Appealing for a shift in emphasis on nuclear safety, the new head of the Nuclear Regulatory Commission warned a gathering of more than 3,000 industry executives, experts and government regulators on Tuesday against relying too heavily on their ability to predict the future, and suggested that when it comes to commercial reactors, the industry and the government should be ready to deal with the unknown.

Speaking two years after the Fukushima Daiichi nuclear power plant disaster in Japan, Allison M. Macfarlane, a geologist who became chairwoman of the five-member commission last July, cited aging nuclear reactors, terrorist attacks and natural disasters, saying, “we don’t know everything about how the Earth behaves, and we must factor this into how we approach nuclear safety.'’

In the past few years the commission has employed a safety technique called probabilistic risk assessment, in which engineers assign probability to equipment failure to determine which accidents are the most likely, and thus which systems and components most deserve improvements.

In her talk on Tuesday morning at the agency’s annual Regulatory Information Conference in Bethesda, Md., Dr. Macfarlane used an oft-repeated quote, attributed to the physicist Niels Bohr: “Prediction is very difficult, especially about the future.” Her field, geology, has traditionally concerned itself with understanding history over long periods, although the nuclear industry, with its focus on earthquake risk and its search for areas to bury nuclear waste, has used that science to make predictions.