This is reactor number 4 of the site where the other reactors are having problems. The Japanese apparently didn’t mention this to anyone until after the second explosion at No. 2.

CBS News –

A fire at a fourth reactor in a quake-damaged nuclear plant sent radiation spewing into the atmosphere Tuesday. Earlier, a third explosion at the plant in four days damaged a critical steel containment structure around another reactor, as Japan’s nuclear radiation crisis escalates dramatically.

From the New York Times –

The problem at the fourth reactor had not been reported before late Tuesday morning. According to officials, a fire broke out at that reactor, which had been offline at the time of the earthquake but was storing spent nuclear fuel.

“No. 4 is currently burning, and we assume radiation is being released. We are trying to put out the fire and cool down the reactor,” the chief government spokesman, Yukio Edano, said at a televised press conference. “There were no fuel rods in the reactor, but spent fuel rods are inside.”

Spent fuel rods, depending on their age, can still emit large amounts of radioactive material and need to remain immersed in cool water. Even so, and despite the fact that the No. 4 reactor was emitting large amounts of radioactive material, Mr. Edano said the reactor “did not pose an imminent threat.”