Flood Hazard Re-Evaluations at U.S. Nuclear Plants

Challenge

After the devastating earthquake and tsunami at Fukushima, Japan, the NRC required all operating U.S. nuclear power plants to re-evaluate their flood vulnerability in order to remain licensed to operate. The studies were to be completed within 1 to 3 years based on the priority ranking set by NRC. Beginning in 2012 and continuing through 2016, GZA was retained by plant owners and nuclear engineering firms to complete several External Flood Hazard Re-evaluations of multiple nuclear power plant sites in coastal and riverine areas throughout the U.S.

Solution

The external flood reevaluations were performed in response to NRC’s 10 CFR part 50, Section 50.54 (f) plant licensing requirements and technical guidance for Post Fukushima Regulatory Response information issued March 12, 2012. GZA’s scope of work included calculation of: Site Specific Probable Maximum Precipitation (PMP), Probable Maximum Flood (PMF); Local Intense Precipitation (LIP) Flooding; Ice Effects Flooding; Probable Maximum Hurricane (PMH); Probable Maximum Storm Surge (PMSS); Probable Maximum Seiche (PMS);Probable Maximum Tsunami (PMT); and Combined Effects Flooding Mechanisms.

GZA worked with subconsultant Applied Weather Associates to develop site-specific precipitation design input. GZA also worked with Dr. Kerry Emanuel of WindRisk Tech in performing a probabilistic analysis of tropical storm and hurricane storm surge using synthetic tropical storm, hurricane tracks and other storm parameters developed by Dr. Emanuel. State-of-the-art technologies included two-dimensional numerical hydrodynamic models to evaluate flow depths and velocities and a high site-specific resolution scale. These model simulations required a high-performance computer cluster to execute and simulate a combined hurricane surge with coincident river flooding and wind-generated wave effects. Beyond-design events included deterministic “Probable Maximum” type storms in addition to probabilistic assessment of extremely low probability events, with annual exceedance probabilities on the order of 1 x 10-6 per year or a one in a million-occurrence interval storm.

Benefit

GZA completed studies at 23 plants over three years in compliance with NRC’s licensing, schedule and technical requirements. All work was performed under the 10CFR50 Appendix B Quality Assurance program. Significant plant safety enhancements resulted from the Post Fukushima studies at U.S. commercial nuclear plants including: adding capabilities to maintain key plant safety functions following a large-scale natural disaster; updating evaluations on the potential impact from flooding events; new equipment to better handle potential reactor core damage events; and strengthening emergency preparedness capabilities.