Fiber-optics temperature and radiation distributed monitoring in nuclear installations (PhD)

Distributed simultaneous temperature and dose measurements are of interest for nuclear industry, particularly for the long-term monitoring of nuclear installations like containment buildings, fuel and waste storage wells [1]. A monitoring system should determine with a required accuracy the location and the intensity of a nuclear radiation source and the temperature distribution. Compared to conventional sensor systems in use, this system should offer

•           a higher spatial accuracy with a minimum of cabling;

•           a highly reliable system with low maintenance requirements;

•           an accurate tool for maintenance and on-going qualification programs in NPPs [11];

•           continuous and real-time information for safety guarding applications.

In recent years distributed temperature measurements systems based on fiber-optics are being employed in various civil applications [2-4]. Some of these could even be used for monitoring large nuclear facilities, since they remain operational at cumulated dose levels of 300 kGy [5]. In most cases such systems apply reflectometric measurements. In the time domain, the Optical Time Domain Reflectometer (OTDR) was proposed to detect perturbations of the attenuation along a fiber. This technique was developed in the optical telecommunication area to monitor the condition of optical networks by addressing the level of losses. It works by correlating the data on the intensity and the arrival time of the reflected signal. As far as an environmental parameter influences the level of transmission in the fiber, the use of an OTDR can also be extended for sensing applications. Recently several ways of distributed or quasi-distributed temperature sensing based on the OTDR technique have been studied at FPMs [6].

Ionizing radiation is also known to induce optical losses, which means that the OTDR can be applied for dosimetry. The feasibility of such distributed fiber optic dosimeter systems has been studied and confirmed [7-8]. However, very little was done to practically realize this approach. The only reported installation was performed to monitor radiation doses in the DESY accelerator [9] and that was done based only on the intensity measurements.

The principal difficulty in the practical realization of a fiber dosimetry system based on induced absorption measurements is related to the dynamic character of the radiation-induced losses in the fiber. Most losses are not stable and anneal with time. Thermal annealing is process with a strong Arrhenius-type temperature dependence. Therefore major efforts were put towards design or selection of a sensor fiber with a minimal annealing rate of the induced absorption. However, despite significant advances [9] the approach remains intrinsically deficient because an uncontrolled temperature change may result in an unpredictable error in the dose evaluation.

Simultaneous with radiation level and temperature measurements offer a solution to the problem. Once the temperature history is recorded, the annealing effect can be accurately taken into account. In this way the emphasis can be shifted from the development of fibers with a temperature-stable radiation-induced loss [10] to a selection of a fiber with a radiation sensitivity relevant for a targeted application.

Solutions alternative to the absorption measurements, like radio-luminescence and optically stimulated luminescence measurements will also studied.

Distributed radiation measurements is a task be which appears in many different area. Developed for a particular application it very likely to be extended to another ones thanks to the possibility to tailor the radiation sensitivity of optical fibers. Therefore, the present PhD can potentially be a part of and bring a significant contribution to various research projects dealing with the presence of radiation fields. Monitoring of nuclear power plants [11] and research nuclear installations, accelerators; cancer radiotherapy is just a few examples.

References

[1] M. Decréton, V. Massaut, and P. Borgermans, "Potential benefit of fibre optics in nuclear applications: the case of decomissioning and waste storage activities.," Optical Fibre Sensing and Systems in Nuclear Environment, Mol, Belgium, 1994, SPIE, Vol. 2425, pp.2-10.

[2] G. R.Williams, G. Brown,W. Hawthorne, A. H. Hartog, and P. C.Waite, “Distributed temperature sensing (DTS) to characterize the performance of producing oil wells,” 2000, Proc. SPIE, vol. 4202, pp. 39–54.

[3] G. Nokes, “Optimising power transmission and distribution networks using optical fiber distributed temperature sensing systems,” Power Eng. J., vol. 13, no. 6, pp. 291–296, 1999.

[4] N. G. Craik, “Detection of leaks in steam lines by distributed fiber-optic temperature sensing (DTS),” in Proc. IAEA Specialists’ Meeting Monitoring and Diagnosis Systems to Improve Nuclear Power Plant Reliability and Safety, 1996.

[5] Fernandez Fernandez A., Brichard B., Berghmans F., Rodeghiero P., Hartog A., Hughes P., "Radiation-Tolerant Raman Distributed Temperature Monitoring System for Large Nuclear Infrastructures", IEEE Transactions on Nuclear Science, 52:6(2005), p. 2689-2694

[6] C. Crunelle, "Development of quasi-distributed and distributed temperature sensors based on optical reflectometry techniques," Service d'Electromagnétisme et de télécommications. Mons: FPMs, 2009, pp. 214.

[7] H. Henschel, O. Kohn, H. U. Schmidt, J. Kirchof, and S. Unger, "Radiation-induced loss of rare earth doped silica fibres," IEEE Trans.Nucl.Sci., vol. 45, pp. 1552-57, 1998.

[8] K. Imamura, T. Suzuki, T. Gozen, H. Tanaka, and S. Okamato, "Application of Nd3+ doped silica fibers to radiation sensing devices," in: Optical Techniques for Sensing and Measurement in Hostile Environments, 1987, Proc. SPIE, Vol. 787, pp. 62-68.

[9] H. Henschel, M. Körfer, J. Kuhnhenn, U. Weinand, and F. Wulf, "Fibre optic radiation sensor systems for particle accelerators," Nucl. Instr. Meth. Phys. Res. A, vol. 526, pp. 537-550, 2004.

[10] P. Borgermans, B. Brichard, F. Berghmans, M. Decreton, K.M. Golant, A.L. Thomashuk, and I.V. Nikolin, "Dosimetry with optical fibres: results for pure silica, phosphorous and erbium doped samples," in: Fiber Optic Sensor Technology, 2001, Proc SPIE, Vol. 4204, pp. 151-160.

[11] M. Van Uffelen, "Electrabel - Tractebel - SCK•CEN Convention: Technical Activity report 2006-2007", Restricted contract report SCK•CEN R-4603, 2009.

The main objective of the PhD is to propose, develop, and test an optical-fiber based distributed temperature-dose measurements system suitable for surveillance of nuclear installation, including next generation systems like MYRRHA.

To achieve this goal innovative solutions for a number of challenging tasks are necessary:

-          improving the understanding of temperature-dependent post-radiation relaxation in optical fibers and development on this basis phenomenological models suitable for the description of the relaxation kinetics with an accuracy required for the practical application

-          design of an experimental set-up for spatially-resolved real time radiation-induced optical attenuation and temperature distributed measurements with optical fibers

-          performing laboratory calibration and field measurements with commercial of-the-shelf and specialty fibers

-          development of a software to process experimental data to find real-time spatially-resolved ionization dose and dose-rate distribution

-          system demonstration in a NPP environment

The achievement of these goals will advance the understanding of fundamental aspects of radiation effects in optical materials and in optical fibers, in particular. The PhD work addresses the need for precise spatially resolved radiation dosimetry, enabling more safe operation of nuclear and ionizing radiation-related installation. It plans to foster breakthroughs in photonics technologies applicable to these fields. The use of an all-fiber approach makes the system intrinsically safe and remotely-controlled, which is extremely important for the targeted hazardous environments. The solutions will have to be highly innovative because no such system is yet existing.