In Japan, after the Great East Japan Earthquake, existing nuclear power plants (NPPs) are being re-examined to ensure they can withstand more intense natural phenomena such as earthquakes, tsunamis, volcanic eruptions, forest fires and tornadoes, without any loss of capability to safely maintain their function. One of the representative tornado-generated missiles (tornado missiles) prescribed by the NRA (Nuclear Regular Authority) is 0.2m wide × 0.3m deep × 4.2m long, 135kg in weight with a maximum velocity of 51m/s; a rectangular steel pipe. Several impact design methodologies have been implemented to the safety evaluation during business licensing activities to restart NPPs. However, there is considerable variation among licensees when implementing these impact design methodologies and no clear guidance explaining the requirements necessary to achieve accurate results for the impact design of tornado missiles.

In response, in 2014, the Japan Society of Mechanical Engineers (JSME) formed a Special Task Group (STG) on codification activities for the purpose of developing a guidance document on the impact design of the system, structure and components (SSCs) of the NPPs against tornado missiles. The current guidance undertaken comprises the following five chapters and appendices.

- Section A: This chapter defines the objectives and scope.

- Section B: This chapter outlines the methodology of how to determine the design tornado intensities and design loads against tornado missiles for SSCs.

- Section C: This chapter provides the methodology on how to select representative tornado missile designs from the plant walk-down investigative results and set the design impact velocity based on several vortex models.

- Section D: This chapter notes empirical formulas for the local damage of steel/concrete panels to prevent penetration or scabbing to which tornado missiles are prone. Methods to set design impact loads for hard and soft missiles are also presented, as well as those used to evaluate structural response using an elastic or elastic-plastic single degree of freedom (SDOF) model subjected to impact loads.

- Section E: Modern explicit dynamics finite element computer codes used to design and evaluate SSCs, such as LS-DYNA, AUTODYN, Abaqus/Explicit are sophisticated and robust enough to produce accurate results for tornado missile impact events. This chapter provides guidance on a computational modelling method for explicit dynamics, including the construction of quality finite element models following strain-based acceptance criteria.

- Appendices: as an example of setting tornado missiles and impact velocity, benchmarking of explicit dynamic codes is introduced.

An overview of the contents of this guidance document will be focused on in this paper.

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