China is on precautionary alert against possible radiation leaks from the deadliest earthquake to hit the country in three decades, according to a government website. The disaster area is home to China's chief nuclear weapons research lab in Mianyang, as well as several secretive atomic sites, but no nuclear power stations. Minister of environmental protection Zhou Shengxian convened an emergency meeting late on Monday, hours after the 7.9 magnitude tremor rocked the southwestern province of Sichuan, and activated the lowest tier of a four-stage system of ranking radiation leaks, the ministry said on its website President Hu Jintao flew to Mianyang today, four days after the quake, which is thought to have killed more than 50,000 persons, state television and the official Xinhua news agency reported, in an indication the risk was low. Xinhua didn't say if Hu had inquired about nuclear facilities there. But nuclear scientists were evacuated from the area as a precaution, a source with knowledge of the evacuation said. "Everyone was evacuated. No one was left," the source, who requested anonymity, told Reuters. A Western expert with knowledge of the Mianyang lab had said it was unlikely it was at serious risk. A Chinese Foreign Ministry spokesman said this week he had not heard of damage to nuclear facilities in the disaster area when asked at a regular news briefing. Meanwhile, China struggled to bury its dead and help tens of thousands of injured and homeless today when a powerful aftershock brought new havoc four days after an earthquake thought to have killed more than 50,000. Premier Wen Jiabao said the quake damage could exceed the devastating 1976 tremor in the northeastern city of Tangshan that killed up to 300,000 people. Wen called on officials to ensure social stability as frustration and exhaustion grew among survivors, many of whom lost everything and were living in tents or in the open air. China put the death toll at just over 22,000 on Friday but has said it expects it to exceed 50,000. About 4.8 million people have lost their homes. Thousands of men, women and children were heading by foot for Mianyang, a city near the epicentre, saying they were abandoning their ruined villages for good. Anger has focused on the state of school buildings, many of which crumpled in Monday's quake, burying thousands of children and prompting the Housing Ministry to order an investigation.