Officials in Japan raised the severity level rating at the stricken Fukushima nuclear plant Saturday, as crews worked desperately to restore power in an effort to cool crippled reactors.

The disaster has been moved from a level 4 to a level 5 on the 7-level INES scale, but some experts believe the situation is more serious. The accident at Pennsylvania’s Three Mile Island was also rated at level 5, while Chernobyl rated 7.  Water is being poured onto the most critically damaged reactor to stave off a meltdown. If that action fails, plant operators may be forced to bury the site with concrete and sand.

Citizens have been advised to evacuate a 12-mile zone around the facility. Those who choose not to leave are being told to stay indoors. Nearly a half-million people remain in rescue shelters in areas ravaged by last week’s earthquake-tsunami. Supplies of food and water are running low and rolling blackouts continue.