Designers, operators and regulators of nuclear power plants believe they need plan only for events that are statistically probable - thus neglecting events that can endanger the public and the environment

Assumptions and calculations of risk are valid only if accident scenarios play out according to predictions. Those assumptions and calculations are sometimes flawed.

Reactor owners and safety officials argue the old "one-size-fits-all" rulebook approach is expensive and inefficient; allowing utilities latitude to estimate risks and respond accordingly saves money and eases regulatory burdens.

Risk-based safety programs lose effectiveness without aggressive regulatory oversight, safety experts say.