Good idea.  I encourage you to keep in mind, a storm shelter is totally different than a safe room.  A storm shelter is hours at most.  

Instead of a bucket thingy, take a look at porta potti.  They're way better, no smell.  

Next, battery operated TV so you can follow the storms progress.  During events, the news usually shows a continuous live radar.  

It's very common for you to lose all utilities and cell service overloads.  

Neat little thing I bought from Amazon is a rechargeable battery fan with light.   Bodies put out a lot of BTUs.   A vent in the ceiling to let heat out would also help but you can't count on power.  

Don't forge to re-enforce your ceiling.  

Tj

First question, pretty simple and kind of a "duh"- but there is someone out there reading this thinking they can do it on a crawlspace...

You are on slab right?

I have built "a few" structures with block, put rebar in each cell and poured solid with grout mix. It's a good way to go, will be bulletproof to at least 308 at 15 yards (I shot my house to test it 25 years ago after building). However I don't think you will get a high PF with it.

For ballistic security, damn good way to go. If your talking radiation you can make it work.

Normally to get higher PF with radiation you are looking at dirt-   18" will give you a PF of 40, 36" will give you a PF of 1,000. Plus a smidge for the poured block (roughly 8 inches solid concrete when done).

Always have another exit if possible.

For just a home "security shelter" type of thing- think about fire. Also look at ways to "assist" people to GTFO of your house while you are in there. There are ways to rig up pepper spray/other releases into the house from inside the shelter. "But Momma's drapes will smell like hell and she'll be pi$$ed!" She will get over it and she'll be alive to complain about it LOL.

Assuming power will still be available, wired (key word) cameras that you can access inside the shelter showing what's going on inside the house. I.e, are those 4 "Scandanvian youths" standing at the door of the shelter figuring out how to get in, or did they grab some of the costume jewelry left out along with the old muskets you left in that obvious "safe"?

You can go as far as two way mirrors, hidden firing ports, etc. but you will lose ballistic integrity with those.

One thing I would always do in FOF CQB was see if those entering would leave a security guy- they usually never do. I can't tell you how many times I've circled a house and pop, pop, pop while a couple guys were fighting through the rest of the structure. A way to do that is very nice.

All this depends on how far you want to go with it. Being as you are considering a major part of a construction project, I figured it would be worth mentioning it.

Oh and get Joel Skousons "Survival Home" I think new editions called "The Secure Home". I think he has or did have a "high security shelters" book also. Dude is old skewl survivalist and architect, being doing this forever.