Design and development of FBG based load cell for weight monitoring of cesium pencil in radiation environment
The study, published in the journal Discover Sensors, presents a specific solution to the frequent failure of conventional resistive strain gauge sensors, which degrade under the combined stress of intense gamma radiation and elevated temperatures. The new sensor utilizes a titanium transducer and optical fiber technology to provide reliable, long-term weight monitoring for nuclear fuel cycle facilities where human access is restricted and environmental conditions are extreme.
The primary motivation for this development stems from the production process of cesium-137 source pencils at the Nuclear Recycle Group. These pencils, which are supplied for use in medical blood irradiators, require the precise pouring of approximately 260 grams of vitrified cesium glass into compact stainless steel containers. Currently, the facility relies on resistive strain gauge-based load cells for this remote weight monitoring. However, these conventional sensors suffer from significant limitations in such harsh settings, including susceptibility to electromagnetic interference, signal degradation, and corrosion. The frequent failures observed in these traditional systems necessitated the engineering of an alternative sensing technology capable of maintaining accuracy and stability under prolonged exposure to radiation and heat.
Fiber Bragg grating sensors emerged as a promising candidate for this application due to their intrinsic immunity to electromagnetic interference and their high radiation tolerance. An FBG consists of a periodic variation of the refractive index along the core of a single-mode optical fiber. When an axial strain is applied to the fiber, it causes a shift in the reflected Bragg wavelength. This shift is linearly proportional to the applied load, allowing for precise quantification of weight. The sensitivity of the sensor is governed by the effective photo-elastic coefficient and the strain transferred from the mechanical host structure to the optical fiber. This mechanism allows for the conversion of mechanical deformation into a measurable optical signal without the need for wired electrical connections that are prone to failure in nuclear environments.
The mechanical design of the load cell features a tapered cantilever structure made from titanium alloy Grade-5. This material was selected for its low Young’s modulus and high yield strength, which provide twice the flexibility and four times the elastic deformation compared to standard stainless steel. The trapezoidal shape of the cantilever beam is optimized to provide larger average strain at the sensor’s location, thereby enhancing the overall sensitivity of the load cell. The total length of the structure is 35 mm, with the fiber mounted across a 6.5 mm distance. The compact dimensions of the sensor, measuring 20 mm by 20 mm, were specifically engineered to fit within the constrained 25 mm by 25 mm space available in the existing cesium indexing system.
To evaluate the performance of the proposed design, the researchers employed finite element analysis to model the strain transfer and optimize the cantilever geometry. The simulation treated the FBG as an equivalent axial spring to account for the mechanical stiffness of the optical fiber. The analysis predicted load sensitivities of 4.82 pm/g, 1.83 pm/g, and 0.92 pm/g for cantilever thicknesses of 0.5 mm, 0.7 mm, and 1.0 mm, respectively. Based on these simulations, the team determined that the 0.5 mm thickness, while offering the highest sensitivity, had a maximum load capacity of only 200 grams. Consequently, prototypes with thicknesses of 0.7 mm and 1.0 mm were selected for fabrication to ensure a measurement range up to 500 grams, which covers the required operational scope for the cesium pencils.
Experimental evaluation of the fabricated prototypes revealed sensitivities of 1.34 pm/g for the 0.7 mm cantilever and 0.76 pm/g for the 1.0 mm cantilever. These experimental values were lower than the simulated predictions, a deviation attributed to incomplete strain transfer through the adhesive bonding layer. The strain transfer from the titanium host to the fiber is governed by the shear-lag effect in the adhesive. Because the adhesive and fiber cladding have finite elastic moduli, they undergo creep and absorb a portion of the applied strain. To mitigate this, the researchers used a thin layer of high-modulus epoxy adhesive, which remains stable up to a 500 kGy gamma dose, to maximize the efficiency of strain transfer to the sensing element.
The linearity and repeatability of the load cell were tested by applying loads from 0 to 500 grams in 100-gram increments. The results demonstrated a high degree of linearity, with fitting determination coefficients of 0.9995 for the 1 mm cantilever and 0.9994 for the 0.7 mm cantilever. In repeatability tests involving five loading and unloading cycles, the maximum deviation from the mean central wavelength was observed to be ± 5 pm. The relative standard deviation for the mean central wavelength shift was 2.38%, indicating that the sensor maintains low error and excellent consistency across multiple operational cycles. This level of precision is critical for the accurate handling of radioactive materials where even small discrepancies in mass can have significant implications.
The FBG-based load cell was integrated into the cesium pencil indexing system using 6-meter-long fiber optic communication cables. The system was placed in close proximity to the existing strain gauge-based system for comparative purposes. The results showed significant agreement between the two measurement techniques, reinforcing the conclusion that the FBG sensor is a robust and dependable alternative to traditional systems.
A key advantage of the FBG system in this environment is its ability to handle radiation-induced wavelength shifts. Gamma exposure induces a radiation-induced Bragg wavelength shift due to the formation of color centers in the fiber core. However, the researchers noted that this shift acts as a common-mode offset affecting both the unloaded reference and the loaded wavelength equally. By automatically acquiring the reference wavelength at system startup, the system effectively cancels out this radiation-induced baseline shift. The load measurement is determined from the differential wavelength shift, ensuring that the accuracy of the weight monitoring is preserved even after significant radiation exposure. The titanium transducer and the epoxy adhesive were also confirmed to remain stable under the site’s annual accumulated dose of 300 to 350 kGy.
The study concludes that the developed FBG-based load cell is suitable for accurate and long-term operation in radiation environments. The sensor achieves a resolution of 2 to 3 grams over a 500-gram range, meeting the precision requirements for cesium pencil indexing. The integration of a dual FBG configuration also allows for temperature compensation and sensor redundancy, which minimizes the need for frequent replacements in harsh operating conditions. This work represents a significant step forward in the development of sensing technologies for nuclear facilities, providing a reliable and compact solution for weight monitoring where conventional electronics fail.
The research highlights the importance of tailoring sensor design to specific environmental constraints.
The authors emphasize that the novelty of the work lies in the specific engineering of a compact, radiation-compatible load cell for integration within a constrained indexing system. The results of this study confirm that optical fiber technology can be effectively adapted for precision weight monitoring in some of the most challenging environments in the industrial world.
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