Development of helium leak detector for canister (Part.4). Evaluation for leak detection method

In the case of the positive pressure design, if a pressure drop of 5 kPa and 60 kPa as the allowable leak amount can be detected within 6 months based on American National Standard and the transportation requirement, it is determined that normal operation can be continued. Also, in the case of the negative pressure design, we should detect leakage into the canister until the internal pressure becomes atmospheric pressure. We performed leak tests under the condition that helium was included in a negatively pressurized canister. The temperature of the heating elements increased because air with low thermal conductivity flew inside the canister. Thus, the temperature of the bottom also increased. The temperature of the lid part increased because helium with high thermal conductivity and low density stayed in the upper part of the canister and the heat of the heating elements spread through the lid. On the other hand, the temperature of the side of the canister body decreased. It was clarified that the higher negative pressure degree is, the larger a temperature change is, on the other hand, the initial temperature of the heating elements do not depend on the negative pressure degree. Therefore, it was proved that the high-sensitive leak detection is possible without allowing gas to leak outside, and the usability of the negatively pressurized canister was verified. In the leak detection method using the temperature change on the canister, both of disturbances by an atmospheric temperature change and by a heat rate change of the heating elements could be removed, so that only the temperature change of the canister surface by the pressure drop could be detected with high accuracy. Also, the method used a self-learning technique, so that its prediction accuracy could be self-improved by using existing data. We verified the effectivity of this technique by using test data. (author)