The water within Reactor Number 3 (where three workers were exposed to high levels of radiation yesterday) is 10,000 times more radioactive than the average water inside a nuclear reactor and contains radioactive iodine that is generated during fission and has a half-life of 8 days. Japanese engineers are pretty sure that this means that fuel rods within the reactor have contributed to fission reactions.
Here’s an important bit: The physical container that holds the fuel rods inside the reactor is very robust and is probably not leaking. However, as you might guess, the reactor container has a number of holes in it for pipes (etc.) that communicate between the inside and outside of the container. The reactor vessel need not be ruptured or broken for stuff to get out. The engineers in Japan think that highly contaminated water probably exited from the reactor containment via these pipes, though they are not sure.
There will be those who will claim that this does not constitute a leak. But it is.
Reactor containers 1 and 2 are also leaking, according to the engineers.
Source: NHK live report. You can also check the IAEA site for updates.
Regarding the workers (from the IAEA site):
three workers at the Fukushima Daiichi nuclear power plant were exposed on 24 March to elevated levels of radiation. The IAEA has received additional information on the incident from the Japanese authorities.
The three were contracted workers laying cables in the turbine building of the Unit 3 reactor. Two of them were found to have radioactivity on their feet and legs.
These were washed in the attempt to remove radioactivity, but since there was a possibility of Beta-ray burning of the skin, the two were taken to the Fukushima University Hospital for examination and then transferred to Japan’s National Institute of Radiological Sciences for further examination. They are expected to be monitored for around four days.
It is thought that the workers ignored their dosimeters’ alarm believing it to be to be false and continued working with their feet in contaminated water.
The Nuclear and Industrial Safety Agency (NISA) of Japan instructed TEPCO to review the radiation control system immediately in order to avoid similar incidents in the future.