BEIJING - An 18-year-old nun at a Tibetan Buddhist convent set herself on fire in southwest China to protest Chinese rule over Tibet, exiles and monitoring sources said on Sunday.Tenzin Choedron, a nun at the Mamae convent in Sichuan province, survived the initial immolation on Saturday evening and was taken away by Chinese police and troops, said Kanyag Tsering, a monk at Kirti Monastery in the capital of the Tibetan government-in-exile in Dharamsala, India."Soldiers surrounded the nunnery and sealed it off, and nothing more is known of the situation inside," Kanyag said from Dharamsala, the seat of the Tibetan government-in-exile.Stephanie Brigden, director of the group Free Tibet, also reported the case.