Fukushima Dai-ichi Unit 1: Risky piece inside a wicked problem

Tepco has released a set of flow charts and simple diagrams called the Roadmap, which lay out a broad summary of its plan to get control of Fukushima Dai-ichi sometime in 2011.

In tackling this wicked problem Tepco plans to fill the reactor pressure vessel (RPV) of Unit 1 with enough water to cover the fuel assembly fully. It's considered a priority because the core of Unit 1 is in the worst shape of all the Dai-ichi reactors, with more than two-thirds of its fuel rods damaged.

See this WSJ article on an engineering dispute about whether the damaged structure of Unit 1 is strong enough to take the full weight of water planned.

(Also, as I mentioned in this post, Unit 1 is unique because the pressure has steadily been rising in its RPV. It now registers 1.12 megapascals -- that's 162 pounds per square inch -- on Pressure Gauge B, which is five times the reading in mid-March and much higher than the pressure in the other damaged reactors. It doesn't mean Unit 1's RPV is in a dangerous state but it ranks as an anomaly.)
 
Since a major roadblock to human intervention is radioactive water in the basements and nothing much can be done until the source is cut off and water is pumped out, Tepco's hope is that repairs in the near future can stop this coolant from dumping into the basement; Tepco wants to keep the reactor coolant in a closed circuit, with heat transferred to the air rather than was water going into basements or the ocean.

Meanwhile, Tepco has been posting still and video shots on this website. The pace of added material has slowed since April 20 but there is good material, lately from cameras on board the T-Hawk ducted-fan drone and ground robots.

Putting Unit 1 in the limelight requires separating it from the others. First, here's the big picture, from the Wikipedia website on the Fukushima crisis. Before the damage they looked like this, from above:

From AsiaCorrespondent.com, an airplane view:

From Wikipedia, the post-explosion view. Unit 1 is the boxlike structure on the far right of the four shown in Reactor Row.

It can be hard to keep them straight in news clips -- I've seen captions that confuse Unit 4 with Unit 1 -- so it helps to remember that Unit 1 has a different appearance from the rest. From most angles,  the lower walls of Unit 1 appear to be mostly intact while the upper walls are missing, leaving only the structural steel showing.

By contrast, Unit 2 still has its walls, and Units 3 and 4 look mostly like skeletons.

Next is a T-Hawk drone-cam view of Unit 1's roof. Presumably the explosion wrecked the deck's trusswork and wall connections and it collapsed onto the upper levels of the massive concrete structure. The roof will be one of many complications in dealing with the spent fuel pool. I'm not sure what the big brown cylinder is on the right -- maybe a heat exchanger?

Here's a clearer view of Unit 1, with Unit 2 on the left:

Here's the control room of Unit 1. Something,  either the earthquake or a later blast, caused all the panels in the false ceiling to come loose. The control room is in a location other than the reactor building.