An independent safety oversight board is warning that a major earthquake could cause a catastrophic fire at the main plutonium laboratory at Los Alamos National Laboratory, which could trigger a massive radiation release.

The four-page letter issued Tuesday by the Defense Nuclear Facilities Safety Board urged Energy Secretary Stephen Chu to make changes to reduce potential hazards.

The board is an auditing agency that oversees federal nuclear programs. It typically examines worst-case scenarios involving national nuclear facilities, then makes recommendations to the Department of Energy on how to minimize risks.

Lab officials released a response, citing several actions taken this year to improve fire safety at the plutonium laboratory building known as Technical Area-55.