NHK offers an interesting reference to unit 5.
NHK: TEPCO studied huge tsunamis in in-house training http://www3.nhk.or.jp/daily/english/20120613_21.html
[Excerpted] Tokyo Electric Power Company says it failed to make use of an in-house study that estimated the extent of damage by huge tsunamis on nuclear reactors.
A group of TEPCO employees conducted a study in 2006 to determine what would happen at the plant's No.5 reactor if it was hit by waves higher than 5.7 meters, the maximum height assumed by the company.
The group estimated that if the waves exceeded 13.5 meters, all power would be lost and it would be impossible to inject water into the reactor.
...But on both occasions, the company failed to make use of the studies' results and did not take any measures against possible disasters."
Majia here: Is this story an indirect strategy for informing us that reactor 5 was damaged?
We have had no official reports for a very long time about the status of reactor 5. Early reports suggested flooding of that unit's basement.
The NRC transcripts had implied that all 6 reactors might melt-down - go to the "final conclusion"
Unit 5 was the same type of Mark 1 reactor as units 1-4. Today I did some research about the plant.
Here is a brief summary of the data contained in an interesting WSJ article (cited below):
The Fukushima Daiichi nuclear power station was Tepco’s first nuclear plant. The plant was built in the 1960s by Ebasco, an American general contractor that no longer exists. Reactors one through five at the site were based on General Electric’s Mark I design.
Kiyoshi Kishi, a former Tepco executive heading Tepco’s nuclear plant engineering was interviewed by the Wall Street Journal about the lack of tsunami precations at the plant. Kishi was quoted as stating that at the time the plant was built, a threat posed by a large tsunami at the site was considered “impossible.”
Later, some precautions were taken to protect the plant against a tsunami with a height of 18.8 feet. The tsunami that hit the plant on March 11 was over twice that height. Tepco engineers interviewed by the Wall Street Journal also reported that the venting systems in reactors one through five were very inefficient.
Flooding of the generators and the poor ventilations systems are cited in the article as the causes of the meltdowns and explosions at the Daiichi units one through four.
The four reactors at the Fukushima Daini complex and building six at the Daiich complex used General Electric’s Mark II system and were purportedly tailored more specifically to meet Japan’s earthquake and tsunami risks.
These reactor buildings at Daini were reported in this article as having shut down safely. However, a status report issued by the IAEA on May 5, 2011, by Deputy Director General and Head of Department of Nuclear Safety and Security, Denis Flory reported that as of April 21st, the exclusion zone around Fukushima Daini plant was reduced from 10 kilometers to 8 kilometers. It is not clear why an exclusion zone around Daini was maintained if the reactors there all shutdown safely.
Norihiko Shirouzu and Chester Dawson. Design Flaw Fueled Nuclear Disaster. The Wall Street Journal (2011, July 1), A1, A12.