If such disaster will happen, your main concern would not be your 12 word seed for a bitcoin wallet, but:

• shelter, water, food
• security, safe place to go
• anything else will be totally useless

So work better on your "bug out" scenario and be prepared.

• move out from big cities. When something like that happen, big cities are first to fall into chaos. Zombies will kill you on the street for a bottle of water.
• make your house a stronghold
• have prepare food and water for at least 1 month
• diesel generator, batteries, solar etc.
• prepare your "plan B" of escape in case your house is compromised, so have a secondary place where you already stockpiled food, water, defensive weapons.

Bitcoin will not help you at all with all the beheaded chickens out there...