During PIV (Physical Inventory Verification), the IAEA has been inspecting the CANDU-Type spent fuels using an optical fiber-based scintillation detector. KINAC has developed a new verification instrument to deal with problems of the existing one such as low sensitivity, heavy and large dimension, and inconvenience-in-use. Our previous studies focused on how to develop the new instrument and had not included its performance tests. Field tests were carried out recently at Wolsung unit 4 to evaluate performance of the existing and new instruments. The objective of this paper is to discuss background noise produced in the optical fiber signal cable itself. The verification equipment for the CANDU-type Heavy Water Reactor spent fuels uses a scintillation detector to bond a scintillation material to the end of an optical signal cable. At this time, the radiation signal obtained by a data acquisition system is the signal generated from the scintillator (p-terphenyl organic scintillator) and the optical signal cable ; The signal produced in the optical cable itself is background noise to degrade the spent fuel verification equipment. To characterize the background radiation noise, the spent fuel bundles at Wolsung Unit 4 were measured using the optical fiber cable without the radiation scintillator. This signal is generated by reaction of the optical cable and the radiation emitted from the spent fuel. From experimental results, it was observed that the background noise signal of the optical cable increased as the optical cable went down in the downward direction, because the cable length irradiated by the radiation increased with the optical cable area in the spent fuel storage pool. Difference in the background noise signal was dependent on the location of the vertical direction and the signal of the new optical cable was up to about 5 times higher than that of the existing cable. While, the new cable has the cross-section area about 3.2 times larger than the old cable. Our past studies showed that total signal amplitude – sum of signals generated from the scintillator and optical fiber - of the new verification instrument was at least about 15 times greater than that of the existing one. Considering the total signal and background noise signal, from this measured results, it was confirmed that the scintillator characteristics – in particular, light output and decay time – has a dominant impact on the signal sensitivity of the newly developed instrument. More details will be discussed at the conference.

• Sung-Woo Kwak(Korea Institute of Nuclear Nonproliferation and Control (KINAC)) corresponding author
• Hojik Kim(Korea Institute of Nuclear Nonproliferation and Control (KINAC))