Complex sentence: Inside the core of a typical pressurized water reactor or boiling water reactor are pencil-thin nuclear fuel rods, each about long, which are grouped by the hundreds in bundles called "fuel assemblies".
Split into simpler sentences: Inside the core of a typical pressurized water reactor or boiling water reactor. These are pencil-thin nuclear fuel rods, each about long, which are grouped by the hundreds in bundles called "fuel assemblies".