Fukushima 1 Nuclear Accident Day 15, 16, 17

Friday, 25 March

Units 1, 2 and 3 had possible breaches in their containment vessels. NISA announced that a breach had likely occurred in the containment vessel of the unit 3 reactor, the only one using a plutonium fuel mix (MOX). Officials suspected it was leaking radiation. Kyodo News reported highly radioactive water was found in the turbine buildings of unit 1 and 2.

US Navy began the transport by barge of 1,890 cubic meters (500,000 gallons) of fresh cooling water to the Fukushima reactors. This fresh water supply is expected to arrive on site within two days.

Japan announced transportation would be provided in a voluntary evacuation zone of 30 kilometres (19 mi).

Tap water was reported to be safe for infants in Tokyo and Chiba but still exceeded limits in Hitachi and Tokaimura.

An analysis of stagnant water in the basement floor of the turbine building of Unit 1 showed heavy contamination.

Nuclide	Concentration (Bq/ml)
38 Cl	1.6×106
74 As	3.9×102
91 Y	5.2×104
131 I	2.1×105
134 Cs	1.6×105
136 Cs	1.7×104
137 Cs	1.8×106
140 La	3.4×102

Saturday, 26 March

Replacing seawater with fresh water for cooling became a priority due to worries that the salt in the seawater was clogging pipes and coating fuel rods, which may have been hindering the cooling process.

It was reported that radiation levels in water in the unit 2 turbine room measured 1 Sv/h, or 10 million times the normal radioactivity of water circulating in an operating reactor, and four times the yearly level allowed for workers. This has since been denied by Japanese officials as a misread by one of the employees working inside the plant, prompting all of the workers to flee before a second reading could be taken to confirm.

Sunday, 27 March

The IAEA announced that workers hospitalized for treatment of radiation burns on Friday had been exposed to between 2 and 6 Sv of radiation at their ankles when standing in water in unit 3.

TEPCO reported measurements of very high radiation levels of 1000 mSv/hr in the basement of the unit 2 turbine building, which officials reported was 10 million times higher than what would be found in the water of a normally functioning reactor. Hours into the media frenzy, the company retracted its report and stated that the figures were not credible. "because the level was so high the worker taking the reading had to evacuate before confirming it with a second reading." Shortly following the ensuing wave of media retractions that discredited the report worldwide, TEPCO clarified its initial retraction; the radiation from the pool surface in the basement of the unit 2 turbine building was found to be "more than 1,000 millisieverts per hour," as originally reported, but the concentration of radioactive substances was only 100,000 times higher than usual, not 10 million. The high radiation levels delayed technicians working to restore the water cooling systems for the troubled reactors.

Japan's Nuclear and Industrial Safety Agency indicated that "The level of radiation is greater than 1,000 millisieverts. It is certain that it comes from atomic fission. But we are not sure how it came from the reactor."

The IAEA reported temperatures at the bottom of the Reactor Pressure Vessel (RPV) at Unit 2 fell to 97 °C from 100 °C on Saturday. Water is being pumped from the turbine hall basement to the condenser in order to allow power restoration activities to continue.

New aerial video recorded on 27 March by a Ground Self-Defense Force helicopter revealed the clearest and most detailed view of the damaged plant to date. Significant observations included:

• White vapor, possibly steam, emanating from the buildings of reactors 2, 3, and 4.
• The roof of the reactor 2 building has been badly damaged but is still intact.
• The reactor 3 building is largely uncovered, its roof blown off in a hydrogen explosion over two weeks prior.
• The walls of the reactor 4 building have also collapsed.