Q&A on latest Japan nuke threat: Radioactive water

The effort to steer Japan's Fukushima Dai-ichi nuclear power plant away from disaster suffered another setback when workers discovered widespread uncontrolled leaking of radioactive water at the six-reactor site.

Q. What's new about this?

A. Since the tsunami knocked out power, plant workers have been intentionally venting radioactive steam into the air to keep overheated reactor cores from bursting. Radioactive steam has also gone aloft from overheated storage pools for used fuel. Now radioactive water has also been discovered at the bottom of turbine buildings at units 1 and 3 adjacent to the reactors. Similar flooding in units 2 and 4 is being checked; it is likely radioactive too.

Q. Where did this radioactive water come from?

A: Plant officials and government regulators say they don't know. It could come from more than one source: A leaking reactor core, associated piping or a spent fuel pool, of which there are seven. Officials won't even rule out the idea that it may have come from overfilling the pools with emergency cooling water. The flooding is deepest at Unit 3 - where it is waist-high - and may have flooded basement areas at other buildings. Equipment in Unit 3 could have sprung a leak on March 14, when a powerful hydrogen gas explosion blew apart that unit's reactor building.

Q. Does the leaking water make meltdown more likely?

A. Probably not at the current rate of leakage. The fuel rods inside units 1, 2 and 3 are believed to be partially melted already. However, with desperate efforts to keep the units cool using sea water, temperatures in recent days have stayed well within a safe zone at all the reactors. If these conditions prevail, there will be no further core melting, despite leakage. The temperatures of the spent fuel pools also have been under control, but occasional spikes have spurred worry.

Q. So why should we care about the leaking?

A. For one thing, it puts more radioactive contamination into the local environment, probably mostly into the ground and sea. It could add more radiation exposure to people near the plant. It has stalled work to restore in-house cooling systems needed to keep the plant safe in the long term. Perhaps the leaking comes from cooling equipment that must be repaired before other work can advance.

Q. When will they get the plant back on a sure footing?

A. It now appears certain to take days, possibly even weeks, to bring Fukushima Dai-ichi under complete control with normal cooling systems in place. And until that happens, the twin specters of total meltdown and spent fuel pool fire will shadow the work and the Japanese nation.