I wonder if a water-filled jacket would be viable. When not in use it would be deflated and rolled up and stored. Between the bomb going off and the fallout arriving, you fetch it out, unroll it, wrap it around your inner sanctum and then fill it with water. If you have a generator and a swimming pool and an electric pump, you'd be self-contained.

The jacket would need to be a few feet thick and quite big, and very strong to support the water. It's harder to engineer than an above-ground swimming pool, but should be doable. The water could be drunk after the fallout has decayed away.

My own plan, such as it is, is to build a lean-to against an inner wall and then pile books against it. I seem to have a surfeit of books. I don't have a family, though.