Power Engineering

Nuclear Safety: Staying ahead of challenges in the U.S.

A technician inspects the jacket water heater on a Cat 3516 diesel generator set.
A technician inspects the jacket water heater on a Cat 3516 diesel generator set.

By Denver Nicks, associate editor

In early March of this year, two studies were released on the same subject with findings that could not have been more divergent.

On March 7, 2013, the Nuclear Regulatory Commission released a report stating that nearly all—99 out of 104—of the commercial nuclear reactors in the United States received passing grades on nuclear safety, and 81 out of the 99 passed with flying colors, meeting every "safety and security performance objective." Three of the worst performing reactors were found to have a "degraded level of performance" requiring additional NRC oversight, and just one had "a safety finding of high significance" requiring yet more oversight and corrective actions. The NRC report seemed to confirm unequivocally that America's nuclear power plants are operating safely.

Just days later, the Union of Concerned Scientists (UCS) released its own study titled "Tolerating the Intolerable," in which 14 nuclear plants were found to have experienced "near miss" scenarios in the last year, in which the risk of a meltdown increased by at least a factor of 10. The media had a field day with the second report, the Atlantic releasing online "A Map of All the U.S. Nuclear Plants That Almost Melted Down in 2012."

One can be forgiven for wondering what's going on here. Two years after the world's worst nuclear disaster in decades is the U.S. nuclear fleet taking reasonable security and safety precautions? Or are Americans narrowly avoiding Fukushimas at a rate of more than once a month?

In truth, the UCS report—an annual publication that defines "near miss" events as those requiring special inspections from the NRC, of which the NRC itself notifies the public—mischaracterizes the nature of regulatory oversight and misrepresents the status of nuclear safety in the U.S. The NRC weighs a number of factors to determine if the odds of damage to a reactor core have increased sufficiently to require them to send in an inspection team but "Sending such a team," an NRC spokesman told Power Engineering, "does not indicate there was ever any realistic threat to public health and safety.

"As a hypothetical" he said, "consider a reactor with all safety-related pumps available. It has a core damage possibility of 1 in 100,000. If an event knocks a pump out of service, the risk could rise to 7 in 100,000. That might require a Special Inspection. If the event knocks out two such pumps and the risk were to rise to 5 in 10,000 that would very likely require a Special Inspection."

Reports from mainstream media outlets, like the Atlantic, that take the UCS's conclusions yet a step further have, frankly, no relationship to reality. "Far from showing lax regulation or oversight," the spokesman said, "the special inspections show the NRC's doing its job to protect the public and the environment by finding and correcting problems early, before they can cause real harm."

A stress analysis of a concrete containment vessel using Abaqus finite element analysis. This graphic shows the deformed shape of a structure subjected to a high level of internal pressure, the contours illustrating the magnitude of displacement.
A stress analysis of a concrete containment vessel using Abaqus finite element analysis. This graphic shows the deformed shape of a structure subjected to a high level of internal pressure, the contours illustrating the magnitude of displacement.

In fact, significant steps have already been taken to apply the lessons learned from Fukushima to making the U.S. nuclear fleet safer. Days after the March 11, 2011, earthquake and tsunami in Japan, the NRC dispatched staff to assist the Japanese government and begin to draw lessons from the event. Four months later, the NRC issued a set of recommendations, which laid the groundwork for a set of full-fledged regulatory requirements issued almost a year to the day after the Fukushima meltdown, on March 12, 2012. As Nuclear Energy Institute (NEI) Chief Nuclear Officer Anthony Pietrangelo wrote in Nuclear Power International magazine, "The industry and the NRC are in broad agreement on the high-priority actions that should be taken at America's reactors. The industry's Fukushima response priority has been to identify those activities that provide maximum tangible safety benefits in the shortest time and implement them first."

Central to the industry's response to the post-Fukushima regulatory regime has been implementation of what the NEI terms the FLEX strategy. Defense in depth against power loss—and subsequent cooling systems failure, as occurred at Fukushima—lay at the foundation of the FLEX strategy. FLEX calls for placing more key backup safety equipment like generators, battery packs, battery chargers, pumps, and air compressors at each reactor and at satellite sites in order to create layers of redundancy.

With respect to backup systems there is often an emphasis on portability, said Dave Hedrick, business development manager for Caterpillar, which produces the generators sought by power producers to comply with safety requirements. Power plant operators have a need to balance generator size with the requirement that the unit be mobile in order to meet both portability specifications and the energy needs for cooling and other essential systems in the event that primary systems fail. To meet their backup power needs, plants have been purchasing generators that run at between 150 KW and 3 MW, and sometimes more, Hedrick said.

Additional aspects of the FLEX strategy include adding more equipment to monitor spent fuel pools to ensure safe temperature and water levels are maintained; training for nuclear plant workers in the use and maintenance of new equipment and communications systems; and establishment of off-site regional support centers, in Memphis and Phoenix, where yet more emergency equipment will be stationed. In addition to these regional support centers, there are 65 facilities across the United States with equipment available to plant operator if needed. In April, AREVA and the Pooled Equipment Inventory Company (PEICo) finalized a contract to operate the regional response centers, managing backup equipment and providing services in fulfillment of the FLEX strategy's regional response requirements.

According to NEI, the NRC's Tier 1 and Tier 2 recommendations have been nearly implemented across the industry. As of March 2013, the NEI says, the industry had purchased over 1,500 units of backup equipment, including 320 emergency diesel generators of various sizes; over 225 pumps that operate up to 5,000 gallons per minute; and hundreds of satellite phones to ensure that communication doesn't break down in the event of a disaster.

Whether building a new plant, uprating an old one, or planning for decommissioning and spent fuel storage, one of the most powerful tools plant operators can use to maintain and improve safety is foresight, the ability to look into potential futures and predict how various structures and pieces of equipment will perform under different circumstances. Fortunately, today we have simulation software that does just that.

A crack propagation analysis using Abaqus extended finite element method simulates crack geometry and growth in a pressure vessel.
A crack propagation analysis using Abaqus extended finite element method simulates crack geometry and growth in a pressure vessel.
A crack propagation analysis using Abaqus extended finite element method simulates crack geometry and growth in a pressure vessel.

The Abaqus software from SIMULIA, for instance, gathers numerous data points to model possible events and changes throughout the lifecycle of a nuclear power plant. Abaqus can model how a part will change with age, the effects, including heat, of the friction between two materials that touch one another, how concrete will change with time and repairs, or the effects of creep (plastic deformations in a material, like steel, under both high temperature and high stress). The software, which is being endlessly tweaked and improved, becoming ever more sophisticated, might be used, for example, to model how a one-of-a-kind event, like an earthquake, would affect a spent fuel storage container. Or it might model how a containment vessel would react if struck by a plummeting airplane. It makes as few assumptions as possible, looking even beyond the specifications of the facility itself.

"You can take into account the properties of the soil as well as that of the structure," said Deepak Datye, an engineering specialist with SIMULIA. "If you have good data on material properties after aging has taken place, these properties can be taken into account in order to predict the structural behavior."

As an FEA (Finite Element Analysis) code, Abaqus looks primarily at mechanical and structural elements. While the code can model some simple fluid movements on its own—like fluid sloshing in a tank, for instance—it is designed to interface with other commercially available CFD (Computational Fluid Dynamics) codes to model the kinds of complex fluid dynamics that occur in a major event like a tsunami.

Perhaps the most controversial aspect of post-Fukushima safety enhancements are the questions surrounding venting and filtration systems in early-model American boiling water reactors (BWR). Some American BWRs are similar to the reactors that melted down at Fukushima, where indecision and delays in getting vents opened led to pressure buildup and hydrogen explosions that greatly exacerbated the severity of the event. Ever since the Japanese disaster, there has been substantial concern that American facilities are not prepared to handle the high pressures that can build up in the early stages of an accident, and that, where vents are in place, they are unfiltered and will release radiation into the air. Furthermore, there's concern that the vents may not even work in the event of a power outage. After years of deliberation, in March 2013 the NRC issued a memorandum that, the NRC says, will make for stronger venting systems," according to a March 19, 2013, NRC blog post.


"In their latest decision, the NRC Commission votes to further strengthen these vents," the post says. "The NRC staff has 60 days to finalize an Order for these enhancements. Generally speaking, these additional requirements mean the vents could handle the pressures, temperatures and radiation levels from a damaged reactor."

In a political victory for the industry and the Nuclear Energy Institute, the commission vowed to use its "rulemaking process to consider the best approach by which these 31 reactors can keep radioactive material from the environment during a severe accident." At least for the time being there will be no one-size-fits all solution imposed on the question of BWR vent filters, which could cost plants between $16 million and $45 million to install. The decision seems to echo the findings of a study published by the Electric Power Research Institute which held that "no system is optimal" for keeping toxic radiation contained in the event of a core meltdown. Instead, says the study, "The best way to avoid radiological release and potential land contamination is to prevent an accident from occurring" in the first place.

As passive safety designs become increasingly sophisticated in Generation III and newer reactors, the need for such numerous, overlapping, and redundant layers of backup safety may decrease over time for newer units. But in all likelihood older designs will continue operating in coming decades and, as has been made painfully clear by the meltdowns at Fukushima, many older nuclear plants were built with unacceptable vulnerabilities and deeply flawed emergency backup systems. The industry has recognized this and is, the NEI says, methodically implementing realistic, sensible solutions to ensure that when things go wrong there are working backup systems in place to ensure that Fukushima is never recreated in the US.

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