Important lessons have been learned from flooding hazard re-evaluations performed in response to the U.S. Nuclear Regulatory Commission's 50.54(f) Letter requesting information to address recommendations of the Near Term Task Force's review of insights from the Fukushima Dai-Ichi accident.

Some United States nuclear plants were initially licensed before completion of modern probable maximum precipitation guidance. Surfacewater flooding hazard evaluations for many U.S. nuclear plants were performed with 1-Dimensional watershed and runoff models, however, modern advances in computer modeling have rendered many basic 1-Dimensional models obsolete. Plant topology changes including building renovation, addition, demolition, and post 2001 security upgrades have significantly changed stormwater runoff patterns in many locations.

The USNRC's 50.54(f) letter requires a flooding hazard re-evaluation to be implemented "using updated seismic and flooding hazard information and present-day regulatory guidance and methodologies".

One of the key lessons learned in this process was that a modern analysis conducted with state-of-the art tools, methodologies and data cannot be expected to result in the same quantification of flooding hazards as initial design/licensing calculations and analyses (some of which were performed 40 to 50 years ago).

Some of the lessons learned from the flooding hazard re-evaluation process relative to project site, regulatory, and environmental subtleties and special concerns are identified below.

Hierarchical Hazard Analysis
The USNRC's Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America (NUREG/CR-7046) advises applicants for NRC license or permit to perform a safety analysis to demonstrate that hazards from natural phenomena do not adversely affect function of the plant's safety-related structures, systems and components (SSC's). Potential flooding hazard contributors including hydrometeorological, seismic, and geographic components should be analyzed and evaluated using a stepwise Hierarchical Hazard Assessment (HHA) approach. This approach begins using the most conservative assumptions regarding input parameters such as rainfall intensity (Local Intense Precipitation-LIP/Probable Maximum Precipitation-PMP), infiltration, and the functionality of passive stormwater systems.

If the most conservative plausible assumptions used in the first phase of the HHA result in flooding hazards leading to adverse effects or exposure to any safety-related SSC, then a more site-specific hazard assessment is performed. This iterative process ultimately provides and demonstrates assurance criterion for safety considering reasonable and justifiable site-specific input.

Present Day Guidance - Rainfall
Accepted guidance for determining Probable Maximum Precipitation (PMP) is contained in Hydrometeorological Report Numbers HMR-51, HMR-52, and HMR-53 (NOAA &US Army Corps of Engineers 1980 - 1982). Nuclear plants initially licensed prior to the release of these documents did not benefit from the research and development which went into these PMP estimates. In the very recent past, scientists have been developing regional site-specific estimates of PMP. The HHA phases of flooding hazard re-evaluation should consider the accepted HMR based PMP and may be allowed to utilize justifiable site-specific PMP to determine rainfall input for LIP calculations.

Modern Analysis Tools and Techniques
Legacy 1-Dimensional watershed (USACE HEC-1) and runoff (USACE HEC-2 or HEC-RAS) models were used for initial licensing Local Intense Precipitation (LIP) studies at many of the US nuclear plants. While these models were state-of-the art in the 1970's, 80's, and 90's, many advances have been made in the field of numeric modeling over the last 20 years. Detailed LiDAR based site topography can be analyzed with 2-Dimensional combined watershed/runoff models (such as FLO-2D) to quantify stormwater runoff depth, duration, and velocity at a very high resolution. This type of 2-Dimensional LIP analysis can often identify areas of concern which are not resolved with a more basic 1-Dimensional analysis.

Site Grading and Topography
Ideal site grading would be designed to elevate the plant Protected Area (PA) and drain all stormwater surface runoff away from critical locations and safety related SSC's. Ideal site grading is not always possible and nuclear plants are typically characterized with somewhat varied topography on or adjacent to the PA. Detailed comprehensive site surveys and modern 2-Dimensional computer models can be used to resolve stormwater flowpaths throughout the PA and then used to identify low spots and areas of potentially hazardous inundation.

Vulnerable Entrances and Penetrations
Depending on plant building design and layout and site topography, some entrances and penetrations may be subject to temporal inundation from LIP. The output from a detailed 2-Dimensional LIP model can be used to identify and evaluate inundation depth and duration at any vulnerable entrances or building penetrations.

Passive Storm Drainage Systems
Sequential iterations of the Hierarchical Hazard Analysis can be used to understand LIP flooding hazards with blocked or partially functional passive surfacewater and stormwater conveyance systems. Time dependent inundation can be evaluated for areas with adverse flooding impacts as LIP generated stormwater is conveyed to and through yard and storm drain systems. The actual status of site storm and yard drains can often be different from the storm plan or as-built drawings. Site walk-downs and storm drain inspections can be used to verify as-built drawings and to inventory areas of necessary on-going maintenance.

Site Improvements
Many plants have undergone some level of site modifications since initial design, licensing, and construction. Building remodels, demolitions, and additions can lead to ground level changes which may impact stormwater flowpaths and ultimately inundation levels. Many changes to access roads or site topography will directly affect stormwater drainage. Post 2001 security additions (Bullet Resistant Enclosures (BRE's), vehicle and delay barriers, storm drain security mesh/grates, etc) can also impact storm drainage and should be inventoried and accounted for within the LIP model.