The International Space Station (ISS) crew had to hide in a special shelter during a massive solar flare, a nuclear scientist said. The sun produced several huge solar flares last week, one of which was the strongest observed in a decade.

“Yesterday [on Sunday], the cosmonauts on the ISS received an ‘alert’ signal, and they had to seek a temporary shelter at the station,” Mikhail Panasyuk, the head of Skobeltsyn Institute Of Nuclear Physics in Moscow, said at a press conference.

On Sunday, a solar flare was reported by scientists across the globe. Called X8.2, it “produced a rapid increase in relativistic proton levels,” according to NASA.

The increase in proton activity coincided with a time at which the ISS was nearer the sun, according to Panasyuk. The proton stream was higher than that of the powerful solar flare that took place on September 7, he said.

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“A powerful proton stream can break through ISS shell,” he added.

Solar flares are huge bursts of radiation released by the sun. The Earth’s atmosphere protects us from the worst effects of the resulting radiation storms, but if the flare is big enough, it can disrupt GPS satellites, certain radio frequencies and other global communications temporarily.

Earlier in September, the sun shot out at least six solar flares, according to NASA data. Global communications and some GPS systems were temporarily affected on the side of the Earth facing the sun at that time.

One of the flares was an X9.3, over four times as powerful as the first and largest solar flare in the current 11-year solar cycle that began in December 2008, according to NASA.

X-class solar flares are the largest explosions that take place within our solar system, shooting out jets of plasma that can reach up to 10 times the size of the Earth in length.