Hi there. I am not sure exactly what scenario you are looking for a solution to...

Well let's see. JAB is right to ask that question. To be the safest in all possible scenarios I guess you would need one of those converted missile silos in USA. Notice I said safest not safe. However everything is not going to happen all at once. So if you take the HIRA (hazard identification and risk assessment) approach see what scenarios are most likely then you will find that there are some shelter options that would offer a solution for multiple scenarios. It's a matter of what you think is probable. If you are worried about flooding do don't build in a flood plain and you should be safe. From a regular flood anyway, not the 2012 end of the world type flood. Prepping is very personal in nature and it depends on what think is important.

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This is just one suggestion.
I think a strong and inexpensive solution is a geodesic dome covered with a couple layers of barrier such as heavy plastic or tarp, then covered over with a foot or so of soil.
If you build like the examples on YouTube with lumber, one of these can be whatever size you want and able to hold up enough soil to protect from radiation.
If it's SHTF and you are binding together trees it can still be accomplished, but with cross binding and more layers.
Any way you can get a heavy layer of protection between you and the radiation will lessen the effects and give you time.
But as mentioned by the other posters here, so much is dependant on the individual situation.

Check this site out for shipping container homes.
http://residentialshippingcontainerprimer.com/conceive%20it

This book has some excellent suggestions on what constitutes a good shelter choice.

"How To Survive The End Of the World As We Know It by J. Wesley Rawles.

http://www.amazon.com/How-Survive-End-World-Know/dp/144186976X

I urge tyou buy this book. I got my copy from Chapters.

and this book

When All Hell Breaks Loose. by Cody Lundin also available at Chapters

These books will answer most of beginners questions about prepping.

I like "quietman"'s suggestion about geodesic domes. It can be built fast and has somewhat of a protection from the radiation with the layers of earth on top. It is nice to have a bunker/silo but those thing cost in the millions which alot of us wish we had but it is not realistic. Alberta has none of those facility built or even for sale. I asked a bunker builder a couple of weeks ago and from my designs he quoted me the cost of 4-6million for only 20-30 people capacity for an underground shelter/bunker. The cost of $100k-300k is more achievable with a group of people but not millions.

While researching on geodesic domes I found out about monolithic domes, which is supposed to be even better. Then I came along and found out about Earthships. Now that seemed perfect, if we all join in and build afew earthships, that would be a very good place to bug out, self sustaining house with indoor garden and all the good stuff and electricity from wind or solar.

the cost can be as much as a regular house or half the price doing your own labor on the main construction. This is a pretty neat find.

Yes I like the Earthship too, but your concearn was nuclear fallout from blown reactors. I think the earthship would lack protection from fallout.
What I think could work is having a dome ready in pieces that could be put together over the earthship once you know fallout is coming. Any layer of protection is a good layer, if built right, soil could be pushed over the temporary dome to further protect you and yours while you wait it out in the earthship.
This same temp dome could be used for a winter shed for keeping things out of the weather, a wind break for the garden or patio etc.
It wouldn't have to sit unused, but be ready for it's intended and hopefully not needed use.