Current boiling water reactors (BWRs) use active safety systems comprised of pumps, valves, motors, and diesel generators. These active safety systems (a) are a major cause of controversy in licensing power reactors because of reliability questions, (b) have high capital costs, and (c) require constant maintenance. An advanced BWR concept with passive emergency core cooling systems (PECOS) offers a basic alternative approach to reactor safety. In the PECOS-BWR, passive emergency core cooling is provided for the first 24 h by gravity flow of stored water released through fluidic valves that have no moving parts. Natural-draft air cooling removes heat from the core for longer periods.