Units 1 and 2: TEPCO has released estimates of the levels of core damage at these two reactors: 70% damage at Unit 1 and 33% at Unit 2. They have also stated that Unit 1 is being adequately cooled.
Outlook: It is difficult to make conjectures at this point about the final disposition of the damaged fuel without further information. However, during our only operating experience with a partially melted and subsequently cooled core, Three Mile Island, the fuel mass was fully contained by the reactor vessel, resulting in minimal radiation release to the public. A decision is currently being made on how to best supply cooling water to Unit 2.
Unit 3: At 8:34 AM JST, white smoke was seen billowing from the roof of Unit 3. The source of this smoke was not investigated because workers were evacuated due to radiation levels. These levels had been fluctuating during the early morning hours before rising to 300-400 millisievert/hr around the time that the smoke appeared. It was unclear at the time whether these rising levels were a result of some new event at Unit 3, or were lingering as a result of Unit 2’s recent troubles.
Outlook: In order to provide some perspective on worker doses to this point, radiation sickness sets in at roughly 1000 millisieverts. A future post will deal further with the health effects of various amounts of radiation. Response to the smoke seen at Unit 3 appears to be in an information gathering phase at this point. Chief Cabinet Secretary Yukio Edano speculated that the smoke from Unit 3 might be the result of a similar wetwell explosion to that at Unit 2, but there is not enough information currently available to support or refute that statement.
Units 4-6: Flames at Unit 4 were reported to be the result of a pump fire, which caused a small explosion that damaged the roof of Unit 4 (See TEPCO’s press release on the most recent fire at http://www.tepco.co.jp/en/press/corp-com/release/11031606-e.html) . Efforts at Units 4-6 are focused on supplying cooling water to the spent fuel storage pools. Temperatures in these pools began to rise in the days after the quake. At the time of the quake, only Unit 4’s core had been fully offloaded to the spent fuel pool for maintenance; roughly 1/3 of the cores of Units 5 and 6 had been offloaded. This explains in part why the temperature in Unit 4’s pool has risen faster than at the other reactors: it has a higher inventory, both in fuel volume and in heat load.
Outlook: The fuel within these pools needs to remain covered with cooling water in order to prevent the low levels of decay heat present from causing it to melt, and also in order to provide shielding. Boiling of the water results in reduction of the water level in the pools, so if/when the pools get hot enough for boiling to begin, water needs to be added to replace what boils off. The staff of Unit 4 plan to begin pumping water to the spent fuel pool from ground level as soon as radiation levels from Unit 3 are low enough for them to return. This pumping operation should be relatively easier than injection of cooling water into the reactor vessels at Units 1-3 because the pools are at atmospheric pressure.
Sources: TEPCO, World Nuclear News
UPDATE (11:48 AM EST): A report by the Federation of Electric Power Companies of Japan indicates that radiation levels as a result of the Unit 4 fire were higher than those reported previously. Radiation levels early this morning at the outside of Unit 3 measured at 400 millisieverts/hr. At the present time however, radiation levels at the boundary of the facility are 1530 microsieverts/hour. We will continue to update as further reliable information is available.