Study of capsule position monitoring methods for ITER neutron activation system

The reliable monitoring of the position of an encapsulated activation sample is essential to ensure the diagnostic accuracy and the maintenance of the ITER neutron activation system (NAS). Conventional methods using optical or electrical detectors to determine the capsule position is difficult to be used in the ITER NAS because of limited space as well as extremely high electromagnetic and radiation environment. In this study, new methods using the flow rate change inside a transfer tube assembly and the propagation characteristics of sound wave are investigated for the reliable determination of the capsule position. Experimental results confirm that the abrupt reduction of flow rate in the transfer tube assembly provides information for the final position of the capsule with a high spatial resolution less than 1 mm. The variation of flow rate is also found to indicate the operational status of the pneumatic transfer system. In the case of capsule lost accident, a laboratory scale test has demonstrated that the exact position of the lost capsule can be determined by the sound wave method in which the time delay between an incident sound signal and a reflected one by the capsule is measured so as to provide the position of the capsule with a spatial resolution of 0.2 m. These two capsule position monitoring methods are expected to improve the accuracy, operational stability, and the ability to handle the accident in the ITER NAS.