what does this mean?
Workers at the Fukushima-1 nuclear plant are trying to remove pools of highly radioactive water that may have seeped from reactor cores, where used fuel rods are stored at four of the six reactors
especially at four of the six reactors

Question

what does this mean?
Workers at the Fukushima-1 nuclear plant are trying to remove pools of highly radioactive water that may have seeped from reactor cores, where used fuel rods are stored at four of the six reactors
especially at four of the six reactors

Answer

In a literal sense, the reporter (记者）wants to emphasize the severity of the accident by associating the number of reactors involved with the source of radioactive leakage which are the reactor cores.  In short, they want to describe how bad this accident is.

On a human level, it means that it will take a long time to clean up,  be extremely expensive and affect thousands of lives.

Answer

'Four of the six reactors' means there are six nuclear reactors (devices which maintain and control the nuclear reaction which produces nuclear energy) and out of the six, four of them have used fuel rod storage which is leaking radioactive water.