US nuclear plants must better prepare for the risk of natural disasters like earthquakes, tsunamis and floods, said a report Thursday on lessons learned from Japan's Fukushima crisis in 2011.

Current approaches to regulating nuclear safety "are clearly inadequate for preventing core-melt accidents and mitigating their consequences," the report said.

As of now, US safety regulations are based on making sure nuclear plants can withstand equipment failures, loss of power and other malfunctions related to the design of the plant, otherwise known as design-basis events.

But history has shown that the biggest nuclear accidents -- including at Fukushima Daiichi, Three Mile Island and Chernobyl -- "were all initiated by beyond-design-basis events," said the report.

Things like natural disasters, human errors and power outages "have the potential to affect large geographical regions and multiple nuclear plants," said John Garrick, a nuclear engineer and co-author of the report.

"These include earthquakes, tsunamis and other geographically extensive floods and such things as geomagnetic disturbances," he told reporters.

Titled "Lessons Learned from the Fukushima Nuclear Accident for Improving the Safety of US Plants," the report was commissioned by Congress from the National Academy of Sciences, a non-governmental group of experts that provides scientific and policy advice.

- Call to update plans -

The March 11, 2011 earthquake and tsunami knocked out power to the Fukushima Daiichi plant, causing severe core damage in three reactors, releases of radioactive material, widespread evacuations and the eventual shutdown of all nuclear power plants in Japan.

The report did not find fault with Japan's actions before or after the incident.

Rather, it called for nuclear plants and US nuclear regulators to actively seek out the latest scientific data on risks and revise their plans accordingly.

The United States operates 100 nuclear power reactors, whose safety procedures are overseen by the Nuclear Regulatory Commission.

Nuclear plants should be ready to respond to a wide-spanning natural disaster that could damage infrastructure and disperse radioactive material beyond their 10-mile (16-kilometer) emergency planning zone, the report said.

It cited a number of off-site events that could interfere with electrical power to nuclear operations, from terrorism to human error to geomagnetic disturbances caused by solar storms that interrupt the electrical grid.

"There is some new evidence now that some of these events are not as rare as perhaps we thought," said Garrick.

- Better understanding of risks -

The report did not include an in-depth examination of US preparedness for a nuclear accident, nor did it set a new safety threshold for whether US nuclear plants should be allowed to operate.

However, it said nuclear plants should examine emergency plans for backup sources of power as well as safety systems for monitoring reactors and spent-fuel pools.

It also recommended improved training for nuclear plant operators who may need to cope with unexpected disasters, and urged the US government to "incorporate modern risk concepts into its nuclear safety regulations."

Risks of natural disaster are not necessarily greater than ever before, but experts now have a better understanding of their potential impacts, said the study authors.

Particular care should be given to treatment and evacuation of children, the elderly and the ill in case of a nuclear accident, the report added.

A number of changes have already been called for in the wake of the Fukushima Daiichi disaster, said study director Kevin Crowley.

"Since the accident, in the United States and in many other countries, there have been a great many efforts to understand the lessons and to implement changes," Crowley told reporters.

"It is really too early to know just how they are going to turn out," he said.