Complex sentence: Criticality is maintained by adding beryllium plates in a tray on top of the core.The core sits in a pool of regular light-water, 2.5 m diameter by 6 m deep, which provides cooling via natural convection.
Split into simpler sentences: Criticality is maintained by adding beryllium plates in a tray on top of the core. The reactor core sits in a pool of regular light-water, 2.5 m diameter by 6 m deep, which provides cooling via natural convection.