Virudhunagar: At least 18 people were charred to death, and six others sustained grievous injuries following a massive explosion at a private firecracker manufacturing unit in Tamil Nadu's Vachakarapatti, police said on Sunday. The explosion occurred at the 'Vanaja' fireworks factory owned by one Muthumanickam, in Kattanarpatti near here. The place is located within the Vachakarapatti police station limits.