Engineers at American nuclear plants have been much better at calculating the risk of an internal problem that would lead to an accident than they have at figuring the probability and consequences of accidents caused by events outside a plant, a report released Thursday by the National Academy of Science said.

Accidents that American reactors are designed to withstand, like a major pipe break, are “stylized” and do not reflect the bigger source of risk, which is external, according to the study. That conclusion is one of the major lessons from the Fukushima Daiichi nuclear accident in Japan in 2011, which began after an earthquake at sea caused a tsunami.

The Nuclear Regulatory Commission calculates which problems are most likely and most troublesome, and aims to find components or systems that should be improved.

B. John Garrick, a nuclear engineering consultant and vice chairman of the two-year study, said that engineers had more experience calculating the probability of failure in a valve or a pipe than in predicting earthquakes or floods. Better predictions of such events were possible, he said.