Concept and Technology Development
Canadian Nuclear Laboratories
Nov 14, 2018
1 - Vibrations of Steam Generator Tubes
Bret Kugelmass: Where did your career in nuclear begin?
Metin Yetisir: Metin Yetisir is a mechanical engineer from Turkey who came to Canada 35 years ago to pursue his Master’s at McMaster University focused on vibrations of steam generator tubes. The nuclear application of his studies was looking into vibrations of steam generator tubes in nuclear reactors. Steam generators, also called boilers, transfer heat that’s generated in the reactor core to the secondary side fluid which turns into steam. The steam is used in turbines to produce electricity. During the process, the secondary side saturation pressure is lower than the primary side, causing different boiling temperatures. The secondary flow that is flowing through the steam generators are going around the tube, which the primary side flow is inside the tubes. The flow velocities for the secondary flow is high and it expands causing turbulence, exciting the tubes to vibrate. Fluid elastic instability happens when the vibration pattern introduced by flow happens at a frequency that matches the natural frequency of the tubes. Turbulent excitation causes vibration at low levels, which causes rubbing and impacting issues at the support locations. When the system becomes fluid elastically unstable, the tubes might rub at the mid-span and collide with each other. All the energy in the tubes causes failure fairly quickly.
2 - Heat Transfer in Heavy Water Reactors
Bret Kugelmass: Tell me about your studies in thermal performance and degradation.
Metin Yetisir: Steam generators have a mechanical aspect, which is vibrations, and a fluid aspect. Metin Yetisir worked in flow-induced vibrations, which required an understanding of fluid flow and mechanical aspects. Yetisir started looking into heat transfer because the steam generator performance affects the performance of the entire power plant. If heat cannot be extracted from the primary side efficiently, the power is not effectively being transferred to the secondary side. In CANDU reactors, this brings the primary side temperature and energy level up in the reactor. When the feed water flow is coming from the turbine side, impurities in the water settles on heat exchanger tubes and creates a layer of crud. This crud could be particulates or dissolved material. There are a lot of advantages to using heavy water since it is a great moderator. In the supercritical water reactor concept, a heavy water moderator is a last resort cooling system. If all power is lost and coolant in the supercritical reactor, the heavy water that is where the fuel channels are sitting provides additional cooling. In pressurized water reactors (PWR), light water is functioning as a coolant and moderator at the same time. In CANDU reactors, a heavy water coolant is located on the primary side, in the fuel channels. The fuel channels are sitting in a vessel which is also filled with heavy water, where it acts as a moderator. Having two separate systems means that, if the primary side coolant is lost, there is still a moderator sitting there as an additional source of coolant that could be used.
3 - Supercritical Water Reactor Concept
Bret Kugelmass: What are the benefits of using water as a coolant and a moderator?
Metin Yetisir: Water-cooled reactor safety systems come down to how much water is available, how quickly it can go into the reactor core, and how quickly it can remove heat. CANDU reactors could be easily modified so everything is based on passive concepts. Passive systems are safer because they don’t rely on components that can break and component performance does not need to be justified. There are advantages of using active systems using pumps. These systems can inject water faster and the speed of it can be controlled, but they do not have the reliability. A passive system has very few components. Active systems have a lot of moving parts on components and a lot of time must be spent to ensure everything is fit for service. Passive systems tend to require a larger space. The trend is moving from active systems to passive systems. Small modular reactors (SMR) aim to be safer and operate with a lot less staff, to make them more economical. Supercritical phase is a single phases that happens past critical point, a particular pressure and temperature point. In supercritical condition, there are clusters of molecules that move like gases but behave like liquids. The density change between the liquid phases and the gas phases is not as big as it is at lower pressures. In the supercritical water reactor concept, there is a fuel channel with a central flow tube that brings the supercritical water in a dense form. As it comes to the bottom, it turns up and goes up through the fuel elements and starts becoming gas-like. By the time it reaches the top of the fuel channel, it has a ten times lower density than its initial density coming down the flow tube. Efficiency is directly related to the steam temperature. There are about 600 supercritical fossil plants available in the world, which is sometimes referred to as “clean coal” due to lower emissions and higher efficiencies.
4 - Advanced Reactor Technology at CNL
Bret Kugelmass: Why don’t any giant supercritical coal companies invest in making a supercritical nuclear power plant?
Metin Yetisir: Normally, the design for a supercritical coal plant is done but the rest of the equipment is purchased from manufacturers who do the turbine and the secondary side equipment. Those technologies are typically not coupled. Metin Yetisir runs the advanced reactor technology group at Canadian Nuclear Laboratories (CNL). Yetisir got involved with supercritical research reactor work in 2009 and they came up with a concept called Canadian Supercritical Water Reactor. The small modular reactors (SMR) are being talked about a lot in Canada and use Generation IV advanced reactor technologies, such as sodium-cooled, molten salt, gas-cooled, and liquid reactor concepts. Labs are shifting gears towards addressing those technology needs. CNL used to be solely focused on CANDU reactors, but is now looking at all reactor technologies that could help the industry within their capabilities. It takes a long time to become really good at all the different technologies. Tritium production and management is an issue in CANDU reactors because of the heavy water. When heavy water is exposed to neutrons, it produces tritium. It is easier to produce tritium from heavy water reactors than light water reactors. One of the isotopes of lithium, which is part of the molten salt reactor, converts to tritium when exposed to neutron bombardment.
5 - Pan-Canadian Small Modular Reactor Roadmap
Bret Kugelmass: How are looking at different advanced nuclear systems and how we understand their inherent challenges?
Metin Yetisir: The requirements for building new nuclear reactors are based on the needs they are being built for and the timelines considered. They may be built to get rid of or minimize nuclear waste. However, all the technologies could be considered. It is unknown, until after getting a lot of experience, which technology will work better. By selecting one favorite technology and dropping the others, there may be missed opportunities to find the best one. There is an ongoing effort called the Pan-Canadian Small Modular Reactor (SMR) Initiative. This initiative brings everyone interested in SMR’s together, including four provincial governments and multiple utilities. The goal of the initiative is to come up with a roadmap towards hows to make SMR’s possible in Canada and create the right environment for it to happen. Having a pan-Canadian approach makes SMR deployment more likely. One thing they are looking at is how the supply chain effect may impact the selection process. The SMR supply chain will be much easier to satisfy than a large reactor since many of the parts are smaller. Some of the issues may come with the fuel that is available, but the industry could mobilize fairly quickly to supply mechanical components. There may be issues with moving reactors to higher temperatures because sensors and monitoring systems may need to be reconsidered, but it provides an opportunity to come up with better techniques. When Metin Yetisir first went to Chalk River, he thought that all the interesting things in nuclear happened in the 1950’s. Many interesting concepts are coming back again and with new ideas. If SMR deployment happens, it will be great for Canada, the United States, and the rest of the world.