Abstract
As mitigation strategies from structural mechanics for Beyond Design Basis Events (BDBE), authors propose failure mitigation methods. After the Fukushima Nuclear Power plant accident, such many efforts were made to mitigate BDBE consequences as portable devices, additional backup facilities and accident managements. In the structural mechanics field, however, efforts were only strengthening to prevent failures for both DBE and BDBE. This approach will lead to limitless requirements for strength and expensive plants.
As the break though approach in structural mechanics for BDBE, authors propose failure mitigation methods by application of the fracture control concept. The fundamental idea is the control of failure order and modes. Preceding failures release loadings and mitigate consequent failures. When preceding failure modes have small effects for safety performance, such as small deformation and clack initiation, and consequent ones are catastrophic modes as collapse and break, fracture control improve safety and resilience.
Failure consequence mitigation can be realized by passive characteristics of structures without additional equipment and electric power. Therefore, this idea enables simple and reliable plants
To demonstrate this idea, failure mitigation methods were applied to next generation fast reactors, where high temperature and low-pressure conditions.
In the case of loss of heat removal accidents, high temperature conditions accelerate creep deformation of structures. When deformation will redistribute loadings and reduce stresses at important portions such as cooling liquid boundaries, consequence to creep rupture of boundaries can be mitigated.
When excessive earthquake, plastic deformation and buckling become dominant, because of thin structures from low pressure. Above failure modes reduce rigidity and natural frequency. When the natural frequency becomes lower than the input frequency, vibration energy is hardly transferred to structures and mitigate consequent failure of structures, such as collapse and break.
