Moving Beyond Traditional Tornado Missile Impact Analysis

On March 11, 2011 a 9.0 magnitude earthquake occurred off the coast of Japan creating a tsunami, which together severely damaged northeastern Japan including the Fukushima Dai-ichi nuclear power plant site. In response to this event, the United States (U.S.) Nuclear Regulatory Commission (NRC) issued Order EA-12-049, 'Issuance of Order to Modify Licenses with Regard to Requirements for Mitigation Strategies for Beyond-Design-Basis External Events'. In order to meet the guidance of the NRC Order EA-12- 049, the a central U.S. nuclear generating station (Plant) was required to demonstrate that the currently installed Borated Water Storage Tank (BWST), Refueling Water Tank (RWT), and Qualified Condensate Storage Tank (QCST) could provide the necessary capacity following a Beyond Design Basis External Event (BDBEE). These tanks were not originally designed to meet current site design basis requirements for tornado wind and missile impacts. To save the Plant from having to build costly new storage tanks off site, the existing tanks were credited as robust for tornado missile impact through advanced finite element impact analyses using the ANSYS Autodyn solver and fluid-structure interaction (FSI). This procedure is the first known NRC acceptance of advanced analysis and FSI for to satisfy the FLEX tornado missile impact criteria. Extensive verification and validation (V and V) was performed to verify perforation of steel plates which included FSI. These V and V analyses were confirmed against large databases of experimental and theoretical results to ensure accurate modeling and results. The use of this advanced analytical method benefitted the Plant in several ways. A new storage tank was not required to be built, saving not only initial construction costs but also ongoing maintenance costs and monitoring activities. By determining specific vulnerable components of each tank a set of modifications and protection schemes were developed. These modifications were much quicker to implement saving the Plant on downtime costs. (authors)