January-February 2019 NPJ

20 NuclearPlantJournal.com Nuclear Plant Journal, January-February 2019 Research & Development Steam Generator Tubes EPRI laboratory research has yielded insights on a vibration mechanism that can cause costly damage in nuclear plant steam generators. The results may lead to new guidance for steam generator design to help avoid these vibrations. Tubes in nuclear plant steam generators prevent radioactive liquid in the primary coolant loop from mixing with nonradioactive liquid in the secondary coolant loop. Tube leakage can result in loss of primary coolant, expensive plant outages, and shutdowns. Tube bundles are subjected to cross- flow of a steam-water mixture, making them susceptible to vibration. In-plane fluid elastic instability, a type of severe vibration demonstrated in laboratories, was observed for the first time in an operating steam generator in 2012 at San Onofre Nuclear Generating Station in California. Unit 3 experienced a small coolant leak, and subsequent tube inspections identified damage caused by in-plane fluid elastic instability. The damagewas a factor in SouthernCalifornia Edison’s decision to permanently shut down San Onofre in 2013. To identify configurations that can cause in-plane fluid elastic instability, EPRI and Canadian Nuclear Laboratories subjected tube bundles to air flow. While air behaves somewhat differently from the steam-water mixture in a steam generator, it enables easier control and adjustment of variables in a laboratory. “In these tests, our objective was to determine which parameters might influence the onset of in-plane fluid elastic instability,” said EPRI Program Manager Helen Cothron. “Air flow tests enable us to vary important parameters, and they are relatively inexpensive compared to steam-water tests.” Researchers examined the interaction between the tubes’ U-bend area and the flat bar supports that stabilize the tubes. They focused on variables such as the number of tubes subjected to air flow, the number of supports, and the gap between the tubes and supports. Key findings:  In-plane fluid elastic instability occurred in a cluster of tubes when the supports failed to stop tube motion in the air flow’s direction.  As the flow velocity increased, the onset of in-plane fluid elastic instability was observed.  Instability rarely occurred where the gaps between tubes and supports were small.  As the flow velocity was reduced incrementally, tubes consistently stabilized at the same velocity.  Out-of-plane fluid elastic instability often preceded in-plane fluid elastic instability. “These results helped the project team develop a test rig for a second round of laboratory tests using a refrigerant instead of air,” said Cothron. “A liquid- vapor refrigerant mixture can simulate steam-water flow in a steam generator more precisely than air.” Researchers conducted the refrigerant flow tests earlier this year and are evaluating the results now. “Steam generator manufacturers were involved in the refrigerant tests,” said Cothron. “They provided input on the development of the test rig, and we will share the results with them.” The results may be used to develop new guidance for the design of new steam generators that can help prevent in-plane fluid elastic instability during operations. Contact: Helen Cothron or Sean Kil, EPRI, email: techexpert@eprijournal.com . Waste Management & Disposal The bulk of the radioactive waste generated by nuclear power plants is classified as low-level waste . In fact, the World Nuclear Association reports that just about 3% of power plant radioactive waste is considered high-level waste (primarily used nuclear fuel). The U.S. Nuclear Regulatory Commission (NRC) divides low-level waste into three classes—A, B, and C— based on the concentration and nature of the radionuclides. ClassA,withthelowestconcentration, comprises materials such as contaminated personal protective equipment, used ion exchange resins from non–reactor coolant systems, and contaminated soil. Given the low radioactivity, relatively low-cost disposal options are available. Class B and C wastes typically comprise filters and ion exchange resins used to capture and remove radionuclides from the reactor coolant system. With disposal requirements more stringent than those of Class A, disposal costs are higher. EPRI estimates that it can be up to 10 times more expensive to dispose of Class B or C waste than Class A waste. It is important to classify waste properly and minimize waste generation, particularly Class B and C waste. Since 2005, EPRI’s Radiation Safety Program has developed guidance and technical solutions to assist utilities with environmentally sound, cost-effective approaches for low-level waste management and disposal. EPRI research has focused on techniques to minimize generation of Class B and C waste and to process wastes to maximize Class A waste volume. As low-level waste disposal facilities faced closures in the mid-2000s, EPRI began investigating ways to help plants produce less Class B and C waste. “Most U.S. plant operators were losing access to disposal facilities that accept Class B and C waste,” said EPRI Radiation Safety Program Manager Phung Tran. “Our research helped them implement practices to minimize generation of this waste, reducing waste storage and disposal costs.” One strategy involved filter replacement. Traditionally, filters have been replaced on a prescribed schedule, serving for 18 to 24 months and often accumulating enough radionuclides for classification as Class B or C waste. EPRI documented approaches for monitoring filters to determine when radionuclides approach levels requiring Class B or C disposal—then replacing the filters before radionuclides reach that level. Since the 1980s, the methodology for classifying low-level nuclear waste has been dictated by the NRC’s Branch Technical Position on Concentrating Averaging and Encapsulation , or BTP. The BTP’s most recent revision in 2015 was informed by EPRI research. “We took into account the latest information on health effects, waste volumes, and disposal site designs to better understand how waste may impact the public many years from now,” said Tran. “This informed a new approach to the concentration averaging methods used to classify waste.”