A section of an underground storage tunnel in a plutonium uranium extraction (PUREX) plant collapsed at the Hanford Nuclear Reservation on Tuesday, prompting authorities to declare emergency at the Washington state site.

Officials were alerted of a cave-in of a 20 feet section of a tunnel that is hundreds of feet long. Located next to the PUREX in the centre of the Hanford site, it is used to store contaminated materials, Xinhua reported.