Vault 6
Vault 6 was one of the Vault series of nuclear Fallout/Blast Shelters constructed by Vault-Tec, despite it's labeling, the vault was actually the 149th to be constructed. This was because it was not until 2001 that the United Nations recieved permission from Japan's government, the Nintendo Kabushiki Gaisha, to construct Vaults in the country. The Vault started construction immediately after being given the go-ahead in 2002, and construction finished in late 2007/early 2008. The vault was constructed in Mount Bandai, a large mountain in Fukushima Prefecture. Vault 6 was one of two Vaults built in Japan, the other being Vault 57, which was finished in 1957, but it was built on the other side of the country. Vault 6 was built to contain 1,200 people, which was far more than usual; as Japan was notorious for it's heavy overpopulation at that time and it was thought that it may have doubled as a place for people to live if it ever got to that point.
When the Great War occured on December 21, 2012, the nearby Fukushima Daiichi suffered a nuclear disaster because of a bombing attack, 1018 people went into the Vault before it was closed. The nuclear meltdown ravaged the area in a way that no-one had expected; with the people outside of the vault suffering a quick, but painful death. For approximately three weeks, the vault worked perfectly; but this was not to last. On January 29th, 2013, the Vault's control system failed, which allowed for the large amount radiation to quickly seep in. Because it was invisible, no-one suspected anything. Some people (mainly scientists), claimed that they could detect radiation, but they were laughed at because there "was no evidence to back them up" (Despite, of course, the small green particles in the air and the humid stench). However, after a small breeze picked up, carrying nearly twice the amount of radiation the Vault was previously subjected to, the population quickly turned into an aggressive pack of extremely irradiated Feral Ghouls.