China
Africa
Explore chapters and articles related to this topic
Fuel Pins, Fuel Rods, Fuel Assemblies, and Reactor Cores
Published in Robert E. Masterson, Nuclear Engineering Fundamentals, 2017
AGRs use a similar refueling method, but the fuel assemblies are positioned vertically rather than horizontally. In recent years, this type of refueling strategy has become known as online refueling. Because the fuel assemblies in a CANDU reactor have a large number of hollow “slots” in a cylindrical or honeycomb arrangement, these fuel assemblies are sometimes called “fuel bundles” instead. A picture of a fuel bundle from a CANDU reactor is shown in Figure 11.11. Most fuel bundles have between 28 and 45 sealed pressure tubes. The pressure tubes contain the fuel, and they also serve the same function as fuel rods do in other reactor designs. In most cases, the fuel pellets inside of the tubes are still cylindrical in shape (see Section 11.4). Except for the use of heavy water as a moderator, CANDU reactors are similar to PWRs in their design. The fission process within the core heats pressurized heavy water in a primary cooling loop that is similar to the primary loop that is used in a conventional PWR.
Nuclear Power in the World Today
Published in Robert E. Masterson, Introduction to Nuclear Reactor Physics, 2017
Because of these trends, the average enrichment of a fuel assembly is much higher today than it was 20 years ago. This trend is likely to continue for the foreseeable future because it is the easiest way to increase the power output from the core, or lengthen the time between refueling cycles for a given power rating. Some reactors, such as the CANDU reactor and the AGR, have been able to avoid this problem entirely because of the unique way in which their fuel assemblies are designed. In these reactors, the fuel within an assembly can be accessed directly without removing the fuel assembly from the core. Thus, the old fuel can be replaced with new fuel while the reactors are still online. This form of refueling is known as continuous refueling or online refueling. The refueling is done using an automated refueling machine such as the one shown in Figure 1.30. The CANDU reactors are currently the most common alternative to conventional PWRs and BWRs when this type of refueling method is required. Traditional reactors such as PWRs and BWRs have to be shut down to remove the depleted fuel. There can also be a significant lead time between when a reactor needs to be refueled and when the fresh fuel needs to be ordered. The actual lead time varies depending upon the fuel supplier that is used, but in general, orders for new fuel assemblies are placed by utilities between 12 and 18 months in advance of the time when the reactor is scheduled to be shut down for refueling. This assumes that one-quarter to one-third of the fuel assemblies are to be replaced at a time. In general, this is needed to get the most energy out of the fuel as possible. Mixing fresh fuel with the burned fuel allows the burned fuel to be burned more completely than if a reactor is run based on just the burned fuel alone. Except for the time interval of about 1 month between the time when a reactor is shut down for refueling, and the time when it can be restarted, it is expected to be up and running and generating useful power all the time. The economics of the nuclear power industry are critically dependent on this basic assumption.
Steady-State Subchannel Thermal-Hydraulic Assessment of Proposed Thorium Fuel Designs
Published in Nuclear Technology, 2018
Canada developed a heavy water nuclear reactor technology for CANDUaCANDU is a registered trademark of Atomic Energy of Canada Limited in Canada. reactors; these types of reactors are more suitable to use a thorium fuel cycle than the light water reactors because of their neutron economy and the online refueling capabilities. Online refueling allows the reactor operator to extract or reload the fuel without shutting down the reactor, thus maximizing the fuel economy. Recently, the Canadian Nuclear Laboratories (CNL) has performed experimental studies to support the viability of fabricating thorium-based fuels. To gain practical experience with thorium, CNL is studying the feasibility of adding thorium-based or thorium-augmented fuels in current Canadian pressure tube–heavy water reactors (PT-HWRs, also known as CANDU). To support this initiative, experimental and analytical studies are being carried out at CNL (Refs. 5, 6, and 7).