What are the key design differences between PWR and BWR fuel assemblies?

The key design differences between Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR) fuel assemblies stem from the different operating conditions and core designs of the two reactor types. In PWRs, the fuel assemblies are typically square and consist of a matrix of fuel rods held together by grids. The fuel rods contain uranium dioxide pellets, and the entire assembly is encased in a zirconium alloy cladding. The fuel assemblies are designed to withstand high pressure and temperature, as the coolant is maintained in a liquid state under high pressure to prevent boiling within the core. Control rods in PWRs are typically inserted from the top of the core, and the fuel assemblies do not have channels for coolant flow. In BWRs, the fuel assemblies are also typically square, but they are enclosed in a channel that directs the flow of coolant through the assembly. This channel ensures that the coolant boils uniformly within the assembly. The fuel rods in BWR fuel assemblies are similar to those in PWRs, but the assemblies are designed to accommodate boiling. Control rods in BWRs are typically inserted from below the core, and the fuel assemblies have channels for coolant flow. BWR fuel assemblies may also contain water rods, which are empty tubes that displace fuel and increase the moderation of neutrons in certain regions of the core. This design difference is due to the fact that the moderator density changes significantly with voiding (steam formation) in a BWR. The channel around the BWR fuel assembly also provides structural support and prevents crossflow between assemblies, which can affect core stability. In summary, the key design differences between PWR and BWR fuel assemblies relate to the presence of channels for coolant flow in BWRs, the method of control rod insertion, and the accommodation of boiling within the BWR fuel assemblies.