The public has become suspicious of nuclear energy partly because of the elaborate methods used to address safety concerns. Most worrisome is the need for external electrical power to supply the pumps used in emergency core cooling systems in the types of reactors now used in the United States. In an accident, this external supply could possibly be disrupted leading to a release of radioactivity. New reactor designs use passive safety to address this problem. Passive safety features can be thought of as characteristics of a reactor that, without operator intervention, will tend to shut or cool a reactor down, keep it in a safe configuration, and prevent release of radioactivity.
Many current problems arise from the huge scale of reactor construction projects.
It runs at lower temperatures and with larger water inventories than current light water reactors, and has a passive emergency core-cooling system that utilizes gravity. It uses passive natural circulation for the removal of heat from the core. Nuclear fission reactors, usually pressurized water reactors with energy conversion based on a steam-turbine cycle, have been used extensively to power ships. Shipboard reactors are constructed smaller than similar installations ashore, with special attention given to maintenance, protection from collision, and leakage. Although the primary use of nuclear power has been in submarines and other military vessels, a few non-military marine installations have been demonstrated.