Vol.:(0123456789) 1 3 Optical Review https://doi.org/10.1007/s10043-019-00538-y SPECIAL SECTION: REGULAR PAPER Development of a fber‑optic remote temperature sensor to monitor water temperature in a spent nuclear fuel pool Sang Hun Shin 1  · Hyungi Byun 1  · Jin Ho Kim 1  · Hyun Young Shin 1  · Si Won Song 1  · Seunghyun Cho 2  · Bongsoo Lee 1 Received: 30 January 2019 / Accepted: 18 August 2019 © The Optical Society of Japan 2019 Abstract In this study, a fber-optic remote temperature sensor (FRTS) for real-time temperature measurements in a spent nuclear fuel pool is developed. The proposed FRTS consists of a fber-optic temperature-sensing probe and an optical time-domain refectometer (OTDR). The probe of an FRTS consists of silicone oil, a fber channel (FC) terminator, a single-mode optical fber, and a copper metal cap. Silicone oil is employed as a temperature-sensing material owing to its temperature-dependent refractive index. The optical powers of the refected light signals (Fresnel refection), which are generated at the interface between the silicone oil and the core of a single-mode optical fber in the distal end of the sensing probe, are measured. The temperature of the water was measured in 5 ℃ increments ranging from 10 to 70 ℃, using a fabricated FRTS and an OTDR. The proposed FRTS could be used to efectively monitor the water temperature of a spent nuclear fuel pool (SNFP) at a nuclear power plant (NPP). Keywords Fiber-optic temperature sensor · Optical time domain refectometer · Water temperature · Remote sensor · Spent nuclear fuel pool 1 Introduction The water temperature of a spent nuclear fuel pool (SNFP) at a nuclear power plant (NPP) needs to be monitored con- tinuously since the spent nuclear fuel continues to produce a lot of radiation and heat, which must be managed to protect workers, the environment and the public. The water tem- perature of an SNFP should be maintained within the range 40–60 ºC according to technical specifcations [1]. The general design of a wet storage facility includes fea- tures such as spent fuel transport cask handling, loading and decontamination systems, radioactive waste treatment and handling systems, personnel support systems, and buildings to house the required equipment. Normally, the size of an SNFP ranges from 10–20 m in length, 7–15 m in width, and 8.5–17 m depth. The storage area varies according to the amount of spent fuel that needs to be stored, which in turn depends on the type and size of the reactor [2]. During abnormal transient or accident situations in nuclear reactors, the operator must be aware of the key ther- modynamic parameters (e.g., temperature, pressure, and water level) in the SNFP to facilitate appropriate action. The Fukushima nuclear accident demonstrates the need to further improve SNFP monitoring instrumentation to be able to withstand better severe accident conditions such as high temperatures and radiation environments [3, 4]. In the past decades, many kinds of sensors have been used for monitoring thermodynamic parameters in an SNFP. However, general electronic or chemical sensors have some problems associated with their use as water temperature monitors of SNFPs, due to the harsh environment with high radiation felds and high humidity of SNFPs; therefore fber- optic remote sensing is essential for monitoring water tem- perature of SNFPs and it is necessary to protect workers [5]. Fiber-optic based sensors ofer several advantages such as a small size, good fexibility, remote operation, immunity to The 11th International Conference on Optics-Photonics Design & Fabrication (ODF’18), Hiroshima, Japan * Bongsoo Lee bslee@cau.ac.kr 1 School of Energy Systems Engineering, Chung-Ang University, Seoul, Korea 2 Department of Organic Materials and Fiber Engineering, Soongsil University, Seoul, Korea