can be in fact very significant in terms of the cross-correlation peak determination. For example, if a noise peak in the cross-correlation becomes higher than a real cross-correlation maximum then the first one will be misinterpreted as a temperature-induced shift. Refractive index change can be considered as a delay change of the backscattered light which can also be wrongly attributed to a temperature influence. It is equivalent to an additional temperature induced shift of the correlation peak. Radiation can also influence the temperature sensitivity. We assume that during one heating-up cycle (about 3 hours) radiation induced change of the refractive index is negligible. This assumption works well for low radiation sensitive fibres such as PS and GD. The procedure described in section 3.1 was used in the temperature measurement experiment under high dose rate gamma radiation in the RITA irradiation facility, SCK·CEN, Belgium 8. Behaviour of the -coefficient under radiation of six different fibres was investigated. Some of the spectral shift traces were very noisy (see Fig. 2b). It seemed natural to attribute it to a radiation effect which dramatically changes the backscattering profile so that the correlation is lost. A more careful investigation demonstrated, nevertheless, that the actual cause is most probably a selective polarisation fading effect. In the OBR a polarisation resolved detection scheme is used to mitigate signal fading due to polarisation misalignment 5. Therefore, if one of polarisations was mitigated then it would result in decrease of cross-correlation peak. However, we are still not sure what could cause this problem. We believe that it didn’t influence our temperature coefficient calculations because all unreliable points were ignored. A similar experiment is planned for a near future to eliminate all uncertainties of the origins of the unreliable points. For purpose of this paper the results of the experiment obtained on PS and GD fibres are mostly interesting. Also the behaviour of the temperature sensitivity coefficient under radiation of the RH fibre is assumed to be similar to one of the radiation hard fibres Type 2 (Manufacture A, doesn’t want to be named) irradiated in the experiment. Results of the experiment are shown in Fig. 3.
Fig. 3 Temperature sensitivity coefficient before in the course of the experiment. Fibres: Manufacture A radiation hard (Type 1), Corning Ge-doped SMF-28 (GD), Oxford Pure Silica (PS).
From Fig. 3 one can notice that GD and PS fibres experienced a slight increase of the absolute value of the coefficient at the start of the irradiation. Subsequent changes during the 7-days long exposure are very small. An abrupt change of the coefficient after the end of irradiation should also be mentioned. This change can be probably related to stresses induced in the fibres as a result of the move of the irradiation container out of the irradiation position.
3.3 Sub-Pile Room temperature measurements The goal of our work is to assess the possibility of using OFDR for temperature monitoring in locations where ionizing radiation is present. The BR2 Sub-Pile Room appears to be a suitable place for simulating such environments, Fig. 4a. When BR2 operates, the dose rates inside the SPR are in a range 0.005 - 5 Gy/h. The actual dose-rate strongly depends on the location. Radiation levels in the SPR are elevated also when the reactor does not operate, but it is man-accessible during those intervals, provided special requirements are fulfilled. In particular restrictions on the work duration inside the SPR are applied. Because of the presence of radiation in the SPR all the equipment was placed outside it and connected to the fibers inside using a dedicated feed-through (see Fig. 4b). Optical switch let to multiplex one OFDR for scanning four fibres. The setup was remotely controlled from a PC using a dedicated LabVIEW program. The backscattering profiles were acquired automatically every hour. Every 24 hours the reference profile was changed. The measurements started two days before the beginning of the cycle and were continued two days after the reactor cycle ended.
Fig. 4 a) BR2 and Sub-Pile Room, b) Experiment setup
Four different optical fibres were used in the experiment: Pure Silica fibre (PS) from Oxford Electronics, Corning Gedoped SMF-28 fibre (GD), Radiation Hard (RH), and Al-doped fibre (AL-doped) from Draka. PS and RH fibre were supposed to show a higher resistivity to radiation effects. GD fibre was used as the most common telecommunication fibre. AL is highly sensitive to radiation fibre and was intended to magnify radiation effects. All the fibres were without jacket. To simplify their installation and to eliminate undesirable external perturbations the fibres were inserted into a polyurethane tube with the inner diameter of about 5 mm and the wall thickness of 1 mm. On the outer side of the tube four thermistors were attached at about 5 meters distance between each other. The length of the fibres was about 20 m. Before installing the fibres inside of SPR the -coefficients were measured again for every fibre to see a possible influence of the tube. To perform the measurements last several meters were coiled into several loops and heated up in a furnace. Then the temperature calibration procedure describe previously was performed. For 3 fibres calculated coefficients were not different from those listed in Table 1. Only GD fibre had slightly different coefficient which was out of the measurement error range, however the difference was still less than 4%. Therefore we concluded that the tube would not considerably influence the measurements. On the next step the tube with the fibres was installed in SPR. The first 5 meters were attached to the wall, next approximately 9 meters crossed the room being thrown over metal tubes of a cooling system and then the remaining 6 meters were attached to a rail of the bridge crane. Unfortunately, after the fibres were installed, an unplanned maintenance work had to be done in the SPR. During the manipulations the tube with the fibres was accidentally torn off at one place. It resulted in a one meter bending of the tube where it was detached from the rail. Unfortunately this modification had critical impact on the temperature retrieving from the second part of the fibre. When the cooling system was turned on, vibration from it was transferred to the detached segment of the fibre so the correlation after ~11 meters was completely lost. Therefore we couldn’t measure temperature after 10 meters. In Fig. 5, 6 spatial distributions measured via GD and RH fibres are shown. The temperature distributions are presented in 3 dimensions where along the Ox axis is distance (measuring fibres in SPR started from 14 meters), Oy axis is time in days, and Oz is temperature. Two black curves are the measurements made by the thermistors attached to the outer side of the tube. Important fact which should be mentioned is that to obtain these temperature distribution surfaces, all the fibre data had to be recalibrated. It was done based on the temperature measurements from the first thermistor. This was
necessary because the measured spectral shift was much higher than expected for these temperatures. The re-calibrated coefficients appeared to be about 2.5 times less than those measured before by means of furnace. For the GD fibre, for 0.29 °C/GHz. example, the new measured coefficient was
Fig. 5 Temperature distribution measured via Corning Ge-doped fibre (GD).
Fig. 6 Temperature distribution measured via Draka Radiation Hard fibre (RH).
After analysing this problem we supposed that the reason for such a big coefficient difference compared to the calibration made before the installation in the SPR might be caused by a higher than expected influence of the tube. As it was mentioned before, in the calibration before the SPR installation we used last several meters of the tube with fibres. Temperature increase lead to expansion of the fibres as well as the tube, for which the expansion coefficient is much higher. Because the tube was not fixed and only last meters were heated up it could stretch independently of the fibres. However, when we mounted the tube inside the SPR we attached it to the walls and metal constructions sometimes bending it. The fibres were not able anymore to expand independently of the tube which eventually caused changes of the temperature sensitivity coefficient. Calibration, thus, has to be performed either after the fibre are installed at the place of measurement, or if the measuring fibre is atteched to a non-flexible framework (needed to protect fibre from undesirable mechanical stress) then the fibre has to be calibrated with taking into consideration the thermo-expansion of the framework. The latter method is more preferable because it can eliminate additional strains applied to the fibre while the installation is made. It is clearly seen from the figures that after the distance of 24 meters the noise increases. This was due to the problem of vibration caused by the accidental fibre detachment described above. On the first five meters one can notice an increase of temperature. This increase corresponds well to the right black curve which is the measured temperature retrieved from
the thermistor. However, data obtained from the second thermistor differ from what we measured using optical fibres. This is not really surprising because due to applied stresses the thermal sensitivity at the thermistor one is not the same as the sensitivity at thermistor 2 location.
4. CONCLUSION In the present work we report on the study of the possibility of performing distributed temperature measurement in a radiation environment using the OFDR technique based on Rayleigh scattering. The results of temperature measurements performed with the fibres inserted in the furnace placed in a closed radiation container demonstrated that at a dose-rate of ~700 Gy/h and the total dose up to 110 kGy such measurements were indeed possible. The effect of radiation on the temperature sensitivity coefficient was below the measurement error. However, we observed unrealistic peaks in the spectral shift distributions. Although it seemed naturally to attribute them to a radiation effect yet after deeper consideration we tend to think that it was caused by a polarisation fading effect. We are planning to repeat the experiment to find out the actual reason. The situation was less evident, when measurements were performed on the fibres exposed in an industrial-like environment, in a room just under the reactor vessel. In particular, mechanical vibrations can be most damaging effect. An accidental fibre detaching from the wall resulted in increase of vibration. As a result the data retrieved from the second half of the fibre were too noisy to allow temperature reconstruction. We faced also a problem that fibres after installation in the SPR the temperature sensitivity coefficients were different from measured in the lab. The changes were probably caused by mounting procedure and are physically related to the high sensitivity of OFDR temperature measurements to strain. This drawback can be overcome by using non-flexible attaching frameworks for fiber protection. We plan to investigate these problems in subsequent experiments. As a conclusion, being theoretically possible, distributed fibre optics temperature measurements based on the OFDR technique present a serious engineering challenge when such measurements are performed in a real and not laboratory environment.
ACKNOWLEDGEMENT This research was partly supported by the FP7 IRSES project, the IAP program VI/10 of the Belgian Science Policy and programs of Russian Federal Agency on Science and Innovation.
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