We simply don’t know. There are enough radionuclides in the outflow to the sea and in the water in the plant that it looks like a leak is possible, but there are too many other things that we don’t know. If there is a leak, it is not a big one.
It’s not a big one, because reactor #3 has been pressurized. If you try to blow up a balloon with a big leak, nothing happens. You can blow up a balloon with a pinhole leak, though. The steel reactor containment vessel is equipped with pressure gauges to measure the pressure. With a big enough leak, the pressure wouldn’t rise, but it has been rising as water is pumped in and turns to steam.
So where is the radioactive water coming from?
The information available doesn’t allow strong conclusions. An enormous amount of seawater has been pumped into the reactors and the spent fuel pools. Some of it is evaporating, but the reactor containment is a closed system. The reactors have been vented when their pressures went too high; fission products would have been released with the steam. If the fuel is damaged, and it almost certainly is in reactors #1, #2, and #3, more fission products will be in the coolant and steam than there are normally. They may condense out in the buildings. Or it is possible that water has been released during the venting.
Standing water in the Unit #3 turbine building has high levels of radiation. Two workers have suffered radiation burns on their feet from stepping in it without protective gear. These burns are being treated as heat burns would be. Although the doses (180 mSv) are in a range that is close to producing symptoms of radiation sickness for whole-body exposures (see chart), exposures to extremities are less likely to do so.
There are different kinds of fission products in the cores and in the spent fuel pools, because the short half-life fission products, like iodine-131, have disappeared from the fuel in the pools. Sampling in Seattle seems to indicate that the releases are coming from the cores.
Electricity has been partially restored to the control rooms. Here’s a photo from the control room for Unit 2. Fresh water is being injected into Units 1, 2, and 3, which removes some of the concern about salt buildup from the use of seawater. Fresh water may also be injected into Unit 4.
There has been some discussion about whether Tepco is holding back information about radiation levels. I’m inclined to think that if information is not being released in an entirely timely manner, it is because the operators’ and managers’ first priority is dealing with the emergency. But Tepco has been less than forthcoming on past accidents, so suspicion remains.
I’m making this assessment from IAEA reports and other sources.