The summer of 2011 was one of the hardest ever on American nuclear power plants. After the Fukushima Daiichi disaster, U.S. nuclear plants were required to inventory and test emergency equipment , and undergo emergency drills to make sure safety systems were in place and working properly. One of the biggest threats to the safety of any nuclear power plant would be a prolonged loss of electrical power because the plants need to be able to continue pumping water over the radioactive fuel to keep it cool. Failure of one or more dams upstream from a nuclear power plant may result in flood levels at a site that render essential safety systems inoperable.

In the immediate aftermath of the Fukushima nuclear disaster, Japanese officials assured everyone that the problems at Fukushima Daiichi were limited and controllable, everything was alright; everything was under control. It took a only took few days for that house of cards to be destroyed, only to be re-built, destroyed, and resurrected many times over, in a perpetually endless cycle.

After extensive flooding both Nebraska nuclear power plants were in shutdown modes for the majority of the summer, wondering if the waters would recede in the near future. The Fort Calhoun nuclear power plant disconnected from the grid because the river water leaked through a cement barrier installed to protect the plant's main transformer. OPPD, the utility which operates the station, invested in about 300 life jackets and a couple hundred pairs of waders for times when employees struggled navigating through the water to check a flood barrier or build more scaffolding. Boats were also used to ferry equipment around the complex.

The NRC Regulatory Guide 1.59 addresses the acceptability of using alternatives to hardened facilities for flood protection.

The importance of considering the performance of hardened protective features was demonstrated during a flood at the Blayais Nuclear Power Plant (France) in 1999. During the event, protection of underground rooms containing safety equipment was insufficient and dikes were found to have insufficient height and shape. Both Units 1 and 2 at the site were severely affected by the floodwaters. An essential service water pump was lost due to the immersion of the motors. Utility galleries, the bottom of the fuel handling building, and rooms containing electrical equipment were also flooded (IAEA 2003).

Moreover, the flood warning system was inadequate and detection of water in affected rooms was difficult (Fraguier 2010). Prairie Island Nuclear Generating Plant has a small margin between flood levels and flood protection elevation (<1.5 ft). By including wave effects, the water level increases to 706.7ft MSL (PINGP 2010, p. 2.4-5) and the margin becomes negative. The NRC admits that this situation would be exacerbated if the flood event is augmented by the flood volume resulting from an upstream dam breach.

Examples of such plants include Prairie Island Nuclear Generating Plant (PINGP 2010, p. 2.4-6), Beaver Valley Power Station (BVPS n.d., p. 5-11), Sequoyah Nuclear Plant (SNP, Sequoyah Nuclear Plant Updated Final Safety Analysis Report, Revision 20 2007, 2.4-6), and Watts Bar Nuclear Plant (WBNP 2010, p. 2.4-8).

A significant contributor to the elevated risk at Fort Calhoun Station comes from its reliance on the placement of temporary barriers to protect the plant during a large external flood event. In the middle of the night, an incident at the plant brought floodwaters literally up to the doors of the nuclear power plant. OPPD, the utility who operates the plant said, a piece of heavy equipment moving sand on the dry side of the water-filled dam, “brushed up” against it, causing it to rupture. Because of the collapsed water-filled dam, river water surrounded the main reactor building, mechanical building, spent fuel pool building and other structures.

Workers at entrances to the buildings in an attempt to prevent water from entering, which failed, and forced work to be focused on pumping water out of the plants turbine building. The dam itself was not part of OPPD's federally required flood defenses, rather an extra line of defense OPPD installed to make it easier to get around the plant's immediate exterior.

“It's pretty jarring to see a boat tied up to the nuclear power plant. ... It's an intense operation going on there, particularly with water surrounding all the buildings,” U.S. Rep. Jeff Fortenberry of Lincoln, who toured the plant.

Nuclear Regulatory Commission Chairman Gregory Jaczko visited both Fort Calhoun and Cooper nuclear power plants in eastern Nebraska one day after the 8-foot-tall, water-filled temporary berm protecting the plant collapsed. The main building at Fort Calhoun is at 1,004 feet above sea level, which is about 2 feet below the level of the Missouri River. "We don't believe the plant is posing an immediate threat to the health and safety of the public," Jaczko said after his inspection.

Fort Calhoun Station is not the only plant to rely on the placement of temporary protective measures. Cooper Nuclear Station, Vermont Yankee Nuclear Power Station , and Arkansas Nuclear One, to name a few, rely on the placement of temporary barriers or connections such as sandbags, wood planks, and temporary power cables. Three Mile Island Nuclear Station is another example of a plant that requires actions on the part of plant personnel (e.g., installation of flood gates and plugging of openings). In addition, a subset of plants have technical specifications in place that require plant shutdown (or placement of the plant in hot standby) when floodwaters reach a predefined threshold.

OPPD’s President said in February 2012, that the utility has no timetable for restarting the plant and admitted “we lost our edge” when it comes to running the reactor. Gary Gates, Omaha Public Power District President, also told the Nuclear Regulatory Commission, “We let the industry down.” That was during a meeting in which for the first time the NRC said Fort Calhoun would have been shutdown even without the flood.

Gates said he feels positive about that meeting. Board members said they remain confident in Gates and his team. "The recovery process is in full swing," Gates said. However, the NRC says OPPD still need to address issues before they resume operations however the NRC would not comment on what those issues were. They did indicate they were present before last year’s breaker room fire and the summer flood.

Problems mitigating potential threats from Probable Maximum Flooding (PMF)

A major factor in the uncertainty associated with reliance on non-passive protective procedures is the amount of time available to take action following notification of a dam breach. The closer the dam is located to a site, the shorter the available response time following a dam breach. Many dams are operated by organizations other than the plant operator. At most nuclear power plants, flood protection dikes, levees, doors, and other features have not been tested against a flood. These features are all susceptible to failure, as demonstrated, and consequently, such features should not be assumed to automatically perform as intended.

For sites located along streams and rivers, nuclear power plant operators must provide an acceptable level of conservatism for estimating the flood levels caused by severe hydrometeorological conditions_ (USNRC 1977, p. 1.59-7). Regulatory Guide 1.59 discusses the _design basis floods that nuclear power plants should be designed to withstand without loss of capability for cold shutdown and maintenance thereof_ (USNRC 1977, p. 1.59-5).

Flooding remains a concern all along, even though the NRC constantly denied it was an issue throughout last summer, which is relentlessly testing the flood-worthiness of American nuclear power plants like never before. Keep in mind that some flooding was deliberately caused by the Army releasing water from reservoirs to protect them from failing. The Nuclear Regulatory Commission announced today, it has started a formal evaluation of potential generic safety implications for dam failures upstream of U.S. commercial nuclear power plants. Based on the screening, the NRC staff has recommended that flooding from upstream dam failure be further evaluated as part of implementing recommendations from the agency’s Japan Near-Term Task Force.

At Oconee Nuclear Station and Fort Calhoun Station, increased flood estimates have led to ongoing regulatory activity. Most sites do not even take into account for dam failures in their analysis,and that the reported margins between flood levels and flood protection is small. Like many sites in the U.S. inventory of nuclear power plants, flood levels at these two stations were based on relatively outdated flood estimation methods and/or probable precipitation estimates. NOAA has not updated Hydrometeorological Report 51 (which covers most of the U.S. east of the 105thmeridian) in over 30 years. Precipitation induced flood estimates for some plants have been based on these older estimates.

The proposal lists 20 currently operating sites where the issue is suggested to be a factor. All of the sites are potentially affected by upstream (flooding) or downstream (loss of ultimate heat sink) dam failures. The scope of this screening analysis is limited to external flooding due to upstream dam failures. Of the 20 sites listed, the proposal provides additional discussion of two sites: Oconee Nuclear Station and Fort Calhoun Station. Regulatory activity related to flooding and/or dam failure analysis recently occurred at these stations.

The upstream dam issue came to the staff’s attention long before the current interest in natural disasters raised by the Japan earthquake/tsunami and reactor accident. Evaluations of dam failure frequencies and possible flood heights at some U.S. nuclear power plants, suggest that flooding effects in some cases may be greater than previously expected. Although the analysis discusses some current regulatory actions regarding flooding issues at specific plant sites, it is silent regarding the performance of those or any other licensees. This screening analysis highlights the Oconee Nuclear Station in South Carolina and the Fort Calhoun Station in Nebraska. Inspections at these plants identified significant findings regarding the adequacy of each licensee’s implementation of their licensing basis for flooding protection. This could potentially initiate actions which could go all the way to changing the licensing basis for US NPPs through Orders or new rules.

Problems at Fort Calhoun Station were recognized because of an NRC inspection that identified an apparent violation of Technical Specification 5.8.1.a for failure to maintain adequate procedures to protect the plant during external flooding events (USNRC 2010b). At Oconee Nuclear Station, attention was drawn to the elevated consequence from external flooding after staff identified a performance deficiency during maintenance activities that involved the installation of temporary electrical cables through an opening in the flood protection wall (USNRC 2006b, p. 1). This performance deficiency was of particular concern when coupled with flooding estimates that are significantly higher than previously assumed (USNRC 2006a). Thus, in these two cases, identification of flood-related issues resulted from particular scrutiny and analysis of flood protection preparations, assumptions, and procedures. It is unlikely that concerns related to dam failure flooding at these two sites would have stood out based on the FSAR and IPEEE documents alone.

Many findings related to the Oconee Nuclear Station were redacted in the report, but some interesting details emerged. A sudden catastrophic failure of the Jocassee Dam is postulated to result in a flood wave that would overtop Keowee Dam as well as overtop the Oconee intake dike and would flood the plant (ONS 1995,5-19). Flooding of the plant yard is expected to inundate the switchyard and eliminate offsite and station power (ONS 1995, 5-23).

Several uncertainties exist with regard to the risk posed to Oconee Nuclear Station due to upstream dam failure. In particular, uncertainty exists about the flood levels at the site that would result from failure of Jocassee Dam. Moreover, hazard due to external flooding was “screened out” in the IPEEE based on a sufficiently small contribution to core damage frequency as calculated at the time. In the Oconee Nuclear Station IPEEE submittal (ONS 1995, p.5.27), the licensee estimates that the conditional core damage frequency resulting from flooding due to failure of the Jocassee Dam is 7.0(10-6)/year (ONS 1995, p. 5-27).

The contribution to core damage frequency from precipitation induced external flooding is considered negligible (ONS 1995, p. 5-18). The licensee operates on the assumption that this external flood core-damage frequency is of the same magnitude as other severe accident events (e.g., earthquakes, fires). Consequently, in the IPEEE, the licensee questionably concluded that external flooding does not pose severe accident vulnerability (ONS 1995, p. 5-27).

The estimate referred to above, relating to the conditional core-damage frequency is based on an estimate (made by the licensee), and does not include failures due to earthquakes (not deemed credible) or overtopping (ONS 1995, p.5-21). This NRC estimate is an order of magnitude larger than the estimate reported in the Oconee Nuclear Station IPEEE submittal. But even the NRC admits that its database is limited with regard to the fact it does not contain data related to failures due to seismic events.

The Watts Bar Nuclear Plant emergency plans indicate that, in the storm contributing to the PMF, _the West Saddle Dike at Watts Bar Dam would be overtopped and breached. No other [dam] failure would occur_ (WBNP n.d., p. 2.4-12). The licensee indicates that _all safety related facilities, systems, and equipment are housed in structures which provide flood protection up to plant grade at Elevation 728ft MSL_ (WBNP 2010, p. 2.4-8). This elevation is substantially below the design basis flood elevation. The plan also states that the Turbine, Control, and Auxiliary Buildings will be allowed to flood, but the licensee indicates that equipment required to maintain plant safety during a flood – and for 100 days following the flood – is _designed to operate submerged, is located above the maximum flood level, or otherwise protected_ (WBNP 2010, p. 2.4-8). The design basis flood described above does not include an upstream dam failure (other than the overtopping/breach of the West Saddle Dike at Watts Bar Dam) from any or the other 12 major dams upstream from Watts Bar Nuclear Plant.

For other plants listed in the Generic Issue proposal, the NRC states that not enough information is available to support a conclusive determination regarding risks posed by external flooding due to dam failure. Without detailed study and interaction with licensees, available information related to external flooding for these sites is generally limited to the FSARs and IPEEE submittals. As illustrated above, exclusive consideration of these documents may not readily indicate a problem exists related to external flooding because of upstream dam failure.

The Nuclear Regulatory Commission had previously cited the Fort Calhoun plant after an aggressive federal inspection of Fort Calhoun in June 2009, for not being adequately prepared for floods and rated the safety violation in the “yellow” category, the second most serious. The agency ordered changes because it said that under the plan in place at the time, a major flood could cause core damage. The NRC review uncovered a mistake in calculating the level of catastrophic flooding that could occur, an error that OPPD has acknowledged. According to the NRC review, OPPD was prepared for flooding up to a level of 1,009 feet above sea level — five feet below what the NRC required. Additionally, the federal agency said, OPPD's plans for protecting the plant to the required 1,014 feet were flawed and subject to failure. An NRC risk analysis released in 2010 determined that under OPPD's now-discarded plans, flooding above 1,010 feet would have led to a 100 percent chance of a fuel damage if the emergency gasoline pumps didn't work.

After initially contesting the findings, the plant’s operators, Omaha Public Power District, said that the problems had been resolved. Gary Gates, the president and chief executive of the utility, said in a statement in March that the company had updated and “tested and retested” the plan to protect the plant, “in the unlikely event of a catastrophic flood.”

During a hearing in Omaha last month some members of the public questioned if the flood has made the ground underneath the Fort Calhoun reactor unstable. OPPD has yet to comment on this statement. The questions that have been raised about the viability of procedures in place at Fort Calhoun Station combined with the additional assessment of Oconee Nuclear Station should push the NRC to systematically investigate existing procedures at other similar sites. This would allow experts to benefit from more accurate and detailed information, including evaluation of whether the licensee can maintain the plant in a safe shutdown condition throughout the duration of a flood event. This would provide a better understanding of the probability that established safety procedures will succeed (or fail) in protecting critical safety equipment.