ABSTRACT
Boiling water reactors (BWR) are the second most popular type of reactor in the world. A BWR is fundamentally different than a pressurized water reactor because the coolant in the core is intentionally designed to boil. When BWRs were first considered as an alternative to Pressurized water reactors (PWR) in the early 1950s, it was believed that hydrodynamic instabilities would occur inside of the core if the water was allowed to boil. BWRs employ a version of the Rankine thermal cycle known as a direct cycle. The water in a BWR begins to develop vapor bubbles about 1/5th of the way up the core, and by the time it reaches the core midpoint, it begins to boil vigorously. BWRs also use cylindrical fuel rods with uranium oxide fuel pellets clad with zirconium metal. BWR fuel assemblies weigh less than PWR fuel assemblies do because they have fewer numbers of fuel rods.
