Japanese Prime Minister Naoto Kan said at a press conference Friday evening that the situation at the Fukushima nuclear plant, where leaks of radioactive materials and other serious problems are still emerging "still does not warrant optimism."

Here's a short summary of the most serious problems: 

Highly radioactive water in the basements of reactor 1, 2 and 3 suggests damage to the reactor pressure vessel

Yesterday three workers were exposed to water containing radioactive materials 10,000 times the normal level in the basement of the turbine building connected to the No. 3 reactor building. They were exposed to high levels of beta radiation after stepping in a puddle of water, which resulted in their being transported to the hospital with beta-burns. Two of them are likely to have suffered internal contamination. Today, highly radioactive water was also found in the basement of the No. 1 and No. 2 reactors' turbine buildings.

The presence of highly radioactive water in the basement suggests that the reactor vessel has been cracked or damaged. The water contains long-lived radioactive isotopes like Cobalt and Caesium that originate from damaged fuel rods in the reactor core or less likely from the spent fuel pool. The presence of large amounts of Iodine-131 with a half-life of only eight days suggests that the contamination originates from the reactor core and not from the spent fuel pool.

The government's Nuclear and Industrial Safety Agency confirmed today that the water appears to have originated from the reactor core. But later they stated that it remains uncertain about how the leakage happened.

Salt encrustments from evaporated seawater could clog cooling pipes and diminish the cooling effect

Huge amount of seawater have been pumped in the reactors and spent fuel pools leaving crystallized salt after evaporation. TEPCO is starting to be concerned that a salt crust on the fuel rods will hamper smooth water circulation in the reactor pressure vessel, thus diminishing the cooling effect. TEPCO announced that they started injecting freshwater already into the No. 1 and No. 3 reactor cores, and the US Military announced plans to bring freshwater by barge next week.

Evacuees might not return to their homes soon

The government told the people living within the 20-30km zone to prepare for evacuation if radiation levels rise and asked them to evacuate voluntarily, because there is “no immediate likelihood that the plant will stop emitting radioactivity." These people have been ordered to stay indoors for almost two weeks, which is of course impossible and they were running out of supplies. A government representative said it "will take months or years for evacuees to return."