Radiation-tolerance qualification for maintenance tasks in the future fusion reactor: the FBG radiation tolerance assessment as case study

The radiation-tolerance assessment of components is required before their use in instrumentation links for the future International thermonuclear fusion reactor (ITER). In this paper, we will detail our methodology to find the most suitable components with regard to the specific ITER requirements and to evaluate the radiation-tolerance of components for the ITER instrumentation links. Our approach aims to identify and describe the main failure mechanisms, to build a radiation-tolerant component database and to develop component validation procedures, a first step towards ITER-dedicated quality assurance procedures for the radiation-tolerance assessment. We illustrate it by considering the in-fibre Bragg grating filter as case study. We summarise our gamma and mixed gamma-neutron irradiation. Finally, we propose a component validation procedure, as first step towards ITER-dedicated quality assurance procedures for the radiation-tolerance assessment of wavelength-selective fibre optic components. I. GENERIC SPECIFICATION FOR THE ITER INSTRUMENTATION DATA LINK The frequent maintenance operations of the future International thermonuclear fusion reactor will have to be performed in a very harsh nuclear environment. Remote-handling technology will therefore play a major role during these maintenance tasks [1]–[5]. Connecting the remote-operated tools with their control unit will require heavy shielded umbilicals. Their management could be eased by using multiplexing techniques. The development of radiation-resistant links is therefore a key issue. Fiber optic technology is being considered since its intrinsic passive Wavelength-Division Multiplexing (WDM) capabilities have shown promising radiation tolerance up to the required MGy dose-levels [6]. Prior to its implementation in the ITER umbilicals, a complete radiation tolerance assessment is required. During the maintenance periods, remote-operated tools will have to operate in an environment characterised by a gamma dose-rate of about 10 kGy/h and temperatures going from 200 ◦C, after the reactor shutdown, down to 50 ◦C after several hours. The required or estimated radiation tolerance level is ranging from 1 MGy up to 100 MGy, depending on the components and their application [4]. Moreover, during the reactor operation, the remote-operated tools and their umbilicals are supposed to be stored in the reactor hall. They will therefore be exposed to neutrons, produced by the deuterium-tritium reaction. Both gamma and neutron irradiation need therefore to be considered during the radiation tolerant assessment procedure. The final specifications are not yet defined at this time and will benefit from the radiation-tolerance assessment program discussed hereafter. II. TOWARDS RADIATION-TOLERANCE ASSURANCE FOR VERY HARSH NUCLEAR ENVIRONMENTS With the generic requirements of the ITER design team, the promising components are selected according to information made available within different radiation effect communities, taking also into account the failure mechanisms reported in the literature for that particular technology. The total dose dependence under gamma radiation is the first step in the radiation-tolerance qualification for maintenance tasks. Afterwards, the temperature, dose rate and recovery effects are evaluated. We then select the most radiation-tolerant component and we proceed with the qualification under mixed neutron-gamma fields. All the irradiation results, obtained since 1993, are stored in a database open for all the fusion community [7]. The successive irradiation tests allow us to adapt existing or develop new test procedures, in order to obtain reproducible and intercomparable results. III. TECHNOLOGICAL GATEKEEPING: FROM ALL-ELECTRICAL LINKS TO OPTO-ELECTRONIC LINKS Digital transmission has been first considered for the ITER instrumentation link [5]. Classical electrical multiplexing techniques have been evaluated under ionising radiation. However, distortion-free data communication requires the use of heavy shieldings against electromagnetic interference. Optical data transmission has been therefore envisaged. The digitization and the multiplexing of multiple analog sensor signals requires the use of radiation-sensitive electronics such as ADC converters and serializers among others. Their radiation-tolerance is at the most 100 kGy at a modulation frequency of 1 MHz [5]. We proposed to address this problem by implementing an analog parallel optical with reduced basic electronic components. This alternative is detailed elsewhere [5]. On the other hand, it is well known that a wavelength dependent attenuation increase occurs in optical fibres exposed to ionising radiation. However, the radiation-induced losses in the optical fibers are maintained at acceptable levels up to tens of MGy, in the infrared window. FBGs are commercially available since mid 1990s [8] and offer intrinsic narrowband wavelength encoding. In the framework of the European fusion development agreement, we decided to evaluate the potentially high radiation-acceptance level of this passive all-fiber component, following the methodology depicted in Fig. 5. A. The FBG radiation tolerance assessment as case study Fibre Bragg gratings are all-passive optic filters written in the core of a single mode optical fibre [8]. Ionizing radiation creates similar defects to those created during the fabrication of the filters by UV exposure. Through the Kramers-Kronig relation, it can be shown that an increase of the attenuation in the UV region, due to ionizing radiation, results in an increase of the fiber core refractive index. The Bragg filter was therefore expected to shift towards higher wavelengths, as its central peak position λB is given by λB = 2neff · Λ, where neff is the effective refractive index and Λ is the grating pitch. This has been confirmed by our irradiation experiments summarised hereafter. 1) Gamma radiation effects: We first showed that FBGs written in the Ge-doped photosensitive fibre exhibit a saturating λB-shift of about 20 pm towards the longer wavelengths, after a total gamma dose of 80 kGy (at 3 kGy/h and a total gamma dose of 1.5 MGy) [9], [10]. We evidenced that the Bragg peak spectral width at Full-Width Half-Maximum (FWHM) and its temperature sensitivity remains stable under gamma radiation, up to 1.5 MGy. Kakuta [11] confirmed our results up to a 10 MGy gamma total dose and a temperature of 400 ◦C and Fujita [12] up to a to 250 kGy gamma total dose and a temperature of 80 ◦C . The radiation sensitivity of FBGs strongly depends on the chemical composition of the fibre and the photosensitisation technique used for writing the FBGs. The sensitivity to gamma radiation of FBGs written in hydrogen-loaded telecom fibres was found to be higher, but the Bragg peak shift saturation still occurs. [9] The lowest radiation sensitivity is achieved with standard highly Ge-doped photosensitive fibre, without any preor post-fabrication treatment. Recently, we evidenced the Bragg peak shift dose-rate dependence of such FGBs, while the FWHM remains unaffected, as it can be shown in Fig. 3 [13]. These experiments allowed us to set-up a qualification procedure for FBGs under ionising radiation. A detailed description of our measurement set-up can be found in a previous paper [14]. 2) Neutron radiation effects: Once the most radiationtolerant FBG under pure gamma radiation, we need to investigate their behaviour under a mixed gamma-radiation field. We therefore inserted FBG filters inside our low flux graphitemoderated air-cooled reactor for 30 month (see Fig. 2 and [14]). The FBG filters written in photosensitive fibre, without any preor post-writing treatment, remain operational due to the saturating property of the Bragg peak shift. These filters still operate satisfactorily after 30 month of continuous irradiation, evidencing the long-term FBG reliability in nuclear Line drivers Fibre WDM coupler Optical WDM link Analog electro-optical conversion