KORIYAMA, Japan —
On Sunday, a few dozen people waited to be checked in a collection of blue tents set up in a parking lot outside a local gymnasium. Fire engines surrounded the scene, with their lights flashing.
Many of the gym's windows were shattered by the quake, and glass shards littered the ground.
A steady flow of people — the elderly, schoolchildren and families with babies — arrived at the center, where they were checked by officials wearing helmets, surgical masks and goggles.
Officials placed five reactors, including Units 1 and 3 at Dai-ichi, under states of emergency Friday after operators lost the ability to cool the reactors using usual procedures.
An additional reactor was added to the list early Sunday, for a total of six — three at the Dai-ichi complex and three at another nearby complex. Local evacuations have been ordered at each location. Japan has a total of 55 reactors spread across 17 complexes nationwide.
Officials began venting radioactive steam at Fukushima Dai-ichi's Unit 1 to relieve pressure inside the reactor vessel, which houses the overheated uranium fuel.
Concerns escalated dramatically Saturday when that unit's containment building exploded.
Officials were aware that the steam contained hydrogen and were risking an explosion by venting it, acknowledged Shinji Kinjo, spokesman for the government's Nuclear and Industrial Safety Agency, but chose to do so because they needed to keep circulating cool water on the fuel rods to prevent a meltdown.
Officials insisted there was no significant radioactive leak after the explosion.
If a full-scale meltdown were to occur, experts interviewed by The Associated Press said melted fuel would eat through the bottom of the reactor vessel, then through the floor of the containment building. At that point, the uranium and dangerous byproducts would start escaping into the environment.
Eventually, the walls of the reactor vessel — six inches (15 centimeters) of stainless steel — would melt into a lava-like pile, slump into any remaining water on the floor, and potentially cause an explosion that would enhance the spread of radioactive contaminants.