NUCLEAR APPLICATIONS OF FIBER BRAGG GRATING SENSORS

WEC CENTRAL & EASTERN EUROPE ENERGY FORUM – FOREN 2012Neptun-Olimp, Romania, 17-21 June 2012

Reference no:

NUCLEAR APPLICATIONS OF FIBER BRAGG GRATING SENSORS

ZBĂGANU ALEXANDRA, POPESCU MIRCEA

R.A.A.N - Sucursala de Inginerie Tehnologică Obiective Nucleare - CITON

Strada Atomiştilor 409, Bucureşti - Măgurele, 077125

Abstract:

Fiber Bragg Grating (FBG) sensors have generated huge interests in the latest years due to their multiple uses in industrial applications. Constant temperature measurements in zones with radiation is a critical problem, because here the conventional thermocouples require special protection, and it is well known that for a high neutron flux they eventualy give in. In this paper we study the possibility of using FBG temperature sensors to measure the temperature in radioactive environments, focusing our attention on the Intermediate Dry Spent Fuel Storage of NPP Cernavodă.

1. Introduction

The nuclear industry shows a growing interest in the possibilities offered by fiber optic technology for both data communication and sensing applications. Temperature measurement by means of fiber optic sensors could improve the monitoring and safety control of nuclear installations, but the major problem in the application of electronic or photonic equipment in the nuclear industry is the presence of ionizing radiation fields. Any electronic or photonic component may suffer from exposure to nuclear radiation, which compromises the reliability of advanced instrumentation systems. Therefore, it is necessary to investigate how the components behave when exposed to radiation and, in most cases, to adapt their design to increase their tolerance to radiation.

The physical fundamentals for FBG were set in 1913 by Sir William Henry Bragg (1862-1942) and Sir William Lawrence Bragg (1890-1971) in the book called "X-rays and Crystal Structure", written in 1915, for which they also won the Nobel prize. They analised the structure of crystals and optical grating reflectors (using interference filters). The technical base of FBG was demonstrated in 1978 by Ken O. Hill in the Applied Physics Letters (1978). Initially, the gratings were fabricated using a visible laser propagating along the fiber core. In 1989, Gerald Meltz, with his colleagues William W. Morey and William H. Glenn, demonstrated the much more flexible transverse holographic technique, where the laser illumination came from the side of the fiber.

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FBG technology is a promising measurement concept for future sensor system applications. The first FBG applications are envisioned for temperature sensors and for strain sensors. Related research is being conducted in many places. Distributed measurements can be easily accomplished with FBG technology by wavelength division multiplexing. With this feature, FBG sensor systems are suitable for applications in plants where explosion proof operation is essential, or in power electronic applications where electro-magnetic interference can be a problem. Also, in process applications, where traditional thermocouples or resistance type of temperature sensors have been used so far, FBG sensor technology can be applied as distributed temperature measurement systems.

2. Theoretical analises

The FBG is an optical device which is formed by exposure to a fringe of UV light. The fundamental principle behind the operation of a FBG is the Fresnel reflection, where light travelling between media of different reflactive indeces may both reflect and refract at the interface. Only the specified wavelength (the Bragg wavelength), related to its grating period, is reflected from a broadband source for an input lightwave. This reflected wavelength, called the Bragg wavelength, is defined by the relationship:

λB=2ne Λ (1)

Where ne = the effective refractive index of the grating in the fiber core

= the period of index modulation (the grating period)

The effective refractive index quantifies the velocity of propagating light as compared to its velocity in vacuum. ne depends not only on the wavelength but also (for multimode waveguides) on the mode in which the light propagates. For this reason, it is also called modal index.

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Figure 1. A Fiber Bragg Grating structure, with refractive index profile and spectral response

The use of a FBG as a temperature sensor is based on the dependence of λBí on the applied temperature. For a restricted temperature interval, this temperature dependence can be approximated with the linear expression:

λB (T )=λB ( T0 )+KT (T−T 0) (2)

where λB (T 0 ) is the Bragg wavelength measured at the temperature of reference T0, and KT is the

temperature sensitivity. If the values of λB (T 0 ) and KT are given, the applied temperature can easily be derived from Eq. (2).

For a temperature change of ∆T, the corresponding wavelength shift ∆λBT is given by:

∆ λBT=λB(1+ξ )Δ T (3)

Where ξ is the fibre thermo-optic coefficient [1].

Figure 2. Radiation-induced Bragg peak shift. The baseline is lowered due to the radiation-induced attenuation in the photosensitive optical fiber.

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3. Advantages of the FBG sensors and their industrial application

Fiber Bragg Grating sensors offer several significant advantages over conventional electrical sensors. The FBG sensor has high accuracy, sensitivity, and immunity to electromagnetic interference, radio-frequency interference, and radiation.

Has the ability to be made into a compact, lightweight, rugged device small enough to embed or laminate into structures or substances to create smart materials that can operate in harsh environments—such as underwater—where conventional sensors cannot work.

The ability to accommodate multiplexing and an inherent low transmission loss at 1550 nanometers. These features allow one to use many sensors on a single optical fiber at arbitrary spacing.

Installation and use are easy. Because the gratings multiplex on a single fiber, one can access many sensors with a single connection to the optical source and detector.

Potential low cost as a result of high-volume automated manufacturing process.

The key advantages of FBG Sensors are:

1. They’re rugged passive components resulting in a high life time (> 20 years) 2. They’re small in size and can be easily embedded into desired areas.3. They form an intrinsic part of the fiber optic cable that can transmit the measurement signal

over several tens of kilometers.4. They show no interference with electromagnetic radiation, so they can function in many

hostile environments where conventional sensors would fail. 5. They don't make use of electrical signals what makes them explosion safe. 6. They’ve the ability to multiplex many sensors using only one optical fibre, driving down

the cost of complex control systems.7. They’re easily installed and virtually maintenance free.8. They’re cost effective and highly reliable.9. They produce fast and accurate measurement.10. They’ve high sensitivity, thus having fast response time.

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Figure 3. Application sectors of FBG sensors – world-wide average [9]

4. Using FBG sensors and optical fibers in nuclear environments

Exposing optical fibres to ionizing radiation results in a wavelength-dependent attenuation increase. This effect limits the radiation-acceptance level of intensity-based fibre optic sensors in nuclear environments. The narrow wavelength encoding of the sensing information, however, helps to avoid the influence of the broadband radiation-induced loss. Therefore, we can rely on Fibre Bragg Grating (FBG) temperature sensors for temperature monitoring purposes.

It was shown previously that the FBG sensitivity to radiation strongly depends on the chemical composition and drawing conditions of the fiber, as well as on the FBG parameteres and writing conditions. FBG temperature sensors written in photosensitive fibre, without any pre- or post-writing treatment, exhibit the higher radiation tolerance in both pure gamma and mixed gamma neutron environments [3].

There are many factors which influence the radiation tolerance of optical fibres. The mostimportant of these are the level and type of dopants and impurities present in the core, the details ofthe fibre drawing process, the wavelength of operation and the environmental conditions in which the fibre is exposed to radiation. The presence of very small quantities of impurities can significantly degrade the radiation tolerance of the fibre. Many commercial multimode fibres contain phosphorous in the core because it lowers the temperature required for deposition during the preform manufacture. This implies a lower humidity in the process and hence results in lower OH (hydroxide) content in the fibre. It is well known that the presence of phosphorous leads to stable defects which cause large increases in the radiation induced absorption (RIA). The presence of alkaline and metal impurities can also lead to a significant increase in RIA. The details of the fibre drawing process determines the number of defects created which can significantly affect the RIA of the fibre. The presence of water in the fibre manufacturing process leads to some OH content in fibre. Higher OH content fibre can have larger RIA for wavelengths around 600 nm but this effect should not be so significant at longer wavelengths.

In [2] there were subjected three Ge-doped multimode fibres, one F-doped multimode fibre and one single-mode fibre to gamma radiation at 22.5 kGy(Si)/hr to an integrated dose of 650 kGy(Si), and also the Infinicor SX+ fibre was subjected to several lower dose rates (1.01, 0.424, 0.343, and 0.0265 kGy(Si)/hr) in order to study the dose rate effects and to provide data at low integrated doses. There was a clear evidence that the damage is lower at the same dose for lower dose rates, which was ascribed to annealing effects. It was shown that the optical fibres which are the most qualified for use use is nuclear environments are the multimode fibres Infinicor SX+ and Draka-RHP-1, and the singlemode fibre SMF-28. The one with the largest RIA from this three is the Infinicor SX+, with an upper limit of 0.41 dB for the estimated total RIA.

Ionizing radiation produces defects in the fibre that change the characteristics of the grating, in concordance with the quantity of the radiation. The use of FBG temperature sensors under radiation depends on the sensitivity of λB (T 0 ) and KT to radiation. These two parameters need to be monitored to evaluate the radiation sensitivity of the FBG temperature sensor. It was shown previously [5] that the change of λB (T 0 ) can be as high as 0.1 nm at a dose level of 0.5 MGy, this shift corresponding to affecting the temperature sensitivity coefficient in high-Ge fibers by gamma radiation, with an apparent temperature change of 10oC. But A. Fernandez Fernandez et al. showed

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in their studies [3], [6], [7] that these results don’t confirm, and the results of their gamma irradiations can be summarized as:

1. The radiation sensitivity of the FBGs strongly depends on the chemical composition of the fiber and the photosensitization technique used for writing the FBGs. The sensitivity to gamma radiation of FBGs written in hydrogen loaded telecom fibers is higher, but the Bragg peak shift saturation still occurs. The peak shift of both FBGs, written in hydrogen-loaded and non-hydrogen loaded Ge-doped fiber saturates at a gamma dose of about 100kGy, while a FBG written in N2-doped fiber shows no saturation up to 1.5 MGy.

2. The lowest radiation sensitivity is achieved using standard highly Ge doped photosensitive fiber, without any pre or postfabrication treatment. Also, such Bragg Gratings have the highest radiation acceptance level when exposed to mixed gamma-neutron fields. The radiation induced peak shift reaches only 20 pm, corresponding to a fictive temperature change of about 2°C. The Bragg peak spectral full width at half maximum (FWHM) remains stable under gamma radiation.

3. FBGs written in radiation-resistant telecom fiber, after photosensitization by hydrogen loading, are more sensitive to gamma radiation. A larger peak shift can be observed and a significative change in the FBG parameters, which are believed to stem from the radiolitic rupture of O–H and the Ge–H bonds, created during the FBG inscription in a H2 loaded fiber.

4. Gamma radiation does not affect the temperature sensitivity KT of the gratings of up to 1.5

MGy.

Therefore, the saturation of the peak shift observed in highly Ge-doped fibres and the stability of the temperature sensitivity under gamma radiation, allow differential temperature measurements using FBG temperature sensors to be radiation-tolerant in ionising environments.

5. Temperature measurements of the module 2 of the dry spent fuel storage facility

Cernavodă Nuclear Power Plant has adopted the following policy regarding spent fuel: - wet storage in spent fuel storage basin near the reactor, for at least 6 years- intermediate storage in Dry Spent Fuel Storage Facility for 50 years.

The storage itself will be up to the final profile of 27 monolithic concrete modules for dry storage, type MACSTOR (AECL), located above ground. Each module encompasses 20 metal enclosures, each storing 10 baskets (60 fuel bundles / basket), giving a storage capacity of 12,000 bundles / module, and covering the amount of the spent fuel in two years of production of a nuclear unit CANDU 6.

Between 2002 and present, four modules from the Dry Spent Fuel Storage Facility were built and put into service.

For Module 2 is necessary an installation for measuring the temperature in various locations of the structure of the module that will provide values of concrete temperature and the storage cylinders from the central zone of the module.

Data collecting is useful for a limited period of time of 2-3 years, in the purpose of checking the thermic regime.

Measuring the temperature requires a number of 14 thermocouples: TE-2A1#1,2; TE-2A2#1,2; TE-2B1#1; TE-2B1#2,3; TE-2B2#2,3;TE-2B2#1; TE-2C6#1,2; TE-2C5#1,2 Fig.4 shows the location of these thermocouples within in the interior of a module of the Dry Spent Fuel Storage Facility.

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Figure 4. Thermocouples mounting for Module 2

The technical solution used for Module 2 is as follows: - Temperatures are measured with thermocouples of Pt 100 (RTD - Resistance

Temperature Detector) which ensures the greatest accuracy in the expected range of temperature

- On each side of the module there is located one box containing resistance/current converters. Converters are supplied in 220V, 50Hz from existing feeds.

- Current signals are transmitted to two recorders located in the body gate of the site and are taken together with other measurements by a wire cable, and then sent to the control room of Unit 1.

6. Temperature monitoring of a dry spent fuel storage facility module using FBG sensors

Starting with the 5th Module, the module type that will be used is MACSTOR 400, instead of MACSTOR 200, which will allow the storage of a double number of fascicles. For the thermic characterization of the new model, a new measurement installation will be needed.

The Fibre Bragg grating temperature measurement system proposed is composed mainly by the temperature sensor probes, the connecting fibre cables, fibre optic connectors, fibre optic transmission, signal processors, communication cables and computer components and it is shown in Figure 5. [8]

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Figure 5. Fiber Bragg grating temperature measurement system, where:1 – temperature sensor2 – connection fibre cable3 – fibre cables connector4 – transmission fibre cable5 – signal processor6 – signal transmission cable7 – optical sensing interrogator8 – master controller

The temperature sensing cable (formed by 1 and 2) is a multipoint temperature sensor. Discrete points along a rugged cable are used to simultaneously measure temperature with sub degree Celsius accuracy over a wide range of temperatures, forming a sensing cable. For this system, a single FBG temperature sensing cable can be used, as shown in figure 5, but several cables can be multiplexed in order to cover a larger area, or for monitoring the temperature of a bigger number of areas, beacuse the maximum number of sensors on a cable is generally 40. In the second module of the intermediate dry spent fuel facility we need to monitor 14 areas, so a single sensing cable is enough.

Because the sensing points in one cable are connected in series, installation is less expensive and less cumbersome than wiring dozens of separate electronic gage networks. If we make a comparison between sensing cables and the conventional thermocouples, we can easily realise that the optical sensing cable is equally accurate, while providing for faster response, with no need for calibration, and no EMI issues. Also, the optical temperature sensing cable is qualified for use in harsh environments and delivers the many advantages inherent to all Fiber Bragg Grating based sensors.

The fibre cable connector (3) is used for multiplexing the fibre sensing cables, when there are more than one. The transmission fibre cable (4) is an optical fibre, resistent to radiation, as shown in chapter 4.

The signal processor (5) is usually an industrial PC and its role is to facilitate communication with the interrogator module. It has to manage power to all the interrogators in the field, to store and transmit sensor data, and to trigger alarms or other external processes.

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Raw or processed data can be exported to other processors via Ethernet, Serial, USB, etc. Also, the signal processing should support remote data transmission through most PC compatible protocols like TCP/IP, RS-232/422/485, Modbus, wireless LAN, etc.

The Optical Sensing Interrogator (7) must employ full spectral scanning and data aquisition, providing measurements with high absolute accuracy, flexible software post-processing, and high dynamic range performance. The interrogator must be suited to measure FBGs. The platform used responds directly to the user commands of the optical interrogator core and outputs sensor wavelength data via Ethernet port and custom protocol. All module settings, sensor calculations, data visualisation, storage, and alarming tasks are run on an external PC or on a sensor processor module. Usually, the sensor manufacturer also provides a specialised software and a suite of tools for data aquisition, computation and analysis for optical sensor networks. An example of such software is shown below in figure 6.

The master controller (8) supervises all of the sensors in the field, gives the alarm command, storages and displays the data in real time. The role of the master controller is to translate the signal wavelength of the probe given by the signal processor into a temperature, according to the spatial location distribution of the temperature measuring points, and to give this results in the layout of a graph, so the users could easily get the characteristics curves. Furthermore, it can set the alarming temperature values according to different requirements, for different cables, in different environments.

Figure 6. Example of a software used to monitor FBG temperature sensors

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7. Conclusions

We studied the possibility of using FBG sensors instead of conventional thermocouples for monitoring the temperature of Module 2 of the intermediate dry spent fuel storage of NPP Cernavodă. The main advantage of the FBG sensors relies on the narrowband wavelength-encoding of the sensing information, making the sensors insensitive to broadband radiation-induced optical power-losses. These sensors showed a very promising radiation tolerance under accelerated gamma radiation, so it can be stated that FBG sensor technology is a very powerful tool and a promising solution to temperature sensing in radioactive environments.

References

[1] Y.-J. Rao, et al., In-fiber Bragg grating sensors, Means. Sci. Technol. 8, 1997.[2] B. QArvidsson, K. Dunn, C. Issever, B.T.Huffman, M. Jones, J. Kierstead, G. Kuyt, T. Liu,

A. Povey, E. Regnier, A.R. Weidberg, A. Xiang and J. Ye, “The radiation tolerance of specific optical fibres exposed to 650 kGy(Si) of ionizing radiation”, http://iopscience.iop.org/1748-0221/4/07/P07010, 2009.

[3] A. Fernandez Fernandez, B. Brichard, P. Borgermans, F. Berghmans, M. Decreton, P. Megret, M. Blondel, A. Delchambre, “Fibre Bragg grating temperature sensors for harsh nuclear environments”

[4] N. Hirayama, Y. Sano, “Fiber Bragg grating temperature sensor for practical use”, ISA Transactions 39, 2000.

[5] P. Niay, P. Bernage, M. Douay, F. Lahoreau, J. Bayon, T. Georges, M. Monerie, P. Ferdinand, S. Rougeault, and P. Cetier, “Behaviour of Bragg gratings, written in germanosilicate fibers, against γ-ray exposure at low dose rate,” IEEE Photonics Technology Letters 6, pp. 1350–1352, 1994.

[6] A. Fernandez Fernandez, A.I. Gusarov, B. Brichard, S. Bodart, K. Lammens, F. Berghmans, M. Decréton, P. Mégret, M. Blondel, A. Delchambre, „Temperature monitoring of nuclear reactor cores with multiplexed fiber Bragg grating sensors”, Optical Engineering, Vol. 41 No. 6, June 2002.

[7] A. Fernandez Fernandez, B. Brichard, F. Berghmans, M. Decréton, „Dose-Rate Dependencies in Gamma-Irradiated Fiber Bragg Grating Filters”, IEEE Transactions on Nuclear Science, Vol. 49, No. 6, december 2002.

[8] A. Fernandez Fernandez, B. Brichard, F. Berghmans, M. Decréton, „Dose-Rate Dependencies in Gamma-Irradiated Fiber Bragg Grating Filters”, IEEE Transactions on Nuclear Science, Vol. 49, No. 6, december 2002.

[9] A. Tang, Y. Zhao, „Fiber bragg grating based fiber-optic temperature measurement system for the high voltage cable”, Journal Of Electrical & Electronics Engineering, vol. 9, no. 2, 2009.

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NUCLEAR APPLICATIONS OF FIBER BRAGG GRATING SENSORS