Study on Post-Buckling Behaviors of Lower Heads for Fracture Control of Reactor Vessels Under BDBE

Abstract

As a lesson learned from the Fukushima nuclear power plant accident, the industry recognized the importance of mitigating accident consequences after Beyond Design Basis Events (BDBE). We propose the concept of applying fracture control to mitigate failure consequences of nuclear components under BDBE. This paper studies post-buckling behaviors of lower heads for fracture control of reactor vessels under BDBE. In the case of a reactor vessel that supports the weight of the vessel at the top, such as a fast reactor, if a loss of cooling accident occurs, the cylindrical body may rupture with large creep deformation due to extremely high temperature. In order to cope with this event, application of fracture control concept is proposed. As a concrete example of the countermeasure, it is considered that reactor vessel lower heads can contact the floor or other structures to relieve the load on the cylindrical body and avoid catastrophic failure of the cylindrical body. In order to achieve this, it is necessary that even if a lower head in contact with the floor or other structures buckles, the subsequent post-buckling behavior is stable to maintain the load carrying capacity, and there is a strength margin before the failure of the lower head. Buckling experiments and analyses were conducted on spherical shells with central cylindrical bodies and smooth spherical shells in contact with a rigid floor. The post-buckling behavior of all the above spherical shells was stable to maintain the load carrying capacity, and they did not fail immediately after buckling occurs. From above results that the load carrying capacity of the lower head is sufficiently maintained after buckling, it was shown that the rupture of the cylindrical body of reactor vessel can be controlled by redistributing the load on the cylindrical body, which is expected to rupture due to extremely high temperature at a loss of cooling function.re.