Why is Fukushima Daini NPP fine while neighboring Daiichi NPP has 3 damaged reactors?

Corrections 28 May: After reviewing the 87-slide TEPCO presentation [PDF] from 24 May I think the main conclusion of this post is incorrect. The tsunami impact on the two sites was different. Ignoring any issues of design errors, my take away is this:

1. I don’t think the updated Daini BWR designs were decisive in the relative outcomes of the two NPP. It isn’t clear to me how the newer designs handled the seismic accelerations — did Daiichi suffer greater damage in critical components? The TEPCO slide #1 compares the earthquake ground motion recordings for the two plants. Daini accelerations were smaller – how significant were the differences?

2. It is clear that the tsunami flooding was significantly different. Daini experienced 14M only on the south side of unit 1 but overall the site was subjected to a 7M inundation. Secondly the flooding depths were less at Daini being sited 12 meters above sea level (O.P.) vs Daiichi 10 meters above sea level (O.P.) Is the 7M vs. 14M local water height difference because the Daini seawall was more effective?

3. The bottom line of course is that Daini retained enough electrical power to operate reactor cooling: one (of four) off-site power lines survived, and 3 of 12 backup diesel generators. We don’t know how Daiichi would have fared if the same level of power supply for cooling had survived there.


Before I read Steve Packard’s The Other Fukushima Nuclear Power Plant, it had not occurred to me to compare the two neighboring Fukushima plants. I created the above public Google Map if you wish to study the siting of the two nuclear parks.

Daini NPP is only 6 miles north of Daiichi NPP. Both plants were struck by the monster earthquake and 14-meter tsunami. The Daiichi NPP suffered three (of six) damaged reactors whereas all of Daini’s four reactors are safely in cold shutdown. What is the difference? Here is an excerpt from Steve’s homework:

(…) Why Daini survived the quake and tsunami so much better than Daiichi:

There’s really only one glaring difference between Fukushima Daiichi and Fukushima Daini: the vintage of the nuclear technology of the plants.    While Fukushima Daiichi was built with reactor designs from the late 1960’s and early 1970’s, Fukushima Daini was built with technology of the early to mid 1980’s.

A comparison of the reactors at Fukushima Daiichi and Daini:

(…) The conclusion that one can draw from the events at the two Fukushima plants is relatively straight forward: While the older BWR-3 and BWR-4 designs are sufficiently safe in most situations, their designs are nowhere near as robust and reliable as newer reactor designs.   Of course, despite holding up so well against forces far beyond what designers had planned, the BWR-5 is, by today’s standards, old technology.   Newer reactors are much safer still and have even more reliable passive-based safety features.

This is all the more reason why we should be building more nuclear plants.   As newer reactors are built we will be able to eventually shut down the older reactors, thus improving economics and safety even further.   The events in Japan do not diminish the picture of safety we have when it comes to new reactors. Rather than assuming that reactors will fail in the manner that they did at Fukushima Daiichi, we should consider how well they held up at Fukushima Daini.   This is far more representative of new reactor designs, although those are even safer and more reliable still.

Read the whole thing – including the comments.