(UPDATE: Operation started late, and it is still on-going as of 11:24AM Pacific Standard Time in the US. See my post.)
The 3.3-ton In-Vessel Transfer Machine (IVTM) dropped back inside the Reactor Vessel of Monju Fast Breeder Reactor in August last year, when they were finishing up on exchanging the fuel rods. The past 2 attempts to pull out the IVTM were unsuccessful.
This time, by removing the "sleeves" (part of the lid) with the 12-meter-long IVTM, they think they can pull the whole thing out and without any air entering the reactor.
According to Fukui Shinbun (6/22/2011), they will pull out very slowly (6 centimeters/minute), and the entire process is expected to last 8 to 9 hours.
The coolant of this reactor is sodium, which burns on contact with air. The reactor uses MOX-fuel.
Information from Monju site on how to pull out the In-Vessel Transfer Machine (blue rod in the diagram):
TV Asahi did a documentary on Monju in June, in which you get to see the inside of Monju:
増殖炉事故 [もんじゅ] 現場で何が・・・内部取材 by sean2010jp
About 3 minutes and 30 seconds into the documentary, a local councilman shouts at the Monju operators after the sodium leak and fire accident in 1995: "A courage to climb the mountain? You need a courage to quit, you idiot!"
About 10 minutes into the documentary, you get to see the inside of the Containment Vessel of Monju. Brightly lit, cavernous hall. We cannot assume the safety of the reactor, but if there is no radiation leak, it means it is safe, says the engineer.
17, 18 minutes into the documentary, you see the public buildings built by Tsuruga City with the money from the government for having nuclear reactors. The documentary then shows the local fishermen reluctant to say anything about Monju. One elderly man says "Safe, of course it's safe." One elderly woman says "I can't answer anything", when asked about the nuclear accident in Fukushima.