Optical fibres – a smarter way to monitor

Optical fibre sensors (OFSs) are increasingly used for structure and process monitoring in many industrial sectors. The use of fibre optics with different sensing devices allows monitoring of a wide range of parameters of concern to the nuclear industry, such as structural integrity of components, levels of hydrogen build up after a fuel accident, and levels of ionising radiation. These sensors can be used for many topologies, such as serial, parallel and star, and form networks (OFSNs).

Most OFS systems in use are based on fibre Bragg grating (FBG) transducers (see panel). An FBG acts as a wavelength selective reflector, and its reflective spectral line is shifted relative to environmental parameters (eg temperature, strain, pressure). FBGs are passive devices; they do not need power supplies, can be used on most surfaces or environmental conditions and can withstand radiation exposure and pressure or temperature extremes.

This monitoring technology offers potential improvements in plant operation and maintenance, availability and safety. OFSs can be retrofitted into existing installations – of particular interest to nuclear plant operators planning to replace their ageing supervision and control systems.

Given this potential, a number of organisations have undertaken R&D to evaluate its use, including CEA-EDF-Framatome in France, EPRI-Tennessee Valley Authority in the USA and CRIEPI in Japan.

CONTAINMENT MONITORING

One potential application is monitoring the nuclear containment building, a structure designed to withstand an hypothetical H2 explosion in case of a nuclear accident. An integrity test is periodically performed on these structures. Nevertheless, a networking approach would considerably enhance reliability and safety and produce savings in maintenance costs.

FBGs may be easily embedded into materials (eg concrete or composite materials) to provide local damage detection and internal strain field mapping with high strain resolution and a large measurement range – just what is needed for developing smart structure technologies. FBGs offer the promise of real-time structural measurements using multiplexed built-in sensor systems. The advantages for nuclear containment monitoring using an OFS are: its passive nature (no need for electricity at the sensing points); large multiplexing capabilities and large measurement range (greater than that of conventional extensometers). A PC-based Fibre Bragg Grating system has been developed by CEA-LETI.

Bragg experiments

An FBG extensometer developed for civil engineering applications is composed of a central metallic rod (0.1 m or 0.2 m length) on which an FBG is bonded; it can be embedded in concrete or anchored on a surface. Compression tests have been done with embedded extensometers in a concrete cylindrical sample, comparing an FBG extensometer, vibrating wire (Telemac C110) and conductive sensor.

Other experiments were carried out to test the FBG’s capability to identify the initiation of cracks on the surface of concrete flooring which were loaded by crack inducing metal plates.

The FBGs performed as well or better than other sensors, as they detect cracks earlier than traditional equipments, provide a measurement intrinsically free of temperature effects on larger base length, can be multiplexed in arrays, and are less intrusive for embedded monitoring.

STEAM PIPE MONITORING

The early detection of leakage is important to avoid fissurisation or pipe breaks which could lead to an emergency situation, such as a loss of coolant accident (LOCA). Improved methods of measuring strain on high pressure components would be of great benefit in assessing remaining life.

The increase in temperature of pipe insulation offers a means of detecting a steam leak. There are several methods available, such as electrical strain gauges and infra-red (IR) cameras. Although inexpensive, electrical strain gauges tend to fail under high temperature, and ageing affects their response; moreover, they need electrical supplies which could be unavailable in the case of an accident. As for the IR camera, these may fail to detect hot spots in the presence of a large amount of steam and need to be frequently recalibrated to quantify the leak rate.

On the other hand, fibres wound around or fixed along pipes are able to remotely detect and locate strains and/or thermal rises (hot spots) due to leaks. Two techniques could be used. The first one is based on temporal reflectometry (OTDR), enabling distributed measurements to be made, and the second uses FBGs which enable quasi-distributed measurements.

OTDR methods consist in an intensity measurement in the time domain of the light backscattered by the fibre. Two phenomena could be used: Raman or Brillouin scattering. Intensive R&D efforts have been made in the last decade on Raman OTDR and some products are now commercially available. They enable an absolute and selective temperature measurement. In theory Brillouin reflectometry enables the determination of both temperature and strain. Nevertheless, this method (so-called B-OTDR) is less mature than the Raman-OTDR and more expensive.

As Bragg temperature sensors are obvious candidates, ways have been investigated to multiplex them. A CEA demonstration system based on spectral encoding (WDM) developed by France’s CEA is able to multiplex tens of sensors.

Unlike the OTDR, which allows distributed (continuous) monitoring, FBGs only allow quasi-distributed measurements. At first glance, OTDR seems better suited to detect potential leaks. But, this requires a continuous scanning of several fibres on many pipes and the amount of data rapidly increases in real time, leading to a quick overflow of data. On the other hand, one could be pragmatic and choose a simpler approach able to detect 95% of the potential events, because preliminary analysis can identify locations where stress concentrations may be expected to occur. Alternatively, a quasi-distributed measurement with FBG sensors localised on specific zones (elbows, soldering, valves, etc) seems a good approach especially if installed on existing pipework.

HYDROGEN RISK

In case of a hypothetical severe accident, the oxidation of zirconium cladding of nuclear fuel rods releases H2 that may lead to a deflagration, as occurred during the TMI accident in 1979, or to a detonation. Moreover, in case of a LOCA, steam is still produced in the building. To mitigate the H2 risk the reactor operators need to monitor water vapour, O2 and N2 concentrations as well as the temperature and pressure.

A practical severe accident monitoring system must have a quick response (of about one minute) and be on stand-by during the entire lifetime of the plant ready to operate at any time, even without power being available inside the containment building.

Optical fibres offer an elegant solution to remote distributed measurement with sensors located in the containment area and the associated instrumentation outside it. Pure silica core fibres should withstand a total dose of nearly 100 Mrad (normal dose rate x 40 years, plus the dose arising from a hypothetical nuclear accident) and ensures that this detection method is free of intensity level variations.

One likely technique to detect diatomic molecules (H2, N2, O2 etc) is Raman spectroscopy. Devices have already been built (eg a bifurcated fibre bundle for in situ remote chemical analysis). The so-called RLFO (Raman laser fibre optics) technique has proved to be efficient in designing versatile, efficient, multipoint, remote, in situ chemical sensors.

Another device based on spontaneous Raman scattering has been designed for in-line sensing of H2 in a mobile launch platform of NASA (high-vibration environment) with a multiple-pass cell. The amount of Raman scattered light is proportional to gas concentration without interference between species by contrast to other methods.

RADIATION MONITORING

To improve the dosimetry monitoring of workers and also to provide remote dosimetry within nuclear zones, a team from LETI (CEA – Technologies Avancees) at Saclay has developed a new approach using optical fibres and luminescence materials based on the phenomenon called optically stimulated luminescence (OSL).

OSL materials trap electrons excited by irradiation on stable electronic levels, like classical thermo luminescent (TL) materials do. But instead of TL materials, where they are released by heat, electrons trapped in OSL materials are released by light and produce a luminescence which is proportional to dose.

The OSL phenomenon offers the same advantages as TL dosimeters (long data storage time, good reproducibility, large dose range, low level of dose achievable etc), and the possibility of a remote optical stimulation instead of post-heating, which is why an optical fibre OSL system can be used for remotely monitoring radiation areas. For dosimetry purposes, a small quantity of OSL material is connected to the end of an optical fibre.

A prototype of an OFS dosimeter developed at the LETI laboratory has been successfully tested in a nuclear zone. It is able to measure both dose and dose rate and provide the measurements in real time, a very useful feature for remote dosimetry. In case of electrical failure, there is no loss of information stored in the passive sensors. This new kind of optical sensor can certainly improve the management of radiation protection in nuclear plants by its real-time display of dose or dose rate.

RADWASTE FACILITIES

Belgium’s SCK-CEN Mol Research Centre is investigating the use of OFS systems for monitoring the pressure, temperature and dose around waste storage canisters for its Hades underground laboratory. In Germany, experiments will be undertaken at the Asse and Gorleben salt mine repositories, using traditional and OFS systems to measure pressure, temperature and strain. In France, Andra (the national radioactive waste management agency) is currently studying underground repository concepts and suitable monitoring equipment jointly with the CEA. Andra also co-operates with Cedra (its Swiss equivalent) in hydrogeology, geochemical and geomechanical studies (the Febex project). Similar studies are being done at the WIPP (Waste Isolation Pilot Plant) facility near Carlsbad, New Mexico and at a site for a deep underground repository in the US.

In France, an alternative to deep underground storage is sub-level storage for which a long-life monitoring system would be needed. The ETLD (Entreposage Très Longue Durée) project consists of sub-level concrete siloes with recirculating air.

All storage scenarios are designed to be reversible. An instrumentation system would be desirable to monitor distributed temperature, dose and the physical integrity of the site. In the future, if it becomes necessary to remove waste canisters from the repository for further recycling, this instrumentation would help anticipate such a decision.

Any proposed monitoring system must be reliable at least during the preliminary phase of storage, ie the first 300 years. This time period is considerably longer than the usual mean-time-to-failure of any existing industrial system – setting a challenge that OFS-based sensors should be able to meet.

MORE APPLICATIONS

For the applications above, FBG-based sensors will provide pressure and temperature measurements. Conversely, remote fibres may be used to determine distributed temperature and irradiated dose by Optical Time Domain Reflectometry of Raman scattering and Rayleigh scattering of silica, respectively. Stokes luminescence (that does not depend on temperature) and anti-Stokes luminescence (that depends on temperature) are separately measured with two detectors and the ratio of the two variables enables the measurement of the temperature, while from the OTDR spectrum the radiation-induced attenuation can be retrieved enabling the radiation dose along the length of the fibre to be inferred using an attenuation-to-dose model.

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