The reluctance of U.S. federal regulators to require operators of nuclear reactors to spend $5 billion to enhance the security of spent fuel rods that are stored underground threatens the country with a potential catastrophe, scientists warned on Friday.

The Nuclear Regulatory Commission greatly underestimated the risk and potential contamination of a nuclear waste fire triggered by a quake or a planned attack, experts wrote in the journal Science.

In 2014, the NRC found the chance of a disaster caused by leaving radioactive waste in storage pools was too remote to warrant the cost of moving it to safer dry casks.