3. Reactor Physics and Reactor Engineering

概要

INTRODUCTION: The accelerator-driven system (ADS) has been developed for transmuting minor acti- nides and long-lived fission products [1-2]. For the ADS system, development of a real-time subcriticality monitor is desired to be developed in order to ensure to keep subcritical condition in any case. Therefore, we are developing a real-time subcriticality monitoring sys- tem. For the ADS experiments in Kyoto University Critical Assembly (KUCA), optical fiber neutron detec- tors were developed. To realize high sensitivity of the detector, neutron scintillator is coated around the wave- length-shifting fiber, which can collect photons from the side surface of the optical fiber. The detector with a long coated region has high sensitivity. In the last year experiments, we used a LiF/Eu:CaF2 eutectics scintillator, because this scintillator material is transparent and can be thick to improve the sensitivity without attenuation loss of the scintillation photons. However, since this scintil- lator has relatively low α/β, which is the ratio of light yield for alpha (high LET) and beta (low LET) particles. Therefore, this detector has relatively high beta-ray sensi- tivity. This detector is influenced from the beta particles, which are emitted from the activated material, such as aluminum sheath of fuel elements, especially under low neutron intensity situation.
　In this study, we attempted to fabricate the optical fiber detector with low beta sensitivity but high neutron sensi- tivity.

EXPERIMENTS: We fabricated the optical fiber neu- tron detector using LiF/ZnS scintillator. This scintillator material has been used for the optical fiber neutron de- tector conventionally. In this study, the detector with quite long sensitive region was realized. Figure 1 shows the fabrication processes of the detector. This detector has a sensitive region of 100 mm long. In addition, since the sensitive region was covered by a stainless steel tube with 3 mm outer diameter, this can easily be inserted into a space between fuel elements. We compared the beta sensitivity of the detectors using LiF/Eu:CaF2 and LiF/ZnS scintillators in the KUCA experiments.

RESULTS: Figure 2 shows the time trends of the measured count rate by the fabricated detectors. We can see that the both detectors have the almost same neutron sensitivity. However, the detector using LiF/ZnS scin- tillator showed lower count rates when the reactor was shutdown. On the other hand, the detector using LiF/Eu:CaF2 scintillator showed the decay component after reactor shutdown.

この論文で使われている画像