More water was sprayed onto the unit no. 3 spent fuel pond and efforts are underway to connect reactor no. 2 to the power grid. Manpower at the plant was ramped up with some 130 operational staff and 320 people now clearing pathways and laying new power lines. 130 firefighter from Tokyo also joined. No other incidents or efforts from the Fukushima Daiichi plant were reported since yesterday.

But what is up with unit no. 4?

Satellite picture March 18 by Digital Globe
Unit no. 4 to no. 1 from left to right
bigger version

I have been harping on over unit no. 4 for days now. I see immediate danger there.

The Japanese authorities have said little about it. There was a fire at no. 4 on the 15th they said, at some pump, and another short fire soon after the first one was out. There were two holes in the reactor building walls they said, 26 square foot each. Yesterday they even downgraded the INES classification of no. 4 from 4 to 3 while upgrading the damage classification of the Daiichi reactors 1 to 3 from 4 to 5. Instead of a "serious accident without significant off-site risk" no. 4 is now only a "serious incident" they claim. Cooling efforts for the spent fuel pond at no. 3 were again made while no efforts are made to cool the pond at no 4.

But all of that can not be right. There ain't just two holes in the unit no. 4 walls as TEPCO initially reported.

Photos and video show all outer metal and reinforced concrete walls down to the first floor severely broken and damaged after the "fire". Between the 16th and 17th the metal roof plating above the unit vanished according to satellite pictures (see yesterday's status post). Brown smoke is coming from the building. No explanation at all has been offered for all of this to happen. The TEPCO reports ignore it. Since yesterday 16:00 local time the Japanese Atomic Industry Forum report table (update 12) says for no. 4 without any further explanation: "Hydrogen from the pool exploded". But the Kyodo News Agency now reports on no. 4:

Renewed nuclear chain reaction feared at spent-fuel storage pool

The spent fuel pond at no. 4 holds about 1,500 fuel assemblies with a total mass of some 250 tons of Uranium fuel. The reactor no. 4 was temporarily unloaded for maintenance in November 2010. Therefore most of the fuel in the pond is not spent but rather fresh and radioactive. Cooling at the pond has ended a week ago right after the quake. The building exploded despite a non-active reactor within it. Why and how did that happen if not for some nuclear incident at the fuel pond?

Usually fuel in a spent fuel pond is moderated and prevented from fission by boron plates between the stored fuel rod assemblies and/or by boron in the cooling water. But if the fuel heats up due to its continuing radioactive decay it could melt and assemble an unmoderated critical mass at the bottom of the pool. Such a mass going back into fission would generate, aside from a lot of radioactivity, enough temperature to eventually burn through the concrete below and would end up in the environment after meeting water underground and exploding it into steam. This was one of the main dangers in Chernobyl and an enormous amount of work was done and serious deadly risk was taken to prevent that.

Today the New York Times writes:

[A] senior Western nuclear industry executive said Friday that there also appeared to be damage to the floor or sides of the spent fuel pool at Reactor No. 4, and that this was making it extremely hard to refill the pool with water. [..]

Engineers said Thursday that a rip in the stainless steel lining of the pool at Reactor No. 4 and the concrete base underneath it was possible as a result of earthquake damage. The steel gates at either end of the storage pool are also vulnerable to damage during an earthquake and could leak water if they no longer close tightly.

The senior executive, who asked not to be identified because his comments could damage business relationships, said that a leak had not been located but that engineers had concluded that it must exist because water sprayed on the storage pool had been disappearing much more quickly than would be consistent with evaporation.

The head of the U.S. nuclear regulator said publicly that the pool at no. 4 was dry. The NYT report tells why that is the case. TEPCO and the Japanese government are very quiet about no. 4. They probably do not know what to do about it. But what to do is easy. Doing it is dangerous though.

A heavy slurry of sand, boron and water must be put into the pond. This would shield the environment from radiation and likely prevent any fission. Drying slurry would probably even seal some of the holes in the pond. Several hundred tons would be needed.

This can be done from helicopters, as it was done in Chernobyl. It would be very dangerous for the pilots but that can not be helped. Alternatively high reaching concrete pumps could be used to deliver the slurry into the pond from the ground below. This would probably also cost the health and even life of emergency workers working near the reactor. Again, this can not be helped.

There are signs that TEPCO and the Japanese might finally start to get it:

In a further sign of spreading alarm on Friday that uranium in the plant could begin to melt, Japan planned to import about 150 tons of boron from South Korea and France to mix with water to be sprayed onto damaged reactors, French and South Korean officials said Friday.

They "plan to import". The boron was needed and offered days ago. Spraying will not help with a leaky no. 4 pool. Slurry is needed. Get it. Get it done. Now.

Additional resources:
AllThingsNuclear Union of Concerned Scientists
Bulletin of the Atomic Scientists
Atomic power review blog
Digital Globe Sat Pictures
IAEA Newscenter
NISA Japanese Nuclear Regulator
Japan Atomic Industry Forum (regular updates)
Japanese government press releases in English
Kyodo News Agency
Asahi Shimbun leading Japanese newspaper in English
NHK World TV via Ustream
Status reports for the German Federal Government by the Gesellschaft für Reaktorsicherheit in German language