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Nuclear Power Plant Safety Upgrades for Extended Continued Operation

Watershed events such as the Fukushima-Daiichi accident threaten to shake public confidence and it is up to the regulators and industry worldwide to provide transparency, education and appropriate improvements to maintain and improve safety margin against safety goals.

By Sean Donnelly

 

Watershed events such as the Fukushima-Daiichi accident threaten to shake public confidence and it is up to the regulators and industry worldwide to provide transparency, education and appropriate improvements to maintain and improve safety margin against safety goals.

Most operating nuclear power plants were designed for an operating life of 30 or 40 years after which the units would be permanently decommissioned. The global nuclear energy industry has seen marginal growth in recent years with approximately 67 new plants under construction as of July 2015, in comparison to 438 operating nuclear reactors, according to numbers from the Nuclear Energy Institute. With many operating plants reaching their end of life in the coming decade, major refurbishments and life extension projects will play a critical role in the stability of nuclear power generation.

Continued operation beyond original design operating life presents a number of technical and regulatory challenges. Plant refurbishments require replacement and/or rehabilitation of major components, including steam generators, turbine generators, instrumentation and control or process computers and reactor components. Obsolescence is a continuous problem and replacement of components often requires significant design changes to accommodate current production models. Qualified components with a finite service life must be replaced, sometimes with great difficulty and cost. However, these types of replacements are required for the maintenance of nuclear safety and efficient operation with minimal downtime due to forced outages. Despite these significant investments in plant life extension, maintenance of nuclear safety margin from the initial plant design and licensing may not be sufficient for extended operation.

Regulator Interface

The regulatory regimes worldwide are adapting to the concept of life extension and longer operating lives. In the United States, plants are initially licenced to operate for 40 years. Current rules allow for life extensions of an additional 20 years and guidance is under development for operating licences of up to 80 years. In Canada, licence renewals for periods up to 10 years have been granted. However, approval of life extensions require that the power plants are deemed to be safe to operate during this period in compliance with modern standards and safety goals.

Plant Design, the Design Basis and Beyond

Modern new build plants have the benefit of the lessons learned from the existing fleet. Next generation and advanced reactor designs include safety designs that provide emergency core cooling and containment features to deal with beyond design basis accidents (BDBAs) and severe accidents, particularly those involving a total loss of on-site electrical power. Existing plants must rely on inherent design safety features and may require modifications to improve robustness in order to meet modern standards and safety goals.

The initial licensing of nuclear power plants worldwide considered design basis accidents which ultimately define the range of analyzed accident conditions; these are considered to be the more probable accident sequences. These accidents consisted of initiating events, coupled with a failure of a safety system that resulted in an accident with its associated consequences (i.e., possible radioactive release). While BDBAs, including severe accidents, were considered in some jurisdictions prior to the Fukushima-Daiichi accident in Japan, following the accident, utilities and regulators worldwide endeavoured to provide increased capability to deal with such events. Utilities and regulators conducted assessments and analysis to analyze the capability of plants to cope with these severe events. Examples include the use of Review Level Conditions in Canada and the European Union Stress Tests. Nuclear safety practitioners have been challenged to identify those sequences that are outside the plant design basis but represent those events that could occur due to initiating events or failures not postulated through conventional deterministic safety analysis. While some of these sequences can be defined, such as more challenging seismic events or floods, others cannot be easily defined. Most utilities have adopted a combination of plant modifications and dedicated portable equipment that provide additional mitigation options for defined beyond design basis accidents, and even for more extreme events whose progression cannot be predicted in advance.

Portable equipment, often called FLEX equipment or Emergency Mitigating Equipment (EME) can be highly effective to mitigate BDBAs, particularly extreme natural disaster events. Portable pumps, generators and uninterruptible power supplies can be stored on-site, but remote from other emergency water and power supplies, or may also be stored in a centralized location. The physical separation of and independence of EME/FLEX from the plant and its structures, systems, and components provides added confidence that an event which disables all mitigating capability (i.e., similar to the Fukushima event) would not occur. Centralized FLEX storage sites have been established in the U.S. to provide the ability to deploy resources to a number of plants within a geographic area. This minimizes costs for individual plants and maximizes the availability of equipment for use should the need arise. Similarly in Canada, support agreements have been reached between utilities and within utility fleets to share their equipment if the need arose. FLEX or EME provides a highly beneficial set of options and helps to bridge the gap between installed accident mitigation capability for DBAs at existing plants and the modern approach to accident management.

Procedural changes have also been made to better address BDBAs. Operating procedures have been extended to provide instructions relevant to these types of events. Symptom-based procedures provide valuable diagnostic capability and flexible accident mitigation actions to cater to such events.

Other modifications have been incorporated worldwide to cater to BDBAs. These events present unique challenges for multi-unit containment designs such as those at the Ontario Power Generation and Bruce Power CANDU stations in Ontario, Canada. At these plants, new systems have been developed and installed for hydrogen control (for example, Passive Autocatalytic Recombiners). Dedicated Containment Filtered Venting Systems for BDBAs have also been installed at certain plants and new techniques for depressurizing containment have been developed. Events that may occur from a shutdown or low power state, or events affecting multiple units or multiple stations are now considered in accident management guidance and emergency procedures.

The Fukushima event also taught the nuclear industry that attention must be given to irradiated fuel bays during extended periods without power for cooling and water makeup. The effects of hydrogen accumulation and bay boil off are severe and although events tend to develop slower than reactor events, mitigation actions can also be very challenging.

While the concepts of BDBA and Severe Accident mitigation represent a new evolution in accident management, the overall principles remain consistent with longstanding nuclear safety principles. Controlling reactively, cooling the fuel, and containing the radioactivity remain fundamental. Enhancements that provide robust capability for fuel cooling without reliance on in-plant supplies (e.g., EME/FLEX) and containing the radioactivity (e.g., procedural enhancements, containment venting systems, hydrogen control measures) help to maintain these safety principles for BDBAs and serve to mitigate the consequences of such events.

Safety Improvement Opportunities and Prioritization

Compliance with modern codes and standards, as well as meeting more stringent safety goals is likely to require significant capital investment by operators of most plants. Environmental requirements also play a significant role for continued operation. Generally, environmental and safety considerations have a strong correlation; an improvement in one often improves aspects of the other. Safety improvements and upgrades form a large portion of capital investment for plant life extensions and should be carefully strategized in order to maximize return on investment. Utilities typically employ cost-benefit analysis (CBA) tools to assist with this strategy and the prioritization of improvements. The CBA process provides for a structured and systematic approach for evaluating alternatives and ensures that the costs of implementing proposed alternatives are commensurate with the benefits gained.

There are many established methodologies for quantifying and ranking safety improvements; most utilities will employ plant Probabilistic Safety Analysis (PSA) models to provide key insights, typically at least a Level 1 and Level 2 PSA will have been conducted. A Level 1 PSA is intended to estimate the frequency of core damage and models all safety functions and postulated failures; whereas a Level 2 PSA is intended to estimate the radioactive release magnitude and timing by analyzing accident progression phenomenology. Some plants may also develop a Level 3 PSA for which the effect of the postulated release and plant damage is assessed. These consequences are monetized and consider injuries and economic losses as well mitigating and aggravating factors such as atmospheric dispersion. By interrogating the plant Level 1 PSA to identify risk-dominant sequences, high-value opportunities can be determined. Utilizing the Level 3 PSA where available, these “offsets” of plant risk can be directly correlated to economic benefit. Utilizing the results of these interrogations and potential risk benefit, conceptual options can be quantitatively evaluated and ranked, providing a valuable tool to utilities when prioritizing capital investments in their plants.

The benefits realized from a safety improvement opportunity (SIO) are typically estimated by calculating the monetary value of the consequences averted by reducing the frequencies of the Fuel Damage Categories (FDCs) and Release Categories (RCs) impacted by the SIO. FDCs and RCs are categories of consequences dictated by the Level 1 and Level 2 PSAs, respectively; certain jurisdictions may use other terminology for these quantities but the intent is the same. The benefits are compared against the costs associated with implementing the change to determine the acceptability of the SIO based on acceptance criteria such as a benefit-cost ratio greater than one. It is possible to help increase the benefit-cost ratio by considering non-safety aspects such as improved economic risk. Any modification which may reduce the forced loss rate, increase generator output, decreased maintenance or operator burden, or minimize outage duration can be highly beneficial and provides added justification for gaining even a modest safety improvement. The CBA is usually expressed in terms of total estimated benefits, total estimated costs, net present value of the SIO and the overall benefit-cost ratio. A good CBA considers the entire life cycle of the SIO, including any associated ongoing deltas in maintenance or decommissioning. Favourable options are then progressed for further development, funding approval, design and implementation as appropriate.

Conclusions

The nuclear power industry is faced with an appropriately high degree of public and regulator scrutiny. Continued safety improvements are key to maintaining public trust and reduce the likelihood and severity of possible accidents. Watershed events such as the Fukushima-Daiichi accident threaten to shake public confidence and it is up to the regulators and industry worldwide to provide transparency, education and appropriate improvements to maintain and improve safety margin against safety goals. Life extensions provide a unique opportunity to examine risk significance and develop appropriate safety improvements to better attain and surpass safety goals. Effective assessment of options and planning of safety improvements is critical to the success of any nuclear plant life extension and long term economic viability.

Author

Sean Donnelly is manager of Station Operations and Licensing for Amec Foster Wheeler’s Nuclear Canada.

 

 

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