Delay impact mechanism of Spent Fuel Cask without assuming content dropping inside packaging

International Atomic Energy Agency (IAEA) regulations for the safe transport of radioactive materials require a 9-m drop test for the Type B packages, such as large spent fuel (SF) transport casks. In vertical drop tests, where the SF cask is in a lid-down orientation, it has been observed that the pullout force of the lid bolts delays the impact from the grounding of the specimen. This phenomenon is known as delayed impact and is of significant concern for the safe transport of radioactive materials. Previously, the delayed impact has been considered to occur because of the delayed collision of the contents in the cask with its lid when there is a gap between the contents and lid. In such case, since the lid of the SF cask is usually fixed at the end of the cask body, the contents push the lid plate to increase the pullout force, leading to the delayed impact. Herein, a new noise reduction method, which is the acceleration integrated velocity approximation (AIVA) method, is proposed to analyze the acceleration data from a 9-m drop test. Based on a reevaluation of the existing acceleration data using the proposed noise reduction method, it is concluded that the delayed impact could occur even when there is no gap between the contents and lid. To evaluate practical SF casks, the mechanical characteristics of the contents are estimated based on the published results of a full-scale drop test, and the delayed impact in a no-gap case is estimated based on the obtained characteristics. In a previous safety analysis of a wet-type SF cask, the delayed impact was not considered to be important, since the contents might be restricted by the water present inside the cask. However, based on this study, it is concluded that the delayed impact caused by the content reaction should be considered even in such no gap cases.