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 discusses example applications of our proposed method to vessels and piping.

As an example for vessels, the fracture controlled method of a fast reactor vessel was proposed to maintain coolant under high pressure and high temperature conditions. One of the important accidents to consider is fuel criticality upon a core meltdown. This accident will cause high temperature and high pressure conditions in the vessel.

To maintain coolant to cover the fuel under above conditions, strength of the upper part of a vessel is controlled to be weaker than the lower part. When the upper parts fail, internal pressure will be released and the lower part will be protected. Coolant of fast reactors is liquid metal with a high boiling point. Therefore, there is no boiling when the pressure drops. As a result, the coolant stays within the vessel for continuous cooling after accidents. If nozzles penetrate only the upper part of a vessel, controlling strengths at the structural discontinuities can realize the proposed method.

As a piping example, the fracture controlled method of main steam piping of a BWR was proposed to avoid collapse and break under excessive earthquake. The piping is constrained by supports to the building. Strength of piping supports should be weaker than that of the piping. If piping supports break before the piping, natural frequencies of piping decrease increasing the frequency ratios. When their frequency ratios are larger than one, the seismic energy hardly transfers to the piping and the piping will not collapse or break. When the primary stresses from dead weight are not negligible, simple supports by wire are enough to protect piping against dead weight.

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