Daily we watch the unfolding story at Fukushima and look for signs the progression of the events there have stabilized. Clearly, recovery, cleanup, and remediation there and throughout the remainder of Japan will continue for a long, long time – even under the best of circumstances,
This set me to thinking about other Fukushima-like "beyond design basis" event scenarios that could challenge the integrity of our nuclear fleet. First, I want to define what I mean by "Fukushima-like".
On March 11, Fukushima Dai-Ich Units 1-3 were operating, and the other three units were shutdown for maintenance and refueling. The "top event" at Fukushima was a Richter 9 earthquake. The safe shutdown earthquake (SSE) for the Fukushima plant was a Richter 8.2 event. Thus the Fukushima quake was a "beyond design basis event".
Within an hour after the quake, a hugh (latest estimate is 14 meters high) tsunami struck the northeastern coast of Japan, visiting death and further devastation on the entire region. I understand from media reports that the Fukushima Dai-Ichi plant was designed for "only" a 6 meter tall tsunami. Another beyond-design-basis event.
Fukushima Dai-Ichi was at that moment damaged, and isolated from outside help - on it's own to deal with the impacts of these two "beyond design basis" events.
It is the combination of the impact of two beyond design basis events, coupled with the utter devastation visited on the entire site and the surrounding region, that defines a "Fukushima-like" event in my mind. Such an event leaves the reactor damaged, constrained by damage to its sister units on the plant site, and isolated from the surrounding region which is, in any event, unable to render assistance to the plant. What a nightmare.
Nuclear power plants are remarkably robust facilities, incorporating multiple transmission lines to provide off-site power, multiple backup station diesel generators in the event all of the transmission lines are severed, and multiple station batteries capable of allowing the plant to remain safely shut down for several hours should the station diesels fail. But the unique characteristic of nuclear power is that one cannot simply "shut it off". Decay power levels in nuclear reactors remain at the "few tenths of a percent" level for a few months after the reactor is "scrammed". Ultimately, long-term accident management depends on the ability of the outside world to provide assistance to recover the decay heat removal function - for both the reactors and any spent fuel stored in fuel pools onsite – before the fuel is significantly damaged. This didn't happen in Fukushima Dai-Ichi 1-3.
So, forgetting "probabilities of occurrence" for a moment, what are other "Fukushima-like" events? In addition to earthquakes and tsunamis, I can only think of two events that meet my definition: hurricanes and solar superstorms. We are all familiar with the impact of hurricanes. Our coastal nuclear power plants (those subject to hurricanes and tsunamis) typically stage backup equipment many miles inland with arrangements for delivery in the event of a major emergency.
Fewer people are aware of the potential for natural solar superstorms (such as the 1859 Carrington Event) to wreck havoc on our entire electrical grid - right down to the electrical systems in our cars, cell phones, radios, etc. According to reliable reports, during the 1859 event, telegraph systems all over North America and Europe failed. Many telegraph operators received harmful shocks. Telegraph systems burst into flames, and some continued to work when disconnected from their battery-driven power sources (see Wikipedia http://en.wikipedia.org/wiki/Solar_storm_of_1859. Given the nature of our electric grid and our electrified society, it's hard to comprehend how such an event would impact life as we know it today – with or without nuclear power plants. Probably deserves some thought...
No nice "wrap-up" on this. Just thinking...